ORCID Profile
0000-0002-9121-9846
Current Organisation
University of Leeds
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Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
Publisher: IEEE
Date: 07-2019
Publisher: IEEE
Date: 10-2011
Publisher: IEEE
Date: 10-2015
Publisher: The Optical Society
Date: 25-06-2019
DOI: 10.1364/OL.44.003314
Publisher: The Optical Society
Date: 12-09-2013
DOI: 10.1364/OE.21.022194
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 09-2018
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: 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: Cambridge University Press (CUP)
Date: 2022
DOI: 10.1017/SUS.2022.17
Abstract: We summarize what we assess as the past year's most important findings within climate change research: limits to adaptation, vulnerability hotspots, new threats coming from the climate–health nexus, climate (im)mobility and security, sustainable practices for land use and finance, losses and damages, inclusive societal climate decisions and ways to overcome structural barriers to accelerate mitigation and limit global warming to below 2°C. We synthesize 10 topics within climate research where there have been significant advances or emerging scientific consensus since January 2021. The selection of these insights was based on input from an international open call with broad disciplinary scope. Findings concern: (1) new aspects of soft and hard limits to adaptation (2) the emergence of regional vulnerability hotspots from climate impacts and human vulnerability (3) new threats on the climate–health horizon – some involving plants and animals (4) climate (im)mobility and the need for anticipatory action (5) security and climate (6) sustainable land management as a prerequisite to land-based solutions (7) sustainable finance practices in the private sector and the need for political guidance (8) the urgent planetary imperative for addressing losses and damages (9) inclusive societal choices for climate-resilient development and (10) how to overcome barriers to accelerate mitigation and limit global warming to below 2°C. Science has evidence on barriers to mitigation and how to overcome them to avoid limits to adaptation across multiple fields.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
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: 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: The Optical Society
Date: 28-03-2019
DOI: 10.1364/OE.27.010221
Publisher: IEEE
Date: 12-2010
Publisher: IOP Publishing
Date: 28-08-2014
Publisher: The Optical Society
Date: 10-03-2015
DOI: 10.1364/OL.40.000994
Publisher: The Optical Society
Date: 30-06-0003
DOI: 10.1364/OL.40.000950
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 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: 06-2019
DOI: 10.1063/1.5094674
Publisher: SPIE
Date: 05-09-2014
DOI: 10.1117/12.2061433
Publisher: IEEE
Date: 07-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 03-2020
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: 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: 29-08-2016
DOI: 10.1364/OE.24.020554
Publisher: IEEE
Date: 12-2014
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: The Optical Society
Date: 22-04-2014
DOI: 10.1364/OL.39.002629
Publisher: IEEE
Date: 10-2013
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: Institution of Engineering and Technology (IET)
Date: 10-2015
DOI: 10.1049/EL.2015.2878
Publisher: The Optical Society
Date: 24-07-2014
DOI: 10.1364/OE.22.018633
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: 2020
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2015
Publisher: The Optical Society
Date: 25-04-2017
DOI: 10.1364/OE.25.010153
Publisher: SPIE
Date: 08-12-2016
DOI: 10.1117/12.2250621
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: 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: 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: MDPI AG
Date: 04-11-2022
DOI: 10.3390/S22218501
Abstract: We have recently reported the self-pulsation phenomenon under strong optical feedback in terahertz (THz) quantum cascade lasers (QCLs). One important issue, however, we left open: the effect of multiple round trips in the external cavity on the laser response to feedback. Our current analysis also casts additional light on the phenomenon of self-pulsations. Using only one external cavity round trip (ECRT) in the model has been the common approach following the seminal paper by Lang–Kobayashi in 1980. However, the conditions under which the Lang–Kobayashi model, in its original single-ECRT formulation, is applicable has been rarely explored. In this work, we investigate the self-pulsation phenomenon under multiple ECRTs. We found that the self-pulsation waveform changes when considering more than one ECRT. This we attribute to the combined effect of the extended external cavity length and the frequency modulation of the pulsation frequency by the optical feedback. Our findings add to the understanding of the optical feedback dynamics under multiple ECRTs and provide a pathway for selecting the appropriate numerical model to study the optical feedback dynamics in THz QCLs and semiconductor lasers in general.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2017
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: American Physical Society (APS)
Date: 08-03-2021
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: 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: SPIE-Intl Soc Optical Eng
Date: 02-10-2014
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 2012
End Date: 2014
Funder: Australian Research Council
View Funded ActivityStart Date: 2016
End Date: 2018
Funder: Australian Research Council
View Funded ActivityStart Date: 2020
End Date: 2022
Funder: Australian Research Council
View Funded Activity