ORCID Profile
0000-0003-4998-7259
Current Organisation
University of Leeds
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: AIP Publishing
Date: 21-03-2013
DOI: 10.1063/1.4795606
Abstract: We report three-well, resonant-phonon depopulation terahertz quantum cascade lasers with semi-insulating surface-plasmon waveguides and reduced active region (AR) thicknesses. Devices with thicknesses of 10, 7.5, 6, and 5 μm are compared in terms of threshold current density, maximum operating temperature, output power, and AR temperature. Thinner ARs are technologically less demanding for epitaxial growth and result in reduced electrical heating of devices. However, it is found that 7.5-μm-thick devices give the lowest electrical power densities at threshold, as they represent the optimal trade-off between low electrical resistance and low threshold gain.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2020
Publisher: Optica Publishing Group
Date: 13-04-2021
DOI: 10.1364/OE.414178
Abstract: We report on both experiments and theory of low-terahertz frequency range (up to 400 GHz) magnetoplasmons in a gated two-dimensional electron gas at low ( K) temperatures. The evolution of magnetoplasmon resonances was observed as a function of magnetic field at frequencies up to ∼400 GHz. Full-wave 3D simulations of the system predicted the spatial distribution of plasmon modes in the 2D channel, along with their frequency response, allowing us to distinguish those resonances caused by bulk and edge magnetoplasmons in the experiments. Our methodology is anticipated to be applicable to the low temperature ( K) on-chip terahertz measurements of a wide range of other low-dimensional mesoscopic systems.
Publisher: Springer Science and Business Media LLC
Date: 06-01-2016
Publisher: Springer Science and Business Media LLC
Date: 18-11-2014
DOI: 10.1038/SREP07083
Publisher: The Optical Society
Date: 04-12-2018
DOI: 10.1364/OL.43.005933
Publisher: Institution of Engineering and Technology
Date: 2018
DOI: 10.1049/CP.2018.0607
Publisher: The Optical Society
Date: 30-07-2019
DOI: 10.1364/OE.27.023164
Publisher: The Optical Society
Date: 20-11-2019
DOI: 10.1364/OL.44.005663
Publisher: AIP Publishing
Date: 28-10-2013
DOI: 10.1063/1.4827886
Abstract: We demonstrate coherent terahertz (THz) frequency imaging using the self-mixing effect in a quantum cascade laser (QCL). Self-mixing voltage waveforms are acquired at each pixel of a two-dimensional image of etched GaAs structures and fitted to a three-mirror laser model, enabling extraction of the litude and phase parameters of the reflected field. From the phase, we reconstruct the depth of the s le surface, and we show that the litude can be related to the s le reflectance. Our approach is experimentally simple and compact, and does not require frequency stabilization of the THz QCL.
Publisher: The Optical Society
Date: 27-06-2014
DOI: 10.1364/OE.22.016595
Publisher: Optica Publishing Group
Date: 16-11-2021
DOI: 10.1364/OE.437861
Abstract: In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal — well known in interband diode lasers — was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback ( C ≤ 1) to fringes modulated by SPs under moderate feedback (1 C ≤ 4.6), and then [under strong feedback ( C 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.
Publisher: IEEE
Date: 09-2014
Publisher: AIP Publishing
Date: 15-10-2018
DOI: 10.1063/1.5051580
Abstract: Current noise fluctuations have been investigated in terahertz (THz) quantum well photodetectors embedded in antenna-coupled photonic architectures and compared with standard substrate-coupled mesa detectors. The noise measurements give a value of the photoconductive gain that is in excellent agreement with that extracted from previous responsivity calibrations. Moreover, our results confirm that the noise equivalent power (NEP) of the antenna-coupled devices is of the order of 0.2 pW/Hz0.5. This low NEP value and the wide band frequency response (∼GHz) of the detectors are ideal figures for the development of heterodyne receivers that are, at present, a valuable technological solution to overcome the current limitation of THz sensors.
Publisher: IOP Publishing
Date: 28-08-2014
Publisher: AIP Publishing
Date: 05-01-2015
DOI: 10.1063/1.4905338
Abstract: We have developed terahertz frequency quantum cascade lasers that exploit a double-periodicity distributed feedback grating to control the emission frequency and the output beam direction independently. The spatial refractive index modulation of the gratings necessary to provide optical feedback at a fixed frequency, and simultaneously, a far-field emission pattern centered at controlled angles, was designed through use of an appropriate wavevector scattering model. Single mode terahertz (THz) emission at angles tuned by design between 0° and 50° was realized, leading to an original phase-matching approach for highly collimated THz quantum cascade lasers.
Publisher: The Optical Society
Date: 10-03-2015
DOI: 10.1364/OL.40.000994
Publisher: AIP Publishing
Date: 11-2021
DOI: 10.1063/5.0065591
Abstract: We present a mid-IR (λ ∼ 8.3 μm) quantum well infrared photodetector (QWIP) fabricated on a mid-IR transparent substrate, allowing photodetection with illumination from either the front surface or through the substrate. The device is based on a 400 nm-thick GaAs/AlGaAs semiconductor QWIP heterostructure enclosed in a nanostructured metal–insulator–metal cavity and hosted on a mid-IR transparent ZnSe substrate. Metallic stripes are symmetrically patterned by e-beam lithography on both sides of the active region. The detector spectral coverage spans from λ ∼ 7.15 to λ ∼ 8.7 μm by changing the stripe width L—from L = 1.0 to L = 1.3 μm—thus frequency-tuning the optical cavity mode. Both micro-FTIR passive optical characterizations and photocurrent measurements of the two-port system are carried out. They reveal a similar spectral response for the two detector ports with an experimentally measured TBLIP of ∼200 K.
Publisher: IEEE
Date: 06-2019
Publisher: Springer Science and Business Media LLC
Date: 05-2019
DOI: 10.1038/S41377-019-0152-Z
Abstract: Random lasers are a class of devices in which feedback arises from multiple elastic scattering in a highly disordered structure, providing an almost ideal light source for artefact-free imaging due to achievable low spatial coherence. However, for many applications ranging from sensing and spectroscopy to speckle-free imaging, it is essential to have high-radiance sources operating in continuous-wave (CW). In this paper, we demonstrate CW operation of a random laser using an electrically pumped quantum-cascade laser gain medium in which a bi-dimensional (2D) random distribution of air holes is patterned into the top metal waveguide. We obtain a highly collimated vertical emission at ~3 THz, with a 430 GHz bandwidth, device operation up to 110 K, peak (pulsed) power of 21 mW, and CW emission of 1.7 mW. Furthermore, we show that an external cavity formed with a movable mirror can be used to tune a random laser, obtaining continuous frequency tuning over 11 GHz.
Publisher: The Optical Society
Date: 04-11-2019
DOI: 10.1364/OE.27.033768
Publisher: Optica Publishing Group
Date: 03-09-2020
Abstract: The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of <!-- 1 H z , robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range ± 0.3 π . These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.
Publisher: The Optical Society
Date: 25-01-2019
DOI: 10.1364/OE.27.002248
Publisher: IEEE
Date: 07-2016
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 08-2015
Publisher: Wiley
Date: 23-12-2019
Publisher: Optica Publishing Group
Date: 03-02-2020
DOI: 10.1364/OE.380656
Abstract: The quantum cascade laser is a powerful solid-state source of terahertz-frequency radiation. However, integrating multiple photonic functions into a monolithic platform in this frequency range is non-trivial due to the scaling of photonic structures for the long terahertz wavelengths and the low frequency tuning coefficients of the quantum cascade lasers. Here, we have designed a simple terahertz-frequency photonic integrated circuit by coupling a racetrack resonator with a ridge laser in the longitudinal direction to design a notch filter. The transmission properties of this filter structure are dependent on the phase matching and losses in the coupled racetrack and results in a comb of stopband frequencies. We have optimized the comb separation by carefully selecting the cavity dimensions of the racetrack resonator to suppress longitudinal modes in the ridge laser enabling single-mode emission. The emission frequencies and output power from laser are controlled through appropriate control of drive currents to the ridge and the racetrack resonator. The emission frequency is electrically tuned over ∼81 GHz exploiting Stark shift of the gain as a function of drive current at the ridge laser, coinciding with an output power variation of ∼27% of the peak power (at a heat sink temperature of 50 K). The output power from the ridge also varied by ∼30% and the frequency was tuned by a further 10 GHz when the driving conditions at the ridge laser are invariant and the current at the racetrack resonator was varied. To our best knowledge, this is the first report of a frequency engineering, tuning and power modulation of terahertz-frequency quantum cascade lasers using a photonic integrated circuit.
Publisher: IEEE
Date: 09-2013
Publisher: The Optical Society
Date: 09-07-2019
DOI: 10.1364/OE.27.020231
Publisher: The Optical Society
Date: 27-11-2017
Publisher: Optica Publishing Group
Date: 21-07-2020
DOI: 10.1364/OE.395227
Abstract: A model based on carrier rate equations is proposed to evaluate the gain saturation and predict the dependence of the output power of a terahertz master-oscillator power- lifier quantum cascade laser (THz-MOPA-QCL) on the material and structure parameters. The model reveals the design rules of the pre lifier and the power extractor to maximize the output power and the wall-plug efficiency. The correction of the model is verified by its agreement with the experiment results. The optimized MOPA devices exhibit single-mode emission at ∼ 2.6 THz with a side mode suppression ratio of 23 dB, a pulsed output power of 153 mW, a wall-plug efficiency of 0.22%, and a low ergence angle of ∼6°×16°, all measured at an operation temperature of 77 K. The model developed here is helpful for the design of MOPA devices and semiconductor optical lifiers, in which the active region is based on intersubband transitions.
Publisher: Springer Science and Business Media LLC
Date: 09-04-2020
DOI: 10.1038/S41377-020-0294-Z
Abstract: Quasi-crystal distributed feedback lasers do not require any form of mirror cavity to lify and extract radiation. Once implemented on the top surface of a semiconductor laser, a quasi-crystal pattern can be used to tune both the radiation feedback and the extraction of highly radiative and high-quality-factor optical modes that do not have a defined symmetric or anti-symmetric nature. Therefore, this methodology offers the possibility to achieve efficient emission, combined with tailored spectra and controlled beam ergence. Here, we apply this concept to a one-dimensional quantum cascade wire laser. By lithographically patterning a series of air slits with different widths, following the Octonacci sequence, on the top metal layer of a double-metal quantum cascade laser operating at THz frequencies, we can vary the emission from single-frequency-mode to multimode over a 530-GHz bandwidth, achieving a maximum peak optical power of 240 mW (190 mW) in multimode (single-frequency-mode) lasers, with record slope efficiencies for multimode surface-emitting disordered THz lasers up to ≈570 mW/A at 78 K and ≈720 mW/A at 20 K and wall-plug efficiencies of η ≈ 1%.
Publisher: AIP Publishing
Date: 07-03-2011
DOI: 10.1063/1.3560980
Abstract: We exploit the modal confinement properties of metal-metal ridge waveguides to investigate the effect of reducing the thickness of the active laser cores in both terahertz and mid-infrared quantum cascade lasers. Devices with active regions over 55 times thinner than the free-space emission wavelength are demonstrated. They show only a modest increase in threshold current density compared with conventional-thickness devices. The limited increase in threshold is possibly due to a parasitic current channel in addition to the radiative current channel. These structures could be useful for the development of ultra-low volume lasers.
Publisher: American Chemical Society (ACS)
Date: 14-05-2020
Publisher: IEEE
Date: 09-2013
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 09-2014
Publisher: The Optical Society
Date: 03-07-2018
DOI: 10.1364/OE.26.018423
Publisher: SPIE
Date: 23-02-2013
DOI: 10.1117/12.2000743
Publisher: Springer Science and Business Media LLC
Date: 28-09-2018
DOI: 10.1038/S42005-018-0059-7
Abstract: Acoustoelectric devices convert acoustic energy to electrical energy and vice versa. Devices working at much higher acoustic frequencies than those currently available have potential scientific and technological applications, for ex le, as detectors in phononics experiments and as transducers in bulk acoustic wave filters at terahertz (THz) frequencies. Here we demonstrated an active acoustoelectronic device based on a GaAs heterostructure: an acoustically gated transistor or phonotransistor. Instead of being controlled in the conventional manner by an electrical signal applied to a metallic or semiconductor gate as in a high electron mobility transistor (HEMT), the drain-source current was controlled by a bulk sub-THz acoustic wave passing through the channel in a direction perpendicular to the current flow.
Publisher: IOP Publishing
Date: 22-08-2012
Publisher: American Chemical Society (ACS)
Date: 19-10-2015
Publisher: Springer Science and Business Media LLC
Date: 20-04-2023
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
Publisher: IEEE
Date: 09-2012
Publisher: Springer Science and Business Media LLC
Date: 08-08-2010
DOI: 10.1038/NMAT2822
Abstract: Surface plasmons have found a broad range of applications in photonic devices at visible and near-infrared wavelengths. In contrast, longer-wavelength surface electromagnetic waves, known as Sommerfeld or Zenneck waves, are characterized by poor confinement to surfaces and are therefore difficult to control using conventional metallo-dielectric plasmonic structures. However, patterning the surface with subwavelength periodic features can markedly reduce the asymptotic surface plasmon frequency, leading to 'spoof' surface plasmons with subwavelength confinement at infrared wavelengths and beyond, which mimic surface plasmons at much shorter wavelengths. We demonstrate that by directly sculpting designer spoof surface plasmon structures that tailor the dispersion of terahertz surface plasmon polaritons on the highly doped semiconductor facets of terahertz quantum cascade lasers, the performance of the lasers can be markedly enhanced. Using a simple one-dimensional grating design, the beam ergence of the lasers was reduced from approximately 180 degrees to approximately 10 degrees, the directivity was improved by over 10 decibels and the power collection efficiency was increased by a factor of about six compared with the original unpatterned devices. We achieve these improvements without compromising high-temperature performance of the lasers.
Publisher: The Optical Society
Date: 27-01-2016
DOI: 10.1364/OE.24.002174
Publisher: SPIE
Date: 27-01-2017
DOI: 10.1117/12.2251405
Publisher: AIP Publishing
Date: 07-03-2016
DOI: 10.1063/1.4943167
Abstract: We demonstrate strong light-matter coupling in three dimensional terahertz meta-atoms at room temperature. The intersubband transition of semiconductor quantum wells with a parabolic energy potential is strongly coupled to the confined circuital mode of three-dimensional split-ring metal-semiconductor-metal resonators that have an extreme sub-wavelength volume (λ/10). The frequency of these lumped-element resonators is controlled by the size and shape of the external antenna, while the interaction volume remains constant. This allows the resonance frequency to be swept across the intersubband transition and the anti-crossing characteristic of the strong light-matter coupling regime to be observed. The Rabi splitting, which is twice the Rabi frequency (2ΩRabi), amounts to 20% of the bare transition at room temperature, and it increases to 28% at low-temperature.
Publisher: Optica Publishing Group
Date: 26-05-2020
DOI: 10.1364/OE.391656
Abstract: We report on the design, fabrication and characterisation of large-area photoconductive THz array structures, consisting of a thin LT-GaAs active region transferred to an insulating substrate using a wafer-scale bonding process. The electrically insulating, transparent substrate reduces the parasitic currents in the devices, allowing peak THz-fields as high as 120 kV cm −1 to be generated over a bandwidth THz. These results are achieved using lower pulse energies than demanded by conventional photoconductive arrays and other popular methods of generating high-field THz radiation. Two device sizes are fully characterised and the emission properties are compared to generation by optical rectification in ZnTe. The device can be operated in an optically saturated regime in order to suppress laser noise.
Publisher: Springer Science and Business Media LLC
Date: 17-07-2012
DOI: 10.1038/NCOMMS1958
Abstract: Symmetric and antisymmetric band-edge modes exist in distributed feedback surface-emitting semiconductor lasers, with the dominant difference being the radiation loss. Devices generally operate on the low-loss antisymmetric modes, although the power extraction efficiency is low. Here we develop graded photonic heterostructures, which localize the symmetric mode in the device centre and confine the antisymmetric modes close to the laser facet. This modal spatial separation is combined with absorbing boundaries to increase the antisymmetric mode loss, and force device operation on the symmetric mode, with elevated radiation efficiency. Application of this concept to terahertz quantum cascade lasers leads to record-high peak-power surface emission (>100 mW) and differential efficiencies (230 mW A(-1)), together with low- ergence, single-lobed emission patterns, and is also applicable to continuous-wave operation. Such flexible tuning of the radiation loss using graded photonic heterostructures, with only a minimal influence on threshold current, is highly desirable for optimizing second-order distributed feedback lasers.
Publisher: IEEE
Date: 10-2011
Publisher: IEEE
Date: 10-2011
Publisher: American Physical Society (APS)
Date: 31-10-2011
Publisher: The Optical Society
Date: 04-11-2015
Publisher: The Optical Society
Date: 25-06-2019
DOI: 10.1364/OL.44.003314
Publisher: AIP Publishing
Date: 06-2021
DOI: 10.1063/5.0046186
Abstract: Near-field microscopy techniques operating in the terahertz (THz) frequency band offer the tantalizing possibility of visualizing with nanometric resolution the localized THz fields supported by in idual resonators, micro-structured surfaces, and metamaterials. Such capabilities promise to underpin the future development and characterization of a wide range of devices, including THz emitters, detectors, optoelectronic modulators, sensors, and novel optical components. In this work, we report scattering-type scanning near-field optical microscopy using a THz-frequency quantum cascade laser (QCL) to probe coherently the localized field supported by in idual micro-resonator structures. Our technique demonstrates deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes in plasmonic dipole antennas and split ring resonator structures. By exploiting electronic tuning of the QCL in conjunction with the coherent self-mixing effect in these lasers, we are able to resolve both the magnitude and the phase of the out-of-plane field. We, furthermore, show that the elliptically polarized state of the QCL field can be exploited for the simultaneous excitation and measurement of plasmonic resonances in these structures while suppressing the otherwise dominant signal arising from the local material permittivity.
Publisher: IEEE
Date: 09-2018
Publisher: AIP Publishing
Date: 30-04-2018
DOI: 10.1063/1.5027202
Abstract: Tapered coplanar waveguides with integrated photoconductors were designed, fabricated, and measured, with pulsed transmission results comparing well with High Frequency Structure Simulator simulations which predict increased confinement and electric field concentration in the tapered region. Devices made with titanium/gold metallisation were used to demonstrate transmission and confinement, while the magnetoresistive properties of devices with cobalt/copper multilayers were used to demonstrate the field concentration. In the latter case, a mathematical framework was developed to understand the relationship between tapering effects and the picosecond magnetoresistance response.
Publisher: Springer Science and Business Media LLC
Date: 12-02-2020
Publisher: Springer Science and Business Media LLC
Date: 06-08-2018
DOI: 10.1038/S41467-018-05601-X
Abstract: Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain s ling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity.
Publisher: IEEE
Date: 09-2019
Publisher: AIP Publishing
Date: 03-03-2014
DOI: 10.1063/1.4866661
Publisher: The Optical Society
Date: 29-08-2017
DOI: 10.1364/OE.25.021753
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2016
Publisher: IEEE
Date: 08-2017
Publisher: IEEE
Date: 09-2013
Publisher: Institution of Engineering and Technology (IET)
Date: 02-2014
DOI: 10.1049/EL.2013.4035
Publisher: OSA
Date: 2015
Publisher: OSA
Date: 2015
Publisher: Springer Science and Business Media LLC
Date: 26-03-2018
DOI: 10.1038/NATURE25790
Abstract: Room-temperature operation is essential for any optoelectronics technology that aims to provide low-cost, compact systems for widespread applications. A recent technological advance in this direction is bolometric detection for thermal imaging, which has achieved relatively high sensitivity and video rates (about 60 hertz) at room temperature. However, owing to thermally induced dark current, room-temperature operation is still a great challenge for semiconductor photodetectors targeting the wavelength band between 8 and 12 micrometres, and all relevant applications, such as imaging, environmental remote sensing and laser-based free-space communication, have been realized at low temperatures. For these devices, high sensitivity and high speed have never been compatible with high-temperature operation. Here we show that a long-wavelength (nine micrometres) infrared quantum-well photodetector fabricated from a metamaterial made of sub-wavelength metallic resonators exhibits strongly enhanced performance with respect to the state of the art up to room temperature. This occurs because the photonic collection area of each resonator is much larger than its electrical area, thus substantially reducing the dark current of the device. Furthermore, we show that our photonic architecture overcomes intrinsic limitations of the material, such as the drop of the electronic drift velocity with temperature, which constrains conventional geometries at cryogenic operation. Finally, the reduced physical area of the device and its increased responsivity allow us to take advantage of the intrinsic high-frequency response of the quantum detector at room temperature. By mixing the frequencies of two quantum-cascade lasers on the detector, which acts as a heterodyne receiver, we have measured a high-frequency signal, above four gigahertz (GHz). Therefore, these wide-band uncooled detectors could benefit technologies such as high-speed (gigabits per second) multichannel coherent data transfer and high-precision molecular spectroscopy.
Publisher: American Chemical Society (ACS)
Date: 26-03-2021
Publisher: American Chemical Society (ACS)
Date: 21-02-2020
Publisher: AIP Publishing
Date: 22-02-2016
DOI: 10.1063/1.4942452
Abstract: The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, attaining a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns.
Publisher: IEEE
Date: 12-2012
Publisher: American Chemical Society (ACS)
Date: 20-07-2017
DOI: 10.1021/ACS.ANALCHEM.7B01235
Abstract: Terahertz-frequency-range measurements can offer potential insight into the picosecond dynamics, and therefore function, of many chemical systems. There is a need to develop technologies capable of performing such measurements in aqueous and polar environments, particularly when it is necessary to maintain the full functionality of biological s les. In this study, we present a proof-of-concept technology comprising an on-chip planar Goubau line, integrated with a microfluidic channel, which is capable of low-loss, terahertz-frequency-range spectroscopic measurements of liquids. We also introduce a mathematical model that accounts for changes in the electric field distribution around the waveguide, allowing accurate, frequency-dependent liquid parameters to be extracted. We demonstrate the sensitivity of this technique by measuring a homologous alcohol series across the 0.1-0.8 THz frequency range.
Publisher: MDPI AG
Date: 20-05-2016
Publisher: MDPI AG
Date: 02-03-2023
DOI: 10.3390/S23052721
Abstract: To reduce the water footprint in agriculture, the recent push toward precision irrigation management has initiated a sharp rise in photonics-based hydration sensing in plants in a non-contact, non-invasive manner. Here, this aspect of sensing was employed in the terahertz (THz) range for mapping liquid water in the plucked leaves of Bambusa vulgaris and Celtis sinensis. Two complementary techniques, broadband THz time-domain spectroscopic imaging and THz quantum cascade laser-based imaging, were utilized. The resulting hydration maps capture the spatial variations within the leaves as well as the hydration dynamics in various time scales. Although both techniques employed raster scanning to acquire the THz image, the results provide very distinct and different information. Terahertz time-domain spectroscopy provides rich spectral and phase information detailing the dehydration effects on the leaf structure, while THz quantum cascade laser-based laser feedback interferometry gives insight into the fast dynamic variation in dehydration patterns.
Publisher: Optica Publishing Group
Date: 21-05-2020
DOI: 10.1364/OE.391073
Abstract: We provide an analysis of the electromagnetic modes of three-dimensional metamaterial resonators in the THz frequency range. The fundamental resonance of the structures is fully described by an analytical circuit model, which not only reproduces the resonant frequencies but also the coupling of the metamaterial with an incident THz radiation. We also demonstrate the contribution of the propagation effects, and show how they can be reduced by design. In the optimized design, the electric field energy is lumped into ultra-subwavelength (λ/100) capacitors, where we insert a semiconductor absorber based on the collective electronic excitation in a two dimensional electron gas. The optimized electric field confinement is exhibited by the observation of the ultra-strong light-matter coupling regime, and opens many possible applications for these structures in detectors, modulators and sources of THz radiation.
Publisher: OSA
Date: 2014
Publisher: AIP Publishing
Date: 08-2013
DOI: 10.1063/1.4816736
Abstract: We present the self-aligned fabrication of on-chip devices in which waveguides, incorporating integrated photoconductive switches, are combined with two-dimensional electron systems to allow probing of the ultrafast (terahertz frequency range) properties of confined semiconductor systems, both at cryogenic temperatures and in high magnetic fields. We demonstrate the direct injection of on-chip terahertz pulses into the mesoscopic system by femtosecond, near infra-red laser excitation of in-plane photoconductive switches formed on an epitaxially grown, low-temperature GaAs layer, which is integrated monolithically with a GaAs/AlGaAs heterostructure containing a two-dimensional electron system. Both the input and output terahertz signals of an on-chip waveguide are s led by altering dynamically the photoconductive excitation/detection arrangement in situ on a single device. We also demonstrate a new method for sub-Kelvin excitation and detection of on-chip terahertz frequency radiation in a 3He/4He dilution refrigerator that allows the photocurrent and detected terahertz transient to be mapped as function of the near-infrared excitation position at the emitter and the detector, respectively. Furthermore, we demonstrate transmission of terahertz transients through a two-dimensional electron system in a coplanar waveguide under magnetic field at temperatures as low as 200 mK.
Publisher: Cambridge University Press (CUP)
Date: 08-05-2019
DOI: 10.1017/S175907871900028X
Abstract: Antenna-pattern measurements obtained from a double-metal supra-terahertz-frequency (supra-THz) quantum cascade laser (QCL) are presented. The QCL is mounted within a mechanically micro-machined waveguide cavity containing dual diagonal feedhorns. Operating in continuous-wave mode at 3.5 THz, and at an ambient temperature of ~60 K, QCL emission has been directed via the feedhorns to a supra-THz detector mounted on a multi-axis linear scanner. Comparison of simulated and measured far-field antenna patterns shows an excellent degree of correlation between beamwidth (full-width-half-maximum) and sidelobe content and a very substantial improvement when compared with unmounted devices. Additionally, a single output has been used to successfully illuminate and demonstrate an optical breadboard arrangement associated with a future supra-THz Earth observation space-borne payload. Our novel device has therefore provided a valuable demonstration of the effectiveness of supra-THz diagonal feedhorns and QCL devices for future space-borne ultra-high-frequency Earth-observing heterodyne radiometers.
Publisher: SPIE
Date: 04-02-2013
DOI: 10.1117/12.2004125
Publisher: OSA
Date: 2018
Publisher: OSA
Date: 2018
Publisher: IOP Publishing
Date: 29-10-2014
DOI: 10.1088/0953-8984/26/47/475801
Abstract: We have investigated experimentally the scaling behaviour of quantum Hall transitions in GaAs/AlGaAs heterostructures of a range of mobility, carrier concentration, and spacer layer width. All three critical scaling exponents γ, κ and p were determined independently for each s le. We measure the localization length exponent to be γ ≈ 2.3, in good agreement with expected predictions from scaling theory, but κ and p are found to possess non-universal values. Results obtained for κ range from κ = 0.16 ± 0.02 to κ = 0.67 ± 0.02, and are found to be Landau level (LL) dependent, whereas p is found to decrease with increasing s le mobility. Our results demonstrate the existence of two transport regimes in the LL conductivity peak universality is found within the quantum coherent transport regime present in the tails of the conductivity peak, but is absent within the classical transport regime found close to the critical point at the centre of the conductivity peak. We explain these results using a percolation model and show that the critical scaling exponent depends on certain important length scales that correspond to the microscopic description of electron transport in the bulk of a two-dimensional electron system.
Publisher: SPIE
Date: 08-12-2016
DOI: 10.1117/12.2250621
Publisher: IEEE
Date: 09-2019
Publisher: AIP Publishing
Date: 21-01-2013
DOI: 10.1063/1.4789535
Abstract: We demonstrate single-mode surface-emitting terahertz frequency quantum cascade lasers utilising non-uniform second-order distributed feedback concentric-circular-gratings. The grating is designed for single-mode operation and surface emission for efficient and directional optical power out-coupling. The devices exhibit single-mode operation over the entire dynamic range with a side-mode-suppression-ratio of around 30 dB at 78 K, and a six-fold rotationally symmetric far-field pattern. In addition, the devices show a peak output power approximately three times higher than in ridge-waveguide lasers of similar size, whilst maintaining similar threshold current densities for the 3.8 THz emission and without remarkably sacrificing the maximum temperature operation performance. Owing to the high symmetry of the structure and the broad area light emission from surface, the devices are potentially very suitable for use as single-mode, high power emitters for integration into two-dimensional laser arrays.
Publisher: IEEE
Date: 09-2013
Publisher: IEEE
Date: 09-2010
Publisher: The Optical Society
Date: 10-02-2015
DOI: 10.1364/OE.23.004012
Publisher: AIP Publishing
Date: 08-08-2011
DOI: 10.1063/1.3623424
Abstract: We demonstrate In-assisted desorption of native GaAs surface oxides at substrate temperatures of 480–550 °C. The oxides are removed through production of volatile Ga and In suboxides, Ga2O, and In2O. Compared to a Ga-assisted desorption process, excess In is easily removed at low substrate temperature, favouring a clean, smooth surface. The feasibility of using In-assisted desorption for the regrowth of high quality quantum dot structures is shown.
Publisher: Springer Science and Business Media LLC
Date: 16-03-2018
DOI: 10.1038/S41467-018-03440-4
Abstract: Terahertz (THz) quantum cascade lasers (QCLs) have undergone rapid development since their demonstration, showing high power, broad-tunability, quantum-limited linewidth, and ultra-broadband gain. Typically, to address applications needs, continuous-wave (CW) operation, low- ergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam ergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. Here, we demonstrate wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies, has led to the achievement of low- ergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.
Publisher: AIP Publishing
Date: 16-06-2014
DOI: 10.1063/1.4884056
Abstract: We demonstrate the operation of coupled-cavity terahertz frequency quantum-cascade lasers composed of two sub-cavities separated by an air gap realized by optical lithography and dry etching. This geometry allows stable, single mode operation with typical side mode suppression ratios in the 30–40 dB range. We employ a transfer matrix method to model the mode selection mechanism. The obtained results are in good agreement with the measurements and allow prediction of the operating frequency.
Publisher: AIP Publishing
Date: 29-02-2016
DOI: 10.1063/1.4943088
Abstract: We report two-dimensional apertureless near-field terahertz (THz) imaging using a quantum cascade laser (QCL) source and a scattering probe. A near-field enhancement of the scattered field litude is observed for small tip-s le separations, allowing image resolutions of ∼1 μm (∼λ/100) and ∼7 μm to be achieved along orthogonal directions on the s le surface. This represents the highest resolution demonstrated to date with a THz QCL. By employing a detection scheme based on self-mixing interferometry, our approach offers experimental simplicity by removing the need for an external detector and also provides sensitivity to the phase of the reinjected field.
Publisher: The Optical Society
Date: 06-01-2017
DOI: 10.1364/OE.25.000486
Publisher: American Chemical Society (ACS)
Date: 02-04-2019
Publisher: Springer Science and Business Media LLC
Date: 15-03-2016
DOI: 10.1038/SREP23053
Abstract: Mode-locked comb sources operating at optical frequencies underpin applications ranging from spectroscopy and ultrafast physics, through to absolute frequency measurements and atomic clocks. Extending their operation into the terahertz frequency range would greatly benefit from the availability of compact semiconductor-based sources. However, the development of any compact mode-locked THz laser, which itself is inherently a frequency comb, has yet to be achieved without the use of an external stimulus. High-power, electrically pumped quantum cascade lasers (QCLs) have recently emerged as a promising solution, owing to their octave spanning bandwidths, the ability to achieve group-velocity dispersion compensation and the possibility of obtaining active mode-locking. Here, we propose an unprecedented compact architecture to induce both frequency and litude self-modulation in a THz QCL. By engineering a microwave avalanche oscillator into the laser cavity, which provides a 10 GHz self-modulation of the bias current and output power, we demonstrate multimode laser emission centered around 3 THz, with distinct multiple sidebands. The resulting microwave litude and frequency self-modulation of THz QCLs opens up intriguing perspectives, for engineering integrated self-mode-locked THz lasers, with impact in fields such as nano- and ultrafast photonics and optical metrology.
Publisher: The Optical Society
Date: 05-02-2018
DOI: 10.1364/OE.26.003814
Publisher: Springer Science and Business Media LLC
Date: 03-07-2023
DOI: 10.1038/S41467-023-39594-Z
Abstract: In the majority of optoelectronic devices, emission and absorption of light are considered as perturbative phenomena. Recently, a regime of highly non-perturbative interaction, ultra-strong light-matter coupling, has attracted considerable attention, as it has led to changes in the fundamental properties of materials such as electrical conductivity, rate of chemical reactions, topological order, and non-linear susceptibility. Here, we explore a quantum infrared detector operating in the ultra-strong light-matter coupling regime driven by collective electronic excitations, where the renormalized polariton states are strongly detuned from the bare electronic transitions. Our experiments are corroborated by microscopic quantum theory that solves the problem of calculating the fermionic transport in the presence of strong collective electronic effects. These findings open a new way of conceiving optoelectronic devices based on the coherent interaction between electrons and photons allowing, for ex le, the optimization of quantum cascade detectors operating in the regime of strongly non-perturbative coupling with light.
Publisher: AIP Publishing
Date: 15-09-2014
DOI: 10.1063/1.4896032
Abstract: A periodically poled lithium niobate (PPLN) crystal with multiple poling periods is used to generate tunable narrow-bandwidth THz pulses for injection seeding a quantum cascade laser (QCL). We demonstrate that longitudinal modes of the quantum cascade laser close to the gain maximum can be selected or suppressed according to the seed spectrum. The QCL emission spectra obtained by electro-optic s ling from the quantum cascade laser, in the most favorable case, shows high selectivity and lification of the longitudinal modes that overlap the frequency of the narrow-band seed. Proper selection of the narrow-band THz seed from the PPLN crystal discretely tunes the longitudinal mode emission of the quantum cascade laser. Moreover, the THz wave build-up within the laser cavity is studied as a function of the round-trip time. When the seed frequency is outside the maximum of the gain spectrum the laser emission shifts to the preferential longitudinal mode.
Publisher: Springer Science and Business Media LLC
Date: 04-05-2016
DOI: 10.1038/SREP24811
Abstract: Diffraction is the ultimate limit at which details of objects can be resolved in conventional optical spectroscopy and imaging systems. In the THz spectral range, spectroscopy systems increasingly rely on ultra-broadband radiation (extending over more 5 octaves) making a great challenge to reach resolution limited by diffraction. Here, we propose an original easy-to-implement wavefront manipulation concept to achieve ultrabroadband THz spectroscopy system with diffraction-limited resolution. Applying this concept to a large-area photoconductive emitter, we demonstrate diffraction-limited ultra-broadband spectroscopy system up to 14.5 THz with a dynamic range of 10 3 . The strong focusing of ultrabroadband THz radiation provided by our approach is essential for investigating single micrometer-scale objects such as graphene flakes or living cells and besides for achieving intense ultra-broadband THz electric fields.
Publisher: IEEE
Date: 08-2017
Publisher: American Chemical Society (ACS)
Date: 12-12-2016
Publisher: AIP Publishing
Date: 20-04-2020
DOI: 10.1063/5.0004591
Abstract: Quantum cascade detectors (QCDs) are unipolar infrared devices where the transport of the photoexcited carriers takes place through confined electronic states, without an applied bias. In this photovoltaic mode, the detector's noise is not dominated by a dark shot noise process, and therefore, performances are less degraded at high temperature with respect to photoconductive detectors. This work describes a 9 μm QCD embedded into a patch-antenna metamaterial, which operates with state-of-the-art performances. The metamaterial gathers photons on a collection area, Acoll, much larger than the geometrical area of the detector, improving the signal to noise ratio up to room temperature. The background-limited detectivity at 83 K is 5.5 × 1010 cm Hz1/2 W−1, while at room temperature, the responsivity is 50 mA/W at 0 V bias. A patch antenna QCD is an ideal receiver for a heterodyne detection setup, where a signal at a frequency of 1.4 GHz and T = 295 K is reported as demonstration of uncooled 9 μm photovoltaic receivers with a GHz electrical bandwidth. These findings guide the research toward uncooled IR quantum limited detection.
Publisher: AIP Publishing
Date: 13-07-2020
DOI: 10.1063/5.0013505
Abstract: We report the realization of controllable linear-to-circular polarization states in single-mode terahertz master-oscillator power- lifier quantum cascade lasers (THz-MOPA-QCLs). The MOPA device contains a first-order distributed feedback (DFB) laser as the master-oscillator, a pre lifier, and a 2D periodical antenna array as the power extractor. The polarization state is determined by the orientation and the phase relationship between the antennas. The antenna array is carefully designed to efficiently extract the THz radiation and not to induce field oscillation in the array or influence the mode oscillation in the DFB section. Each demonstrated device exhibits single-mode emission with a side mode suppression ratio of ∼26 dB and a single-lobed beam with a low ergence of ∼23°×30°. Realized in different devices, the degree of linear or circular polarization reaches as high as 97.5% or 99.3%. Both the operation frequency and the polarization state of the radiation are lithographically tunable.
Publisher: AIP Publishing
Date: 20-03-2023
DOI: 10.1063/5.0142359
Abstract: Terahertz (THz) quantum cascade lasers (QCLs) have been shown to emit peak powers greater than 1 W from a single facet in a single plasmon geometry. However, this is typically achieved by increasing the laser ridge width, resulting in higher-order transverse modes, limiting the achievable power density. Here, we control and fully suppress these modes through thin metallic side-absorbers, showing laser action solely on the fundamental transverse mode operation without sacrificing high THz peak powers. This leads to enhanced power densities and electric fields of up to 1.8 kW/cm2 and 1.17 kV/cm, respectively, opening up the possibility of applying THz QCLs as pump sources for investigations of nonlinear THz physical phenomena.
Publisher: Springer Science and Business Media LLC
Date: 03-03-2021
DOI: 10.1038/S41467-021-21659-6
Abstract: Millimeter wave (mmWave) generation using photonic techniques has so far been limited to the use of near-infrared lasers that are down-converted to the mmWave region. However, such methodologies do not currently benefit from a monolithic architecture and suffer from the quantum defect i.e. the difference in photon energies between the near-infrared and mmWave region, which can ultimately limit the conversion efficiency. Miniaturized terahertz (THz) quantum cascade lasers (QCLs) have inherent advantages in this respect: their low energy photons, ultrafast gain relaxation and high nonlinearities open up the possibility of innovatively integrating both laser action and mmWave generation in a single device. Here, we demonstrate intracavity mmWave generation within THz QCLs over the unprecedented range of 25 GHz to 500 GHz. Through ultrafast time resolved techniques, we highlight the importance of modal phases and that the process is a result of a giant second-order nonlinearity combined with a phase matched process between the THz and mmWave emission. Importantly, this work opens up the possibility of compact, low noise mmWave generation using modelocked THz frequency combs.
Publisher: Springer Science and Business Media LLC
Date: 11-02-2020
DOI: 10.1038/S41467-020-14662-W
Abstract: The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast litude modulation that can be controlled using the QCL drive current or strain pulse litude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.
Publisher: Springer Science and Business Media LLC
Date: 30-11-2018
DOI: 10.1038/S41467-018-07629-5
Abstract: The original version of this Article contained an error in the Acknowledgements, which incorrectly omitted the following: ‘We also acknowledge support from the Australian Research Council’s Discovery Projects Funding Scheme (Grant DP 160 103910).’ This has been corrected in both the PDF and HTML versions of the Article.
Publisher: The Optical Society
Date: 18-01-2018
DOI: 10.1364/OE.26.001942
Publisher: The Optical Society
Date: 16-12-2013
DOI: 10.1364/OE.21.031872
Publisher: AIP Publishing
Date: 02-12-2019
DOI: 10.1063/1.5116351
Abstract: We demonstrate a sensitive and compact terahertz heterodyne detection system based on a quantum cascade laser (QCL) as a local oscillator and a hot electron bolometer (HEB) as a mixer. It relies on an original optical coupling scheme where the terahertz (THz) signal to be detected and the local oscillator (LO) signal are coupled to the HEB from both sides of the integrated lens/antenna mixer. The THz signal of interest impinges on the front side through the silicon lens while the LO onto the rear (air) side. This concept allows us to remove the beam splitter usually employed in terahertz heterodyne receivers. The mixer consists of a Niobium Nitride HEB with a log-spiral planar antenna mounted on the flat side of a hyperhemispherical silicon lens. The local oscillator of the heterodyne detector is a low power consumption and low beam ergence 3rd-order distributed feedback laser with single mode emission at the target frequency of 2.7 THz. The coupling between the QCL and the HEB has been further optimized, using a dielectric hollow waveguide that reliably increases the laser beam directivity and permits us to pump the HEB into its most sensitive state through the air side of the planar antenna. We have measured a noncorrected double sideband receiver noise temperature of 880 K at 2.7 THz.
Publisher: AIP Publishing
Date: 03-04-2017
DOI: 10.1063/1.4979536
Abstract: Interdigitated photoconductive (iPC) switches are powerful and convenient devices for time-resolved spectroscopy, with the ability to operate both as sources and detectors of terahertz (THz) frequency pulses. However, reflection of the emitted or detected radiation within the device substrate itself can lead to echoes that inherently limit the spectroscopic resolution achievable for their use in time-domain spectroscopy (TDS) systems. In this work, we demonstrate a design of low-temperature-grown-GaAs (LT-GaAs) iPC switches for THz pulse detection that suppresses such unwanted echoes. This is realized through the growth of a buried multilayer LT-GaAs structure that retains its ultrafast properties, which, after wafer bonding to a metal-coated host substrate, results in an iPC switch with a metal plane buried at a subwavelength depth below the LT-GaAs surface. Using this device as a detector, and coupling it to an echo-less iPC source, enables echo-free THz-TDS and high-resolution spectroscopy, with a resolution limited only by the temporal length of the measurement governed by the mechanical delay line used. As a proof-of-principle, the 212-221 and the 101-212 rotational lines of water vapor have been spectrally resolved, demonstrating a spectral resolution below 10 GHz.
Publisher: American Chemical Society (ACS)
Date: 04-02-2021
Publisher: Springer Science and Business Media LLC
Date: 20-12-2021
DOI: 10.1038/S41377-021-00685-5
Abstract: The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.
Publisher: IEEE
Date: 09-2013
Publisher: Springer Science and Business Media LLC
Date: 03-08-2017
DOI: 10.1038/S41598-017-07432-0
Abstract: The effects of optical feedback (OF) in lasers have been observed since the early days of laser development. While OF can result in undesirable and unpredictable operation in laser systems, it can also cause measurable perturbations to the operating parameters, which can be harnessed for metrological purposes. In this work we exploit this ‘self-mixing’ effect to infer the emission spectrum of a semiconductor laser using a laser-feedback interferometer, in which the terminal voltage of the laser is used to coherently s le the reinjected field. We demonstrate this approach using a terahertz frequency quantum cascade laser operating in both single- and multiple-longitudinal mode regimes, and are able to resolve spectral features not reliably resolved using traditional Fourier transform spectroscopy. We also investigate quantitatively the frequency perturbation of in idual laser modes under OF, and find excellent agreement with predictions of the excess phase equation central to the theory of lasers under OF.
Publisher: AIP Publishing
Date: 03-03-2005
DOI: 10.1063/1.1872213
Abstract: We have optimized the molecular-beam epitaxy growth conditions of self-organized InAs∕GaAs quantum dots (QDs) to achieve a low density of dots emitting at 1300 nm at low temperature. We used an ultralow InAs growth rate, lower than 0.002ML∕s, to reduce the density to 2dots∕μm2 and an InGaAs capping layer to achieve longer emission wavelength. Microphotoluminescence spectroscopy at low-temperature reveals emission lines characteristic of exciton-biexciton behavior. We also study the temperature dependence of the photoluminescence, showing clear single QD emission up to 90 K. With these results, InAs∕GaAs QDs appear as a very promising system for future applications of single photon sources in fiber-based quantum cryptography.
Publisher: Springer Science and Business Media LLC
Date: 27-08-2020
DOI: 10.1038/S41467-020-18004-8
Abstract: Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field litudes owing to the weak light-matter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic single-chip design.
Publisher: IEEE
Date: 08-2017
Publisher: American Chemical Society (ACS)
Date: 07-2020
Publisher: American Association for the Advancement of Science (AAAS)
Date: 04-10-2019
Abstract: We bring to light the complex interplay of optical nonlinearities in quantum structures.
Publisher: AIP Publishing
Date: 09-2021
DOI: 10.1063/5.0056487
Abstract: The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.
Publisher: IEEE
Date: 09-2019
Publisher: IEEE
Date: 08-2017
Publisher: The Optical Society
Date: 30-01-2015
DOI: 10.1364/OE.23.002720
Publisher: The Optical Society
Date: 09-2017
Publisher: The Optical Society
Date: 27-06-2014
DOI: 10.1364/OL.39.003962
Publisher: Springer Science and Business Media LLC
Date: 03-07-2019
DOI: 10.1038/S41467-019-10913-7
Abstract: Miniaturized frequency comb sources across hard-to-access spectral regions, i.e. mid- and far-infrared, have long been sought. Four-wave-mixing based Quantum Cascade Laser combs (QCL-combs) are ideal candidates, in this respect, due to the unique possibility to tailor their spectral emission by proper nanoscale design of the quantum wells. We demonstrate full-phase-stabilization of a QCL-comb against the primary frequency standard, proving independent and simultaneous control of the two comb degrees of freedom (modes spacing and frequency offset) at a metrological level. Each emitted mode exhibits a sub-Hz relative frequency stability, while a correlation analysis on the modal phases confirms the high degree of coherence in the device emission, over different power-cycles and over different days. The achievement of fully controlled, phase-stabilized QCL-comb emitters proves that this technology is mature for metrological-grade uses, as well as for an increasing number of scientific and technological applications.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
Publisher: AIP Publishing
Date: 03-06-2013
DOI: 10.1063/1.4808385
Abstract: We demonstrate the generation of high order terahertz (THz) frequency sidebands (up to 3rd order) on a near infrared (NIR) optical carrier within a THz quantum cascade laser (QCL). The NIR carrier is resonant with the interband transition of the quantum wells composing the QCL, allowing the nonlinearity to be enhanced and leading to frequency mixing. A phonon depopulation based QCL with a double metal cavity was used to enhance the intracavity power density and to demonstrate the higher order sidebands. The 1st order sideband intensity shows a linear dependence with THz power corresponding to a single THz photon, while the second order sideband has a quadratic dependence implying a two THz photon interaction and hence a third order susceptibility. These measurements are compared to the photoluminescence and the QCL bandstructure to identify the states involved, with the lowest conduction band states contributing the most to the sideband intensity. We also show that the interaction for the second order sideband corresponds to an enhanced direct third order susceptibility χ(3) of ∼7 × 10−16(m/V)2, two orders of magnitude greater than the bulk value.
Publisher: OSA
Date: 2018
Publisher: IEEE
Date: 08-2017
Publisher: IOP Publishing
Date: 14-03-2019
Publisher: Optica Publishing Group
Date: 02-03-2023
DOI: 10.1364/BOE.480615
Abstract: Early detection of skin pathologies with current clinical diagnostic tools is challenging, particularly when there are no visible colour changes or morphological cues present on the skin. In this study, we present a terahertz (THz) imaging technology based on a narrow band quantum cascade laser (QCL) at 2.8 THz for human skin pathology detection with diffraction limited spatial resolution. THz imaging was conducted for three different groups of unstained human skin s les (benign naevus, dysplastic naevus, and melanoma) and compared to the corresponding traditional histopathologic stained images. The minimum thickness of dehydrated human skin that can provide THz contrast was determined to be 50 µm, which is approximately one half-wavelength of the THz wave used. The THz images from different types of 50 µm-thick skin s les were well correlated with the histological findings. The per-s le locations of pathology vs healthy skin can be separated from the density distribution of the corresponding pixels in the THz litude–phase map. The possible THz contrast mechanisms relating to the origin of image contrast in addition to water content were analyzed from these dehydrated s les. Our findings suggest that THz imaging could provide a feasible imaging modality for skin cancer detection that is beyond the visible.
Publisher: American Chemical Society (ACS)
Date: 10-02-2017
Publisher: The Optical Society
Date: 25-01-2018
DOI: 10.1364/PRJ.6.000117
Publisher: AIP Publishing
Date: 08-09-2020
DOI: 10.1063/5.0018865
Abstract: Scanning-probe-assisted mid-infrared nano-spectroscopy is employed to reveal the polaritonic dispersion of in idual MIM (metal-insulator-metal) square patch antennas whose modes can be strongly coupled to a mid-infrared intersubband transition. The patch antenna side length L sets the resonances between λ = 5.5 μm and 12.5 μm. The active region consists of a highly doped AlInAs/InGaAs/AlInAs single quantum well that presents an intersubband transition at 1190 cm−1 (λ = 8.4 μm). When the patch antenna optical resonance approaches and matches the intersubband transition frequency (L ∼ 1.8 μm), a clear anticrossing behavior—evidence of strong coupling—is observed in the near-field scattering phase spectra of in idual antennas. The measured Rabi splitting is 4.5 THz. The near-field scattering spectra agree with the far-field extinction spectra acquired on arrays of identical antennas.
Publisher: IEEE
Date: 10-2011
Publisher: AIP Publishing
Date: 15-11-2010
DOI: 10.1063/1.3514549
Abstract: We investigate the effect of the arsenic source (As2 and As4) on the optical properties of InGaAs quantum rods (QRs) grown by molecular beam epitaxy. Owing to differences in the In and Ga diffusion lengths under As2 and As4 fluxes, photoluminescence (PL) peak energies of the QR s les depend strongly on the As source when similar growth conditions are used. A marked improvement in the PL intensities from QR s les grown using As4 is achieved. However, for both As2 and As4, an increase of the As overpressure results in a PL intensity degradation, probably due to the formation of nonradiative recombination centers.
Publisher: IEEE
Date: 10-2019
Publisher: SPIE
Date: 06-09-2019
DOI: 10.1117/12.2530727
Publisher: Springer Science and Business Media LLC
Date: 15-06-2017
DOI: 10.1038/NCOMMS15763
Abstract: Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. Here we report the fabrication of THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z -scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. These results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold s les and ultra-high speed communications, providing unprecedented compactness and resolution.
Publisher: Institution of Engineering and Technology (IET)
Date: 06-2015
DOI: 10.1049/EL.2015.1137
Publisher: Springer Science and Business Media LLC
Date: 30-11-2018
DOI: 10.1038/S41598-018-36015-W
Abstract: A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper
Publisher: Wiley
Date: 07-2017
Publisher: Springer Science and Business Media LLC
Date: 09-02-2023
DOI: 10.1038/S41467-023-36418-Y
Abstract: Topological cavities, whose modes are protected against perturbations, are promising candidates for novel semiconductor laser devices. To date, there have been several demonstrations of topological lasers (TLs) exhibiting robust lasing modes. The possibility of achieving nontrivial beam profiles in TLs has recently been explored in the form of vortex wavefront emissions enabled by a structured optical pump or strong magnetic field, which are inconvenient for device applications. Electrically pumped TLs, by contrast, have attracted attention for their compact footprint and easy on-chip integration with photonic circuits. Here, we experimentally demonstrate an electrically pumped TL based on photonic analogue of a Majorana zero mode (MZM), implemented monolithically on a quantum cascade chip. We show that the MZM emits a cylindrical vector (CV) beam, with a topologically nontrivial polarization profile from a terahertz (THz) semiconductor laser.
Publisher: IEEE
Date: 07-2019
Publisher: The Optical Society
Date: 02-05-2018
DOI: 10.1364/OL.43.002225
Publisher: OSA
Date: 2014
Publisher: IOP Publishing
Date: 05-08-2013
Publisher: The Optical Society
Date: 07-11-2013
Publisher: The Optical Society
Date: 06-03-2015
DOI: 10.1364/OE.23.006915
Publisher: American Chemical Society (ACS)
Date: 19-06-2018
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 08-2015
Publisher: The Optical Society
Date: 28-03-2019
DOI: 10.1364/OE.27.010221
Publisher: American Physical Society (APS)
Date: 19-12-2022
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 05-2019
Publisher: IEEE
Date: 08-2015
Publisher: The Optical Society
Date: 30-06-0003
DOI: 10.1364/OL.40.000950
Publisher: The Optical Society
Date: 11-11-2016
DOI: 10.1364/OE.24.026986
Publisher: Springer Science and Business Media LLC
Date: 24-04-2020
DOI: 10.1038/S42005-020-0344-0
Abstract: Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes.
Publisher: IEEE
Date: 08-2015
Publisher: AIP Publishing
Date: 03-10-2016
DOI: 10.1063/1.4963891
Abstract: We study the emission properties of an electroluminescent THz frequency quantum cascade structure embedded in an array of patch antenna double-metal microcavities. We show that high photon extraction efficiencies can be obtained by adjusting the active region thickness and array periodicity as well as high Purcell factors (up to 65), leading to an enhanced overall emitted power. Up to a 44-fold increase in power is experimentally observed in comparison with a reference device processed in conventional mesa geometry. Estimation of the Purcell factors using electromagnetic simulations and the theoretical extraction efficiency are in agreement with the observed power enhancement and show that, in these microcavities, the overall enhancement solely depends on the square of the total quality factor.
Publisher: Optica Publishing Group
Date: 2020
DOI: 10.1364/CLEOPR.2020.C12B_4
Abstract: In the present work, the drought response in Tiger grass (Thysanolaena latifolia) plants has been investigated by monitoring water status using THz QCL based Laser Feedback Interferometry imaging technique.
Publisher: American Chemical Society (ACS)
Date: 13-06-2018
Publisher: Wiley
Date: 16-12-2020
Publisher: Institution of Engineering and Technology (IET)
Date: 10-2018
DOI: 10.1049/EL.2018.6062
Publisher: Springer Science and Business Media LLC
Date: 19-12-2014
DOI: 10.1038/NCOMMS6884
Abstract: Quasi-crystal structures do not present a full spatial periodicity but are nevertheless constructed starting from deterministic generation rules. When made of different dielectric materials, they often possess fascinating optical properties, which lie between those of periodic photonic crystals and those of a random arrangement of scatterers. Indeed, they can support extended band-like states with pseudogaps in the energy spectrum, but lacking translational invariance, they also intrinsically feature a pattern of ‘defects’, which can give rise to critically localized modes confined in space, similar to Anderson modes in random structures. If used as laser resonators, photonic quasi-crystals open up design possibilities that are simply not possible in a conventional periodic photonic crystal. In this letter, we exploit the concept of a 2D photonic quasi crystal in an electrically injected laser specifically, we pattern the top surface of a terahertz quantum-cascade laser with a Penrose tiling of pentagonal rotational symmetry, reaching 0.1–0.2% wall-plug efficiencies and 65 mW peak output powers with characteristic surface-emitting conical beam profiles, result of the rich quasi-crystal Fourier spectrum.
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 05-2013
Publisher: Royal Society of Chemistry (RSC)
Date: 2013
DOI: 10.1039/C3LC50485A
Abstract: We investigate the effect of substrate thickness on the transmission bandwidth of on-chip terahertz-frequency-range planar Goubau lines both experimentally and theoretically. The bandwidth and frequency resolution are improved through substrate thinning and geometry modifications (reducing reflections arising from the THz photoconductive generators and detectors). We demonstrate that the enhanced bandwidth (2 THz) and resolution (3.75 GHz) allows this type of on-chip waveguide to be used for spectroscopic measurements of polycrystalline materials from cryogenic (4 K) to room temperature (292 K) by recording vibrational absorption spectra from overlaid s les of lactose monohydrate.
Publisher: IEEE
Date: 05-2011
Publisher: AIP Publishing
Date: 05-12-2016
DOI: 10.1063/1.4969067
Abstract: We report on the realization of a monolithically integrated master-oscillator power- lifier architecture in a terahertz quantum cascade laser (THz-QCL) with a metal-metal waveguide. The master-oscillator section is a first-order distributed feedback (DFB) laser. Instead of using a thick anti-reflection coating, we exploit a diffraction grating together with an absorbing boundary in the power- lifier section to efficiently extract the laser radiation and suppress the self-lasing in it. The devices demonstrate a stable generation and power lification of single-mode emission. The lification factor is about 5, and the output power is approximately twice that of the standard second-order DFB lasers fabricated from the same material. Emission beam pattern with a ergence angle of ∼18 × 40° is achieved. Our work provides an avenue for the realization of single-mode THz-QCLs with high output power and good beam quality.
Publisher: The Optical Society
Date: 20-02-2015
DOI: 10.1364/OE.23.005190
Publisher: Institution of Engineering and Technology (IET)
Date: 06-2017
DOI: 10.1049/EL.2017.0662
Publisher: Springer Science and Business Media LLC
Date: 12-06-2023
DOI: 10.1038/S41377-023-01200-8
Abstract: One of the most exciting breakthroughs in physics is the concept of topology that was recently introduced to photonics, achieving robust functionalities, as manifested in the recently demonstrated topological lasers. However, so far almost all attention was focused on lasing from topological edge states. Bulk bands that reflect the topological bulk-edge correspondence have been largely missed. Here, we demonstrate an electrically pumped topological bulk quantum cascade laser (QCL) operating in the terahertz (THz) frequency range. In addition to the band-inversion induced in-plane reflection due to topological nontrivial cavity surrounded by a trivial domain, we further illustrate the band edges of such topological bulk lasers are recognized as the bound states in the continuum (BICs) due to their nonradiative characteristics and robust topological polarization charges in the momentum space. Therefore, the lasing modes show both in-plane and out-of-plane tight confinements in a compact laser cavity (lateral size ~3λ laser ). Experimentally, we realize a miniaturized THz QCL that shows single-mode lasing with a side-mode suppression ratio (SMSR) around 20 dB. We also observe a cylindrical vector beam for the far-field emission, which is evidence for topological bulk BIC lasers. Our demonstration on miniaturization of single-mode beam-engineered THz lasers is promising for many applications including imaging, sensing, and communications.
Publisher: AIP Publishing
Date: 28-12-2015
DOI: 10.1063/1.4938207
Abstract: Tunable oscillators are a key component of almost all electronic and photonic systems. Yet, a technology capable of operating in the terahertz (THz)-frequency range and fully suitable for widescale implementation is still lacking. This issue is significantly limiting potential THz applications in gas sensing, high-resolution spectroscopy, hyper-spectral imaging, and optical communications. The THz quantum cascade laser is arguably the most promising solution in terms of output power and spectral purity. In order to achieve reliable, repeatable, and broad tunability, here we exploit the strong coupling between two different cavity mode concepts: a distributed feedback one-dimensional photonic resonator (providing gain) and a mechanically actuated wavelength-size microcavity (providing tuning). The result is a continuously tunable, single-mode emitter covering a 162 GHz spectral range, centered on 3.2 THz. Our source has a few tens of MHz resolution, extremely high differential efficiency, and unprecedented compact and simple design architecture. By unveiling the large potential that lies in this technique, our results provide a robust platform for radically different THz systems exploiting broadly tunable semiconductor lasers.
Publisher: American Chemical Society (ACS)
Date: 31-12-2018
Publisher: OSA
Date: 2015
Publisher: Optica Publishing Group
Date: 05-07-2023
DOI: 10.1364/OE.490217
Abstract: The response of terahertz to the presence of water content makes it an ideal analytical tool for hydration monitoring in agricultural applications. This study reports on the feasibility of terahertz sensing for monitoring the hydration level of freshly harvested leaves of Celtis sinensis by employing a imaging platform based on quantum cascade lasers and laser feedback interferometry. The imaging platform produces wide angle high resolution terahertz litude and phase images of the leaves at high frame rates allowing monitoring of dynamic water transport and other changes across the whole leaf. The complementary information in the resulting images was fed to a machine learning model aiming to predict relative water content from a single frame. The model was used to predict the change in hydration level over time. Results of the study suggest that the technique could have substantial potential in agricultural applications.
Publisher: OSA
Date: 2014
Publisher: The Optical Society
Date: 12-2016
DOI: 10.1364/OE.24.028583
Publisher: AIP Publishing
Date: 20-04-2015
DOI: 10.1063/1.4918983
Abstract: We report on the implementation of 5 THz quantum well photodetector exploiting a patch antenna cavity array. The benefit of our plasmonic architecture on the detector performance is assessed by comparing it with detectors made using the same quantum well absorbing region, but processed into a standard 45° polished facet mesa. Our results demonstrate a clear improvement in responsivity, polarization insensitivity, and background limited performance. Peak detectivities in excess of 5 × 1012 cmHz1/2/W have been obtained, a value comparable with that of the best cryogenic cooled bolometers.
Publisher: AIP Publishing
Date: 21-12-2020
DOI: 10.1063/5.0033367
Abstract: We demonstrate a high-temperature performance quantum detector of Terahertz (THz) radiation based on three-dimensional metamaterial. The metamaterial unit cell consists of an inductor-capacitor (LC) resonator laterally coupled with antenna elements. The absorbing region, consisting of semiconductor quantum wells, is contained in the strongly ultra-subwavelength capacitors of the LC structure. The high radiation loss of the antenna allows strongly increased collection efficiency for the incident THz radiation, while the small effective volume of the LC resonator allows intense light-matter coupling with a reduced electrical area. As a result, our detectors operate at much higher temperatures than conventional quantum well detectors demonstrated so far.
Publisher: The Optical Society
Date: 12-09-2016
DOI: 10.1364/OE.24.021948
Publisher: IEEE
Date: 09-2019
Publisher: American Chemical Society (ACS)
Date: 03-12-2020
Location: United Kingdom of Great Britain and Northern Ireland
Location: Switzerland
Location: China
No related grants have been discovered for Lianhe Li.