Optical Cooling and Superradiance in Fluorescent Solids

Lead halide perovskite nanocrystals are fluorescent materials that are being extensively studied for their unusual interactions with light. They have near-unity photoluminescence quantum yields, and show efficient anti-Stokes photoluminescence (ASPL), emitting photons with a higher energy than that of the absorbed photons. Together, these properties make them suitable for optical cooling. In this thesis, I will report and theoretically rationalize unexpectedly high ASPL efficiencies in cesium lead bromide nanocrystals. I will show that resonant multiple-phonon absorption into the polaron state is the microscopic mechanism of ASPL in this material. Laser radiation has been used in all demonstrated cases of optical refrigeration with the intention of minimizing the entropy of the absorbed photons. I show that as long as the incident radiation is unidirectional, the loss of coherence does not significantly affect the cooling efficiency. Using a general formulation of radiation entropy as the von Neumann entropy of the photon field, I show how the cooling efficiency depends on the properties of the light source such as wavelength, coherence, and directionality. My results suggest that the laws of thermodynamics permit optical cooling of materials with incoherent sources such as light emitting diodes and filtered sunlight almost as efficiently as with lasers. When lead halide perovskite nanocrystals cooled and self-assembled into superlattices, they cooperatively emit coherent radiation, a phenomenon known as superradiance. However, nanocrystal size fluctuations and thermal decoherence strongly suppress superradiance. I will show how optimizing the geometrical arrangement of nanocrystals makes superradiant emission robust to disorder and thermal noise. Finally, I also point out mathematical similarities between superconductivity and superradiance, which may be used in future to predict new phenomena in superradiant systems.