锗半导体纳米晶的激光冷却
Laser Cooling of Germanium Semiconductor Nanocrystals
摘要：
通过反斯托克斯光致发光的物质的激光冷却已经在凝聚介质中成功地实现，特别是利用实现亚100 K固态光制冷的稀土掺杂系统，其中光的发射频率由于热振动能量的吸收而超过撞击激光的发射频率。研究表明，半导体中的激光冷却具有实现低至~ 10 K的温度的潜力，并且其直接集成可以迎来独特的高性能纳米结构半导体器件。虽然最近已经报道了纳米结构II-VI族半导体的激光冷却，但是间接带隙半导体（例如IV族硅和锗）的激光冷却仍然是一个主要挑战。在这里，我们报告的异常观察占主导地位的反斯托克斯光致发光锗纳米晶体。我们将这一结果归因于超高纯度锗的汇流，高密度电子空穴等离子体的产生，非极性间接带隙半导体中纵向和横向光学声子的固有简并性，以及相称的空间限制效应。在高激光强度下，激光冷却的晶格温度可低至~ 50 K。
英文摘要：
Laser cooling of matter through anti-Stokes photoluminescence, where the emitted frequency of light exceeds that of the impinging laser by virtue of absorption of thermal vibrational energy, has been successfully realized in condensed media, and in particular with rare earth doped systems achieving sub-100K solid state optical refrigeration. Studies suggest that laser cooling in semiconductors has the potential of achieving temperatures down to ~10K and that its direct integration can usher unique high-performance nanostructured semiconductor devices. While laser cooling of nanostructured II-VI semiconductors has been reported recently, laser cooling of indirect bandgap semiconductors such as group IV silicon and germanium remains a major challenge. Here we report on the anomalous observation of dominant anti-Stokes photoluminescence in germanium nanocrystals. We attribute this result to the confluence of ultra-high purity nanocrystal germanium, generation of high density of electron-hole plasma, the inherent degeneracy of longitudinal and transverse optical phonons in non-polar indirect bandgap semiconductors, and commensurate spatial confinement effects. At high laser intensities, laser cooling with lattice temperature as low as ~50K is inferred.
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