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We report programmable control of pulses from a quantum cascade laser. A programmable arbitrary waveform generator is used to control the emission of the lasers by gain-switching. Quantum cascade lasers with both multi-mode and single-mode emission are studied. Laser emission is coherently detected using a phase resolved injection seeding technique based on time domain spectroscopy. For the single mode laser, pulse widths of 500–4000 ps are demonstrated, with time-bandwidth products remaining close to, or below, unity for all pulse widths. Additionally, sequences of two pulses are demonstrated, where the pulse widths, delay, and amplitude of each of the pulses are programmed electronically. A range of pulse parameters are explored. This approach allows for the generation of arbitrary terahertz pulse sequences, which will enable coherent multidimensional spec-troscopy and applications requiring optimized pulse shapes.

Objectives To examine job satisfaction, well-being, job stress, and burnout among laboratory professionals. Methods The study utilized a cross-sectional survey design. The survey was administered online via the American Society for Clinical Pathology’s survey tool, to elicit information about job satisfaction, well-being, job stress, and burnout among medical laboratory professionals. Results Although this survey shows high job satisfaction among respondents, overall job-related stress is high and burnout is prevalent. The majority of the respondents rated their work-life balance as “fair.” The main contributing factors to job stress, burnout, and work-life balance are quantity of workload and understaffing. Conclusions Based on the results of this survey, creating targeted interventions may help improve the quality of well-being programs for laboratory professionals. A comprehensive wellness program developed at the institutional, local, and national levels may improve morale and alleviate the recruitment and retention challenges faced by medical laboratory professionals.

Frequency tuning in terahertz frequency quantum cascade lasers is challenging because of low thermal and current tuning coefficients. Moreover, photonic designs like Vernier selection based sampled gratings, used in telecom lasers to tune emission frequency, are unsuitable due to the long terahertz wavelengths and will require impractically long cavities (>15 mm). Here, we report the first wideband frequency tuning from a monolithic device exploiting Vernier selection rules using a coupled-cavity laser with a defect engineered photonic lattice. A precisely positioned defect lattice allows us to engineer the free spectral range and finesse of one of the cavities, similar to a sampled grating but using shorter cavity lengths (

Objectives To determine the extent and distribution of workforce shortages within the nation’s medical laboratories. Methods The survey was conducted through collaboration between the American Society for Clinical Pathology Institute for Science, Technology, and Public Policy in Washington, DC, and the Evaluation, Measurement, and Assessment Department and Board of Certification in Chicago, IL. Data were collected via an internet survey distributed to individuals who were able to report on staffing and certifications for their laboratories. Results The coronavirus disease 2019 (COVID-19) pandemic disrupted the staffing of clinical laboratories and the stream of incoming graduates entering the workforce. Results show decreased vacancy rates for the majority of laboratory positions across all departments surveyed. The overall anticipated retirement rates continue to decline, which suggests that the field has already lost personnel with vast amounts of experience. Conclusions Addressing the current and future needs of the laboratory workforce requires a collective effort by numerous groups of stakeholders at all levels, including the laboratory employers, laboratory training programs, health care executives/hospital administrators, and professional organizations. The time is now to address the future shortage of laboratory professionals and to create a resilient clinical laboratory professional workforce.

We have developed precision micromachining techniques for integration of terahertz quantum cascade lasers (QCLs) with waveguides and feedhorns for use as local oscillators in satellite-borne receivers. We demonstrate these techniques using QCLs at 3.4 THz and 4.7 THz, as well as the first QCL with a monolithically integrated power modulator.

We report all-electronic coherent scattering-type scanning near-field microscopy (s-SNOM) using a terahertz-frequency quantum cascade laser. By exploiting the coherent self-mixing effect in these lasers, in conjunction with electronic frequency tuning of the laser, we demonstrate spatial mapping of both the amplitude and the phase of the scattered field with deeply subwavelength resolution. We apply our technique for coherent microscopy of a phonon-resonant crystal. The extracted amplitude and phase parameters reveal clear contrast when compared to both metallic and nonresonant dielectric materials and show excellent agreement with those calculated using a finite-dipole model of the near-field interaction between the s-SNOM tip and the resonant sample in the Reststrahlen band. Our technique paves the way for fast nanoscale-resolved mapping of the dielectric function of solid state systems and optoelectronic nanodevices at terahertz frequencies.

We present a novel approach in optical processing by accelerating Fourier transform through integration of silicon photonics and free-space optics. We introduce Fourier transform based multiply and accumulate operations, that require less number of operations compared to the traditional multiply and accumulate operations used in computer architecture.

Objectives To inform the pathology and laboratory field of the most recent national wage data. Historically, the results of this biennial survey have served as a basis for additional research on laboratory recruitment, retention, education, marketing, certification, and advocacy. Methods The 2019 Wage Survey was conducted through collaboration of the American Society for Clinical Pathology (ASCP) Institute of Science, Technology, and Policy in Washington, DC, and the ASCP Board of Certification in Chicago, Illinois. Results Compared with 2017, results show an overall increase in salaries for most laboratory occupations surveyed except cytogenetic technologists, laboratory information systems personnel, and performance improvement or quality assurance personnel. Geographically, laboratory professionals from urban areas earned more than their rural counterparts. Conclusions As retirement rates continue to increase, the field needs to intensify its efforts on recruiting the next generation of laboratory personnel. To do so, the report urged the field to highlight advocacy for better salaries for laboratory personnel at the local and national levels when developing recruitment and retention strategies.

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. However, they are frequently not as heightened as predicted, limiting their exploitation in nanostructured THz nonlinear optics. Here, we show that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive interference effects. Using THz quantum-cascade-lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an interplay of the second-order nonlinear contributions of multiple light and heavy hole states. As well as of importance to engineer the resonant optical properties of nanostructures, this framework can be employed as a novel, sensitive tool to elucidate the bandstructure properties of complex materials.

Objectives The purpose of this study was to align the current experiences and best practices in revised reporting (issuing of addenda and amendments) in pathology. Pathology specialties explored in the survey include anatomic pathology, surgical pathology, cytopathology, and hematopathology. Methods The study used a cross-sectional design in which an online revised reporting survey was deployed to a large national sample represented by pathologists, pathology residents, pathology fellows, pathology managers, and laboratory directors. Results Qualitative and quantitative results from this survey highlight significant variation in standards for creating, issuing, and tracking quality indicators related to addenda and amendments. The most notable findings were a lack of standardization and the potential for widespread adoption of revised reporting best practices within and between pathology services. Conclusions Survey insight provides the potential for improving patient safety outcomes, engaging with consumers of our reports, providing a current state view of revised reporting, and assessing the attitudes of pathologists and laboratory professionals on how their individual approaches and team-based workflows achieve revised reports. The data generated from this survey will provide patient safety opportunities associated with accurate pathology reporting and will encourage further development of optimal pathology revised reporting guidelines.

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. These resonant nonlinearities continually attract attention, particularly in newly discovered materials. However, they are frequently not as heightened as currently predicted, limiting their exploitation in nanostructured nonlinear optics. Here, we present a clear-cut theoretical and experimental demonstration that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive, as well as constructive, interference effects in complex systems. Using terahertz quantum-cascade-lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an unexpected interplay of the second-order nonlinear contributions of multiple light and heavy hole states.

Objectives The study explored job stress, burnout, work-life balance, well-being, and job satisfaction among pathology residents and fellows. The aims were to examine the prevalence and sources of stress and burnout, as well as identify resources to promote work-life balance and well-being and prevent burnout. Methods The study used a cross-sectional survey deployed online to a large national sample of pathology residents and fellows. Results Job stress and burnout were prevalent, with more than a third of the respondents reporting that they were currently experiencing burnout. The respondents, particularly residents, were struggling with academics, and higher percentages were struggling with work-life balance and emotional well-being. Overall, the majority of respondents who rated their work-life balance indicated that it was poor or fair. Among the factors contributing to job stress and burnout and detracting from work-life balance, workload was the leading factor. Conclusions The factors contributing to job stress and burnout included organizational factors such as workload, value, and aspects of the learning environment, as well as personal factors such as work-life integration. One of the overarching implications is the need to address a range of interdependent considerations in designing resources to reduce job stress, promote work-life balance, and prevent burnout.

Objectives To examine job satisfaction, well-being, job stress, and burnout among pathologists. Methods The study utilized a cross-sectional survey design. The survey was administered online via the American Society for Clinical Pathology’s (ASCP’s) survey tool to elicit information about job satisfaction, well-being, job stress, and burnout among pathologists. Results Job satisfaction is high and well-being is rated fair to good by most respondents. However, feelings of anxiety or worry about work, high levels of stress, and burnout are prevalent among pathologists. The main contributing factor to job stress, burnout, and work-life balance is quantity of workload. Conclusions Creating targeted interventions based on the results of this survey may help improve the type and quality of wellness programs for pathologists. Trust among team members, managers and clinicians, and institutions can help reduce stress and increase collaboration, engagement, and motivation.

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.

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 amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, 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., The typical electronic modulation of terahertz quantum cascade lasers is fundamentally limited at fast timescales by device properties. Here the authors propose and study an alternative, acoustic mechanism for modulating such THz QCLs at high speed.

To determine the extent and distribution of workforce shortages within the nation's medical laboratories.The 2016-2017 Vacancy Survey was conducted through collaboration between the American Society for Clinical Pathology's Institute of Science, Technology,Policy in Washington, DC, and the Evaluation, Measurement, and Assessment division and Board of Certification in Chicago, Illinois. Data were collected via an internet survey that was distributed to individuals who were able to report on staffing and certifications for their laboratories.Results of the 2016-2017 Vacancy Survey shows decreased vacancy rates for laboratory positions across all departments surveyed compared with 2014. While overall, the data show that vacancy rates are decreasing, overall retirement rates and certification requirements are higher.Focus on the qualifications and certification status of laboratory professionals would be crucial factors in addressing the needs of the laboratory workforce. The field needs to intensify its efforts on recruiting the next generation of laboratory personnel.

We describe a new approach to modelling the optoelectronic properties of a terahertz-frequency quantum cascade laser (THz QCL) based on a quantum transmission line modelling (Q-TLM) method. Parallel quantum cascade transmission line modelling units are employed to describe the dynamic optical processes in a nine-well THz QCL in both the time and frequency domains. The model is used to simulate the current-power characteristics of a QCL device and good agreement is found with experimental measurements, including an accurate prediction of the threshold current and emitted power. It is also confirmed that the Q-TLM model can accurately predict the Stark-induced blue shift of the emission spectrum of the THz QCL with increasing injection current. Furthermore, we establish the new Q-TLM model to describe the properties of a THz QCL device incorporating a photonic lattice patterned on the laser ridge, by linking the transmission line structure to each scattering module. The predicted effects of the lattice structure on the steady-state emission spectra of the THz QCL, including the side-mode suppression, are found to be in good agreement with experimental results. Our Q-TLM modelling approach is a promising tool for the future design of THz QCLs and analysis of their temporal and spectral behaviors.

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. These resonant nonlinearities continually attract attention, particularly in newly discovered materials. However, they are frequently not as heightened as currently predicted, limiting their exploitation in nanostructured nonlinear optics. Here, we present a clear-cut theoretical and experimental demonstration that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive, as well as constructive, interference effects in complex systems. Using terahertz quantum cascade lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an unexpected interplay of the second-order nonlinear contributions of multiple light and heavy hole states. As well as of importance to understand and engineer the resonant optical properties of nanostructures, this advanced framework can be used as a novel, sensitive tool to elucidate the band structure properties of complex materials., Science Advances, 5 (10), ISSN:2375-2548

We demonstrate a new method for high-speed modulation of terahertz emission and electronic transport of a Ga(Al)As quantum cascade laser using coherent acoustic phonon pulses. The modulation, which is on the order of 6%, can be partially explained by a perturbation-theory analysis. The 100 GHz are possible.

We report measurements of switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. We observe hopping between lasing modes on picosecond-timescales and temporal evolution of transient multi-mode emission into steady-state single mode emission.

ObjectiveAdoption of pre-exposure prophylaxis (PrEP) to prevent HIV infection has been slow. The purpose of this study was to evaluate knowledge, attitudes and referral practices for PrEP among non-prescribing providers, who may play key role.MethodsWe performed a cross-sectional survey on PrEP knowledge, attitudes, and referral practices among 66 non-prescribing HIV prevention providers (1st August to 31st December, 2016), in Philadelphia, followed by qualitative interviews with 12 of them (5th April to 10th May, 2017).ResultsParticipants had a mean age of 36 years, with 62% females. Majority were HIV case managers and rapid testers. For half of the respondents, PrEP eligibility screening was part of rapid HIV testing at their organization, 40% never had PrEP training and only 27% indicated personally screening clients for eligibility. Qualitative data revealed that participants held positive attitudes about PrEP and perceived organizational support, but had concerns about potential negative impacts and barriers to routine HIV screening.ConclusionResults highlight the importance of training non-prescribing HIV prevention providers about PrEP, addressing their concerns, and incorporating PrEP screening and referral into routine HIV testing.

Introduction/Objective Diagnostic errors in pathology may have adverse impact on patient outcomes and are often rectified through revised reports (RR). Improving patient outcomes with accurate RR is a tangible yet challenging benefit to assuring continuous quality improvement (CQI). Assessment and elevation of RR optimization requires counterbalance of workflow complexity in the diagnostic reporting domain. Implications inform best-practice guidelines for pathology RR and exemplify improved patient outcomes by driving down negative impacts from diagnostic errors. Methods A “Survey for RR in Pathology: Reality & Best Practices” was sent via email to relevant stakeholders. The 8-item survey was designed by the National Pathology Quality Registry team & ASCP’s Institute for Science, Technology & Policy. The model included quantitative and qualitative feedback to probe current experiences with RR. The survey was open April 1-30, 2019, via Key Survey and used snowball sampling. Results Key results illuminate necessity for RR standardization. Survey findings represent 172 respondents. Ninety- two percent of respondents indicated report accuracy as a major indication for optimizing RR practices & positively impacting patient care. Pathology practices assure appropriate RR by notifying a care provider when a change in diagnosis necessitates RR (89%) & 86% of respondents indicate delineation of RR types (e.g. addenda, amendment). Still 54% of respondents see inappropriate RR use with lack of notification to care providers and 48% indicate no delineation of RR types. This balance-counterbalance highlights deviations from optimized RR and a need for guidelines. Effects on patient care or impact to a patient’s treatment plan was indicated by 43% who affirmed stratification of diagnostic discrepancies as major vs. minor. Solely focusing on changes in diagnosis (benign vs. malignant) was heralded by 19% of respondents as a reason to categorize diagnostic discrepancies. Forty-two percent of respondents indicate data-driven CQI in the RR domain. Conclusion Identified RR practice gaps decrease diagnostic accuracy, confirming the need for optimal RR guidelines. RR guidelines should focus on standardized nomenclature; active dialogue between laboratory team & clinical care partners; streamlined workflows to assure accuracy; & valuing transparency to derive improved patient outcomes based on high-quality diagnostic pathology RR.

To inform the pathology and laboratory field of the most recent national wage data. Historically, the results of this biennial survey have served as a basis for additional research on laboratory recruitment, retention, education, marketing, certification, and advocacy.The 2017 Wage Survey was conducted through collaboration between the American Society for Clinical Pathology's (ASCP's) Institute of Science, Technology, and Policy in Washington, DC, and the ASCP Board of Certification in Chicago, IL.Compared with 2015, results show an overall increase in salaries for most of the laboratory occupations surveyed except histotechnologists and pathologists' assistants. Geographically, laboratory professionals from urban areas earn more than their rural counterparts.Survey results encourage laboratory professionals to be actively engaged in advocating for the profession in the workforce and educational training programs. Awareness of the career opportunities and value of the profession is needed to strengthen the future of the field.

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modelling and the spectral characteristics are modelled using a transfer matrix model. Continuous frequency tuning of ~19 GHz is demonstrated while maintaining an output power of ~4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically-controlled continuous tuning performance from a THz QCL without significant change in output power.

We report electrically-controlled continuous frequency tuning over ∼20 GHz by exploiting the additive Vernier tuning effect in a THz QCL based on a longitudinally-coupled Y-branched waveguide.

The authors demonstrate a broadband, heterogeneous terahertz frequency quantum cascade laser by exploiting an active region design based on longitudinal optical-phonon-assisted interminiband transitions. They obtain continuous wave laser emission with a threshold current density of ~120 A/cm 2 , a dynamic range of ~3.1, and an emission spectrum spanning from 2.4 to 3.4 THz at 15 K.

We propose a laser feedback interferometer operating at multiple terahertz (THz) frequency bands by using a pulsed coupled-cavity THz quantum cascade laser (QCL) under optical feedback. A theoretical model that contains multi-mode reduced rate equations and thermal equations is presented, which captures the interplay between electro-optical, thermal, and feedback effects. By using the self-heating effect in both active and passive cavities, self-mixing signal responses at three different THz frequency bands are predicted. A multi-spectral laser feedback interferometry system based on such a coupled-cavity THz QCL will permit ultra-high-speed sensing and spectroscopic applications including material identification.

In [1], the affiliation for Andrew Grier was incorrect. The correct affiliation where his contribution was made is as follows: A. T. Grier was with the School of Electronic and Electrical Engineering, University of Leeds, LS2 9JT Leeds, U.K. (e-mail: atgrier4@gmail.com).

We report on the high detection sensitivity of a laser feedback interferometry scheme based on a terahertz frequency quantum cascade laser (QCL). We show that variations on the laser voltage induced by optical feedback to the laser can be resolved with the reinjection of powers as low as ∼-125 dB of the emitted power. Our measurements demonstrate a noise equivalent power of ∼1.4 pW/√Hz, although, after accounting for the reinjection losses, we estimate that this corresponds to only ∼1 fW/√Hz being coupled to the QCL active region.

Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented using a transgenic mouse model. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We demonstrate the generation of short terahertz pulses from spectrally broad metal-metal quantum cascade lasers at 77 K via active mode-locking, and show the limiting role of phase-matching between the terahertz pulse and the microwave modulation. Furthermore a new concept of THz pulse dispersion control is proposed to go beyond the limitation of the current modulation scheme.

We demonstrate terahertz (THz) apertureless near-field microscopy exploiting the self-mixing effect in a quantum cascade laser (QCL). A THz wave is scattered by a sharp needle positioned above an object and coupled back into the QCL cavity resulting in detection of the THz near-field signal through the self-mixing effect. Using this technique we demonstrate two-dimensional imaging at 2.53 THz with a spatial resolution of 1 μm — the highest image resolution achieved with a THz frequency QCL to date. This method offers an experimentally simple approach to coherent, high-resolution THz imaging.

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 sampling 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., Single-mode, tuneable monolithic semiconductor lasers are important light sources for integrated photonics. Here, Kundu et al. observe the switch-on dynamics and mode competition of a terahertz quantum cascade laser and explain the behaviour with a carrier and photon transport model.

We demonstrate a gas spectroscopy technique, using self-mixing in a 3.4 terahertz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional terahertz instrumentation (detectors, mixers, or spectrometers) for system pre-calibration or spectral analysis. By contrast, our system self-calibrates the laser frequency (i.e., with no external instrumentation) to a precision of 630 MHz (0.02%) by analyzing QCL voltage perturbations in response to optical feedback within a 0–800 mm round-trip delay line. We demonstrate methanol spectroscopy by introducing a gas cell into the feedback path and show that a limiting absorption coefficient of ∼1×10⁻⁴ cm⁻¹ is resolvable.

International audience; The far-field emission profile of terahertz quantum cascade lasers (QCLs) in metal-metal waveguides is controlled in directionality and form through planar horn-type shape structures, whilst conserving a broad spectral response. The structures produce a gradual change in the high modal confinement of the waveguides and permit an improved far-field emission profile and resulting in a four-fold increase in the emitted output power. The two-dimensional far-field patterns are measured at 77 K and are agreement in with 3D modal simulations. The influence of parasitic high-order transverse modes is shown to be controlled by engineering the horn structure (ridge and horn widths), allowing only the fundamental mode to be coupled out.

We demonstrate the generation of 11ps terahertz pulses from metal-metal (MM) quantum cascade lasers (QCLs) at 77K via active mode-locking. Contrary to popular belief that a long gain recovery time is required, we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic microwave modulation and the generated THz pulses in the QCL. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting pulse generation.

We demonstrate high-performance THz QCLs lasing at 2.2, 2.53, 3.5 and 4.7 THz, which target absorption lines of water, methane, hydroxyl and atomic oxygen respectively. Reliable single-mode targeting of gas species is obtained through the use of a photonic lattice design. A highly reproducible micro-machined waveguide block yields narrow beam-divergence and enables future integration of a complete THz heterodyne system including local-oscillator, mixer, and feed-horn.

Injection seeding of terahertz quantum-cascade-lasers with metal-metal waveguides is demonstrated at liquid nitrogen temperatures through injection of phase-locked terahertz pulses. Coherent detection and modelocking of the QCL are demonstrated with the generation of 11ps pulses.

We present experimental evidence for a photothermoelastic response in zincblende crystals illuminated by quantum cascade laser sources in the frequency range 2.2–2.9 THz. Results obtained using an optically balanced sampling arrangement indicate a mechanism whereby the stress distribution established through localized heating of the crystal induces a change in optical birefringence via the photoelastic response of the crystal. A full mathematic model of this photothermoelastic mechanism in (110)-orientated crystals is presented, and shown to agree well with experimental measurements of the magnitude, and the orientational and spatial dependencies of the sampled signal in ZnTe and GaP crystals.

We investigate the photothermoelastic response of ZnTe and GaP crystals irradiated by THz-frequency radiation from a quantum cascade laser. We present a full theoretical description of this interaction that agrees well with the measured response.

We demonstrate quasi-continuous tuning of the emission frequency from coupled cavity terahertz frequency quantum cascade lasers. Such coupled cavity lasers comprise a lasing cavity and a tuning cavity which are optically coupled through a narrow air slit and are operated above and below the lasing threshold current, respectively. The emission frequency of these devices is determined by the Vernier resonance of longitudinal modes in the lasing and the tuning cavities, and can be tuned by applying an index perturbation in the tuning cavity. The spectral coverage of the coupled cavity devices have been increased by reducing the repetition frequency of the Vernier resonance and increasing the ratio of the free spectral ranges of the two cavities. A continuous tuning of the coupled cavity modes has been realized through an index perturbation of the lasing cavity itself by using wide electrical heating pulses at the tuning cavity and exploiting thermal conduction through the monolithic substrate. Single mode emission and discrete frequency tuning over a bandwidth of 100 GHz and a quasi-continuous frequency coverage of 7 GHz at 2.25 THz is demonstrated. An improvement in the side mode suppression and a continuous spectral coverage of 3 GHz is achieved without any degradation of output power by integrating a π-phase shifted photonic lattice in the laser cavity.

We explain the origin of voltage variations due to self-mixing in a terahertz (THz) frequency quantum cascade laser (QCL) using an extended density matrix (DM) approach. Our DM model allows calculation of both the current–voltage (I–V) and optical power characteristics of the QCL under optical feedback by changing the cavity loss, to which the gain of the active region is clamped. The variation of intra-cavity field strength necessary to achieve gain clamping, and the corresponding change in bias required to maintain a constant current density through the heterostructure is then calculated. Strong enhancement of the self-mixing voltage signal due to non-linearity of the (I–V) characteristics is predicted and confirmed experimentally in an exemplar 2.6 THz bound-to-continuum QCL.

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 amplitude is observed for small tip-sample separations, allowing image resolutions of ∼1 μm (∼λ/100) and ∼7 μm to be achieved along orthogonal directions on the sample 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.

We have investigated the thermo-optical (TO) response of a ZnTe crystal to THz QCL radiation in the frequency range ~2.2-2.9 THz. The TO response was investigated for variations in driving current, QCL polarisation direction and pulse modulation frequency. We show that the anisotropy of the TO coefficients should be taken into account to correctly describe the experimental response.