ORCID Profile
0000-0002-3950-4359
Current Organisation
University of Leeds
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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: SPIE
Date: 08-07-2021
DOI: 10.1117/12.2592548
Publisher: Wiley
Date: 10-01-2012
Publisher: Springer Science and Business Media LLC
Date: 06-01-2016
Publisher: IEEE
Date: 10-2011
Publisher: IEEE
Date: 09-2013
Publisher: IEEE
Date: 10-2015
Publisher: The Optical Society
Date: 04-12-2018
DOI: 10.1364/OL.43.005933
Publisher: The Optical Society
Date: 12-09-2013
DOI: 10.1364/OE.21.022194
Publisher: The Optical Society
Date: 30-07-2019
DOI: 10.1364/OE.27.023164
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: The Optical Society
Date: 20-11-2019
DOI: 10.1364/OL.44.005663
Publisher: Optica Publishing Group
Date: 27-04-2020
DOI: 10.1364/OE.390433
Abstract: The typical modal characteristics arising during laser feedback interferometry (LFI) in multi-mode terahertz (THz) quantum cascade lasers (QCLs) are investigated in this work. To this end, a set of multi-mode reduced rate equations with gain saturation for a general Fabry-Pérot multi-mode THz QCL under optical feedback is developed. Depending on gain bandwidth of the laser and optical feedback level, three different operating regimes are identified, namely a single-mode regime, a multi-mode regime, and a tuneable-mode regime. When the laser operates in the single-mode and multi-mode regimes, the self-mixing signal litude (peak to peak value of the self-mixing fringes) is proportional to the feedback coupling rate at each mode frequency. However, this rule no longer holds when the laser enters into the tuneable-mode regime, in which the feedback level becomes sufficiently strong (the boundary value of the feedback level depends on the gain bandwidth). The mapping of the identified feedback regimes of the multi-mode THz QCL in the space of the gain bandwidth and feedback level is investigated. In addition, the dependence of the aforementioned mapping of these three regimes on the linewidth enhancement factor of the laser is also explored, which provides a systematic picture of the potential of LFI in multi-mode THz QCLs for spectroscopic sensing applications.
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: MDPI AG
Date: 09-03-2016
DOI: 10.3390/S16030352
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
Publisher: The Optical Society
Date: 23-09-2013
DOI: 10.1364/OE.21.022988
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: IOP Publishing
Date: 28-08-2014
Publisher: IEEE
Date: 09-2013
Publisher: The Optical Society
Date: 10-03-2015
DOI: 10.1364/OL.40.000994
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: IEEE
Date: 09-2012
Publisher: AIP Publishing
Date: 19-04-2016
DOI: 10.1063/1.4946845
Abstract: We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAs material found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and material doping level were also studied.
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: IEEE
Date: 07-2016
Publisher: IEEE
Date: 08-2017
Publisher: American Chemical Society (ACS)
Date: 07-2020
Publisher: IEEE
Date: 12-2012
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: Optica Publishing Group
Date: 24-03-2009
DOI: 10.1364/OL.34.001030
Abstract: We report the generation of Bessel beams using polytetrafluoroethene conical lenses and a quantum cascade laser emitting at 2.8 THz. The formation of a central beam spot that retains its size over distances exceeding the characteristic Rayleigh range is demonstrated, and the power transport properties of these beams are compared with those obtained using parabolic reflectors. These lenses could provide an attractive alternative to parabolic reflectors for terahertz imaging applications where a large depth of focus and/or efficient and controllable coupling of radiation onto a small target are desirable.
Publisher: The Optical Society
Date: 29-08-2016
DOI: 10.1364/OE.24.020554
Publisher: IEEE
Date: 12-2014
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: 22-04-2014
DOI: 10.1364/OL.39.002629
Publisher: The Optical Society
Date: 09-2017
Publisher: IEEE
Date: 10-2013
Publisher: AIP Publishing
Date: 22-06-2009
DOI: 10.1063/1.3158592
Publisher: AIP Publishing LLC
Date: 2015
DOI: 10.1063/1.4915165
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
Publisher: Institution of Engineering and Technology (IET)
Date: 10-2015
DOI: 10.1049/EL.2015.2878
Publisher: AIP Publishing
Date: 27-07-2020
DOI: 10.1063/5.0014251
Abstract: Photonic engineering of the terahertz emission from a quantum cascade laser (QCL) is fundamental for the exploitation of this unique source in a myriad of applications where it can be implemented, such as spectroscopy, imaging, and sensing. Active control of the frequency, power, polarization, and beam profile has been achieved through a variety of approaches. In particular, the active control of the emitted frequency, which is difficult to determine a priori, has been achieved through the integration of a photonic structure and/or by using external cavity arrangements. In this work, an external cavity arrangement, which implements a metamaterial/graphene optoelectronic mirror as an external feedback element, is proposed and demonstrated. The reflectivity and dispersion properties of the external active mirror were tuned via electrostatically gating graphene. It was possible to electronically reproduce the mode-switch occurring in a QCL emitting ∼2.8 THz by mechanically changing the external cavity length formed by an Au mirror. The external cavity arrangement was investigated and described in the framework of the self-mixing theory. These results open a way for the all-electronic engineering of the QCL emission by the use of a fast reconfigurable external mirror. This approach can uniquely address both power and frequency control, with ∼100 MHz reconfiguration speeds, using an integrated external element. Furthermore, the metamaterial/graphene mirror's strong dispersive properties might be implemented for the active mode locking of THz QCLs. Finally, this approach offers a unique opportunity to study the laser dynamics and mode competition in THz QCLs in the self-mixing feedback regime.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2015
Publisher: IEEE
Date: 09-2014
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: SPIE
Date: 23-02-2013
DOI: 10.1117/12.2000743
Publisher: Optica Publishing Group
Date: 15-08-2022
DOI: 10.1364/OE.467673
Abstract: Lasers that can emit two photons from a single electron relaxation between two states of the same parity have been discussed since the early days of the laser era. However, such lasers have seen only limited success, mainly due to a lack of suitable gain medium. We propose that terahertz (THz) frequency quantum cascade lasers (QCLs) are an ideal semiconductor structure to realize such two-photon emissions. In this work, we present a THz QCL heterostructure designed to emit two resonant photons from each electronic relaxation between two same-parity states in the active region. We present coupled Maxwell-Bloch equations that describe the dynamics of such a two-photon laser and find analytical solutions for the steady-state light intensity, the steady-state energy-resolved carrier densities, and the total threshold carrier density. Due to the two-photon emission from each excited state relaxation and an increased photon-driven carrier transport rate, our simulations predict a significant enhancement of light intensity in our designed resonant two-photon THz QCL when compared to an exemplar conventional THz QCL structure.
Publisher: IOP Publishing
Date: 22-08-2012
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2016
Publisher: AIP Publishing
Date: 02-11-2009
DOI: 10.1063/1.3253714
Publisher: IEEE
Date: 08-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2018
Publisher: AIP Publishing
Date: 02-12-2013
DOI: 10.1063/1.4839535
Abstract: In this Letter, we demonstrate the self-mixing effect in an interband cascade laser. We show that a viable self-mixing signal can be acquired through the variation in voltage across the laser terminals, thereby removing the need for an external detector. Using this interferometric technique, we have measured the displacement of a remote target, and also demonstrated high resolution imaging of a target. The proposed scheme represents a highly sensitive, compact, and self-aligned sensing technique with potential for materials analysis in the mid-infrared.
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: 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: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
Publisher: IEEE
Date: 07-2019
Publisher: The Optical Society
Date: 02-05-2018
DOI: 10.1364/OL.43.002225
Publisher: The Optical Society
Date: 25-06-2019
DOI: 10.1364/OL.44.003314
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2018
Publisher: The Optical Society
Date: 07-11-2013
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: The Optical Society
Date: 25-04-2017
DOI: 10.1364/OE.25.010177
Publisher: American Chemical Society (ACS)
Date: 19-06-2018
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: IEEE
Date: 09-2019
Publisher: The Optical Society
Date: 29-08-2017
DOI: 10.1364/OE.25.021753
Publisher: The Optical Society
Date: 30-06-0003
DOI: 10.1364/OL.40.000950
Publisher: AIP Publishing
Date: 05-08-2013
DOI: 10.1063/1.4818584
Abstract: Joule heating causes significant degradation in the power emitted from terahertz-frequency quantum-cascade lasers (THz QCLs). However, to date, it has not been possible to characterize the thermal equilibration time of these devices, since THz power degradation over sub-millisecond time-scales cannot be resolved using conventional bolometric or pyroelectric detectors. In this letter, we use a superconducting antenna-coupled niobium nitride detector to measure the emission from a THz QCL with a nanosecond-scale time-resolution. The emitted THz power is shown to decay more rapidly at higher heat-sink temperatures, and in steady-state the power reduces as the repetition rate of the driving pulses increases. The pulse-to-pulse variation in active-region temperature is inferred by comparing the THz signals with those obtained from low duty-cycle measurements. A thermal resistance of 8.2 ± 0.6 K/W is determined, which is in good agreement with earlier measurements, and we calculate a 370 ± 90-μs bulk heat-storage time, which corresponds to the simulated heat capacity of the device substrate.
Publisher: Institution of Engineering and Technology (IET)
Date: 02-2014
DOI: 10.1049/EL.2013.4035
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2013
Publisher: OSA
Date: 2015
Publisher: The Optical Society
Date: 11-11-2016
DOI: 10.1364/OE.24.026986
Publisher: IEEE
Date: 08-2015
Publisher: AIP Publishing
Date: 06-2019
DOI: 10.1063/1.5094674
Publisher: IEEE
Date: 08-2015
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: 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: The Optical Society
Date: 17-10-2014
DOI: 10.1364/BOE.5.003981
Publisher: OSA
Date: 2016
Publisher: The Optical Society
Date: 09-07-2012
DOI: 10.1364/OE.20.016662
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: IEEE
Date: 08-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 03-2016
Publisher: AIP
Date: 2011
DOI: 10.1063/1.3666407
Publisher: OSA
Date: 2014
Publisher: AIP Publishing
Date: 20-04-2015
DOI: 10.1063/1.4918993
Abstract: Terahertz-frequency quantum cascade lasers (THz QCLs) based on bound-to-continuum active regions are difficult to model owing to their large number of quantum states. We present a computationally efficient reduced rate equation (RE) model that reproduces the experimentally observed variation of THz power with respect to drive current and heat-sink temperature. We also present dynamic (time-domain) simulations under a range of drive currents and predict an increase in modulation bandwidth as the current approaches the peak of the light–current curve, as observed experimentally in mid-infrared QCLs. We account for temperature and bias dependence of the carrier lifetimes, gain, and injection efficiency, calculated from a full rate equation model. The temperature dependence of the simulated threshold current, emitted power, and cut-off current are thus all reproduced accurately with only one fitting parameter, the interface roughness, in the full REs. We propose that the model could therefore be used for rapid dynamical simulation of QCL designs.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 03-2013
Publisher: OSA
Date: 2018
Publisher: The Optical Society
Date: 15-02-2012
DOI: 10.1364/OL.37.000731
Publisher: Institution of Engineering and Technology (IET)
Date: 06-2017
DOI: 10.1049/EL.2017.0662
Publisher: The Optical Society
Date: 24-07-2014
DOI: 10.1364/OE.22.018633
Publisher: The Optical Society
Date: 25-04-2017
DOI: 10.1364/OE.25.010153
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: SPIE
Date: 05-10-2007
DOI: 10.1117/12.739281
Publisher: AIP Publishing
Date: 22-08-2011
DOI: 10.1063/1.3629991
Abstract: There has been growing interest in the use of terahertz (THz) quantum cascade lasers (QCLs) for sensing applications. However, the lack of compact and sensitive THz detectors has limited the potential for commercial exploitation of sensors based on these devices. We have developed a self-mixing sensing technique in which THz QCLs are used for both generation and interferometric sensing of THz radiation, eliminating the need for a separate detector. Using this technique, we have measured the displacement of a remote target, both with and without opaque (in the visible spectrum) materials in the beam path and demonstrated a stand-off distance of up to 7 m in air.
Publisher: IEEE
Date: 09-2012
Publisher: The Optical Society
Date: 12-2016
DOI: 10.1364/OE.24.028583
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 Physical Society (APS)
Date: 08-03-2021
Publisher: The Optical Society
Date: 05-02-2018
DOI: 10.1364/OE.26.003814
Publisher: The Optical Society
Date: 12-09-2016
DOI: 10.1364/OE.24.021948
Publisher: The Optical Society
Date: 07-2011
DOI: 10.1364/OL.36.002587
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.
Location: United Kingdom of Great Britain and Northern Ireland
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