Laser Cooling of Semiconductors: Ultrafast Carrier and Lattice Dynamics

In modern manufacturing industries, high power lasers are routinely used for precision cutting and welding of even the toughest metals and compounds. It is perhaps from this point of view, that a concept of laser cooling of matter appears so counter-intuitive. Despite this and in the context of atomic gases, the field of laser cooling has achieved temperatures in the sub-nano Kelvin range and has been decorated with two Nobel Prizes. In gases, thermal energy is stored in the translational motion of the atoms, which in the process of cooling is removed via spontaneous emission of the atoms, excited by carefully arranged laser beams. In the field of laser cooling of solids, the reduction of the lattice temperature occurs by anti-Stokes photoluminescence, accompanied by phonon annihilation. This process has been remarkably successful in rare-earth doped fluoride glasses and crystals, culminating with a recent demonstration of cryogenic operation down to a temperature of 90 K. Theoretical investigations predict that even lower temperatures can be achieved by laser cooling of semiconductors. Due to the general compatibility with high-speed electronics applications, their successful laser cooling would have a large impact and trigger many applications. Despite numerous studies and in contrast to rare-earth doped hosts, to date no net-cooling has been demonstrated in gallium arsenide (GaAs), one the most widely available high-purity direct-gap semiconductor. All the previous characterization and cooling attempts have been based on steady-state measurements, i. e. on timescales much longer than any relevant physical processes in a solid. This dissertation takes a different approach. Using time-resolved spectroscopy of the lattice temperature and carrier dynamics, a new dimension is added to the observations. Different time scales, ranging from femtoseconds to microseconds are associated with different microscopic processes of the laser cooling cycle, including those that have precluded observation of bulk cooling to date. By capturing the entire time span of the system’s dynamics which follows after a laser cooling excitation, the underlying physics of the process can now be examined. New insights into fundamental and unanswered questions such as on the origin of the parasitic background absorption signal, a phenomenological term which describes the production of heat, and the origin of the below-gap absorptions states are gained and described in detail.