ORCID Profile
0000-0002-5493-6352
Current Organisation
University of Leeds
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Publisher: The Optical Society
Date: 04-11-2015
Publisher: IEEE
Date: 07-2019
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: American Physical Society (APS)
Date: 12-12-2022
Publisher: The Optical Society
Date: 09-2017
Publisher: IEEE
Date: 09-2013
Publisher: IEEE
Date: 09-2018
Publisher: The Optical Society
Date: 30-07-2019
DOI: 10.1364/OE.27.023164
Publisher: IEEE
Date: 10-2019
Publisher: American Physical Society (APS)
Date: 12-06-2013
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: 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: Institution of Engineering and Technology (IET)
Date: 06-2017
DOI: 10.1049/EL.2017.0662
Publisher: American Physical Society (APS)
Date: 12-09-2022
Publisher: The Optical Society
Date: 25-04-2017
DOI: 10.1364/OE.25.010177
Publisher: IEEE
Date: 09-2014
Publisher: American Chemical Society (ACS)
Date: 19-06-2018
Publisher: AIP
Date: 2013
DOI: 10.1063/1.4848506
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: 08-2015
Publisher: IEEE
Date: 09-2014
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: Springer Science and Business Media LLC
Date: 22-12-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 03-2015
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 05-2019
Publisher: Institution of Engineering and Technology (IET)
Date: 02-2014
DOI: 10.1049/EL.2013.4035
Publisher: OSA
Date: 2015
Publisher: The Optical Society
Date: 11-11-2016
DOI: 10.1364/OE.24.026986
Publisher: IEEE
Date: 08-2015
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: 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: The Optical Society
Date: 12-2016
DOI: 10.1364/OE.24.028583
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2018
Publisher: American Chemical Society (ACS)
Date: 21-02-2020
Publisher: IEEE
Date: 09-2012
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: 10-2019
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: The Optical Society
Date: 28-08-2012
DOI: 10.1364/OE.20.020855
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: 08-2017
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: American Chemical Society (ACS)
Date: 07-2020
Publisher: The Optical Society
Date: 05-02-2018
DOI: 10.1364/OE.26.003814
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Joshua Freeman.