This review focuses on theoretical foundations, experimental implementation and an overview of experimental results of the thermoreflectance spectroscopy as a powerful technique for temperature monitoring and analysis of thermal processes in semiconductor lasers. This is an optical, non-contact, high spatial resolution technique providing high temperature resolution and mapping capabilities. Thermoreflectance is a thermometric technique based on measuring of relative change of reflectivity of the surface of laser facet, which provides thermal images useful in hot spot detection and reliability studies. In this paper, principles and experimental implementation of the technique as a thermography tool is discussed. Some exemplary applications of TR to various types of lasers are presented, proving that thermoreflectance technique provides new insight into heat management problems in semiconductor lasers and in particular, that it allows studying thermal degradation processes occurring at laser facets. Additionally, thermal processes and basic mechanisms of degradation of the semiconductor laser are discussed.

The development of charge coupled device thermoreflectance (CCD TR) instrumentation for accurate and rapid evaluation of the thermal characteristics of quantum cascade lasers is demonstrated. The thermal characterization of such devices provides a mode for comparing different operating conditions, geometries and device designs. The method allows for registration of the high-resolution maps of the temperature distribution in a time not exceeding several seconds. The capabilities of the CCD TR are compared with standard TR spectroscopy.

In this paper, we report on the experimental investigation of the thermal performance of lattice matched AlInAs/InGaAs/InP quantum cascade lasers. Investigated designs include double trench, single mesa, and buried heterostructures, which were grown by combined Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE) techniques. The thermal characteristics of lasers are investigated by Charge-Coupled Device CCD thermoreflectance. This method allows for the fast and accurate registration of high-resolution temperature maps of the whole device. We observe different heat dissipation mechanisms for investigated geometries of Quantum Cascade Lasers (QCLs). From the thermal point of view, the preferred design is the buried heterostructure. The buried heterostructures structure and epi-layer down mounting help dissipate the heat generated from active core of the QCL. The experimental results are in very good agreement with theoretical predictions of heat dissipation in various device constructions.

We investigate molecular beam epitaxy (MBE) growth conditions of micrometers-thick In_0.52Al_0.48As designed for waveguide of InGaAs/InAlAs/InP quantum cascade lasers. The effects of growth temperature and V/III ratio on the surface morphology and defect structure were studied. The growth conditions which were developed for the growth of cascaded In_0.53Ga_0.47As/In_0.52Al_0.48As active region, e.g., growth temperature of T_g = 520 °C and V/III ratio of 12, turned out to be not optimum for the growth of thick In_0.52Al_0.48As waveguide layers. It has been observed that, after exceeding ~1 µm thickness, the quality of In_0.52Al_0.48As layers deteriorates. The in-situ optical reflectometry showed increasing surface roughness caused by defect forming, which was further confirmed by high resolution X-ray reciprocal space mapping, optical microscopy and atomic force microscopy. The presented optimization of growth conditions of In_0.52Al_0.48As waveguide layer led to the growth of defect free material, with good optical quality. This has been achieved by decreasing the growth temperature to T_g = 480 °C with appropriate increasing V/III ratio. At the same time, the growth conditions of the cascade active region of the laser were left unchanged. The lasers grown using new recipes have shown lower threshold currents and improved slope efficiency. We relate this performance improvement to reduction of the electron scattering on the interface roughness and decreased waveguide absorption losses.

In this work design, fabrication and characterization of mid-infrared AlInAs/InGaAs/InP QCLs taper quantum cascade lasers are presented. In order to increase output power while keeping good beam quality, taper design of resonator was used. We have studied devices emitting at 4.5 μm with different shapes (linear, concave and convex) and angles of taper waveguide section, to optimize the output beam parameters, namely ${M^{2}}$ and brightness. Experimental results demonstrate that convex geometry taper laser with 1.7° taper shows the best performance, the highest output power up to 5 W and the smallest horizontal beam divergence of 5.6° in the fundamental mode. This design is also characterized by the highest brightness.