Optical Refrigeration in Semiconductor Nanocrystals

The concept of solid-state optical refrigeration (laser cooling) is premised on removing thermal energy from a material through its anti-Stokes photoluminescence (ASPL). Since the first report of ASPL-induced cooling in 1995, multiple experiments have now unambiguously confirmed solid state laser refrigeration and cooling to cryogenic temperatures in rare-earth-doped glasses and crystals. A record cooling temperature of 91 K has recently been achieved with a Yb3+-doped yttrium lithium fluoride (YLF) crystal.

Going to lower temperatures, however, requires changing cooling media. This stems from the thermal depopulation of atomic levels at low temperatures, which shuts off optical cooling cycles. Numerous cooling attempts have therefore been made with alternate, condensed phase media such as solutions of rhodamine 101. Recent focus has shifted to semiconductors given Fermi-Dirac statistics that guarantee populated valence bands at very low temperature. This results in minimum cooling floors as low as 10 K.

This PhD thesis begins by presenting an overview of the theoretical model in semiconductor laser cooling, which highlights key factors and concepts crucial to establishing valid laser cooling claims. The thesis then compares three different methods of all-optical thermometry: pump-probe luminescence thermometry (PPLT), differential luminescence thermometry (DLT), and up-conversion emission thermometry. Additionally, the thesis summarizes the longstanding debate in the community regarding whether excitation energy dependencies exist in semiconductor nanocrystals. Finally, the thesis introduces a non-contact method that could be used to directly detect defects in semiconductor nanocrystals.