The results on fabrication and optical characterization of lattice-matched to InP quantum-cascade laser emitting at $8 \mu \mathrm{m}$ are reported. The high current dynamic range is observed for lasers with four cleaved facets.

The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range (5.7-6.7)o. Keywords: ring cavity, grating, focused ion beam (FIB) etching, superlattices, quantum-cascade laser, epitaxy, indium phosphide.

This study uses a numerical model to analyze the dynamics of high-power semiconductor lasers pumped by a high-repetition rate pulse sequence. It explores the distribution of photons and gain along the laser cavity and proposes an approach to optimize laser parameters for maximum efficiency and stability. The repetition rate range of interest spans from sub-MHz to several GHz.

Quantum cascade lasers (QCLs) have received enormous attention from the scientific community due to their broad range of applications in a wide variety of industries, agriculture, healthcare, environmental protection, and many other scientific and technical fields. In this article, in addition to a review of the main applications and the state of research and development of high -power QCLs in the mid -infrared range, we consider the features of their manufacturing technology that make it possible to obtain a high peak power and discuss the effect of overheating of the active region on the output optical power and spectral characteristics. A comparison is made of the characteristics of QCLs with the same cavity parameters but with different active regions made on the basis of substrate -matched or strained heteropairs, which provides a different energy barrier between the upper laser level and the continuum. It is shown that the use of strained heteropairs in the active region of a QCL provides an almost twofold increase in the characteristic temperature T0 as well as a significantly higher efficiency and an increase in the maximum output optical power to over 21 W, which is a world record for a single stripe QCL with a 8 pm spectral range.

A series of low-voltage thyristor current switches based on (Al)GaAs/GaAs homo- and heterostructures with a volume charge region formed in the lightly doped p-GaAs base layer have been developed. The transient processes characteristics in pulse generation mode of nanosecond duration have been studied. It has been shown that the use of a wide-bandgap barrier based on AlGaAs at the n‑emitter/p-base junction allows reducing the minimum control current amplitude from 30 to 3 mA, and the turn-on delay time can be shortened to 6 ns. For the developed thyristor switches, a minimum transition time of 3.7–3.9 ns was demonstrated when operating in a circuit with a 1 nF capacitive load. In a circuit with a nominal 1 Ω resistive load, the thyristor switches provided a peak current of 17.5 A with a pulse duration of 3.7 ns.

In the scope of a computational experiment, high-contrast gratings (HCG) formed on a silicon- on-insulator (SOI) platform within vertical-cavity surface-emitting lasers (VCSELs) were studied for multispectral laser sources. A simulation model for spectral characteristics calculation is proposed, which includes two heterogeneously integrated parts of the VCSEL: 1) the lower output mirror based on a HCG grating in the silicon layer of the SOI surrounded by air cavities to enhance the contrast of the HCG; 2) the semiconductor VCSEL structure with an air aperture for current and optical confinement. Comparative analysis results of the spectral characteristics of VCSEL-SOI structures for zeroth, first, and second-order modes, which can be excited in the air aperture of the VCSEL, are presented. It is demonstrated that the HCG, acting as one of the cavity mirrors, effectively discriminates the VCSEL higher-order modes. An algorithm for calculating HCG parameters that ensure the maximum reflectivity at a fixed thickness of the silicon layer of the SOI is developed.

A model of a two-dimensional photonic crystal (PC) for lasers with vertical radiation output is developed. The influence of geometrical parameters of the PC on the characteristics of mode structures is analysed. Calculations show that PCs based on holes in the shape of a rectangular isosceles triangle are the most preferable by their characteristics for creating surface-emitting lasers.

The study investigates microstripe bars of optically isolated single-mode lasers based on heterostructures with double asymmetry, operating under sub-nanosecond current pulse pumping conditions. For microstripe bars with different filling densities of the emitting aperture, the effect of time delay dispersion of various stripes’ turn-on is demonstrated, with a maximum difference up to 50 ps. The developed microstripe bar designs demonstrate stable zero mode lasing. The microstripe bar consisting of 10 stripes with a 6 μm width and a stripe period of 20 μm demonstrates pulses with a peak power of 3 W and a duration of 140 ps under 0.4 ns current pulses pumping.

The development of high-power mid-infrared laser sources is highly desired for a number of applications in free-space optical communication, laser imaging, detection, and ranging (LIDAR) and environmental monitoring. Quantum cascade lasers (QCLs) hold solid position among these technologies, however up to now their highest powers are demonstrated in 4.5 - 5 $\mu \mathrm{m}$ spectral range [1], while the results at other mid-IR wavelength may differ by an order of magnitude. In this work, we consider three different designs of high-power QCLs grown by a two-stage MBE and MOCVD epitaxy. The quality of all fabricated heterostructures is similar to that of structures produced solely by the MBE technique. The active region remains the same as in [2] in all three types of structures, while the main difference lies in the design of the upper cladding and contact layer. In particular, we discuss two structures with thick uniformly doped InP upper cladding together with InP or InGaAs contact layer (Types I and II correspondingly), and design with gradient doping of the InP upper cladding accompanied by InGaAs contact layer (Type III). All three structures were subjected to post-growth processing and fabrication of QCL chips with 40 and 60 $\mu \mathrm{m}$ stripes and 3–5 mm cavity lengths. All samples were tested under 150 ns pulsed pumping with a 12 kHz repetition rate. Our experiments show that QCLs based on both structures with InGaAs contact layer with uniform and gradient cladding doping (Types II and III) demonstrate better efficiency while the lasers based on Type I design with InP contact layer and uniformly doped upper cladding feature improved power characteristics resulting in the record-high power value $> 16\ \mathrm{W}(> 8\mathrm{W}/\text{facet})$ . We claim the latter is directly related to the better thermal conductivity of InP contact layer comparing to InGaAs counterpart. This was confirmed by the chirp measurements demonstrating the lower heating rate of the active region in structure with InP contact layer (Type I), see Fig. 1a. At the same time, in our experiments InP contact layer had lower electrical conductivity, that finally affected the laser efficiency as shown in Fig 1.b.

The results of studies of 7.5-8.0 μm range surface-emitting ring quantum-cascade lasers are presented. A second-order diffraction grating with a calculated coupling coefficient of ~9 cm-1 is formed on the entire surface of the ring cavity by focused ion beam milling. Surface-emitting lasing at room temperature near 7.75 μm with a threshold current density of ~8 kA/cm2 and an outer radius of the ring cavity of 202 μm is demonstrated. The results of studying the intensity distribution in the far-field near the normal to the surface showed the presence of two maxima. It is shown that the implemented coupling coefficient is not sufficient to ensure single-mode lasing in the studied ring quantum-cascade lasers. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide, focused ion beam milling.

Photoactivated current switches for pumping diode laser stacks with ns and sub-ns current pulses are studied. Test optical pulse: 860 nm, 50 ps leading edge, peak power up to 9.3 W. Samples were tested at up to 50 V bias. Photoresponse amplitude was up to 17 V (340 mA) with linear dependence on pulse power. Photoresponse leading edge duration was ~80 ps (20-80% level).

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. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide.

We analyze the influence of optical coatings on the electro-optical characteristics of quantum cascade lasers. We compare light-current characteristics of devices without and with different combinations of optical coatings. The highest output power is achieved with combination of anti- and high-reflection coatings, while the lowest threshold with partial-high- and highreflection coatings.

The results of experimental studies on semiconductor heterostructures with InGaAs quantum wells $\mathbf{(} \mathbf{9 8 0}-\mathbf{9 9 5 n m}$) grown by selective MOCVD epitaxy are presented. It is shown that the wavelength of photoluminescence varies across the window and depends on the thickness of the waveguide layer.

We study quantum-cascade lasers with active region designs based on strained and lattice-matched heterostructures. Lasers based on strained well/barrier pairs demonstrate improved efficiency, temperature stability and record-high optical power.

Measurements of the output optical power, laser-oscillation spectra, optical-pulse duration, and switching-on delays of semiconductor lasers–thyristors with a strip width of 200 μm and a length of 980 μm were performed in the operating temperature range from 20 to 70°C at a nominal value of the discharge capacitor of 22 nF and a control-current amplitude of 10.4 mA. It is shown that lasers–thyristors have high temperature stability. When the devices were heated from 20 to 70°C, the level of reduction of the peak output power did not exceed 15

We present a study of quantum cascade laser dynamical properties accounting for the Joule heating released in the active region. In particular, we study the QCL emitting at 8 mu m in the pulsed pumping mode and present experimental measurements, as well as a theoretical description of the QCL build-up time, showing the features appearing due to the Joule heating released inside the active region.

We study generation of random bit sequences (RBS) with quantum-cascade laser (QCL) and quantum-cascade detector. We show that QCL emission intensity randomly varies due to lateral modes competition and can be converted into RBS.

The lasing at $7.59 \mu \mathrm{m}$ wavelength was achieved in quantum-cascade surface emitting lasers with increased radius of ring cavity. The large mode number in single-mode spectrum was caused by over coupling between laser core and second-order Bragg grating.

The possibility of realizing single-mode emission in quantum-cascade lasers due to modulation of output optical losses in a Fabry–Perot cavity is demonstrated. For the active region of the 7.5–8.0 μm spectral range, the two- phonon resonance design was used, thus, 50 stages and waveguide layers based on indium phosphide made it possible to realize single-mode 7.765 μm lasing at the temperature of 292 K. Side-mode suppression ratio was about 24 dB and remained the same with an increase in the current pumping up to 1.2 of the threshold current values. The coefficient of wavelength shift with temperature (temperature tuning) in the single-mode lasing regime was 0.56 nm/K.