Laser Refrigeration by an Ytterbium-Doped NaYF₄ Microspinner

Thermal control of liquids with high (micrometric) spatial resolution is required for advanced research such as single molecule/cell studies (where temperature is a key factor) or for the development of advanced microfluidic devices (based on the creation of thermal gradients at the microscale). Local and remote heating of liquids is easily achieved by focusing a laser beam with wavelength adjusted to absorption bands of the liquid medium or of the embedded colloidal absorbers. The opposite effect, that is highly localized cooling, is much more difficult to achieve. It requires the use of a refrigerating micro-/nanoparticle which should overcome the intrinsic liquid heating. Remote monitoring of such localized cooling, typically of a few degrees, is even more challenging. In this work, a solution to both problems is provided. Remote cooling in D₂ O is achieved via anti-Stokes emission by using an optically driven ytterbium-doped NaYF₄ microparticle. Simultaneously, the magnitude of cooling is determined by mechanical thermometry based on the analysis of the spinning dynamics of the same NaYF₄ microparticle. The angular deceleration of the NaYF₄ particle, caused by the cooling-induced increase of medium viscosity, reveals liquid refrigeration by over -6 K below ambient conditions.