Towards Laser Cooling of Semiconductors

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This dissertation reports on novel theoretical concepts as well as experimental efforts toward laser cooling of semiconductors. The use of quantum well system brings the opportunity to engineer bandstructure, effective masses and the spatial distribution of electrons and holes. This permits the incorporation of novel quantum mechanical phenomena to manipulate the temperature change of the material upon light-matter interaction. Inspired by the fact that Coulomb interaction can lead to blueshift of radiation after photo-absorption, the theory of Coulomb assisted laser cooling is proposed and investigated for the first time. In order to design suitable multiple quantum well (MQW) structures with Coulomb interaction a Poisson-Schrödinger solver was devised using MATLAB software. The software is capable of simulating all III-V material compositions and it results have been confirmed experimentally. In the next step, different MQW designs were proposed and optimized to exploit Coulomb interaction for assisting of optical refrigeration. One of the suitable designs with standard InGaAsP/InAlAs/InP layers was used to grow the MQW structures using metal organic vapor deposition (MOCVD). Novel techniques of fabrication were implemented to make suspended structures for detecting ultralow thermal powers. By fabricating accurate thermometers, the temperature changes of the device upon laser absorption were measured. The accurate measurement of the temperature encouraged us to characterize the electrical response of the device as another important tool to promote our understanding of the 4 underlying physical phenomena. This is in addition to the accurate spectral and time-resolved photoluminescence measurements that provided us with a wealth of information about the effects of stress, Auger recombination and excitonic radiance in such structures. As the future works, important measurements for finding the quantum efficiency of the devices via electrical characterization and pump-probe spectroscopy are explained. To achieve that, the fabrication of the first suspended devices with electrical contacts is suggested and examined. The technique of back focal plane imaging for characterization of PL extraction efficiency and frequency analysis of temperature response for determining quantum efficiency are also proposed. We believe that the efforts and the proposed measurements can elucidate the most important mechanisms that play a fundamental role for laser cooling of semiconductors

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  • 02/26/2018
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