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We report on the implementation of a terahertz two-photon quantum cascade laser operating in a continuous wave mode. Lasers that can emit two photons as a result of the relaxation of a single electron between two states of the same parity have been discussed since the early days of the laser era, but implementation has been hampered by the lack of a suitable gain medium. The semiconductor structure of a quantum cascade laser seems to be an ideal medium for realizing such two-photon emission. Our work demonstrates dual-band laser radiation in the range of 3.1–3.9 THz (104–130 cm−1) at temperatures up to 90 K. [ABSTRACT FROM AUTHOR]

The influence of argon and helium on the rovibrational kinetics of carbon dioxide (CO2) and CO in low-temperature conversion plasma is investigated. With this objective, a combined experimental and computational study is conducted, applying quantum cascade laser infrared absorption spectroscopy to a pulsed DC CO2 glow discharge with varying noble gas admixture and modeling it with a two-term Boltzmann solver. Time-resolved rovibrational temperatures and dissociation fractions are presented, exhibiting an increase in rotational–vibrational non-equilibrium and an increasing CO2 conversion with argon (Ar) and helium (He) admixtures. Results are discussed in the context of energy transfer processes for collisions involving electrons, corroborated by electron-kinetic modeling, and heavy particle collisions. With noble gas addition, an increase in the electron number density, promoting excitation, and the high-energy tail of the electron energy distribution function are found. Penning ionization processes are proposed as an explanation for the increase in conversion, showing higher conversion for Ar due to the lower excitation thresholds and, therefore, larger state population. In the context of rovibrational kinetics, processes leading to the gain or loss of vibrational energy of CO2 are analyzed, pointing out subtle differences in, for example, relaxation rate coefficients between Ar and He. However, the cooling of the gas through conductive heat transfer is identified as the most important influence of the Ar and He admixture, as it keeps the relaxation rate for vibrational quenching low. [ABSTRACT FROM AUTHOR]

Bistability in the current–voltage characteristics of semiconductor superlattices and quantum cascade laser structures has the potential for wide-ranging applications, particularly in sensing systems. However, the interdependency of applied bias and current injection in conventional two-terminal structures has led to complications in analysis and rendered the bistability phenomenon difficult to implement in practical applications. Here, we report a new kind of electronic bistability coupled to optical switching in a resonant tunneling bipolar superlattice transistor. This bistability manifests as sharp discontinuities in the collector current with extremely small variations of the applied voltage, which arise from unstable tunneling transmission across the hetero-barrier between the two-dimensional electron gas (2DEG) at the edge of the transistor base and the collector superlattice structure. The electronic transitions between high and low quantum mechanical transmissions are demonstrated to be caused by self-consistent variations of the internal electric field at the heterointerface between the 2DEG and the superlattice. They are also present in the base current of the three-terminal device and result in sharp switching of near-infrared spontaneous light emission output from an interband radiative recombination process with a peak emission wavelength of 1.58 μ m. A comprehensive quantum mechanical theoretical model accounting for the self-consistent bistable tunneling transmission is in quantitative agreement with the experimental data. The measured peak transconductance sensitivity value of 6000 mS can be used in the highly sensitive detector and non-linear device applications. [ABSTRACT FROM AUTHOR]

A band design of a quantum cascade laser with a generation frequency higher than 6 THz and an active region based on four GaAs/Al0.14Ga0.86As quantum wells is proposed. Calculations were carried out based on the solution of the Schrödinger equation taking into account the dephasing of quantum states, as well as a closed system of balance equations. The temperature dependences of the gain at frequencies of 6.3–6.6 THz were calculated for the proposed quantum cascade laser with a double metal waveguide. Features of the proposed laser structure include two injection quantum wells and the suppression of non-radiative transitions between laser levels. According to calculations, this provides the maximum operating temperature of up to 81 K at 6.4 THz. The results of this study open up the way for quantum cascade lasers based on GaAs/AlGaAs to operate at frequencies above 6 THz. [ABSTRACT FROM AUTHOR]

Terahertz (THz) quantum cascade lasers (QCLs) are technologically important laser sources for the THz range but are complex to model. An efficient extended rate equation model is developed here by incorporating the resonant tunneling mechanism from the density matrix formalism, which permits to simulate THz QCLs with thick carrier injection barriers within the semi-classical formalism. A self-consistent solution is obtained by iteratively solving the Schrödinger–Poisson equation with this transport model. Carrier–light coupling is also included to simulate the current behavior arising from stimulated emission. As a quasi-ab initio model, intermediate parameters, such as pure dephasing time and optical linewidth, are dynamically calculated in the convergence process, and the only fitting parameters are the interface roughness correlation length and height. Good agreement has been achieved by comparing the simulation results of various designs with experiments, and other models such as density matrix Monte Carlo and non-equilibrium Green's function method that, unlike here, require important computational resources. The accuracy, compatibility, and computational efficiency of our model enable many application scenarios, such as design optimization and quantitative insights into THz QCLs. Finally, the source code of the model is also provided in the supplementary material of this article for readers to repeat the results presented here, investigate, and optimize new designs. [ABSTRACT FROM AUTHOR]

Molecular lasers pumped by quantum cascade laser (QCL) open new possibilities for THz generation and its numerous applications, in particular, for high resolution molecular spectroscopy. In this article, a THz water laser pumped by a mid-infrared QCL was demonstrated using the broad tunability of the pump laser. Twenty D 2 O laser lines were measured under a continuous wave pumping regime, in a spectral range expending from 63 to 177 cm − 1 (1.9–5.3 THz), and with an output power ranging from tens to hundreds of μ W. This letter contains a description of the experimental setup used to produce the THz laser radiation and a comparison of the measured output power with a molecular gain factor used to sort out the most favorable laser lines. In addition to the measured laser transitions, a complete list of laser frequencies together with their corresponding molecular gain is given in the supplementary material, for both H 2 O and D 2 O isotopologues excited in their bending and stretching vibrational states. [ABSTRACT FROM AUTHOR]

Spintronic terahertz (THz) emitters have been shown to be a cost-efficient source for use in time-domain THz spectroscopy. The use of external magnetic fields to control the polarity of the THz emission provides an opportunity to measure the magnetization of spintronic materials as well as shaping THz emission. Here, we demonstrate an efficient method of measuring magnetic hysteresis with material sensitivity and speed several orders of magnitude greater than typical magnetometry methods. In addition, we utilize the rapid control of material magnetization for lock-in detection in time-domain THz spectroscopy of spintronic emitters. The ability to rapidly control and measure the material magnetization on very small volumes provides an opportunity to study magnetic hetero-structures with sub-micron spatial resolution. [ABSTRACT FROM AUTHOR]

In this paper, polarization-stable and spectrally single-mode operation of oxide-confined multi-junction vertical-cavity surface-emitting lasers (VCSELs) are achieved by using a non-circular integrated anti-phase transverse mode filter. A record-high single-mode output power is obtained by radially reducing effective mirror reflectivity at the surface areas where the higher order modes are expected to lase, leading to an increase in the threshold gain for these transverse modes, all the while minimal losses are introduced to the fundamental mode. The polarization control of such devices is achieved by using a non-circular mode filter in order to perturb the cylindrical symmetry of the cavity. An elliptically etched surface relief is shown to be effective in realizing a single-mode single-polarization device over a wide range of operating bias currents and temperatures. The continuous-wave light–current–voltage characteristics of 8-junction 940 nm VCSELs show single-mode output power >13 mW at room temperature, spectrally measured with a side-mode suppression ratio of >30 dB. This single-mode performance can be achieved depending on the ratio between the relief opening and the oxide aperture size. Additionally, the single-polarization behavior of such devices (with an extinction ratio of >15 dB) is shown to be dependent on the orientation of the elliptical mode filter, where the light is mainly polarized along the minor axis of the elliptical feature. Overall, combined with a multi-junction active region, the elliptical model filter is able to achieve highly efficient single-mode linearly polarized VCSELs. [ABSTRACT FROM AUTHOR]

We fabricated a photoconductive antenna (PCA) with Er-doped InAs quantum dot (QD) layers on a GaAs substrate and demonstrated THz wave generation with 1.5 μm pump. In terahertz time-domain spectroscopy (THz-TDS), PCAs require high mobility and short lifetimes of photocarriers since the carriers need to respond rapidly to optical pulses for broad THz generation and detection with a high time resolution. In this study, to achieve PCA operation at an optical communication wavelength band of 1.5 μm, a substrate for a PCA was fabricated using stacked Er-doped-InAs QD layers. Then, photocurrent measurements were performed to observe optical absorption in the 1.5 μm band. The results indicated the high capability of the PCA for THz wave generation and detection, in fact, which enabled us to generate the THz wave in the 1.5 μm band optical system. [ABSTRACT FROM AUTHOR]

This study presents both numerical modeling and experimental fabrication of three different partial-reflection (PR) coatings each optimized for quantum cascade lasers (QCLs) that emit radiation in the mid-infrared range. A novel double-layer PR coating comprising silicon dioxide (SiO2) and silicon nitride (Si3N4) was proposed as a potential solution for compatibility with QCLs fabrication processes. Subsequently, the PR coating was compared with two well-known PR coatings: a single layer of aluminum oxide (Al2O3) and a single layer of yttrium oxide (Y2O3). The coatings were designed to reduce the reflectivity of the front laser mirror from 30% to approximately 13%. The thickness of the dielectric layers was optimized for lasers emitting at 4.4 μm, with applicability in the 2.5–6 μm range. The proposed double-layer coating achieved the desired reflectivity while reducing the total coating thickness by 120 nm. By using the presented coatings it will be possible to increase the optical power of Mid-Infrared QCLs. [ABSTRACT FROM AUTHOR]

Light polarization control is a target in photonics, and this paper provides a comprehensive review of research from various groups on the silicon‐on‐insulator (SOI) platform. It draws comparisons between devices such as polarization splitters (PS), polarizers, and polarization splitters/rotators (PSR). These devices are fabricated using various technologies, including silicon nanowires, ridge waveguides, hybrid plasmonic waveguides, and subwavelength grating (SWG) waveguides. A detailed review of polarizers used as cleanup filters in splitters is initiated. Subsequently, various polarization splitters utilizing asymmetric directional couplers (ADCs), which typically exhibiting low extinction ratios (ERs), are delved. To enhance ERs, a detailed comparison of methods outlined in the literature is provided. One notable method includes integrating on‐chip polarizers at both ports to eliminate unwanted light fractions and achieve exceptionally high ERs. Furthermore, SWG‐based polarizers and splitters commonly face issues with Bragg reflections that can affect other photonic devices and lasers and ways to minimize unwanted polarization back reflections in SWG‐designed polarization control devices are examined. Finally, emerging applications in mid‐infrared (MIR) sensing are explored, highlighting the necessity of polarization rotators for on‐chip transverse electric (TE) operation, since quantum cascade lasers, the primary sources in this range, emitting radiation in the (TM) mode. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) in the mid-infrared (MIR) hold significant potential for widespread applications in both military and civilian contexts. However, the utility of present single-chip QCLs is hampered by issues such as low output power and subpar beam quality. This study addresses these limitations by employing spectral beam combining (SBC) based on a diffraction grating, with an aim to enhance both power and beam quality of MIR QCLs. Coaxial power synthesis of three single-chip QCLs (around 4.75 μm) is achieved experimentally, importantly, the beam quality did not decrease after combining, essentially maintaining the same quality as before combining. It is proposed that there are four main factors affecting the combining efficiency, namely operating optical power (or driving current) of the chips, individual differences in QCL chips, diffraction grating, and reflectivity of feedback mirror, and their effects on the combining efficiency are discussed separately. This study confirms that SBC is an effective way to obtain high-power and high-beam quality MIR QCL sources, and lays a research foundation for more beam spectral combining. [ABSTRACT FROM AUTHOR]

The terahertz frequency region of the electromagnetic spectrum is crucial for understanding the formation and evolution of galaxies and stars throughout the universe's history, as well as the process of planet formation. Detecting the unique spectral signatures of molecules and atoms requires terahertz spectrometers, which must be operated in space observatories due to water vapor absorption in the Earth's atmosphere. However, current terahertz spectrometers face challenges such as low resolution, limited bandwidth, large volume, and complexity. In this paper, the issues of size and weight are addressed by demonstrating a concept for a centimeter‐sized, low‐weight terahertz spectrometer using a metasurface. The design of the metasurface spectrometer is first discussed for the 1.85 to 2.4 THz range, followed by its fabrication. Next, an array of quantum cascade lasers operating at slightly different frequencies around 2.1 THz is utilized to characterize the spectrometer. Finally, a spectrum inversion method is applied to analyze the measured data, confirming a resolution R (λ/Δλ) of at least 273. This concept can be extended to other application areas, such as planetary observations and various wavelengths in the far‐infrared (FIR) and near‐infrared (NIR) ranges. [ABSTRACT FROM AUTHOR]

A longwave infrared quantum cascade laser with a peak wavelength of 10.5 μm is reported. The active region features a multi-miniband transition design for broad tuning operation. In pulsed mode and continuous wave mode, the maximum power of the Fabry-Pérot laser is 1.4 W and 0.32 W at room temperature, respectively. In the Littrow external cavity configuration, the wavelength of laser can be tuned in the ranges of λ ~ 8.59–9.67 μm and 10.20–10.68 μm at one fixed voltage of 12.6 V and 10.4 V, respectively, in pusled mode. [ABSTRACT FROM AUTHOR]

We present a high performance top emitting room temperature monolithic terahertz (THz) quantum cascade laser based on difference frequency generation (DFG) and a buried modulated distributed feedback grating. Using a semi-insulating InP:Fe substrate combined with an index-coupled grating could minimize the internal losses for the THz mode. The laser consists of two separate transversely superimposed gratings. The first grating provides selective feedback for the mid-infrared pump modes and is realized in an index-coupled DFG grating designed for single-mode operation for two distinct MIR wavelengths. The second metallic grating for THz extraction is carefully aligned to the spatial distribution of the MIR modes. The resulting device lases pulses up to 150 °C in the single mode. The maximum THz output power at RT is 100 μW measured at 3.37 THz. A THz wavelength tuning of around 0.065 nm/K was achieved through thermal tuning of the MIR pump lasers. [ABSTRACT FROM AUTHOR]

In this study, we introduce an automated design method for Quantum Cascade Laser (QCL) active region (AR) structure employing a generative neural network, termed an inverse network, which creates the structure designs based on specific k · p metric inputs related to device performance. The training dataset, derived from an earlier study, was selectively filtered to remove entries affected by energy-level hybridization or splitting, yielding ∼300 000 valid entries. A pre-trained forward network that processes QCL-AR structures and returns corresponding k · p metrics serves as the evaluator for the inverse network, supplanting traditional loss functions such as mean squared error or mean absolute error. This strategy overcomes the problem of non-uniqueness in the mapping from k · p metrics to QCL-AR structures. The inverse network incorporates a random layer, allowing it to produce a variety of QCL-AR structures from identical predicted metrics, thereby increasing the model's practicality. Performance testing indicates high accuracy in the metrics of the generated QCL-AR structures, with the coefficient of determination, R2 scores, for key energy-level differences between the upper-laser (ul) level and the lower-laser level, E43, and between the next-higher-energy level above the ul level and the ul level, E54, of 0.9153 and 0.9701, respectively; and for the electron lifetimes τ43 and τ54 of 0.9568 and 0.9175. As an example, we show how the network generates a QCL-AR structure with the potential for low threshold-current density by suppressing shunt-type carrier leakage from the ul level through a higher energy AR state. [ABSTRACT FROM AUTHOR]

Frequency combs are powerful tools for many applications and high performances are achieved by stabilizing these lasers. For operation in the mid‐infrared, quantum cascade lasers (QCL) are ideal candidates as they present numerous advantages. However, stabilized QCL‐combs lack a detailed characterization of their noise properties due to the sensitivity limits of current analyzing techniques. To overcome these challenges, what is believed to be the first tightly locked dual QCL‐comb system is developed. Its use is twofold. First, phase noise analysis of the dual‐comb signal shows residual phase noise below 600 mrad for all comb lines, and the comb coherence as well as the performances of the repetition frequency locking mechanism is characterized. Second, coherent averaging with a 7 × 105 Hz1/2 figure‐of‐merit system is demonstrated, which is compatible with high‐precision spectroscopy. [ABSTRACT FROM AUTHOR]

Interband cascade lasers typically have significantly lower threshold current and power consumption than quantum cascade lasers. They can also have advantages regarding costs and compactness with the photonic integration onto silicon substrates by epitaxial growth. This research introduces a novel examination of the relative intensity noise and the modulation dynamics of a silicon-based Fabry–Perot interband cascade laser emitting at 3.5 μm. The investigation delves into crucial parameters, such as relaxation oscillation frequency, differential gain, gain compression, and K-factor. The resonance patterns identified in relative intensity noise curves can provide essential insights for the thorough characterization of high-defect mid-infrared semiconductor structures intended for high-speed applications. Moreover, this study demonstrates the feasibility of reaching 10 Gbit/s free-space transmission using a silicon-based interband cascade laser in conjunction with an interband cascade infrared photodetector. [ABSTRACT FROM AUTHOR]

We analyze the main physical processes in quantum cascade lasers with a spatial separation between the region of photon radiation and longitudinal optical (LO) phonon emission, which facilitates the depopulation of the lower level of the optical transition. Our objective is to identify the reasons for the reduction of population inversion at low photon energy and explore methods to enhance it. The expression for population inversion is derived from an equation for a simplified density matrix. This approach allows us to consider the coherence of tunneling between different levels and comprehend its influence on transition probabilities in a straightforward manner. We have found out that the energy uncertainty principle is the fundamental factor limiting population inversion in terahertz lasers. By optimizing the tunneling matrix element between the two regions and the LO phonon emission time, it is possible to significantly increase the population inversion. The optimal value for the matrix element is smaller than its maximum possible value, while the optimal LO phonon emission time exceeds the time achieved during LO phonon resonant emission. [ABSTRACT FROM AUTHOR]

Portable terahertz (THz) frequency comb sources are highly desired for applications in rotational molecular spectroscopy and sensing. To date, direct THz quantum cascade laser (QCL) frequency comb generation is not achievable at room temperature. However, THz comb generation based on intracavity difference frequency generation (DFG) in mid-infrared (mid-IR) QCLs is a promising alternative. Here, we present a numerical study of THz DFG-QCL comb formation in mid-IR QCLs based on a self-consistent multi-domain simulation approach. The dynamical simulations are performed using our open-source software tool mbsolve, which provides a flexible and efficient codebase for solving the generalized full-wave Maxwell–Bloch equations. Here, DFG in the active region of a dual-wavelength mid-IR QCL is considered for the generation of THz radiation. The mixing process and, thus, THz generation require a high second-order intersubband nonlinear susceptibility in the QCL active region and can be obtained by targeted quantum engineering. The associated nonlinear effects are included in the Hamiltonian of our Maxwell–Bloch simulation approach. All necessary input parameters for the description of the quantum system are determined self-consistently using our in-house ensemble Monte Carlo software tool for stationary carrier transport simulations. Notably, such simulations require a full-wave Maxwell–Bloch solver that does not employ the common rotating wave approximation, as a broadband optical field extending from the THz to the mid-IR region is investigated. Our modeling approach and the obtained simulation results for two THz DFG-QCL comb setups are validated against experimental data, showing reasonable agreement. Furthermore, we obtain a locked frequency modulated comb state for mid-IR and THz regimes. [ABSTRACT FROM AUTHOR]

The proposed Minkowski fractal antenna design achieves wideband and continuous frequency tuning in a multiple-input and multiple-output (MIMO) antenna system. By manipulating the fractal geometry of the unit antenna element, the resonance frequency of the antenna can be adjusted simply by changing its electrical length. The Minkowski fractal operator generates an increasing current path, resulting in a leftward frequency shift as the antenna side length increases with each iteration. In the first iteration, the proposed fractal antenna demonstrated a 97.9% continuous frequency shift from 0.204 to 0.404 THz with maximum return loss values of −31.23 and −21.6 dB, respectively. In the second iteration, a 38.6% continuous resonance frequency shift from 0.413 to 0.578 THz was achieved with return loss values of −18.22 and −40.47 dB, respectively. The maximum tunable bandwidths of the first and second iterations were approximately 0.2 and 0.16 THz, respectively. The proposed correlation between the dimensions of a single antenna and its resonance frequency provides the foundation for designing and implementing MIMO antenna systems in high-speed wireless devices, cognitive radio, and other multiband MIMO applications. A 2 × 2 MIMO antenna system has been designed from the results of the proposed single antenna to achieve multiband operation or frequency tuning through selective switching of the antenna feed. [ABSTRACT FROM AUTHOR]

Membranes are crucial for the functionality of membrane proteins in several cellular processes. Time-resolved infrared (IR) spectroscopy enables the investigation of interaction-induced dynamics of the protein and the lipid membrane. The photoreceptor and proton pump bacteriorhodopsin (BR) was reconstituted into liposomes, mimicking the native purple membrane. By utilization of deuterated lipid alkyl chains, corresponding vibrational modes are frequency-shifted into a spectrally silent window that allows us to monitor lipid dynamics during the photoreaction of BR. Our home-built quantum cascade laser (QCL)-based IR spectrometer covers all relevant spectral regions to detect both lipid and protein vibrational modes. QCL-probed transients at single wavenumbers are compared with the previously performed step-scan Fourier-transform IR measurements. The absorbance changes of the lipids could be resolved by QCL-measurements with a much better signal-to-noise ratio and with nanosecond time resolution. We found a correlation of the lipid dynamics with the protonation dynamics in the M intermediate. QCL spectroscopy extends the study of the protein's photocycle toward dynamics of the interacting membrane. [ABSTRACT FROM AUTHOR]

The combination of speed and resolution is a fundamental figure of merit for Fourier transform spectroscopy. Here, we show that, by combining a fast rotational delay line with a strongly modulated quantum cascade laser (QCL), it is possible to perform broadband spectroscopy over a range of 175 cm−1, with a total effective integration time of about 6 s and a resolution of 0.03 cm−1. Remarkably, even though the resolution of the measurement is limited by the source, the interleaving technique can still be applied when the QCL is not in a frequency comb regime, considerably broadening the bandwidth of the instrument. [ABSTRACT FROM AUTHOR]

We present mutual optical coupling in terahertz (THz) quantum cascade wire laser arrays that are flip-chip bonded to a dielectric substrate. The mounting substrate is patterned for individual electrical contacting of each wire laser of the array. The resulting sandwich-like structure supports wire laser modes with a significant part propagating outside the cavity and mediates the long range coupling. The evanescent field part of the modes couples to the adjoining ridge, which, in turn, leads to mutual optical injection-locking between them. We demonstrate this effect for both geometrically similar and dissimilar wire lasers when biased in pulsed operation with temporally overlapping bias pulses. Finite element simulations confirm our measurement results. By applying time-shifted bias pulses to individual array elements, a controllable optical injection seeding of the wire cavity is achieved. We observe intensity modification of the laser modes with changing bias pulse overlap as a result of the injection locking. By choosing both the physical spacing of the laser ridges and the intensity of the seeding laser correctly, the relative intensities of the favored lasing modes are enhanced up to 95 percent. Understanding the coupling in THz wire laser arrays is important for future device improvements in terms of higher continuous-wave operating temperatures through better thermal dissipation, and higher output power and an improved far field due to controlled coupling of their modes. [ABSTRACT FROM AUTHOR]

A new antenna array architecture is proposed here to overcome a current bottleneck in phased arrays concerning the enhancement of radiation efficiency and avoidance of scan blindness. In contrast to the conventional approach of using patches with circular or square geometries that exploit 90 ∘ or 180 ∘ rotational symmetry, this work proposes to use waveguide radiating elements based on 120 ∘ symmetry. To implement such symmetry, the tri-ridge aperture is proposed, and its capability to scan widely within a broad frequency bandwidth is demonstrated. A modal analysis is performed to explain the physical phenomena underlying such superior performance. A successful experimental validation is provided by means of a monolithic prototype built in metal additive manufacturing. The measured results show, for the first time, that it is possible to achieve high radiation efficiency and circular polarization when scanning widely in a broad bandwidth. Such an accomplishment constitutes a major achievement in the field of active electronically steered phased arrays, and impacts significantly the capabilities and potential of modern radar and communication systems. [ABSTRACT FROM AUTHOR]

Cryogenic ion vibrational predissociation (CIVP) spectroscopy is an established and valuable technique for molecular elucidation in the gas phase. CIVP relies on tunable lasers, wherein among typical laser schemes, the application of mid-infrared continuous-wave quantum cascade laser (cw-QCL) is the most robust and elegant solution, as we have recently demonstrated. However, potential challenges arise from an inhomogeneous character across laser power tuning curves. A large laser power output could have undesired consequences, such as multiphoton absorption or saturation effects. Significant variations in laser power tuning curves could potentially alter the shape of the investigated band, particularly for diffuse bands. In this study, we have developed and introduced an automatic variable laser power attenuator designed to keep the laser power output uniform at a user-defined value across the entire available spectral range. We demonstrated the application of this attenuator in obtaining CIVP spectra of a model compound with a diffuse N–H–N band. This approach enhances the reliability of measuring diffuse bands and overall applicability of cw-QCL. [ABSTRACT FROM AUTHOR]

Fluorotellurite glasses heavily doped with Er3+ are prepared in less demanding conditions as compared with fluoride glasses. Specifically, the effects of PbF 2 added in the system of AlF 3 –MgF 2 –CaF 2 –SrF 2 –BaF 2 –TeO 2 (AMCSBYT) are systematically studied on the correlation between glass structure and mid-infrared (MIR) luminescent properties of Er3+. These PbF 2 -modified glasses with an improved thermal stability and a noticeably reduced phonon energy as compared with telluride glasses exhibit an intense MIR emission at 2.72 μm and a long radiative decay lifetime. Calculation and analysis based on the Judd-Ofelt theory provide deeper insights into the interdependence of luminescent properties and glass structure. Numerical simulation is conducted for a MIR fiber laser based on the experimental data derived from the proposed glass. The 2.72 μm lasing output of 0.68 W is predicted with a slope efficiency of 3.38 %. It is expected that the performance of the fiber laser can be further optimized such that the output power can be increased over 1.0 W. The results emphasize the potential of the PbF 2 -modified fluorotellurite glass as a promising gain material for 2.72 μm fiber lasers. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) is a pyramidal symmetric top molecule with inversion motion and it exists in two distinct nuclear spin isomeric forms, ortho-NH3 (K = 3n) and para-NH3 (K ≠ 3n). Here, a pair of electric dipole-allowed [RQ(4,3) and PP(2,2)] and weak forbidden [OP(3,3) and SQ(8,1)] transitions of gas-phase NH3 were probed in the ν4 fundamental vibrational band occurring at 6.2 µm mid-IR interference-free spectral region using an external-cavity quantum cascade laser coupled cavity ring-down spectrometer. We have experimentally achieved the ortho-to-para ratio (OPR) of (1.03 ± 0.24) and (1.08 ± 0.14) at room temperature (296 K) for the allowed and forbidden transitions of NH3, respectively. Furthermore, our experimental findings confirm that the nuclear spin-symmetry is conserved under the nonreactive perturber-induced collisional processes. This study paves the way to directly probe the nuclear spin-isomers of gas-phase NH3 and thus may lead to an in-depth understanding of nuclear spin-isomerism associated with various physical and chemical processes. This work shows the high-resolution probing of ortho- and para-nuclear spin-isomers of NH3 in the gas phase at room temperature using quantum cascade laser coupled cavity ring-down spectroscopy at 6.2 µm mid-IR spectral region. [ABSTRACT FROM AUTHOR]

Ammonia (NH3) is a pyramidal symmetric top molecule with inversion motion and it exists in two distinct nuclear spin isomeric forms, ortho-NH3 (K = 3n) and para-NH3 (K ≠ 3n). Here, a pair of electric dipole-allowed [RQ(4,3) and PP(2,2)] and weak forbidden [OP(3,3) and SQ(8,1)] transitions of gas-phase NH3 were probed in the ν4 fundamental vibrational band occurring at 6.2 µm mid-IR interference-free spectral region using an external-cavity quantum cascade laser coupled cavity ring-down spectrometer. We have experimentally achieved the ortho-to-para ratio (OPR) of (1.03 ± 0.24) and (1.08 ± 0.14) at room temperature (296 K) for the allowed and forbidden transitions of NH3, respectively. Furthermore, our experimental findings confirm that the nuclear spin-symmetry is conserved under the nonreactive perturber-induced collisional processes. This study paves the way to directly probe the nuclear spin-isomers of gas-phase NH3 and thus may lead to an in-depth understanding of nuclear spin-isomerism associated with various physical and chemical processes. This work shows the high-resolution probing of ortho- and para-nuclear spin-isomers of NH3 in the gas phase at room temperature using quantum cascade laser coupled cavity ring-down spectroscopy at 6.2 µm mid-IR spectral region. [ABSTRACT FROM AUTHOR]

Rationale: Dual clumped isotope paleothermometry determines carbonate formation temperatures by measuring the frequency of 13C–18O (∆638) and 18O–18O (∆828) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high‐precision measurements of 18O–18O clumping using current techniques requires large sample sizes and long acquisition times. Methods: We developed a mid‐infrared isotope ratio laser spectrometer (IRLS) for simultaneous measurement of the isotopologue ratios ∆638 and ∆828 in gas‐phase carbon dioxide (CO2) at room temperature. Our IRLS uses a single laser scanning from 2290.7 to 2291.1 cm−1 and a 31 m pathlength optical cell, and it simultaneously measures the five isotopologues required for calculating ∆638 and ∆828: 16O12C16O, 16O13C16O, 16O12C18O, 16O13C18O, and 18O12C18O. In addition, our IRLS can measure 16O12C17O, enabling ∆17O analysis. Results: At ~20°C and a CO2 pressure of ~2 Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues 16O13C18O and 18O12C18O, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for 16O12C16O, 16O13C16O, and 16O12C18O. This yielded precisions of 0.348‰ (∆638) and 0.302‰ (∆828) within 25 s. Simulated sample–reference switching highlights the potential of our system and the need for further development. Conclusions: We demonstrated simultaneous measurements of ∆638 and ∆828 in CO2 to precisions of <0.35‰ within 25 s using a room‐temperature, single‐laser IRLS. Future developments on better resolving 16O12C16O and 16O13C16O peaks and system temperature control could further improve the measurement precision. [ABSTRACT FROM AUTHOR]

Rationale: Dual clumped isotope paleothermometry determines carbonate formation temperatures by measuring the frequency of 13C–18O (∆638) and 18O–18O (∆828) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high‐precision measurements of 18O–18O clumping using current techniques requires large sample sizes and long acquisition times. Methods: We developed a mid‐infrared isotope ratio laser spectrometer (IRLS) for simultaneous measurement of the isotopologue ratios ∆638 and ∆828 in gas‐phase carbon dioxide (CO2) at room temperature. Our IRLS uses a single laser scanning from 2290.7 to 2291.1 cm−1 and a 31 m pathlength optical cell, and it simultaneously measures the five isotopologues required for calculating ∆638 and ∆828: 16O12C16O, 16O13C16O, 16O12C18O, 16O13C18O, and 18O12C18O. In addition, our IRLS can measure 16O12C17O, enabling ∆17O analysis. Results: At ~20°C and a CO2 pressure of ~2 Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues 16O13C18O and 18O12C18O, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for 16O12C16O, 16O13C16O, and 16O12C18O. This yielded precisions of 0.348‰ (∆638) and 0.302‰ (∆828) within 25 s. Simulated sample–reference switching highlights the potential of our system and the need for further development. Conclusions: We demonstrated simultaneous measurements of ∆638 and ∆828 in CO2 to precisions of <0.35‰ within 25 s using a room‐temperature, single‐laser IRLS. Future developments on better resolving 16O12C16O and 16O13C16O peaks and system temperature control could further improve the measurement precision. [ABSTRACT FROM AUTHOR]

This paper introduces a novel, cost-effective solution designed to achieve large absorption bandwidth within the THz spectrum employing a miniaturized, single-layer metamaterial structure. The designed structure features a single circular ring composed of an ohmic resistive sheet with notably higher sheet resistance than traditional metallic resonators. This distinctive design is implemented on a lossy dielectric polyimide substrate with a backing of metallic gold. Our developed absorbing structure demonstrates the capability to achieve a substantial absorption bandwidth ranging from 3.78 to 4.25 THz, maintaining a consistent absorption rate of over 90%. Moreover, we conducted an analysis to assess its absorption performance under various sheet resistance values within the top layer. Additionally, we characterized its angular stability and polarization insensitivity through oblique incident and polarization angle analysis. Finally, an RLC circuital and interference theory approach is adopted to justify its simulated results. The proposed absorber shows potential for a broad spectrum of applications, encompassing communication, imaging, and diverse integrated circuits operating within the THz band. [ABSTRACT FROM AUTHOR]

This paper introduces a novel, cost-effective solution designed to achieve large absorption bandwidth within the THz spectrum employing a miniaturized, single-layer metamaterial structure. The designed structure features a single circular ring composed of an ohmic resistive sheet with notably higher sheet resistance than traditional metallic resonators. This distinctive design is implemented on a lossy dielectric polyimide substrate with a backing of metallic gold. Our developed absorbing structure demonstrates the capability to achieve a substantial absorption bandwidth ranging from 3.78 to 4.25 THz, maintaining a consistent absorption rate of over 90%. Moreover, we conducted an analysis to assess its absorption performance under various sheet resistance values within the top layer. Additionally, we characterized its angular stability and polarization insensitivity through oblique incident and polarization angle analysis. Finally, an RLC circuital and interference theory approach is adopted to justify its simulated results. The proposed absorber shows potential for a broad spectrum of applications, encompassing communication, imaging, and diverse integrated circuits operating within the THz band. [ABSTRACT FROM AUTHOR]

Optical frequency combs with equidistant frequency modes have revolutionized metrology and spectroscopy. The most widespread combs consist of periodic pulse trains generated by mode-locked lasers. However, it has recently been demonstrated that most semiconductor lasers based on Fabry–Pérot cavities, such as quantum well laser diodes, quantum cascade lasers, and quantum dot lasers, can enter an unconventional regime without traditional mode-locking mechanisms. The time-domain profile of these self-locked combs features a frequency-modulated (FM) wave with quasi-continuous-wave intensity and near-linear frequency chirp. The observation of the FM mode of operation in lasers with significantly different dynamics suggested that this mode is a fundamental operating state of semiconductor lasers, stemming from a deeper underlying mechanism. Thanks to recent theoretical and experimental advances, the origin of FM behavior has become clear. In this Perspective, we discuss the current status of FM combs in semiconductor lasers based on Fabry–Pérot cavities, focusing on their physical origin, modeling, characterization, bandwidth enhancement, and potential in future applications. [ABSTRACT FROM AUTHOR]

A novel pulsed power source capable of nanosecond pulses with burst frequencies up to 1 MHz is employed to create atmospheric pressure pulsed plasma in pure CO2 gas. The short bursts contain up to four nanosecond pulses. The CO2 conversion and corresponding energy efficiency are measured ex-situ with Fourier-transform infrared absorption spectroscopy. Trends in the absorption line profile of in-situ quantum cascade laser infrared absorption spectroscopy indicate an elevated vibrational temperature of CO2 with an increasing number of pulses per burst. The key result of this paper is that the dissociation energy efficiency is higher when operating the plasma in burst mode. Furthermore, a larger number of pulses in a burst is associated with a further increase of the dissociation efficiency. The highest efficiency measured is (17.7 ± 0.3) % for single pulses spaced 2 ms apart, and (20.0 ± 0.3) % for bursts of three pulses, with an in-burst frequency of 1 MHz and bursts spaced 4 ms apart. [ABSTRACT FROM AUTHOR]

Ten to 25 μm‐long‐wavelength mid‐infrared laser has broad application prospects in fields such as military, industrial, medical, and spectroscopy. Currently, lasers capable of emitting 10–25 μm‐long‐wavelength mid‐infrared radiation include CO2 lasers, solid‐state lasers, quantum cascade lasers, infrared supercontinuum light sources, and free electron lasers. This review summarizes the development status of these five types of lasers, providing a detailed analysis and summary of their operating principles, advantages, limitations, and future outlooks. The aim is to offer valuable reference information for related research and to promote the further development and application of tunable mid‐infrared laser at 10–25 μm. [ABSTRACT FROM AUTHOR]

A low‐cost single frequency laser, emitting in the mid‐infrared spectral region and dissipating minimal electrical power, is a key ingredient for the next generation of portable gas sensors for high‐volume applications involving chemical sensing of important greenhouse and pollutant gases. Herein, a Quantum Cascade Surface Emitting Laser (QCSEL) is proposed, which is implemented as a short linear cavity with high reflectivity coated end‐mirrors to suppress any edge emission and employs a buried semiconductor diffraction grating to extract the light from the surface. By wafer‐level testing, the cavity length scaling is investigated, mirror reflectivities larger than 0.9 are extracted, and a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 µm is achieved. Finally, single‐mode emission with a side‐mode suppression ratio larger than 33 dB is demonstrated for a 248 µm short cavity, which is mounted with the epitaxial layer up and operated in continuous wave at 20 ∘C$^{\circ}{\rm C}$. [ABSTRACT FROM AUTHOR]

A tunable terahertz metamaterial sensor based on aluminum and silicon has been proposed in this paper. Two identical aluminum crosses are placed on either side of the silicon layer. Two substructures are formed by changing certain structural parameters, and two different resonant modes are enhanced directionally in these substructures, respectively. The refractive index sensing range for both resonant modes is 1.0–1.8. The sensitivity and figure of merit (FOM) of the electric dipole resonant mode which is centered at 2.333 THz are 117.67 and 1.07 GHz/RIU. As for the Fano mode which is centered at 3.381 THz, the calculated values are 81 and 2.53 GHz/RIU. Both resonant modes can achieve over 98% opacity. More significantly, an enhancement method of sensing capacity is realized based on perturbation theory. By placing polytetrafluoroethylene (PTFE) plates on the ends of the upper surface aluminum cross, the sensitivity of both resonant modes is improved. The optimized structure provides a sensitivity of 132.75 and 105.17 GHz/RIU for the two resonant modes, respectively. In addition, the effects of PTFE layer thickness on sensing performance are also discussed. This work opens up new prospects in the design of terahertz metamaterial sensors and provides a new method of enhancing sensing capacity. [ABSTRACT FROM AUTHOR]

For the first time, we demonstrate the hybrid integration of dual distributed feedback (DFB) quantum cascade lasers (QCLs) on a silicon photonics platform using an innovative 3D self-aligned flip-chip assembly process. The QCL waveguide geometry was predesigned with alignment fiducials, enabling a sub-micron accuracy during assembly. Laser oscillation was observed at the designed wavelength of 7.2 μm, with a threshold current of 170 mA at room temperature under pulsed mode operation. The optical output power after an on-chip beam combiner reached sub-milliwatt levels under stable continuous wave operation at 15 °C. The specific packaging design miniaturized the entire light source by a factor of 100 compared with traditional free-space dual lasers module. Divergence values of 2.88 mrad along the horizontal axis and 1.84 mrad along the vertical axis were measured after packaging. Promisingly, adhering to i-line lithography and reducing the reliance on high-end flip-chip tools significantly lowers the cost per chip. This approach opens new avenues for QCL integration on silicon photonic chips, with significant implications for portable mid-infrared spectroscopy devices. [ABSTRACT FROM AUTHOR]

This study conducts a comparative analysis, using non-equilibrium Green's functions (NEGF), of two state-of-the-art two-well (TW) Terahertz Quantum Cascade Lasers (THz QCLs) supporting clean 3-level systems. The devices have nearly identical parameters and the NEGF calculations with an abrupt-interface roughness height of 0.12 nm predict a maximum operating temperature (Tmax) of ~ 250 K for both devices. However, experimentally, one device reaches a Tmax of ~ 250 K and the other a Tmax of only ~ 134 K. Both devices were fabricated and measured under identical conditions in the same laboratory, with high quality processes as verified by reference devices. The main difference between the two devices is that they were grown in different MBE reactors. Our NEGF-based analysis considered all parameters related to MBE growth, including the maximum estimated variation in aluminum content, growth rate, doping density, background doping, and abrupt-interface roughness height. From our NEGF calculations it is evident that the sole parameter to which a drastic drop in Tmax could be attributed is the abrupt-interface roughness height. We can also learn from the simulations that both devices exhibit high-quality interfaces, with one having an abrupt-interface roughness height of approximately an atomic layer and the other approximately a monolayer. However, these small differences in interface sharpness are the cause of the large performance discrepancy. This underscores the sensitivity of device performance to interface roughness and emphasizes its strategic role in achieving higher operating temperatures for THz QCLs. We suggest Atom Probe Tomography (APT) as a path to analyze and measure the (graded)-interfaces roughness (IFR) parameters for THz QCLs, and subsequently as a design tool for higher performance THz QCLs, as was done for mid-IR QCLs. Our study not only addresses challenges faced by other groups in reproducing the record Tmax of ~ 250 K and ~ 261 K but also proposes a systematic pathway for further improving the temperature performance of THz QCLs beyond the state-of-the-art. [ABSTRACT FROM AUTHOR]

This work proposes a colored intelligent thermal control emitter characterized by adjustable structural color development with a reflection spectrum in the visible light range and intelligent thermal control with temperature switching in the mid infrared wavelength band. In the beginning, we investigate an intelligent thermal control emitter based on a multilayer membrane formed by the phase change material vanadium dioxide, metal material Ag, and dielectric materials Ge and ZnS. By incorporating the thermochromic material vanadium dioxide, the structure exhibits a high emissivity of ε H = 0.85 in the range of 3–13 μm wavelength when in the high temperature metallic state, enabling efficient heat radiation. In the low temperature dielectric state, the structure only has a low emission capacity of ε L = 0.07, reducing energy loss. Our results demonstrate that the structure achieves excellent emission regulation ability (Δ ε = 0.78) through the thermal radiation modulation from high to low temperature, maintaining effective thermal control. Furthermore, the thermal control emitter exhibits selective emission of peak wavelength due to destructive interference and shows a certain independence on polarization and incidence angle. Additionally, the integration of structural color adjustment capability enhances the visual aesthetics of the human experience. The proposed colored intelligent thermal control structure has significant potential for development in aesthetic items such as colored architecture and energy-efficient materials. We propose a colored intelligent thermal control emitter characterized by adjustable structural color development with a reflection spectrum in the visible light range and intelligent thermal control with temperature switching in the mid infrared wavelength band. By incorporating the thermochromic material vanadium dioxide, our results demonstrate that the structure achieves excellent emission regulation ability (Δ ε = 0.78) through the thermal radiation modulation from high to low temperature, maintaining effective thermal control. Additionally, the integration of structural color adjustment capability enhances the visual aesthetics of the human experience. The proposed colored intelligent thermal control structure has significant potential for development in aesthetic items such as colored architecture and energy-efficient materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Optical resonators are fundamental building blocks of photonic systems, enabling meta-surfaces, sensors, and transmission filters to be developed for a range of applications. Sub-wavelength size (< λ/10) resonators, including planar split-ring resonators, are at the forefront of research owing to their potential for light manipulation, sensing applications and for exploring fundamental light-matter coupling phenomena. Near-field microscopy has emerged as a valuable tool for mode imaging in sub-wavelength size terahertz (THz) frequency resonators, essential for emerging THz devices (e.g. negative index materials, magnetic mirrors, filters) and enhanced light-matter interaction phenomena. Here, we probe coherently the localized field supported by circular split ring resonators with single layer graphene (SLG) embedded in the resonator gap, by means of scattering-type scanning near-field optical microscopy (s-SNOM), using either a single-mode or a frequency comb THz quantum cascade laser (QCL), in a detectorless configuration, via self-mixing interferometry. We demonstrate deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes resolving both amplitude and phase of the supported modes, and unveiling resonant electric field enhancement in SLG, key for high harmonic generation. [ABSTRACT FROM AUTHOR]

Electrically tunable graphene‐metal metasurfaces with controllable optical properties have attracted interest for straightforward manipulation of free space light. Their resonance tuning range depends on graphene's electrical transport characteristics, which are affected by its quality, operating conditions, and the device design. An important example of the latter is the direct contact of metallic antennas with the graphene layer that limits the extent to which a bias voltage can tune the metasurface's permittivity. In this work, this issue is resolved in a straightforward and fabrication‐efficient way for graphene‐metal hybrid metasurfaces with multiple plasmonic resonances. It is demonstrated that the incorporation of a 10 nm Al2O3 barrier layer enhances the tuning range of mid‐infrared resonances compared to metasurfaces without barrier layer, i.e., from 300 to 700 nm for a 7.3 µm resonance and from 110 to 140 nm for a 4.7 µm resonance. The improved tunability of the metal/dielectric/graphene metasurface can be attributed to the reduced electrical coupling between metal and graphene, as confirmed by an equivalent circuit model. These results bring closer the use of active metasurfaces based on two‐dimensional materials under ambient conditions, with possible applications as optical filters, modulators, and information processing devices that require dynamic control of light. [ABSTRACT FROM AUTHOR]

The investigation of tuneable and energy-efficient terahertz (THz) generation has become a prominent field of study due to its significant ramifications in various disciplines, including chemical analysis and medical research. This study selects two co-propagating Hermite-cosh-Gaussian (HchG) laser beams with positively chirped frequencies for analysis. The laser beam exhibits interaction with a collisionless underdense plasma in the presence of a static transverse magnetic field. The interaction between laser and plasma exhibits nonlinear characteristics, leading to the creation of THz radiation with high energy efficiency. The primary objective of this study is to conduct an analytical examination of the correlation between THz conversion efficiency, normalised transverse distance, and plasma frequency. Additionally, the analysis will consider various laser parameters, including the Hermite polynomial mode index (s), decentred parameter (a), and frequency chirp (b). The results show that in off-resonant condition THz conversion efficiency quickly decreases and approaches to zero for normalised THz frequencies. The observed phenomenon entails an augmentation in the normalized amplitude of terahertz (THz) waves, accompanied by a displacement of the peak towards greater normalized transverse distance values. This occurrence is contingent upon the variation of the Hermite polynomial mode index within the range of 0 t o 2. As the chirp parameter (b) is increased from 0.0011 t o 0.0099 , the normalised terahertz (THz) amplitude exhibits an increase with respect to the normalised transverse distance for values of s equal to 0,1 a n d 2. The proposed methodology demonstrates significant use in generating high-intensity, adjustable, and energy-efficient terahertz (THz) radiation sources through the manipulation of the decentred parameter and Hermite polynomial mode index values. [ABSTRACT FROM AUTHOR]

Allan–Werle-plots are an established tool in infrared absorption spectroscopy to quantify temporal stability, maximum integration time and best achievable precision of a measurement instrument. In field measurements aboard a moving platform, however, long integration times reduce time resolution and smooth atmospheric variability. A high accuracy and time resolution are necessary as well as an appropriate estimate of the measurement uncertainty. In this study, Allan-Werle-plots of calibration gas measurements are studied to analyze the temporal characteristics of a Quantum Cascade Laser Absorption Spectrometer (QCLAS) instrument for airborne operation. Via least-squares fitting the individual noise contributions can be quantified and different dominant regimes can be identified. Through simulation of data according to the characteristics from the Allan-Werle-plot, the effects of selected intervals between in-flight calibrations can be analyzed. An interval of 30 min is found sufficient for successful drift correction during ground operation. The linear interpolation of the sensitivity increases the accuracy and lowers the measurement uncertainty from 1.1 % to 0.2 % . Airborne operation yields similar results during segments of stable flight but suffers from additional flicker and sinusoidal contributions. Simulations verify an appropriate interval of 30 min in airborne operation. The expected airborne measurement uncertainty is 2.45 ppbv. [ABSTRACT FROM AUTHOR]

Multiband infrared photodetectors (PDs) have garnered considerable attention due to their potential in various fields such as biomedical applications, optical communications, and military operations. However, the lattice mismatch of mature bulk semiconductors with varying bandgaps in the same PD pixel has impeded the advancement of multiband infrared detectors. Colloidal quantum dots (CQDs), with adjustable bandgaps, wide spectral range, and easy solution processing are promising materials for the fabrication of multiband infrared detectors. Herein, HgTe CQDs are used with varying bandgaps to construct a three‐band infrared detector with short‐wave infrared of 1.7 and 2.4 μm and midwave infrared of 4 μm. The utilization of Ag2Te nanocrystals to bring in spatially stable doping is the key to achieving three‐band infrared detectors. Through adjusting bias polarity, the detectors switch between a three‐band mode and a dual‐band mode. The detector exhibits exceptional detection performance, with detectivity above 3 × 1010 Jones in three‐band mode. [ABSTRACT FROM AUTHOR]

The use of photonic crystal (PhC) to form photonic bandgaps is expected to be applied to optical communications, and various studies have been conducted on this technology. In a previous study, the current‐injected Circular Defect (CirD) in 2D PhC laser is proposed. At that time, the dry‐etching process involves three steps. However, three‐step dry etching cannot be used to simultaneously form drainage trench and PhC air holes. In this study, a one‐step dry‐etching process for CirD lasers is conducted. Optical measurements are performed on the successfully fabricated samples. As a result, the first lasing of a CirD laser with a contact layer and an upper cladding layer fabricated with a one‐step dry etching is confirmed. [ABSTRACT FROM AUTHOR]

The B-BOP detector is composed of polarization sensitive bolometer arrays in the 70–350 µm spectral range with a goal sensitivity around one attoWatt/sqrt (Hz). A polarized infrared calibration set-up is required to evaluate its performances. The calibration source is based on 6-ring-shaped, concentric blackbody-like emitters that can be individually controlled. The total emitting surface is therefore tunable from 9 up to 1200 mm2. The temperature is tunable from 1 to 40 K without disturbing the dilution cryostat and the working temperature of the detector arrays. The present paper describes the source, the rotating polarizer at low temperature, the modulation means, and the complete optical tube leading polarized infrared photons for characterizing detectors down to femtowatt level. Details of the design and realization are discussed. [ABSTRACT FROM AUTHOR]

Terahertz generation through a photoconductive antenna is quite popular, mainly due to the advancement of nanotechnology and material research. Based on numerical modelling, a thorough study of the interdigitated photoconductive antenna (IPCA) in the terahertz (THz) frequency region has been conducted. To comprehend the capacitive behaviour of the gap between the interdigitated electrodes, an equivalent circuit model for IPCA based on its electrical perspective with matching expressions has been solved. This work explores the performance of IPCA with variations in structural parameters of their interdigitated electrode geometry in the active region. The interdigitated elemental periodicity, width and length of the IPCA are varied by keeping the interdigitated gap constant between the elements. The temporal behavior of the generated electric field and its corresponding spectral response of the IPCAs with variations in the interdigitated electrode geometry have been compared. Moreover, the coupling behavior of the elements has been observed in the generated radiation. Finally, the results prove that the THz amplitude, frequency and efficiency can be enhanced by optimizing the structural parameters of the interdigitated electrode elements in the active region. This analysis is very useful when doing practical experiments in spectroscopy, imaging and communication applications. [ABSTRACT FROM AUTHOR]

Efficiently fabricating a cavity that can achieve strong interactions between terahertz waves and matter would allow researchers to exploit the intrinsic properties due to the long wavelength in the terahertz waveband. Here we show a terahertz detector embedded in a Tamm cavity with a record Q value of 1017 and a bandwidth of only 469 MHz for direct detection. The Tamm-cavity detector is formed by embedding a substrate with an Nb5N6 microbolometer detector between an Si/air distributed Bragg reflector (DBR) and a metal reflector. The resonant frequency can be controlled by adjusting the thickness of the substrate layer. The detector and DBR are fabricated separately, and a large pixel-array detector can be realized by a very simple assembly process. This versatile cavity structure can be used as a platform for preparing high-performance terahertz devices and opening up the study of the strong interactions between terahertz waves and matter. Here the authors report a terahertz detector with a Q value of 1017, embedded in a Tamm cavity and offers a 469 MHz bandwidth. It features an Nb5N6 microbolometer in an Si/air DBR and metal reflector, with tunable resonant frequency via substrate layer thickness. [ABSTRACT FROM AUTHOR]