UC Berkeley

This dissertation will delve into the concept laser cooling in solid-state semiconductors, showcasing a remarkable application of photonics operating at the thermodynamic limits. Originally conceptualized by Pringsheim in 1929, laser cooling has attracted significant attention ever since. While its success has been demonstrated in atomic systems, enabling groundbreaking insights into quantum mechanics, it was not until 1995 that the first successful of solid-state laser cooling was realized in rare-earth-based materials. This achievement paved the way to explore a broader range of solid-state materials. In solid-state laser cooling, the process involves the absorption of sub-bandgap photons, coupled with the absorption of phonon energy, to excite a valence electron to the conduction band. An anti-Stokes photoluminescence (ASPL) emission is followed sequentially, which completes the cycle of phonon annihilation. This cycle leads to a reduction in the material’s temperature. Despite extensive research, confirmed instances of laser cooling in semiconductor materials remain elusive.

Here, our exploration will begin with a thorough examination of semiconductor nanomaterial candidates. Among extensively researched nanomaterials, lead-halide perovskite nanocrystals in particular exhibit exceptional optoelectronic properties owing to their efficient up-conversion and near-unity photoluminescence quantum yield, both essential factors for realizing laser cooling, positioning them as a prime material for advancing the field of solid-state laser cooling.

The investigation will start with examination of the sequential processes crucial for solid-state laser cooling. Initially, the up-conversion efficiency will be quantified, followed by a comprehensive comparative analysis of different halide perovskite nanocrystals. Then rigorous investigations of phonon modes in pure and mixed halides will be followed to reveal the dynamics in steady-state and excited states. After up-conversion efficiency quantification and electron-phonon dynamics investigation, we will study the quantum confinement effect on the radiative lifetimes in perovskite nanocrystals. The ASPL process relies on the emission of photons upon exciton radiative recombination. Thus, it is essential to shorten the radiative lifetime to reduce the likelihood of non-radiative pathways, which results in undesired heating.

After demonstrating the promising potential of lead-halide perovskite nanocrystals for solid-state laser cooling applications, we will conduct experimental laser cool trials. We will develop a special setup that will directly and precisely measure any temperature changes. The setup will incorporate a highly sensitive infrared camera, enabling direct monitoring of temperature variations upon photoexcitation of the nanocrystals with sub-bandgap photons. We then will document our laser cooling attempts and analyze the outcomes. This analysis will include a comparison of different sample preparation methods and their impact on achieve a net laser cooling. Through this comprehensive thesis, we aim to contribute invaluable insights into the practical realization of laser cooling using lead-halide perovskite nanocrystals.