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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 samples (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 samples were well correlated with the histological findings. The per-sample locations of pathology vs healthy skin can be separated from the density distribution of the corresponding pixels in the THz amplitude–phase map. The possible THz contrast mechanisms relating to the origin of image contrast in addition to water content were analyzed from these dehydrated samples. Our findings suggest that THz imaging could provide a feasible imaging modality for skin cancer detection that is beyond the visible.

Theoretical investigation of electro-absorption modulators in the mid-infrared range (>∼2 μm) is performed using asymmetric intra-step quantum wells based on Ge1−η1Snη1/Ge1−η2Snη2 heterostructures with SiGeSn outer barriers. After exploring the parameter space of the Sn content difference and width ratio of the intra-layers, a linear and much larger Stark shift is realized, compared to that of a square quantum well, without an increase of the total structure width. A modulator based on an optimized intra-step quantum well structure with a total well width of 12 nm is theoretically predicted to have both a larger peak shift per unit applied field and a larger absorption change than a 12 nm square quantum well device. By analyzing the device performance based on the two figures of merit: (1) absorption change per applied field and (2) absorption change per applied field squared, and taking 10 dB extinction ratio, a 44% higher bandwidth per volt and 46% lower power consumption per bit are achieved in intra-step than in a square well. Although the swing voltage for a square quantum well can be reduced by using a larger on-set applied field and performance could be improved, we found that the intra-step quantum well using zero on-set still retains its advantages when compared to the square quantum well which uses a 0.5 V on-set voltage.

This present collaborative research, undertaken in two different hemispheres, in an effort to address the challenge of early structural and sub-surface assessment of heritage marble architectures, like the Taj Mahal, using two complementary non-contact, non-invasive imaging techniques in the THz spectral range. In our previous work, it was already demonstrated that the complementary techniques of broadband Terahertz Time Domain Imaging (THz-TDI) and microRaman spectroscopy are successful in probing volume and surface damage in marble with Pietra-dura work. In the present work, the unique combination of THz-TDI and highly sensitive THz-Laser Feedback Interferometry (THz-LFI) have been explored to study sub-surface damage and irregularities of marble structures with Pietra-dura motif. These optical techniques hold immense possibility in large-scale architectural restoration projects as they collectively provide accurate structural depth profile up to several inches into the volume of the marble including the strain generated within the structure leading to potential cracks.

A direct bandgap can be engineered in Ge-rich group-IV alloys by increasing Sn content and by introducing tensile strain in GeSn. Here, we combine these two routes in quantum well (QW) structures and systematically analyze the properties of SiGeSn/GeSn quantum wells for a range of Sn content, strain, and well width values, within realistic boundaries. Using the k ⋅ p method, and including L-valley within the effective mass method, we find that 13–16 nm is a preferred range of well widths to achieve high gain for tensile-strained SiGeSn/GeSn quantum wells. Within the range of the well widths, a loss ridge caused by inter-valence band absorption and free carrier absorption is found in the region of parameter space where Sn content and strain in the well are related as Sn ( % ) ≈ − 7.71 e x x ( % ) + 17.13. Limited by a practical strain boundary of 1.7%, for a 14 nm quantum well, we find that 7.5 ± 1 % Sn and 1 ± 0.2 % strain is a promising combination to get a good net gain for photon transition energy higher than ∼0.42 eV. A maximum utilization of strain is preferred to obtain the best gain with lower energies (

We provide an experimental evidence of transient instabilities (TIs) in a terahertz (THz) quantum cascade laser (QCL) under optical feedback, in contrast to the widely accepted claim that THz QCLs are ultrastable against feedback. The TIs appear as periodic oscillations in emitted power or terminal voltage of the laser with an increasing oscillation frequency as feedback increases. The absence of relaxation oscillations and low linewidth enhancement factor in THz QCLs makes them a platform uniquely suitable for exploring external-cavity-related dynamics in semiconductor lasers. This work opens a pathway to a THz sensing and imaging modality based on these TIs, which has much reduced complexity compared to existing approaches using laser feedback interferometry.

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.

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.

Over the last decade, terahertz (THz) time-domain spectroscopy has been investigated as a technique for assaying the ethanol content of liquid solutions-indeed, operating at THz frequencies addresses some of the challenges that traditional optical refraction measurements face, such as delineation between sugar-ethanol content, florescence, and problems arising from carbonation or other dissolved gasses. In this article, we propose an alternative system and method for assaying ethanol content of liquid solutions at THz frequencies, which employs a laser feedback interferometer built around a 2.6-THz quantum cascade laser. The system is tested against a series of controlled water-ethanol solutions, as well as a series of commercially available beverages. The accuracy of the estimated ethanol content compares favorably to THz time-domain spectroscopy techniques.

Terahertz quantum cascade-lasers (QCLs) are able to produce higher optical output power at higher temperatures when operated in pulsed mode. Predicting a laser's behavior under pulsed operation in order to achieve performance requirements is, however, a nontrivial exercise: the complex and nonlinear interplay between current, electric field, and thermal transients gives rise to complex responses in both optical output power and emission frequency. In applications where it is important to predict and control these behaviors, establishing the link between current drive, emission frequency, and optical output power is necessary. In this paper, we demonstrate, via a realistic laser-specific model, that by appropriate manipulation of the drive pulse we can not only obtain a higher optical output at increased operating temperature but also both extend and linearize a QCL's frequency sweep. We suggest that consideration of laser behavior through realistic and comprehensive modeling is not only useful but is also required in any pulsed application in which emission frequency change is likely to affect performance.

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.

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.

In this work we present a generalization of the Liouvillian superoperator for periodic quantum systems that can be formulated through partitioned Hamiltonians. We formulate a compact algebraic form of the superoperator that allows efficient numerical implementation along with the possibility of further generalization and the inclusion of the system’s boundary effects (i.e. device contacts). We apply this formalism to Quantum Cascade Laser structure where we compare the second nearest and the nearest on approximation, and present the laser dynamics that is independent from the number of states considered.

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.

Quantum cascade lasers (QCLs) are high-power sources of coherent radiation in the midinfrared and terahertz (THz) bands. Laser feedback interferometry (LFI) is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and LFI is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. LFI with QCLs has been demonstrated for a wide range of applications, including the measurement of internal laser characteristics, trace gas detection, materials analysis, biomedical imaging, and near-field imaging. This article provides an overview of QCLs and the LFI technique, and reviews the state of the art in LFI with sensing using QCLs.

We report a coherent terahertz (THz) imaging system that utilises a quantum cascade laser (QCL) operating in pulsed-mode as both the source and detector. The realisation of a short-pulsed THz QCL feedback interferometer permits both high peak powers and improved thermal efficiency, which enables the cryogen-free operation of the system. In this work, we demonstrated pulsed-mode swept-frequency laser feedback interferometry experimentally. Our interferometric detection scheme not only permits the simultaneous creation of both amplitude and phase images, but inherently suppresses unwanted background radiation. We demonstrate that the proposed system utilising microsecond pulses has the potential to achieve 0.25 mega-pixel per second acquisition rates, paving the pathway to video frame rate THz imaging.

We consider the influence of additional carrier confinement, achieved by application of strong perpendicular magnetic field, on inter Landau levels electron relaxation rates and the optical gain, of two different GaAs quantum cascade laser structures operating in the terahertz spectral range. Breaking of the in-plane energy dispersion and the formation of discrete energy levels is an efficient mechanism for eventual quenching of optical phonon emission and obtaining very long electronic lifetime in the relevant laser state. We employ our detailed model for calculating the electron relaxation rates (due to interface roughness and electron-longitudinal optical phonon scattering), and solve a full set of rate equations to evaluate the carrier distribution over Landau levels. The numerical simulations are performed for three- and four-well (per period) based structures that operate at 3.9 THz and 1.9 THz, respectively, both implemented in GaAs/Al0.15Ga0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band nonparabolicity is accounted for. (C) 2016 Elsevier B.V. All rights reserved.

The optimal design of asymmetric quantum well structures for generation of entangled photons in the mid-infrared range by spontaneous parametric downconversion is considered, and the efficiency of this process is estimated. Calculations show that a reasonably good degree of entanglement can be obtained, and that the optical interaction length required for optimal conversion is very short, in the few μm range.

The state of the art terahertz-frequency quantum cascade lasers have opened a plethora of applications over the past two decades by testing several designs up to the very limit of operating temperature, optical power and lasing frequency performance. The temperature degradation mechanisms have long been under the debate for limiting the operation up to 210 K in pulsed operation in the GaAs/AlGaAs material system. In this work, we review the existing designs and exploit two main temperature degradation mechanisms by presenting a design in which they both prove beneficial to the lasing operation by dual pumping and dual extracting lasing levels. We have applied the density matrix transport model to select potential candidate structures by simulating over two million active region designs. We present several designs which offer better performance than the current record structure.

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.

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 amplitude (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.

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).

A prospective comparative study assessing the importance of the intra-operative dynamic rotational tracking-especially in the treatment of astigmatisms in corneal refractive Excimer laser correction-concerning clinical outcomes is presented. The cyclotorsion from upright to supine position was measured using iris image comparison. The Group 1 of patients was additionally treated with cyclorotational control and Group 2 only with X-Y control. Significant differences were observed between the groups regarding the mean postoperative cylinder refraction (p < 0.05). The mean cyclotorsion can be calculated to 3.75 degrees with a standard deviation of 3.1 degrees. The total range of torsion was from -14.9 degrees to +12.6 degrees. Re-treatment rate was 2.2% in Group 1 and 8.2% in Group 2, which is highly significant (p < 0.01). The investigation confirms that the dynamic rotational tracking system used for LASIK results in highly predictable refraction quality with significantly less postoperative re-treatments.

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 recent years remarkable progress has been made in filling the “terahertz gap” of compact sources from the “optical” side using quantum cascade lasers and difference frequency generation systems. On the “electronic” side the range up to 0.5 THz is covered by several devices like Gunn diodes, IMPATT diodes and HEMT transistors. However for the most difficult central range 0.5–2 THz, which includes frequencies for detection of explosives and several airborne threats, along with the popular semi-compact Schottky diodes based frequency multiplier systems, the resonant tunneling diodes (RTD) as micrometer-size range ultrafast electronic devices able to operate at room temperature may offer a real solution for the design of compact portable equipment. It has been predicted that frequency limit for operation of the RTDs may extend up to 3 THz if the problem of the low, microwatt-range output power could be solved. Over the last 10 years remarkable progress has been achieved in increasing the output power of RTDs by almost 2 orders of magnitude, and in extending the operation frequencies from earlier 0.7 THz range to the values near 2 THz, thus making RTDs competitive with the Schottky diode based multipliers. The chapter compares the RTDs with other compact THz sources and discusses the design approaches that have yielded remarkable increase of power and frequency of RTDs.

One of the challenges in the design of metamaterials’ unit cells is the reduction of losses caused by the metallic inclusions. In order to overcome this obstacle, it has been proposed to use the active medium as the unit cell. Quantum cascade lasers are great candidates for the active medium materials since they are able to provide high values of optical gain. In this paper we investigate and compare two quantum cascade structures optimized for emission frequencies lower than 2 THz and simulate the effect of a strong magnetic field applied perpendicularly to the layers. Comprehensive description of conduction-band nonparabolicity is used to calculate the electronic structure, and subsequently evaluate the longitudinal optical phonon and interface roughness scattering rates and solve the system of rate equations which govern the distribution of carriers among the Landau levels. Once we assess the degree of population inversion, we have all the necessary information about the permittivity component along the growth direction of the structure and may determine the conditions under which the structure displays negative refraction.

Regardless of the huge advances made in the design and fabrication of mid-infrared and terahertz quantum cascade lasers, success in accessing the ~3-4 mm region of the electromagnetic spectrum has remained limited. This fact has brought about the need to exploit resonant intersubband transitions as powerful nonlinear oscillators, consequently enabling the occurrence of large nonlinear optical susceptibilities as a means of reaching desired wavelengths. In this work, we present a computational model developed for the optimization of second-order optical nonlinearities in In0.53Ga0.47As/Al0.48In0.52As quantum cascade laser structures based on the implementation of the Genetic algorithm. The carrier transport and the power output of the structure were calculated by self-consistent solutions to the system of rate equations for carriers and photons. Both stimulated and simultaneous double-photon absorption processes occurring between the second harmonic generation-relevant levels are incorporated into rate equations and the material-dependent effective mass and band non-parabolicity are taken into account, as well. The developed method is quite general and can be applied to any higher order effect which requires the inclusion of the photon density equation. [Projekat Ministarstva nauke Republike Srbije, br. III 45010]

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.

Optical feedback effects in lasers may be useful or problematic, depending on the type of application. When semiconductor lasers are operated using pulsed-mode excitation, their behavior under optical feedback depends on the electronic and thermal characteristics of the laser, as well as the nature of the external cavity. Predicting the behavior of a laser under both optical feedback and pulsed operation therefore requires a detailed model that includes laser-specific thermal and electronic characteristics. In this paper we introduce such a model for an exemplar bound-to-continuum terahertz frequency quantum cascade laser (QCL), illustrating its use in a selection of pulsed operation scenarios. Our results demonstrate significant interplay between electro-optical, thermal, and feedback phenomena, and that this interplay is key to understanding QCL behavior in pulsed applications. Further, our results suggest that for many types of QCL in interferometric applications, thermal modulation via low duty cycle pulsed operation would be an alternative to commonly used adiabatic modulation.

The fifth International School and Conference on Photonics—Photonica 2015 held on 24 August–28 August 2015 in Belgrade, Serbia, was one of the largest events dedicated to the celebration of the UNESCO proclaimed International year of light and light based technologies (IYL 2015) in southern Europe. In a year in which many magnificent scientific anniversaries related to great achievements in light science from the last thousand years coincided, the Vinca Institute of Nuclear Sciences (Belgrade, Serbia), the Serbian Academy of Sciences and Arts, the Optical Society of Serbia and Aston University (Birmingham, UK) tried to contribute to this global initiative which highlighted to the mankind the immense importance of light and related technologies in everyday life by organizing this respectable scientific and educational meeting. The conference was attended by a record number of 222 registered participants where, for the first time, domestic researchers have been outnumbered by foreign ones from 30 countries and all continents. Almost eighty PhD and master students had the luck and privilege to get special postal envelopes dedicated to the IYL 2015 and to listen to six illuminating tutorial talks, seven red-hot keynote talks and 21 invited lectures from

We describe the theoretical and experimental studies of GaN/AlGaN based resonant tunnelling diodes, and in particular analyse the effects and typical values of interface roughness, and then discuss the implications of these, realistic material quality parameters on performance of unipolar optoelectronic devices.

The authors acknowledge support the European Cooperation in Science and Technology (COST) Action BM1205. This research was supported under Australian Research Council’s Discovery Projects funding scheme (DP 120 103703) and the State of Bavaria. Y.L.L. acknowledges support under the Queensland Government’s Smart Futures Fellowships programme. P.D. acknowledges support from the EPSRC (UK). S. H. gratefully acknowledges support by the Royal Society and the Wolfson Foundation. In this work, we demonstrate a simple square-wave electrical modulation scheme for imaging with laser feedback interferometry (LFI). Distinct advantages of this scheme include (a) the straightforward creation of the modulating signal, even for high-current lasers, and (b) its natural suitability for lock-in detection. We compare this simple scheme against two established imaging modalities for LFI: mechanical modulation using an optical chopper and the swept-frequency feedback interferometry approach. The proposed scheme lends itself to high frequency modulation which paves the way for high frame-rate LFI imaging with no motion artefacts using off-the-shelf equipment. Publisher PDF

Considerable interest exists for sensing and imaging technologies in the terahertz (THz) spectral range, in particular for the interrogation of materials of an organic or biological nature. Development in THz quantum cascade lasers is seeing higher operating temperatures and peak output powers in pulsed mode, accentuating their place as the preferred source of coherent THz frequency radiation. Technological development of interferometric sensing schemes continues to take advantage of practical improvements in THz quantum cascade lasers. In this Summary, we give a brief overview of some recent developments in this regard.