Anti-Stokes Cooling for Fluidics

温度在物理、化学、生物和工程中起着无处不在的作用。温度的变化可以引起相变，增强或抑制化学反应，加速或减缓生物体的代谢过程。许多制造方法依赖于明确定义的温度才能继续进行。温差驱动着热力机。然而，温度通常仅作为确保热平衡的系统的全局参数来控制，例如通过电冰箱或热板。最近，金属纳米颗粒被证明是有效的光控纳米热源，允许在样品中受控良好的位置远程注入热量。在这一动态发展的热塑性体学领域，局部温度增量已被用于诱导热潜水溶质漂移或在小长度尺度上传输和操纵生物对象的热渗透液体流动的流体应用中。但光也可以用于制冷应用，如原子或微机械系统的冷却和陷阱，以探索其量子力学基态。然而，大多数冷却实验都是在真空中进行的，与热水浴完全隔绝。在这个项目中，我们的目标是将纳米晶体的激光制冷带到液体环境中，用于流体应用。掺Yb的纳米晶体应该借助非弹性反斯托克斯散射过程进行光学冷却，就像最近在D2O和真空中所展示的那样。我们将通过研究不同的表面钝化方法将这种纳米晶体的适用性扩展到水环境中，以排除与表面相关的快速激发态失活，这被认为是阻止水中有效冷却的过程之一。利用拉曼测温仪，我们将测量纳米晶体的温度，并确定冷却过程的效率。然后，我们将使用纳米晶体作为流体应用的冷点，测量由局部温度梯度产生的热渗透界面流动。与光控热源相结合，进一步探索热电场的可控产生。除了这些流体应用，在凝聚系统中局部冷却的能力也将为生物物种的扰动开辟许多新的可能性。

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.