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This study conducts experimental and numerical investigations on a stagnation-point, reverse-flow combustor to examine the effect of its design features on the combustion behavior and emission characteristics of non-premixed, turbulent hydrogen/air flames. The study covers a wide range of equivalence ratios, from ultra-lean to stoichiometric, and thermal power inputs varying between 5 kW and 20 kW. The main motivation behind this design is to achieve passive recirculation of flue gas within the combustor to enhance dilution of the incoming charge, thus potentially leading to relatively uniform thermal fields, lower temperatures, and suppressed NO x emissions. This design concept is implemented in a micro gas turbine – a practical energy conversion device tapped to play a major role in decarbonizing CHP systems suitable for small-scale industries. Optical chemiluminescence measurements, in addition to thermocouple-based temperature and Quantum Cascade Laser (QCL) spectroscopy-based exhaust emissions measurements, were conducted for all the test cases. Spatial OH* signal intensity is used as heat release signature to identify distinct features of the test flames, allowing for useful information on the nature and dynamics of the combustion process within the combustor. The results show stable hydrogen/air flames for all the operating conditions; however, a more distributed combustion is observed at higher powers for all the given equivalence ratios. Relatively higher NO x emissions at φ >0.2 suggest the lower achieved mixing between burnt and fresh gases, particularly in the non-preheated case, prevent a thorough dilution of the fresh mixture. The latter is necessary to achieve low-temperature, uniform thermal fields – an essential for low thermal NO x emissions in gas turbine combustors. Further, CFD results show that preheating considerably changes flow field dynamics and recirculation pattern for better mixing; however, this also dramatically increases thermal NO x emission, with the lowest levels observed at φ = 0.2, while the peak occurs at φ = 0.4. Finally, a 'brute-force' sensitivity analysis shows that thermal NO and NNH pathways contribute the most to NO formation in hydrogen/air flames. • The viability of hydrogen as a carbon-free alternative fuel for micro gas turbines is studied. • Preheating extends hydrogen flame stability at ultra-lean conditions. • Fuel injection velocity strongly affects the extent of charge dilution. • Effectiveness of recirculation depends upon the degree of mixing. • Thermal NO and NNH pathways contribute the most to NO formation in H 2 flames. [ABSTRACT FROM AUTHOR]

Off-axis integrated cavity output spectroscopy (OA-ICOS) has attracted much interest because it potentially allows highly sensitive field measurements with robust optical alignment. In this paper, a novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of multi-component gases N2O, H2O and CO in the atmosphere based on a mid-infrared quantum cascade laser (QCL) and OA-ICOS. In order to improve the energy utilization and increase the signal-to-noise ratio (SNR) of the signal, a new type of optical path structure of the laser re-injection method is adopted. Furthermore, the system performance can be effectively improved by using a new intervention method of injecting radio frequency (RF) white noise into a laser driver to suppress cavity mode noise and combining the wavelength modulation method (WMS). We compared the sensitivity of the second harmonic signal demodulation between the re-injection method and the standard OA-ICOS, and the SNR increased by 2.68 times compared to the latter. Analysis of the spectral measurements with Allan variance indicates that within an integration time of 1 s, the measurement accuracy of N2O, H2O, and CO is 6.71 ppb, 13.945 ppm, and 1.81 ppb, respectively, and within an integration time of 820 s, the measurement accuracy of N2O, H2O, and CO can be further improved to 1.26 ppb, 2.089 ppm, and 172 ppt, respectively. Our approach represents an underlying analytical method that provides guidelines for monitoring of representative gases in the atmosphere, industrial processes, emergency safety, etc. [ABSTRACT FROM AUTHOR]

We present a design for a quantum photodetector operating in the terahertz range, at 3.45 THz (15 meV, 87 μm). Our device relies on biased GaAs/AlGaAs heterostructure, designed to exploit LO phonon scattering as an extraction mechanism. In our design, the external potential due to the applied bias forms an extraction miniband and allows accommodating an LO phonon transition (36 meV) and use it as an extraction mechanism, even though its energy exceeds the detector's absorbing transition at 15 meV. Spectral-resolved measurements performed on arrays of patch antenna microcavities reveal a peak photocurrent at the designed photon energy with a responsivity of 80 mA/W at 20 K. The maximum operating temperature of the photodetector is found to be 40 K. Detector characterizations were performed both with a black-body source as well as with a terahertz quantum cascade laser emitting at 3.5 THz. [ABSTRACT FROM AUTHOR]

Assessing the nature of topological quantum materials, and in particular probing the existence of topological surface states, is a very challenging task. Terahertz (THz) frequency scattering near‐field optical microscopy has emerged as an effective technique to investigate the presence of massless surface carriers by locally probing collective surface excitations, i.e., plasmon polaritons, whose dispersion critically depends on the density and nature of surface carriers. Here, thin (14–19 nm) films of Bi2Se3 are experimentally investigated through a combination of x‐ray diffraction, Hall‐bar magneto‐transport, and near‐field detectorless optical holography at THz frequencies, from 2 to 4.3 THz. The dispersion of surface plasmon polaritons are determined for different Bi2Se3 film thicknesses, proving the presence of massless surface carriers. The results open intriguing opportunities in THz nano‐plasmonics and topological nano‐photonics including the development of superlenses and metasurfaces, making use of plasmon polaritons. [ABSTRACT FROM AUTHOR]

In this paper, unlike many researches that focus on beam splitters based on gratings with symmetrical structures, an asymmetric grating with double-groove structure is proposed. Based on the double-groove grating, multi-port beam splitters including dual-port, three-port, and four-port, under normal incidence working at terahertz frequency are designed for both transverse-electric and transverse-magnetic polarizations. Rigorous couple-wave analysis and simulated annealing algorithm are employed to get the optimal parameters of the grating and accelerate the computing process. For the purpose of understanding the phenomenon of the beam incident, the finite element method is also applied to get the simulation result. It is proven that the proposed beam splitters all present great efficiency and uniformity on the energy distribution. High manufacturing tolerance of the beam splitters is also analyzed, which allows for greater flexibility in the fabrication process. Compared with beam splitters based on single-groove grating, multi-port beam splitters based on double grooves show great performance and universality. [ABSTRACT FROM AUTHOR]

We report on a lightwave-driven scanning tunneling microscope based on a home-built microscope and a compact, commercial, and cost-effective terahertz-generation unit with a repetition rate of 100 MHz. The measurements are performed in an ultrahigh vacuum at temperatures between 8.5 and 300 K. The cross-correlation of the pump and probe pulses indicates a temporal resolution on the order of a picosecond. In terms of spatial resolution, CO molecules, step edges, and atomically resolved terraces are readily observed in terahertz images, with sometimes better contrast than in the topographic and (DC) current channels. The utilization of a compact, turn-key terahertz-generation system requires only limited experience with optics and terahertz generation, which may facilitate the deployment of the technique to further research groups. [ABSTRACT FROM AUTHOR]

Freely engineering the operation frequency of frequency comb sources is crucial for various applications, e.g., high-precision spectroscopy, ranging, communications, and so on. Here, by employing band structure simulations, group velocity dispersion (GVD) analysis, and experimental verifications, we demonstrate that the operation frequency of terahertz (THz) quantum cascade laser frequency combs can be engineered from 4.2 to 4.0 THz. First of all, from the viewpoint of the band structure engineering, we shift the frequency corresponding to the optical transitions in the active region from 4.2 to 4.0 THz by slightly altering the thicknesses of quantum wells. Meanwhile, a GVD analysis is applied to evaluate the potential comb performance. Finally, experimental characterizations, e.g., emission spectra, inter-mode beatnote, dual-comb operation, are performed to validate the exceptional comb operation at 4.0 THz. The advancement in simulations and experimental results present a comprehensive method to customize the desired THz radiative frequency for comb generation, which facilitates the practical development of broadband, high-precision THz comb sources. [ABSTRACT FROM AUTHOR]

Far-infrared (FIR) wavelength light provides a wealth of information about astrophysical processes. Large field-of-view FIR spectrometers face a variety of challenges that can be bypassed by utilizing on-chip filter bank technology. To this end, we have been developing microwave kinetic inductance detectors (MKIDs) to couple to on-chip spectrometric technologies. We utilize a unique two-silicon wafer design to create a waveguide for an on-chip spectrometer between 1.4 and 2 THz. The spectrometer's principle of operation is electromagnetic radiation traveling down a center waveguide before being picked off by a resonance cavity and absorbed by a detector. The MKIDs are designed to couple directly to the waveguide using an annular antenna shape for the inductor. Alignment of the two-wafer design is achieved with two dowel pins and a hole and slot pattern in our custom designed box. We present the design and early test data from a 18-channel prototype device. The prototype yielded the full 18 resonances with quality factors Q c of about 5 × 10 4 and Q i of about 1 × 10 4 . We plan to use the technology we developed to create an n × n pixel device targeting R = 100 . [ABSTRACT FROM AUTHOR]

In this communication, a hybrid MIMO antenna (combination of graphene and ceramic) is designed and investigated in THz frequency range. Circular Polarization is achieved by feeding the cylindrical ceramic using moon shaped aperture with L-shaped Microstrip line. Mutual coupling between the antenna ports has been reduced to − 25 dB by using the concept of polarization diversity. Graphene material is used to introduce the re-configurability in the proposed radiator. For avoiding the difficulty of simulation of radiator at THz regime (very high simulation time), machine learning algorithms i.e. Artificial Neural Network (ANN), Random Forest and XGBoost are utilized to predict the reflection coefficient and axial ratio of designed antenna. The designed radiator works from 2.4 to 3.1 THz with 3-dB axial ratio from 2.6 to 3.0 THz. Good value of diversity parameters and stable radiation features make the designed radiator appropriate for various wireless applications in THz frequency band. [ABSTRACT FROM AUTHOR]

Actuators play a crucial role in microelectromechanical systems (MEMS) and hold substantial potential for applications in various domains, including reconfigurable metamaterials. This research aims to design, fabricate, and characterize structures for the actuation of the EMA. The electromagnetic actuator overcomes the lack of high drive voltage required by other actuators. The proposed actuator configuration comprises supporting cantilever beams with fixed ends, an integrated coil positioned above the cantilever's movable plate, and a permanent magnet located beneath the cantilever's movable plate to generate a static magnetic field. Utilizing flexible polyimide, the fabrication process of the EMA is simplified, overcoming limitations associated with silicon-based micromachining techniques. Furthermore, this approach potentially enables large-scale production of EMA, with displacement reaching up to 250 μm under a 100 mA current, thereby expanding their scope of applications in manufacturing. To demonstrate the function of the EMA, we integrated it with a metamaterial structure to form a compact, tunable terahertz absorber, demonstrating a potential for reconfigurable electromagnetic space. [ABSTRACT FROM AUTHOR]

In recent years, the development of terahertz (THz) technology has attracted significant attention. Various tunable devices for THz waves (0.1 THz–10 THz) have been proposed, including devices that modulate the amplitude, polarization, phase, and absorption. Traditional metal materials are often faced with the problem of non-adjustment, so the designed terahertz devices play a single role and do not have multiple uses, which greatly limits their development. As an excellent phase change material, VO2's properties can be transformed by external temperature stimulation, which provides new inspiration for the development of terahertz devices. To address these issues, this study innovatively combines metamaterials with phase change materials, leveraging their design flexibility and temperature-induced phase transition characteristics. We have designed a THz intelligent absorber that not only enables flexible switching between multiple functionalities but also achieves precise performance tuning through temperature stimulation. Furthermore, we have taken into consideration factors such as the polarization mode, environmental temperature, structural parameters, and incident angle, ensuring the device's process tolerance and environmental adaptability. Additionally, by exploiting the principle of localized surface plasmon resonance (LSPR) accompanied by local field enhancement, we have monitored and analyzed the resonant process through electric field characterization. In summary, the innovative approach and superior performance of this structure provide broader insights and methods for THz device design, contributing to its theoretical research value. Moreover, the proposed absorber holds potential for practical applications in electromagnetic invisibility, shielding, modulation, and detection scenarios. [ABSTRACT FROM AUTHOR]

We demonstrate a method for the realization of highly nonlinear optical 4-(4-dimethylaminostyryl)- 1-methylpyridinium tosylate (DAST) two-dimensional structures by a double-step technique. The desired polymeric structures were first fabricated by using the multiple exposure of the two-beam interference technique, and the DAST nanoscrystals were then prepared inside the air-voids of these photoresist templates, resulting in nonlinear periodic structures. The nonlinear properties were characterized by optical and scanning microscopies, as well as by second-harmonic generation technique. This nonlinear modulation is very promising for the enhancement of nonlinear conversion rates, such as terahertz generation, by using the quasi-phase matching technique. [ABSTRACT FROM AUTHOR]

Recent advancements in terahertz (THz) wave technology have highlighted the criticality of circular-polarization detection, fostering the development of more compact, efficient on-chip THz circular-polarization detectors. In response to this technological imperative, we presented a chiral-antenna-integrated GaAs/AlGaAs quantum well (QW) THz detector. The chiral antenna selectively couples the incident light of a specific circular-polarization state into a surface-plasmon polariton wave that enhances the absorptance of the QWs by a factor of 12 relative to a standard 45° faceted device, and reflects a significant amount of the incident light of the orthogonal circular-polarization state. The circular-polarization selectivity is further enhanced by the QWs with a strong intrinsic anisotropy, resulting in a circular-polarization extinction ratio (CPER) as high as 26 at 6.52 THz. In addition, the operation band of the device can be adjusted by tuning the structural parameters of the chiral structure. Moreover, the device preserves a high performance for oblique incidence within a range of ±5°, and the device architecture is compatible with a focal plane array. This report communicates a promising approach for the development of miniaturized on-chip THz circular-polarization detectors. [ABSTRACT FROM AUTHOR]

The photonic crystal surface-emitting laser (PCSEL) has attracted much attention due to the advantages of a small far-field divergence angle and high output power. Here, we report a high-power terahertz (THz) photonic crystal laser with high beam quality through the optimization of the absorption boundary condition and the introduction of the symmetrically distributed electrodes. Single-mode surface emission at 3.4 THz with the maximum peak output power of 50 mW is demonstrated. Meanwhile, a high symmetric far-field pattern with C6 symmetry and a small divergence angle is achieved. In this device, the integration of the stable single-mode operation, high beam quality and high output power is realized, which may have great significance for practical applications. [ABSTRACT FROM AUTHOR]

Low efficiency of Terahertz (THz) radiation and radiation power have limited the development of THz Science and Technology. Thus, with the aims of achieving greater photoconductive current and improving radiation characteristics of THz photoconductive antenna (PCA), this study utilized Extraordinary Optical Transmission (EOT) of light passing through various subwavelength metal structures to control and restrain the light wave in subwavelength scale. Furthermore, by grooving the grating electrode structure, the influence of metal grating's EOT on the transmission field of PCA were investigated and analyzed. Simulation results show that the effect of local electric field enhancement is significant. When the incident power is 0.1 W, the peak value of the local electric field reaches 7. 0 4 × 1 0 7 V/m. In addition, comparing to the grating electrode with no groove structure in which the field intensity was less than 4. 2 8 × 1 0 6 V/m, the local electric field increased by 16.4 times, respectively. Correspondingly, the photocurrent intensity of the improved photoconductive plasmonic structure is increased by 72.3 times. In conclusion, the improved plasma photoconductive structure was shown to obviously enhance the transmission field strength of semiconductor materials and the current of the PCA, and accordingly, to improve the THz radiation capability. [ABSTRACT FROM AUTHOR]

We present the simulation, design, fabrication, and characterization of planarized double metal quantum cascade lasers based on InGaAs/GaAsSb. Intended for astrophysical heterodyne measurements and having the cavity embedded in benzocyclobutene, the devices are equipped with thermal bridges on either side of the ridge, in order to improve the heat dissipation. The lasers are shown to vertically emit a single mode around 26 μm in pulsed operation, with peak powers of ≈ 30 μW and a current density threshold of Jth = 3.7 kA/cm2. Maximum operation temperature is around 170 K, with the maximum supported duty cycle being extended from the initial 15% to about 30% with the help of the improved thermal management technique. [ABSTRACT FROM AUTHOR]

This study demonstrates the conversion of metallic titanium (Ti) to titanium oxide just by conducting electrical current through Ti thin film in vacuum and increasing the temperature by Joule heating. This led to the improvement of electrical and thermal properties of a microbolometer. A microbolometer with an integrated Ti thermistor and heater width of 2.7 µm and a length of 50 µm was fabricated for the current study. Constant-voltage stresses were applied to the thermistor wire to observe the effect of the Joule heating on its properties. Thermistor resistance ~14 times the initial resistance was observed owing to the heating. A negative large temperature coefficient of resistance (TCR) of −0.32%/K was also observed owing to the treatment, leading to an improved responsivity of ~4.5 times from devices with untreated Ti thermistors. However, this does not improve the noise equivalent power (NEP), due to the increased flicker noise. Microstructural analyses with transmission electron microscopy (TEM), transmission electron diffraction (TED) and energy dispersive X-ray (EDX) confirm the formation of a titanium oxide (TiOx) semiconducting phase on the Ti phase (~85% purity) deposited initially, further to the heating. Formation of TiOx during annealing could minimize the narrow width effect, which we reported previously in thin metal wires, leading to enhancement of responsivity. [ABSTRACT FROM AUTHOR]

High-quality optical ring resonators can confine light in a small volume and store it for millions of roundtrips. They have enabled the dramatic size reduction from laboratory scale to chip level of optical filters, modulators, frequency converters, and frequency comb generators in the visible and the near-infrared. The mid-infrared spectral region (3−12 μm), as important as it is for molecular gas sensing and spectroscopy, lags behind in development of integrated photonic components. Here we demonstrate the integration of mid-infrared ring resonators and directional couplers, incorporating a quantum cascade active region in the waveguide core. It enables electrical control of the resonant frequency, its quality factor, the coupling regime and the coupling coefficient. We show that one device, depending on its operating point, can act as a tunable filter, a nonlinear frequency converter, or a frequency comb generator. These concepts extend to the integration of multiple active resonators and waveguides in arbitrary configurations, thus allowing the implementation of purpose-specific mid-infrared active photonic integrated circuits for spectroscopy, communication, and microwave generation. Multifunctional active mid-infrared ring resonators and directional couplers with quantum cascade laser cores allow electrical control of resonant frequency and quality factors, tunable filtering and frequency comb generation. [ABSTRACT FROM AUTHOR]

In the last decade, intense research has been witnessed on developing compact, terahertz (THz) driven electron accelerators, producing electrons with a sub-MeV—few tens of MeV energy. Such economical devices could be used in scientific and material research and medical treatments. However, until now, the extremely high-energy THz pulses needed by the THz counterparts of the microwave accelerators were generated by optical rectification (OR) of ultrafast Ti:sapphire or Yb laser pulses. These lasers, however, are not very effective. Because of this, we use numerical simulations to investigate the possibility of generating high-energy THz pulses by the OR of pulses produced by CO2 lasers, which can have high plug-in efficiency. The results obtained supposing optical rectification (OR) in GaAs demonstrate that consideration of the self-phase-modulation (SPM) and the second-harmonic-generation (SHG) processes is indispensable in the design of CO2 laser-based THz sources. More interestingly, although these two processes hinder achieving high laser-to-THz conversion efficiency, they can still surpass the 1.5% value, ensuring high system efficiency and making the CO2 laser OR system a promising THz source. Our finding also has important implications for other middle-infrared laser-pumped OR-based THz sources. [ABSTRACT FROM AUTHOR]

There is a growing interest in spoof surface plasmon polariton (SSPP) structures at terahertz (THz) frequencies for applications such as filtering, sensing, and communications. However, to date, there are limited experiments that confirm SSPP characteristics at THz frequencies. The majority of literature focuses on simulation or verification by device scaling to Gigahertz (GHz) frequencies where standard vector network analyzers are readily available. This paper presents the first experimental verification of SSPP characteristics at THz frequencies in a guided wave system using coplanar strip (CPS) feedlines. Specifically, we design three SSPP structures with varying band-edge frequencies (1.04 THz, 0.63 THz, and 0.53 THz), then fabricate and verify the low-pass transmission characteristics using a modified THz-time-domain spectrometer (THz-TDS) system. We find strong agreement between simulation, theory, and experiment. [ABSTRACT FROM AUTHOR]

Defects within perovskite have been known to act as the nonradiative recombination centers, negatively impacting the carrier transport, which degrades the photovoltaic performance of perovskite solar cells (PSCs). Therefore, preparing a high‐quality perovskite film is of vital significance. To this end, a room‐temperature molten salt, dimethylamine formate (DMAFa), is introduced into perovskite precursor solution to regulate the crystallization process of CsPbI3 films. DMAFa can coordinate with Pb2+ as HCOO−‐Pb2+ in the early stages, then HCOO−‐Pb2+ is gradually displaced by I−‐Pb2+ due to its decomposition during the subsequent annealing, thus delaying the crystallization rate, meanwhile, the DMA+ can interact with the uncoordinated Pb2+ to passivate defects of perovskite films, thereby, forming a high‐quality CsPbI3 film with large grain size and low‐defect density. As a result of this strategy, the power conversion efficiency is increased to 20.40%, and the open‐circuit voltage is up to 1.21 V. These findings indicate that the introduction of DMAFa offers a fundamental way to achieve high‐performance CsPbI3 PSCs. [ABSTRACT FROM AUTHOR]

We report a mid-infrared quartz-enhanced photoacoustic sensor for highly sensitive nitrous oxide (N2O) detection using a 4.56 µm quantum cascade laser (QCL) with a butterfly package type. This new type of QCL features a compact size (30 mm × 12.7 mm × 13 mm), low threshold current (< 120 mA) at room temperature, and relatively high emission power (> 100 mW), which are suitable for portable trace gas sensor development. In this work, the QCL beam is directly coupled to the QEPAS detection module by using only one focusing lens. By measuring N2O mixtures with different concentrations (120 ppb to 3.16 ppm), the developed sensor shows a good linearity (R-square value of 0.999) and a minimum detection limit of below 1 ppb. The sensor has been successfully deployed for ambient air measurement, demonstrating the applicability and promising potential of using this QCL with butterfly package for various gas sensing applications. [ABSTRACT FROM AUTHOR]

In this study, we endeavored to create and simulate a D‐type flip‐flop utilizing two‐dimensional photonic crystals featuring a square lattice. This flip‐flop, composed of just two intersecting waveguides, is both compact and straightforward. Constructed from 10 × 10 dielectric rods suspended in air, it boasts low output power in logic mode 0, creating a significant distinction between optical powers in modes 0 and 1, lending it a logical aspect. Optimized for operation at a communication wavelength of 1.55 µm, this flip‐flop proves suitable for optical communication systems. The third window optical fiber communication system is suitable for 1.55 μm wavelength because the fiber has the lowest loss. Our analysis involved calculating the band structure via the plane wave expansion method and determining the optical power distribution using the finite difference time domain method. [ABSTRACT FROM AUTHOR]

This paper presents two waveguide quadrature hybrids in the WR‐3 band, which are designed to be fabricated by computer numerical control milling process with relaxed branch width requirements. An E‐plane four‐branch directional coupler gives an amplitude imbalance less than 0.8 dB and a phase difference between 88° and 94° from 240 to 324 GHz. Another design is an H‐plane bandpass coupler composed of cavity resonators, showing a bandwidth of 2% near 300 GHz. [ABSTRACT FROM AUTHOR]

The temperature measurement of Sulfur dioxide (SO2) is a crucial parameter for accurately determining its concentration, particularly in high‐temperature environment. Herein, it is realized by using three different techniques, which are direct absorption spectroscopy, wavelength modulation spectroscopy and non‐calibration wavelength modulation spectroscopy based on the bispectral lines temperature measurement method. The bispectral lines of SO2 locates at 1396.844 and 1396.921 cm−1, respectively. The gas temperature in the absorption cell was also measured with K‐type thermocouple and compared with these three spectral measurements. The linearities corresponding to these three techniques are 99.71%, 99.8%, and 99.88%, respectively. Their average relative error between the spectral measurement results and the linear fitting value of the three techniques is 1.15%, 0.86%, and 0.57%, respectively. It is illustrated the feasibility of the bispectral lines method for high‐temperature measurement. These investigations provide methods for temperature measurement in high‐temperature coal‐fired boilers, potentially for investigating other sulfide gas molecules such as SO3 and H2S. [ABSTRACT FROM AUTHOR]

Thermal considerations affect the performance of most microsystems. Although surface techniques can give information on the thermal properties within the material or about buried heat sources and defects, mapping temperature and thermal properties in three dimension (3D) is critical and has not been addressed yet. Infrared thermography, commonly used for opaque materials, is not adapted to semi-transparent samples such as microfluidic chips or semiconductor materials in the infrared range. This work aims at answering these needs by using the variations of transmittance with temperature to obtain information on the temperature within the thickness of the sample. We use a tunable mid-infrared light source combined with an infrared camera to measure these variations of transmittance in a glass wafer. We couple this technique with a thermal model to extract the thermotransmittance coefficient—the coefficient of temperature variation of the transmittance. We then introduce a semiempirical model based on Lorentz oscillators to estimate the temperature-dependent optical properties of our sample in the mid-IR spectral range. Combined with the measurement, this paper reports the spectroscopic behavior of the thermotransmittance coefficient in the mid-IR range and a way to predict it. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) are a prominent semiconductor laser source operating in the mid-infrared and terahertz regimes. As is typical with semiconductor lasers, QCLs usually monolithically integrate the active gain material and the resonator. Hence, over nearly 30 years of QCL development, resonator geometries have developed alongside active region designs. Disk and ring geometries, in particular, have long been recognized for their unique attributes, which have, in turn, contributed to the demonstration of ultra-small cavities as well as surface emission from QCLs. In recent years, ring geometries have witnessed a resurgence as promising platforms for frequency comb and soliton generation as well as mid-infrared photonic integration. In this Perspective, we describe the attributes that make ring and disk QCLs unique by discussing key demonstrations. We present recent results, which indicate that these devices are poised to become building blocks of highly integrated, next-generation spectrometers operating in the mid-infrared. We discuss promising avenues for future research centered around monolithic ring and disk-type QCLs in applications ranging from gas sensing and spectroscopy to quantum optics and non-Hermitian photonics. [ABSTRACT FROM AUTHOR]

Mid‐infrared (MIR) pulsed lasers operating at 2–5 µm have important applications in communication, sensing, and medicine. However, the lack of robust MIR saturable absorbers (SAs) remains a major obstacle. Here, a semiconductor material cadmium oxide (CdO) film with high laser‐induced damage threshold (800 nm, 134 mJ cm−2) and broadband saturable absorption (2.0–3.9 µm) is investigated. The effective tuning of the nonlinear optical response of CdO is demonstrated by adjusting the carrier concentration via a tungsten (W)‐doping scheme. The saturable absorption is improved and carrier relaxation process is accelerated after increasing the W‐dopant level. Based on these findings, the robust CdO‐SAs with customizable parameters are realized and Q‐switched lasers with decreasing pulse width from 372 to 254.3 ns are obtained at 2 µm. The design flexibility provided by CdO opens up a large parameter space that enables the continuous improvement of compact and high‐performance MIR ultrafast lasers. [ABSTRACT FROM AUTHOR]

In this paper, a spoof surface plasmon polaritons based reconfigurable bandstop filter is designed and simulated at a resonant frequency of 0.216 THz. The proposed filter design comprises corrugated L-shaped resonators that are excited using a spoof transmission line. The designed filter is simulated using Co-EM simulation and it is evident from the simulation results that the designed filter can be tuned from 0.216 to 0.219 THz by employing a variable capacitor. Microwave frequency design studies are conducted to validate the designed methodology experimentally, and the BSF is fabricated on a 1.52 mm thick Rogers R4003C substrate using an SMV 1430 varactor diode. The measured results indicate that raising the reverse bias voltages across the varactor diode from 0.5 to 10 V, shifts the resonance tuned over a frequency range of 0.03 GHz with 0.22 GHz absolute bandwidth. The designed THz filter holds the potential to be employed as a compact filter in polymer industries for spectroscopic imaging. [ABSTRACT FROM AUTHOR]

A bridge monitoring approach is proposed for the distributed detection and quantification of crack opening displacements (CODs) along the lengths of large bridges. The proposed method does not require a priori knowledge of traffic loads and is applicable under normal operational conditions of bridges. A damage index is introduced, using the difference between crack-induced strain peak values and bridge base strains caused by moving loads. The applicability of the proposed approach for quantification of the detected cracks is demonstrated in terms of CODs at a number of locations in a multispan bridge. Six field tests were performed on a 332 m, five-span precast post-tensioned concrete box girder bridge for this purpose. The bridge was tested under operational conditions; thus, random daily traffic loads were employed as external forces. Two distributed optical fiber sensors based on the Brillouin scattering system were adhered to the external and internal surfaces of the box girder bridge. Fiber Bragg grating crack sensors were also employed for verification of the COD measurements, based on direct acquisition of distributed strains over the 332-m length of the bridge. By using this approach, it is possible to detect and quantify cracks with CODs larger than 5 µm. [ABSTRACT FROM AUTHOR]

We hereby offer a comprehensive analysis of various factors that could potentially enable terahertz quantum cascade lasers (THz QCLs) to achieve room temperature performance. We thoroughly examine and integrate the latest findings from recent studies in the field. Our work goes beyond a mere analysis; it represents a nuanced and comprehensive exploration of the intricate factors influencing the performance of THz QCLs. Through a comprehensive and holistic approach, we propose novel insights that significantly contribute to advancing strategies for improving the temperature performance of THz QCLs. This all-encompassing perspective allows us not only to present a synthesis of existing knowledge but also to offer a fresh and nuanced strategy to improve the temperature performance of THz QCLs. We draw new conclusions from prior works, demonstrating that the key to enhancing THz QCL temperature performance involves not only optimizing interface quality but also strategically managing doping density, its spatial distribution, and profile. This is based on our results from different structures, such as two experimentally demonstrated devices: the spit-well resonant-phonon and the two-well injector direct-phonon schemes for THz QCLs, which allow efficient isolation of the laser levels from excited and continuum states. In these schemes, the doping profile has a setback that lessens the overlap of the doped region with the active laser states. Our work stands as a valuable resource for researchers seeking to gain a deeper understanding of the evolving landscape of THz technology. Furthermore, we present a novel strategy for future endeavors, providing an enhanced framework for continued exploration in this dynamic field. This strategy should pave the way to potentially reach higher temperatures than the latest records reached for Tmax of THz QCLs. [ABSTRACT FROM AUTHOR]

In this research, we demonstrate a wavelength-stable continuous wave (CW) distributed feedback (DFB) quantum cascade laser (QCL) emitting at 7.2 μm using a surface metal grating approach without epitaxial regrowth. The deep metal grating provides an appropriate DFB coupling coefficient and enhanced thermal extraction, resulting in improved lasing performance and the realization of impressive wavelength stability. Quantitatively, the temperature tuning coefficient of the single-mode emission is only 0.54 nm/°C from 20 °C to 70 °C, and the current tuning coefficient of the single-mode emission is 3.2 nm/A from 1.0 A to 1.6 A. A DFB-QCL with a 2 mm cavity length exhibits a low threshold current of 0.6 A and a power of 1.1 W with a slope efficiency of 1 W/A in the CW mode at 300 K. A single-mode operation with a side mode suppression ratio of 33 dB and a single-lobed far-field without beam steering is obtained in the working temperature range of 20–70 °C The improved wavelength stability using a deep surface metal grating approach promises simplified fabrication, which is meaningful for the commercial applications of QCLs. [ABSTRACT FROM AUTHOR]

A tunable near-infrared surface plasmon resonance (SPR) biosensor based on gate-controlled graphene plasmons is numerically investigated by using the finite element method (FEM) and the transfer matrix method (TMM). The novel properties of chemical potential sensing make the proposed sensor promising in the application of ultra-sensitive and highly specific biosensing technology. The sensitivity of chemical potential sensing in wavelength interrogation mode can be calculated to be 1.5, 1.89, 2.29, 3.21, 3.73 and 4.68 nm/meV, respectively, at the resonance wavelengths of 1100, 1200, 1310, 1550, 1700 and 1900 nm. The figure of merit (FOM) achieves 129.3, 101.1, 84.5, 67.7, 69.5 and 59.7 eV−1, respectively, at these resonance wavelengths. The sensitivity of chemical potential sensing in gate voltage interrogation mode also can be calculated to be 156.9822, 143.6147, 131.0779, 111.0351, 101.3415 and 90.6038 mV/meV, respectively, at the incident wavelengths of 1100, 1200, 1310, 1550, 1700 and 1900 nm. The FOM achieves 135.6, 103.0, 88.9, 62.2, 66.6 and 61.5 eV−1, respectively, at these incident wavelengths. Theoretical estimates suggest that the limit of detection (LOD) of the sensor's DNA sensing can reach the level of femtomolar or even attomolar, comparable to and even lower than that of 2D nanomaterial-enhanced metal SPR sensors with AuNPs as a sensitivity enhancement strategy. The feasibility of preparation and operation of this new concept SPR biosensor is also analyzed and discussed. [ABSTRACT FROM AUTHOR]

Mid-IR spectroscopy is a powerful and label-free technique to investigate protein reactions. In this study, we use quantum-cascade-laser-based dual-comb spectroscopy to probe protein conformational changes and protonation events by a single-shot experiment. By using a well-characterized membrane protein, bacteriorhodopsin, we provide a comparison between dual-comb spectroscopy and our homebuilt tunable quantum cascade laser (QCL)-based scanning spectrometer as tools to monitor irreversible reactions with high time resolution. In conclusion, QCL-based infrared spectroscopy is demonstrated to be feasible for tracing functionally relevant protein structural changes and proton translocations by single-shot experiments. Thus, we envisage a bright future for applications of this technology for monitoring the kinetics of irreversible reactions as in (bio-)chemical transformations. [ABSTRACT FROM AUTHOR]

This paper discusses the terahertz electromagnetic response of metallic gratings on anisotropic dielectric substrates. The metallic gratings consist of parallel gold stripes. Utilizing numerical simulations, we observe that it is possible to excite a series of resonant modes in these structures. These modes are affected differently by the different indices on the anisotropic substrate. An analytical model is discussed to show that modes associated with transmission peaks are due to the excitation of (a) Fabry–Pérot modes with polarization along the grating and/or (b) waveguide modes with polarization perpendicular to the grating. It is observed that the resonance associated with the TM1,1 mode is a narrow linewidth resonance which, in some particular circumstances, becomes nearly independent of substrate thickness. Therefore, from the spectral position of this resonance, it is possible to extract the out-of-plane component of the substrate refractive index with very small uncertainty. Based on this observation, we demonstrate the refractive index characterization of several lossless semiconductor substrates through frequency-domain polarized terahertz transmission measurements in the frequency range of 0.2–0.6 THz at normal incidence. The reliability of the technique is demonstrated on well-known materials, such as high-resistivity silicon and sapphire substrates. This technique is also applied for the characterization of a Fe-doped β-Ga2O3 single-crystal substrate. [ABSTRACT FROM AUTHOR]

This document is a summary of a special topical collection in the journal APL Photonics on the advancements and applications of optical frequency combs. Optical frequency combs are a technology that combines ultrafast laser technology and single frequency laser stabilization techniques to generate stable optical and microwave signals for high-resolution time and frequency applications. The collection includes articles on various topics such as laser ranging, terahertz photonics, molecular referencing, optical communications, and space-based time/frequency dissemination. The articles highlight the impact and innovation that frequency comb technology brings to each field. The collection demonstrates the transformative potential of optical frequency combs and the ongoing research to further expand their applications. [Extracted from the article]

This article discusses the exploration of infrared (IR) and terahertz (THz) quantum technologies and their potential applications in communication, sensing, and imaging. The focus is on the exploitation of novel materials and the harnessing of intersubband transitions, which involve the excitation of electrons between quantized energy levels within quantum wells or superlattices. The article also highlights the importance of selecting and engineering materials with tailored properties for efficient operation in these spectral ranges, including traditional semiconductors and emerging materials such as graphene, transition metal dichalcogenides (TMDs), and topological insulators. The article concludes by emphasizing the significant advancements that can be achieved through the synergistic interplay between material science, quantum physics, and device engineering in the field of IR and THz quantum devices. [Extracted from the article]

We present a method to perform accurate and efficient simulations of photovoltaic quantum cascade detector (QCD) structures based on the ensemble Monte Carlo (EMC) approach. Since the photocurrent is typically orders of magnitude smaller than the pump current in a quantum cascade laser (QCL), a direct application of QCL simulation techniques is not sufficiently accurate. We demonstrate that by exploiting thermodynamic equilibrium relations for the electron populations and scattering rates, the EMC method can be adapted to yield reliable results for the essential QCD figures of merit, such as responsivity and specific detectivity. The modeling approach is validated against available experimental results for various mid-infrared and terahertz QCD designs and furthermore compared to simulations based on the non-equilibrium Green's function method. [ABSTRACT FROM AUTHOR]

The third-order tunneling current model, reflecting the effect of carrier leakage from bound to continuum states, is modeled based on the second-order tunneling current and semi-classical leakage current theories and is incorporated in a simulation based on the rate equation. GaAs/AlGaAs-based two-well resonant-phonon terahertz quantum cascade lasers, published in the past, are theoretically examined by this method. Based on the latest recorded device, the influence of barrier height is investigated by changing the Al-composition from 12.5% to 45% with a fixed-layer thickness. As a result, there are carrier dynamics that diminish the carrier density of lasing subbands in both high and low barrier conditions. On the one hand, carrier leakage into continuum states increases drastically below 25% Al-composition. On the other hand, in high barrier conditions, substantial carriers accumulate in the ground state due to a strong confinement effect in addition to oscillator strength lowering. [ABSTRACT FROM AUTHOR]

We report room temperature (RT) continuous-wave (CW) lasing of quantum well (QW) lasers grown on (001) Si substrates emitting at 980 nm. Two different QW structures, including conventional compressively strained InGaAs/GaAs QWs and strain-compensated InGaAs/GaAs/GaAsP QWs, were investigated. Photoluminescence properties and device performance of both structures on native GaAs and (001) Si substrates are discussed. By adding GaAsP barriers to the InGaAs/GaAs QWs, the lowest threshold current density of ridge waveguide edge-emitting QW lasers obtained on Si is 550 A/cm2, measured on a 10 μm × 2 mm device at RT. The working temperature of the InGaAs/GaAs/GaAsP QW lasers grown on Si can be over 95 °C in the CW mode. This work suggests a feasible approach to improve the 980 nm laser performance on Si for monolithic optoelectronic integration. [ABSTRACT FROM AUTHOR]

A simple optically transparent multi-port dual polarized wideband THz-MIMO antenna is presented in this study. The main focus of the work is to design an optically transparent antenna using indium tin oxide (ITO) as conducting material on polyimide substrate. Further, multi elements are integrated in the configuration without compromise of the MIMO performance of the antenna. The antenna development process begins with design of single element. A circular monopole radiator with an etched curved-shaped slot is developed first. The bottom of the antenna is integrated with partial ground plane with inverted L-shaped strip. This configuration of the single element antenna is provided wideband in THz frequency range. Further, two-port (parallel and orthogonally placed element scenarios) MIMO antenna is implemented to investigate the MIMO performance stability. The orthogonal arrangement of antenna element is accounted as a best arrangement to extend antenna into four-port configuration. Proposed four-port antenna performance is evaluated in terms conventional parameters (gain, efficiency and radiation patterns) and MIMO parameters (ECC, TARC, Diversity gain, and CCL). and four-port MIMO configurations. The antenna has offered wide bandwidth (0.55–10.6 THz). The gain of the antenna is 7.1 dBi over the interested frequency range. The circular polarization characteristics is exhibited by axial ratio which is shown 3-dB ARBW for 1–2.1 THz. The evaluated MIMO performance results port isolation, ECC, TARC, DG, and CCL are − 20 dB, 0.01457, − 25 dB, 10, 0.07322 respectively. Comparatively, the proposed MIMO antenna systems performance results and wide operating bandwidth in THz range is appropriate for optically transparent systems and sensing, Industry 4.0, IoT and 6G applications. [ABSTRACT FROM AUTHOR]

Introduction: Metamaterials consist of periodic arrangements of artificial subwavelength units that possess electromagnetic properties not present in natural media. It has attracted more interest due to its ability to alter electromagnetic radiation in a flexible manner, which has resulted in the development of multiple radio frequency devices based on metamaterials. Metamaterials with the required frequency band for electric or magnetic resonance can be made using unit cell structure. The incident electromagnetic wave will enter the metamaterials and be kept there in the absence of reflection. Methods: This paper proposes a novel broadband THz absorber filter based on graphene for emerging applications. The proposed structure comprised of three parts. The top layer consists of graphene, the middle layer consists of dielectric and the bottom layer is made up of gold. Results: The proposed structure is experimentally designed and validated using the COMSOL simulator. Discussion: Simulation results show that the proposed absorber has better performance as compared with existing methods. [ABSTRACT FROM AUTHOR]

A spectrometer based on silicon nanosandwiches has been proposed to detect complications caused by COVID-19. Operating in the mode of a balanced photodetector, the silicon nanosandwich is both a source of terahertz irradiation and a receiver of reflected and/or radiated from biological tissue. It has been demonstrated that recording the current–voltage characteristics of a silicon nanosandwich made it possible to analyze changes in the thyroid gland, thereby determining the degree and nature of changes caused by the COVID-19 disease. [ABSTRACT FROM AUTHOR]

The past decade witnessed the emergence of a new generation of room-temperature infrared detectors based on low-dimensional solids. Among these are topological insulating materials. The present work aims to evaluate this class of photodetectors in the so-called high-temperature infrared (high operating temperature) photodetector family. Their performance, such as current responsivity and detectivity, are compared with available HgCdTe photodiodes, interband quantum cascade photodetectors, colloidal quantum dot detectors, and two-dimensional transition metal dichalcogenides. [ABSTRACT FROM AUTHOR]

The possibility of fabrication of 4.6 μm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA/cm2. [ABSTRACT FROM AUTHOR]

We have demonstrated a quantum cascade laser (QCL) with a generation frequency of about 3.8 THz, grown by metal-organic vapor phase epitaxy. The multilayer heterostructure for QCLs consists of 185 repetitions of an active module containing four GaAs/Al0.15Ga0.85As quantum wells. The threshold current and threshold voltage of the fabricated QCL were 2.25 kA/cm2 and 19.7 V, respectively. The QCL oscillations were carried out in the multimode regime, and the detection of terahertz radiation continued with an increase in the laser temperature up to 60 K. [ABSTRACT FROM AUTHOR]

A tunable quantum cascade laser was scanned over the P(20) line in the 1 ← 0 transition band of CO at 2059.91 cm−1. Line strengths were measured in mixtures with helium and argon at room temperature (296 K) and broadening parameters at high temperatures between 1100 and 2800 K, and for pressures from 4.7 kPa up to 147.4 kPa behind reflected shock waves. Two mixtures were used to study the broadening coefficients: 0.006 CO/He and 0.006 CO/0.01 H2/Ar, for which CO vibrational equilibrium is ensured by He in the former mixture and by 1% H2 in the latter one. Direct measurements of γ CO - He T 0 and γ CO - Ar T 0 at room temperature were obtained, and the broadening coefficients, defined as γ T = γ T 0 T 0 / T n , were fully investigated by extrapolating the temperature-dependent coefficients n CO - He and n CO - Ar from the new, high-temperature scanning data. For validation purposes, two calibrations from the literature that characterize the P(20) line absorption coefficient (k υ) with a temperature-dependent equation using mixtures of CO perturbated in 0.2 He/Ar bath are compared with our computed k υ obtained via the linestrength taken from the HITRAN database and the line-shape function modeling with a pseudo-Voigt profile that depends on the new collisional broadening coefficients. Good agreement is observed. The CO–He broadening was found to not be identical to CO–Ar broadening, and when He is assumed to be Ar in the broadening effect analyses, large discrepancies can appear in the k υ quantifications, therefore in the CO time histories measured in a shock tube. The simulated CO absorption coefficients that account for He-related broadening are necessary for accurate CO measurements to validate modern detailed kinetics mechanisms. [ABSTRACT FROM AUTHOR]

Recently, high-throughput quantum cascade laser-based vibrational circular dichroism (QCL-VCD) technology has reduced the measurement time for high-quality vibrational circular dichroism spectra from hours to a few minutes. This study evaluates QCL-VCD for chiral monitoring using flow-through measurement of a changing sample in a circulating loop. A balanced detection QCL-VCD system was applied to the enantiomeric pair R/S-1,1′-bi-2-naphthol in solution. Different mixtures of the two components were used to simulate a racemization process, collecting spectral data at a time resolution of 6 min, and over three concentration levels. The goal of this experimental setup was to evaluate QCL-VCD in terms of both molar and enantiomeric excess (EE) sensitivity at a time resolution relevant to chiral monitoring in chemical processes. Subsequent chemometric evaluation by partial least squares regression revealed a cross-validated prediction accuracy of 2.8% EE with a robust prediction also for the test data set (error = 3.5% EE). In addition, the data set was also treated with the least absolute shrinkage and selection operator (LASSO), which also achieved a robust prediction. Due to the operating principle of LASSO, the obtained coefficients constituted a few discrete spectral frequencies, which represent the most variance. This information can be used in the future for dedicated QCL-based instrument design, gaining a higher time resolution without sacrificing predictive capabilities. Graphical abstract This is a visual representation of the abstract. [ABSTRACT FROM AUTHOR]