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Bound states in the continuum (BICs) are observed in optical cavities composed of a high refractive index periodic structure embedded in significantly lower refractive index surroundings, enabling vertical confinement of the grating modes. Here, we propose a vertically nonsymmetric configuration, implemented on a high refractive index bulk substrate with a one-dimensional grating positioned on a distributed Bragg reflector (DBR). In this configuration, the grating modes are leaky, which could prohibit the creation of a BIC if the grating was implemented on uniform substrate. However, the judiciously designed DBR on which the grating is implemented reflects nonzero diffraction orders induced by the grating. We found that the laterally antisymmetric optical modes located at the center of the Brillouin zone of this structure create BICs that are robust against changes in the grating parameters, as long as the DBR reflects the diffraction orders. The configuration enables a high degree of design freedom, facilitating the realization of very high quality factor cavities in conventional all-semiconductor technology.

The focused ion beam (FIB) technique was used to fabricate a nanothermocouple (with a 90 nm wide nanojunction) based on a metal–semiconductor (Pt–Si) structure, which showed a sensitivity up to 10 times larger (with Seebeck coefficient up to 140 µV/K) than typical metal–metal nanothermocouples. In contrast to the fabrication of nanothermocouples which requires a high-tech semiconductor manufacturing line with sophisticated fabrication techniques, environment, and advanced equipment, FIB systems are available in many research laboratories without the need for a high-tech environment, and the described processing is performed relatively quickly by a single operator. The linear response of the manufactured nanothermocouple enabled sensitive measurements even with small changes of temperature when heated with a stream of hot air. A nonlinear response of the nanothermocouple (up to 83.85 mV) was observed during the exposition to an argon-laser beam with a high optical power density (up to 17.4 Wcm−2), which was also used for the laser annealing of metal–semiconductor interfaces. The analysis of the results implies the application of such nanothermocouples, especially for the characterization of laser beams with nanometer spatial resolution. Improvements of the FIB processing should lead to an even higher Seebeck coefficient of the nanothermocouples; e.g., in case of the availability of other suitable metal sources (e.g., Cr).

Semiconductor planar microcavities significantly enhance the interaction between light and matter and are thus crucial as a fundamental research platform for investigations of quantum information processing, quantum dynamics, and exciton-polariton observations. Microcavities also serve as a very agile basis for modern resonant-cavity light-emitting and detecting devices now in large-scale production for applications in sensing and communication. The fabrication of microcavity devices composed of both common materials now used in photonics and uncommon or arbitrary materials that are new to photonics offers great freedom in the exploration of the functionalities of novel microcavity device concepts. Here we propose and carefully investigate two unique microcavity designs. The first design uses a monolithic high-index-contrast grating (MHCG) and a distributed Bragg reflector (DBR) as the microcavity mirrors. The second design uses two MHCGs as the microcavity mirrors. We demonstrate by numerical analysis that MHCG-DBR and MHCG-MHCG microcavities, whose lateral radial dimension is 16 μm, reach very large quality factors at the level of 104 and nearly 106, as well as purposely designed wavelength tuning ranges of 8 and 60 nm in both configurations, respectively. Our MHCG-MHCG microcavities with a very small size of 600 nm in the vertical dimension show extremely large quality factors, which can be explained by treating the optical modes as quasi-bound states in a continuum (BICs). Moreover, we verify our theoretical analysis and calibrate our simulation parameters by comparing to the experimental characteristics of an electrically injected MHCG-DBR microcavity vertical-cavity surface-emitting laser (VCSEL) emitting at a peak wavelength of about 980 nm. We use the calibrated parameters to simulate the emission characteristics of electrically injected VCSELs in various MHCG-DBR and MHCG-MHCG microcavity configurations to illustrate the influence of microcavity designs and their quality factors on the predicted lasing properties of the devices.

Lasers emitting mid-infrared (MIR) beams have become indispensable for spectroscopy, free space communication or remote security measures. To the one of the most promising families of the lasers suitable for these applications certainly belongs a group of the Quantum Cascade Lasers (QCL). However, among the conditions they must satisfy there is a high enough radiance of the beam they emit. Radiance depends in a complicated way on the laser output power and optical quality of the laser beam. This paper has been devoted to a description and a short analysis of the factors that decide about radiance of so far developed QCLs. Literature concerning both single devices and QCL arrays operating in beam combining systems have been examined and results described. The survey may be useful for estimation of how far the QCLs have come of age.

In this paper, we present a theoretical and experimental analysis of mid-infrared (4.7 μm) quantum cascade laser waveguides with different taper waveguides geometries: linear, convex, and concave tapers. We have calculated and measured brightness of the optical beam and maximal power density on the laser front mirror for different widths and lengths of the taper. Convex geometry shows the best performance and the highest powers were obtained.

We experimentally demonstrate and elucidate by numerical simulations that breaking circular symmetry of large apertures of vertical-cavity surface-emitting lasers (VCSELs) significantly enhances their emission properties by increasing the optical density of states. Specifically, deformed shapes of circular oxide apertures of VCSELs enhance stimulated emission and suppress undesired non-radiative recombination contributing to an increase in output optical output power of more than 60% and in quantum efficiency of more than 10%. Our example deformed VCSEL structures demonstrate that the optical density of states appears to be of high importance for conventional optoelectronic devices in accordance to the predictions of quantum electrodynamics theory.

We propose a grating that design is inverted with respect to a design of conventional high-contrast grating (HCG) in which low refractive index grating is implemented on high refractive index cladding. We show that inverted HCG can achieve power reflectance of nearly 100% even if the refractive index of the grating is as low as 1.8. Inverted HCG facilitates implementation of highly reflecting mirrors composed of etched SiN, HfO or 3D micro-printed IP-Dip on semiconductors such as GaAs that processing is less technologically demanding with respect to processing of HCGs or monolithic HCGs.

The present paper provides a theoretical and experimental analysis of mid-infrared (4.5 µm) quantum cascade laser Fabry-Perrot waveguides with various sidewall geometries. Calculations, processing and measurements have been carried out for varied sidewall angles of quantum cascade lasers, comparing the double trench geometry (wet etched) with single ridge geometry (dry etched). The latter is shown to operate in the fundamental transverse mode and for larger waveguide widths. Furthermore, the optimum sidewall angle is shown to be about 64o.

We investigate growth conditions for strain-compensated In 0.67 Ga 0.33 As/In 0.36 Al 0.64 As/InP quantum cascade lasers (QCLs) by solid-source molecular beam epitaxy (SSMBE). The extensive discussion of growth procedures is presented. The technology was first elaborated for In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As material system lattice matched to InP. After that QCLs with lattice matched active region were grown for validation of design and obtained material quality. The next step was elaboration of growth process and especially growth preparation procedures for strain compensated active regions. The grown structures were examined by HRXRD, AFM, and TEM techniques. The on-line implementation of obtained results in subsequent growth runs was crucial for achieving room temperature operating 4.4-μm lasers. For uncoated devices with Fabry-Perrot resonator up to 250 mW of optical power per facet at 300 K was obtained under pulsed conditions. The paper focuses on MBE technology and presents developed algorithm for strain-compensated QCL growth.

In this paper, we present experimental results of spontaneous emission clamping in the threshold for vertical-cavity surface-emitting lasers (VCSELs) with oxide current confinement. We show that the spontaneous emission not wholly clamps in the threshold. We propose a new method for determining the threshold current value using the study of the clamping phenomena. This method has an advantage over the commonly used methods in the accuracy because the current of the spontaneous emission clamping is betted defined than the current of the slope change of the stimulated emission light-current curve. The estimated uncertainty of the method is no more than 20 µA.

In this paper, we present experimental results of current-induced polarization change and transverse mode change in mid-infrared quantum cascade lasers. The polarization of laser beam was determined by measuring of far-field distributions of polarization projection on the linear polarization basis. The measured amount of TE-polarized light is associated with transversal mode structure, which was determined based on the far-field power distributions. The TE polarized light contribution in the beams varies from 6 to 19%. This quantity is anti-correlated to the fundamental transverse mode contribution.

In this paper we present results of numerical calculations of transverse modes in folded waveguides of mid-infrared quantum cascade lasers. We show that, depending on the waveguide geometry, the transversal mode distribution along the waveguide is unchanged or there is switching between the fundamental and the first-order transverse mode.

This paper presents impact of front and back laser mirror ion cleaning on quantum cascade laser characteristics. Cleaning of the front mirror makes the beam more symmetric and increases external differential quantum efficiency, while cleaning of the back mirror reduces the electrical resistance, increases the threshold current and slightly affects the optical polarization of the beam.

In this paper we investigate chip bonding technology of GaAs/AlGaAs quantum cascade lasers (QCLs). Its results have strong influence on final performance of devices and are essential for achieving room temperature operation. Various solders were investigated and compared in terms of their thermal resistance and induced stress. The spatially resolved photoluminescence technique has been applied for a device thermal analysis. The soldering quality was also investigated by means of a scanning acoustic microscopy. The particular attention has been paid to Au–Au die bonding, which seems to be a promising alternative to the choice between hard and soft solder bonding of GaAs/AlGaAs QCLs operating from cryogenic temperatures up to room temperatures. A good quality direct Au–Au bonding was achieved for bonding parameters comparable with the ones typical for AuSn eutectic bonding process. High performance room temperature operation of GaAs/AlGaAs QCLs has been achieved with the state-of-the-art parameters.

In this paper we present calculated mode distributions in double-trench mesa waveguide of a quantum cascade laser (QCL) and far-field distributions of the QCL beam—both calculated and experimental. We have computed electric field distribution in the waveguide, confinement factor and mode absorption for different transversal modes and far-field distributions based on them. The parameters of the modes were calculated for various sizes: width and length, of the QCL waveguide. The maximal mesa width for single-transverse-mode operation was determined to be about 0.7 times wavelength. This value and the mode discrimination depend only slightly on the resonator length. To verify experimentally the results of the simulations we have measured power distribution in the far field by using goniometric profiler. The measured distribution agrees well with the results of the numerical calculations.

for electrical insulation. The quantum cascade laser structures have been grownbymolecularbeamepitaxy,withRiberCompact21Treactor. Thestringentrequirements—placedparticularlyon the epitaxial technology — and the influence of technological conditions on the device structure propertieswerepresentedanddiscussedindepth.PACSnumbers:42.55.Px,81.15.Hi,85.60.

Facets of semiconductor optical amplifiers (SOA) designed for external cavity lasers must be coated with an antireflection (AR) film of high quality and extremely low reflectance. Therefore measurements of facet reflectance play a crucial role in the fabrication of such AR coating. The reflectance can be estimated by studying the optical power reflected from the Fabry–Perot cavity formed for that purpose by the examined SOA facet and the end of a single-mode fibre. We have made analysis of practical suitability of such measurement method. Theoretical calculations show that, for the low reflectance coatings, losses due to light coupling into optical fibre cannot be omitted in the analysis of the experimental results. To verify this conclusion a theoretical model was tested for a low reflectance surface and we have found that the relative error of the measurements supported by the theoretical model is on the order of 8%.

The intra-pulse measurements of the beam steering in an AlGaAs/GaAs quantum cascade laser are presented in this paper. The experimental results are explained by a two-mode theoretical model. The near field of the laser radiation is calculated according to the effective index method and transposed to the far field numerically according to Huygens principle. The maximal observed value of deflection of the beam has been found to be \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$17^{\circ }$$\end{document}17∘. For supply currents in the range from 1.6 to 2.4 times the threshold the beam steering occurs only on one side of the resonator axis, and stays is the same for all current values. For higher supply current, it occurs alternately on both sides and exhibits a bistability. The time period of the beam direction change has been found to be about 40 ns for the lowest current and was decreasing with the current increase to about 20 ns.

Quantum cascade lasers (QCL’s) have proven their usefulness as light sources in many applications, like remote gas sensing, molecular spectroscopy or free-space communication. In most cases the high-quality low-divergence beam is desired. This work presents the theoretical analysis of QCL’s beam divergence. The electromagnetic field in the resonator is calculated according to effective index method. Theoretical results are compared with measurements.

The paper presents results of experimental investigations of spatial distribution of radiation emitted by quantum cascade lasers. Measurements have been performed by means of a unique goniometric profilometer specially de- signed for the large angle laser beams. The advantages and limitations of the set-up and the applied experimental method are discussed. The obtained results have enabled the analysis of dependence of geometry of the beam on the geometry of the laser structure and on the mount method of the laser chips. The angular divergence of the beams has also been tested as a function of laser power supply.

The development of (λ ∼ 9. 4 μ m) GaAs-based quantum cascade lasers (QCLs) operating at the room temperature is reported. The laser design followed an “anticrossed-diagonal” scheme of Page et al. [1]. The QCL GaAs∕Al0.45Ga0.55As heterostructures were grown by solid source (SS) MBE. The double trench lasers were fabricated using wet etching and Si3N4 for electrical insulation. Double plasmon confinement with Al-free waveguide has been used to minimize absorption losses. Optical and electrical properties of resulting devices are presented and discussed.

The basic device characterization of (λ∼9.4µm) GaAs-based quantum cascade lasers (QCLs) operating up to 260K is reported. The laser design followed an “anticrossed-diagonal” scheme. The QCL structures were grown by MBE, with Riber Compact 21T reactor. The influence of injector doping on electrical and optical properties of the lasers has been clarified.

The development of (λ∼9.4µm) GaAs-based quantum cascade lasers (QCLs) operating over 260K is reported. The laser design followed an “anticrossed-diagonal” scheme of Page et al. [1]. The QCL GaAs/Al 0.45 Ga 0.55 As heterostructures were grown by solid source (SS) MBE. The double trench lasers were fabricated using wet etching and Si 3 N 4 for electrical insulation. Double plasmon confinement with Al-free waveguide has been used to minimize absorption losses. Optical and electrical properties of resulting devices are presented and discussed.

In this paper we analyse the influence of structure parameters of the step-like GaInNAs/GaAs quantum well on its emitted wavelength and optical gain spectrum. The calculated red-shift is in a fairly good agreement with experimental data. We also show that the symmetric structure seems to be the optimal one in respect both of the maximal red-shift and the maximal gain value.

Tailoring of optical mode profiles of high-power diode lasers is directly demonstrated by means of near-field photocurrent spectroscopy. Three double barrier separate confinement heterostructures with different confinement geometries are designed and their optical mode profiles are studied both theoretically and experimentally. The near-field spectroscopic results clearly resolve the intended variation in optical mode width. A remaining discrepancy between the designed and experimentally measured mode profiles, manifesting itself in the reduction of their evanescent tails, is attributed to the structure of the antiguiding barrier. The results demonstrate that near-field field photocurrent spectroscopy is a powerful, nondestructive, and quantitative technique for optical waveguide inspection in high-power diode lasers.

The quantum cascade lasers (QCLs) are unipolar semiconductor light sources based on intersubband transition and on tunneling. Active region of the QCL is formed by alternating coupled quantum wells and superlattice regions (injectors). The whole active region has thickness of several micrometers and contains hundreds of nanometer semiconductor layers. Thermal conductivity of such structure can be even ten times smaller than thermal conductivity of bulk material [1, 2]. For that reason thermal management is crucial issue in designing, fabrication and testing of quantum cascade lasers. Several methods to investigate thermal properties of QCLs have been proposed recently. There are based on studies of emitted wavelength [3] or on photoluminescence [4–6]. Using those methods temperature of active region can be estimated. Contrary to photoluminescence method, wavelength method can measure rapid temperature changes during pulse excitation. Disadvantage of that method is a fact that only structures whit good optical properties can be studied. Therefore it cannot be used as a diagnostic tool. In this paper we propose an alternative method of QCL temperature estimation based on electrical properties study. Measured temperature is the temperature of the whole structure. This method allows for measurement of rapid temperature changes and does not need good optical properties of the laser. Another advantage is the fact that no sophisticated experimental setup is needed.

A new method to calculate the near field of widely divergent laser beams based on a given far field is proposed. It is assumed that the field of radiation consists of several transversal modes and the far field is determined by the Huygens principle. The idea consists in employing the least squares method to find a minimum of the difference between the measured far field and the far field calculated under the assumption that is consists of several transversal modes with assumed weights. To find this minimum the Levenberg–Marquardt algorithm is used. Thus the problem is dealt beyond the paraxial approximation. The method is illustrated by its results for experimental mid-infrared quantum cascade laser beams and for the beams obtained by simulations.

The quantum cascade lasers (QCLs) are unipolar devices based on intersubband transitions and tunnelling transport [1, 2]. The GaAs-based QCLs have proved to be an effective source of laser radiation in mid-infrared (MIR) as well as far-infrared (FIR) regions. They find application in the gas sensing systems (e.g., for detecting CO2, NO, CH4) [3], medical diagnostics [4] and environment monitoring [5]. The complex design and operating principle require the technology of quantum cascade lasers to ensure the highest accuracy, uniformity and repeatability of both epitaxy and device processing [6]. High requirements for epitaxy are connected with ultrathin layers of periodic active region, where the wave functions are to be properly engineered [7–9]. The thickness accuracy of the structure should in general be better than 3%. Another crucial issue is the optimisation of injector doping, which is necessary to assure a large enough dynamic range of laser operation [10, 11]. In order to restrict the lateral current spreading in the device it is necessary to form a good quality deep mesa structures. Since the available gain is usually limited, the device design should minimize the radiation losses. The mesa is etched through active region and reduction of the optical losses requires a smooth waveguide sidewalls [12]. There is a number of QCLs designs reported in the literature [6, 13]. Sirtori et al. [6]. have presented the ridge structure and double trench structure, in which ion implantation was used for electrical isolation instead of dielectric layers. The optimum one seems to be the double trench construction which insures a good overlapping of the lateral mode with the gain region and enables stable epi-down mounting necessary for efficient cooling of the device [14]. The construction in which the extended proton-implanted regions were applied in the area defined by the two trenches, was the another presented solution. In this case the channel for current injection was narrower than the ridge width, which minimizes absorption of generated radiation in dielectric layers insulating mesa sidewalls [6, 15]. As a rule the relatively high voltages as well as current densities are necessary to polarize QCLs. That indicates demand for low resistance ohmic contacts in order to reduce the device serial resistance. These contacts should be characterized by thermal stability, low depth of metal diffusion into semiconductor layers and lateral uniformity of metal–semiconductor interfaces. In this paper we discuss some issues fabrication technology of Al0.45Ga0.55As/GaAs QCL operating at λ ≈ 9.4 μm. The devices lased up to 262 K (−11 ◦C) with optical powers over 1 W at 77 K, threshold current density values of 7 kA cm−2 and differential efficiencies above 0.6 W/A. Since lasers were uncoated, the quoted values should be understood.