The aim of this work was to model, fabricate, and optimize anti-reflective coatings for long-wave quantum cascade lasers emitting radiation in the wavelength range of 4 - 6 µm. Using the transition matrix method implemented in the Mathematica program, a number of simulations were performed, in which various variants of anti-reflective coatings made of materials such as aluminum oxide, yttrium oxide, magnesium fluoride, silicon oxide, and silicon nitride were considered. The model uses refractive indices determined on the basis of ellipsometric measurements in the spectral range of 2 - 15 µm. Based on the results of the simulation, three types of anti-reflection coatings were proposed, so that they could reduce the value of the reflection coefficient from the front mirror of the quantum cascade laser from 30% to about 10%. Two of them were in the form of a single layer of alumina or yttrium oxide, while the third was a two-layer structure of silicon oxide and silicon nitride.
The next stage of research was the optimization of the processes of physical electron beam deposition and inductively coupled plasma–chemical vapor deposition in order to obtain layers of the desired thickness, uniformity, and optical properties. Using Fourier spectroscopic measurements in the spectral range of 3 - 10 µm, the values ​​of the obtained reflection coefficients of the anti-reflective layers deposited on the laser mirror were positively verified and turned out to be close to the assumed values. In order to integrate the proposed anti-reflective coatings with quantum cascade lasers, research was carried out to improve the adhesion of the coating to the laser mirror, including tests using oxygen plasma and ammonia. The last stage was to measure the current-voltage characteristics and the dependence of the optical power on the current intensity for lasers before and after covering the front mirrors with anti-reflective coatings.
The results obtained as part of this master's thesis confirmed the optimization and effective implementation of three types of anti-reflection coatings for long-wave quantum cascade lasers emitting radiation in the wavelength range of 4 – 6 µm. The reflection coefficient of the front laser mirror was reduced from 30% to ~ 15%, which in the future should translate into a significant increase in the power of the quantum cascade lasers under development. In addition, it was found that the proposed two-layer AR coating of silicon oxide and silicon nitride is fully compatible with the technological process used in the production of the laser, and has excellent adhesion to the substrate and stress resistance. To the best of our knowledge, it has no equivalent in the literature.