Anti-Stokes Cooling for Fluidics

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Temperature plays a ubiquitous role in physics, chemistry, biology, and engineering. Changing temperature may induce phase transitions, enhance or inhibit chemical reactions as well as speed up or slow down metabolic processes in organisms. Many fabrication methods rely on a well-defined temperature to proceed. Temperature differences drive thermodynamic machines. Yet, temperature is often only controlled as a global parameter of a system ensuring thermal equilibrium, e.g., by electric refrigerators or hot plates. Recently, metal nanoparticles have been shown to be effective light-controlled nano heat sources allowing to inject heat remotely at well-controlled positions in the sample. In this dynamically developing field of thermoplasmonics, local temperature increments have been employed, for example, in fluidic applications inducing thermophoretic solute drifts or thermo-osmotic liquid flows transporting and manipulating biological objects on small length scales. But light can be also used for refrigeration applications as demonstrated in the cooling and trapping of atoms or micromechanical systems to explore their quantum mechanical ground state. Most of those cooling experiments, however, proceed in vacuum, well isolated from a thermal bath. Within this project, we aim to bring laser refrigeration of nanocrystals to liquid environments for use in fluidic applications. Ytterbium-doped nanocrystals shall be optically cooled with the help of inelastic anti-Stokes scattering processes as recently demonstrated in D2O and vacuum. We will extend the applicability of such nanocrystals to an aqueous environment by studying different ways of surface passivation to exclude fast surface-related excited-state deactivation, which is supposed to be one of the processes preventing efficient cooling in water. Using Raman thermometry, we will measure the temperature of the nanocrystals and determine the efficiency of the cooling process. We will then use the nanocrystals as cold spots for fluidic applications, measuring thermo-osmotic interfacial flows created by the local temperature gradients. In combination with optically controlled heat sources, we will further explore the controlled generation of thermo-electric fields. Besides these fluidic applications, the ability to cool locally in condensed systems will open up a number of new possibilities also for the perturbation of biological species.