Hexagonal sodium yttrium fluoride (β-NaYF4) crystals are currently being studied for a wide range of applications including color displays, solar cells, photocatalysis, and bio-imagβing. β-NaYF4 has also been predicted to be a promising host material for laser refrigeration of solids. However, due to challenges with growing Czochralski β- NaYF4 single-crystals, laser refrigeration of bulk β-NaYF4 has not yet been achieved6. Recently hydrothermal processing has been reported to produce Yb-doped β-NaYF4 nanowires (NWs) that undergo laser refrigeration during single-beam optical trapping experiments in heavy water. The local refrigeration of the individual nanowire is quantified through the analysis of its Brownian motion through the analysis of forward scattered light that is focused onto a quadrant photodiode. The individual β-NaYF4 nanowires show maximum local cooling of 9°C below ambient conditions. Here we present the emission lifetime for the 4S3/2 – 4I15/2 transition for Er(III) ions in Yb/Er-codoped -NaYF4 NW ensembles was measured to be (220 ± 6) μs using a an electron multiplying charge coupled device (EMCCD) as a detector with high spatial resolution. This lifetime is consistent with values reported in the literature.
Single-beam laser-tweezers have been demonstrated over the past several decades to confine nanometer-scale particles in three dimensions with sufficient sensitivity to measure the spring constants of individual biological macromolecules including DNA. Large laser-irradiance values (on the order of MW/cm2) commonly are used to generate laser traps which can lead to significant laser-heating within the 3D optical potential well. To date, laser-refrigeration of particles within an aqueous medium has not been reported stemming primarily from the large near-infrared (NIR) optical absorption coefficient of liquid water (0.2 cm-1 at lambda = 1020nm). In this paper we will detail the methods on how single-beam laser-traps can be used to induce and quantify the refrigeration of optically trapped nanocrystals in an aqueous medium. Analysis of the Brownian dynamics of individual nanocrystals via forward light scattering provides a way to determine both a relative and absolute measurement of particle’s temperature. Signal analysis considerations to interpreting Brownian motion data of trapped particles in nonisothermal aqueous environments, or so-called hot Brownian motion, are detailed. Applications of these methods to determining local laser-refrigeration of laser trapped nanoparticles in water show promise at realizing the first observation of particles undergoing cold Brownian motion.