In this work, we provide a simplified theoretical analytical estimation of the quantum cascade laser build-up time, accurately taking into account the main effects: the QCL overheating during the pump pulse and the photon mode filling effect. The non-trivial interplay of the mentioned effects brings about a variety of possible experimental build-up time behaviors. The latter range from decreasing curvature for low-power devices to non-monotonous function for devices performing at high power.

The correct accounting for the thermal effects is always a challenge when one needs to make quantitative predictions for any laser applications. In such complicated devices as quantum cascade lasers, the temperature strongly affects the operational conditions. In particular, it prevents reaching the CW mode as well as effective device performance in the pulsed regime. The rate equations are the most effective and simple way to model the lasing dynamics. However, the conventional approaches consider a finite number of population levels and generalize the obtained results to an infinite number of cascades. The latter may lead to unavoidable non-physical results and difficulties in making quantitative predictions. In this work, we modify the conventional three-level rate equation approach by adding a self-heating description and applying it to the calculation of the QCL dynamics. Our results highlight the significant influence of temperature on threshold characteristics and build-up time, while also integrating electronic effects into the overall description of QCL behavior.

In this work we revealed the fundamental origins of the lasing wavelength chirp caused by the quantum cascade laser (QCL) active region (ARn) overheating and proved our results with the density functional theory simulations and the normal dispersion theory. We demonstrated that the chirp is related to the growth of the optical path, which is governed by a thermally stimulated increase of the refractive index within the medium. This change is induced by the amplitude of the ion oscillations, which increases with temperature and alters the electronic structure-related parameters, namely the bandgap and the interband transition strength. The concurrent thermal variation of these parameters plays a primary role in the chirp formation. Our analysis justifies and simplifies the experimental method used to determine the temperature within the QCL ARn, a task that is generally nontrivial.

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.

We theoretically investigate droplet quasi-Bessel beams for different shapes of the round-tip axicon. Exact solutions for the Fresnel diffraction integrals describing the axial distribution of the electric field amplitude behind the axicon are demonstrated. The analysis of the exact solutions shows that the period of "light droplets" is not a constant value, but depends on the axial distance and on the deviation of the axicon surface from the conical shape far from the rounded region. The predicted effect can be applied for the reconstruction of the exact shape of the axicon surface without 3D scanning.

We study the QCL active region overheating and discuss the effects of nonequilibrium heat dissipation on laser performance. We show that the effective thermal management fundamentally depends on the active region material properties.

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.

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.

The results on fabrication and experimental study of ring cavity surface-emitting quantum-cascade lasers with staircase-like distributed feedback grating are presented. The grating with a gradient in etching depth of the slits along the ring cavity was formed by direct high-vacuum ion-beam lithography. The lasing was observed close to 7.9 μm at 293 K with maximum output power about 15 mW. Analysis of the near and far field patterns showed that created ring quantum-cascade lasers operated on a high order transverse mode.

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.

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.

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.

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.

In this paper we present the study of the features of epitaxial growth highly stressed superlattices based on highly stressed thin InGaAs/InAlAs layers on InP substrates by the molecular beam epitaxy. It was shown that the growth rates of the InGaAs and InAlAs bulk layers lattice-matched to InP substrates do not allow us to precisely determine the growth rates of thin highly stressed In0.36Al0.64As/In0.67Ga0.33As strain compensated superlattices and the error is about 10 percent. The effect is related to the difference in the growth temperatures of InGaAs and InAlAs bulk layers, which affects the intensity of indium evaporation from the growth surface.

We provide the ab-initio analysis of the refractive index thermal variation and justify the method of QCL active region overheating determination via wavelength chirp measurements.

Quantum cascade lasers are of high interest in the scientific community due to unique applications utilizing the emission in mid-IR range. The possible designs of QCL are quite limited and require careful engineering to overcome some crucial disadvantages. One of them is an active region (ARn) overheat, that significantly affects the laser characteristics in the pulsed operation mode. In this work we consider the effects related to the non-equilibrium temperature distribution, when thermal resistance formalism is irrelevant. We employ the heat equation and discuss the possible limitations and structural features stemming from the chemical composition of the AR. We show that the presence of alloys in the ARn structure fundamentally limits the heat dissipation in pulsed and CW regimes due to their low thermal conductivity. Also the QCL post-growths affects the thermal properties of a device only in (near)CW mode while it is absolutely invaluable in the pulsed mode

We present the results on the fabrication and time-resolved spectral characterization of 5.2 um range quantum-cascade laser. 40 stages of active region are based on strain-compensated In0.6Ga0.4As/In0.44Al0.56As heteropair. The mutli-mode lasing was demonstrated in the 5.09-5.21 um spectral range for lasers with four-cleaved facets. The estimated values of the threshold current density and voltage at 80 K were about 1.1 kA/cm 2 and 15 V, respectively. The maximal lasing temperature for lasers with four-cleaved facets was about 210 K.