Quantum-Confined Nanocrystal Materials for Anti-Stokes Optical Coolers

ABSTRACT: Nanocrystals (NCs) offer many potential benefits for optical refrigeration; these quantum-confined materials have discrete energy levels, large optical transition dipole moments, and high photoluminescence quantum efficiency. Using well-characterized upconverting ion-doped metal oxide NCs and core-shell semiconductor nanocrystals as a baseline, a design of experiments (DOE) will be conducted to optimize the coupling and population of the energy carriers (photons, electrons, and phonons), including the photon-exciton and exciton-phonon couplings, the ion-dopant concentration, the phonon density of states, the photon population, the host material and size, the core-shell architectures, the NCs"surface properties, and the ion doping concentration. This will make efficient optical cooling possible and eliminate non-radiative Auger-recombination paths. The coupling and populations will be studied using time-resolved photoluminescence (TRPL) and photoluminescence excitation (PLE), femtosecond pulse probe spectroscopy, and ultraviolet photoelectron spectroscopy (UPS) and X ray photoelectron spectroscopy (XPS) measurements to characterize the NC materials so that the nanostructured materials can be fabricated and demonstrated in working optical cryocoolers capable of cooling power>200 mW. In Phase II, the macroscopic properties of the device will be optimized in a series of fiber optics and optical cavities, so that improvements in efficiency and mass can be demonstrated for space applications. BENEFIT: While the full capabilities, performance, and cost for this technology are not yet fully understood, a low-cost and high-performance cooling system capable of being conformably coated onto a substrate has the potential to unleash a new era in electronics, communications, and sensors. Additionally, the same material sets can be used for thermoelectric generation. We foresee the following cooling applications as high-value target markets: military infrared sensors, silicon chip industry, and optoelectronics.