Optical refrigeration and superradiance in fluorescent solids

Abstract: Lead halide perovskite nanocrystals and rare-earth-doped solids are fluorescent materials that are known for their unusual interactions with light. They have near-unity photoluminescence quantum yields, and can emit photons with a higher energy than that of the absorbed photons, making them suitable for optical refrigeration. For example, Yb:YLF was cooled from room temperature to 91 K with a laser, and was recently used to cool a payload. In this seminar, I will show that one can optically cool these materials with incoherent light sources like filtered sunlight and LEDs just as efficiently as with lasers. When nanocrystals of such materials are 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.