IRFP: Ultrafast Spectroscopy of Urbach Tail Excitations in Semiconductors: Unraveling Dynamics of the Anti-Stokes Cooling Cycle

国际研究奖学金项目使美国科学家和工程师能够在国外进行9至24个月的研究。该计划的奖励为联合研究提供了机会，并利用独特或互补的设施、专业知识和国外的实验条件。该奖项由国际科学与工程办公室和材料研究部的电子和光子材料项目共同资助。该奖项将支持Denis V. Seletskiy博士与德国康斯坦茨大学的Alfred Leitenstorfer教授进行为期24个月的研究。光学制冷需要激光激发低于其平均发光能量的固体，自然在其吸收光谱的尾部（乌尔巴赫尾）。在这个过程中，每发射一个高能光子，就有一个晶格振动量子湮灭，这符合能量守恒的要求。如果这一过程的量子效率高，激光将使固体冷却。虽然稀土掺杂绝缘体已经被冷却到低温，但迄今为止还没有观察到半导体的冷却，尽管十多年来取得了广泛的研究进展。这项研究首次在超快时间尺度上研究了半导体乌尔巴赫尾的激励动力学，为这些激励如何与固体的内部自由度耦合提供了见解。更具体地说，通过瞬态吸收光谱和相干反斯托克斯拉曼光谱技术同时测量载流子密度和晶格温度的时间演变。由于乌尔巴赫尾的信号很小，需要进行高灵敏度的实验。通过采用高度可配置的超稳定光纤飞秒光谱系统，实现了前所未有的灵敏度，该系统由德国康斯坦茨大学Leitenstorfer教授的团队首创。结合新颖的探测方案，这些高分辨率实验为乌尔巴赫尾态的动力学和随后的激光冷却周期提供了新的见解。更好地理解乌尔巴赫尾态将促进我们对凝聚态物理中低能激发的基本相互作用的认识。UT态超快动力学的新知识可能对电信行业产生影响，也可能有利于光伏元件、高量子效率发光二极管和超低电压晶体管等节能技术的发展。对激光冷却周期的进一步了解将推动对小型化全固态制冷机的研究，其潜在应用范围从电子和太空组件的快速和局部可寻址冷却到使用激光冷却超导量子干涉探测器的新型医疗设备的脑成像。这些技术的持续发展对于解决我们社会日益增长的健康、通信和能源需求至关重要。最后，康斯坦茨大学Leitenstorfer教授团队最先进的实验设施和专业知识对这项研究的成功以及PI的专业发展至关重要。该奖项还将帮助Seletskiy博士扩大他的潜在合作者网络以及他的科学范围，为他未来的职业生涯的成功提供宝贵的支持。

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award is co-funded by the Office of International Science and Engineering and by the Electronic and Photonic Materials Program in the Division of Materials Research. This award will support a twenty-four-month research fellowship by Dr. Denis V. Seletskiy to work with Prof. Alfred Leitenstorfer at the University of Konstanz, in Germany. Optical refrigeration requires laser excitation of a solid below its mean luminescence energy, naturally in the tail of its absorption spectrum (Urbach tail). In this process, one quantum of lattice vibration is annihilated for every emitted high energy photon, as required by energy conservation. If quantum efficiency of this process is high, the laser light will cool the solid. While rare-earth doped insulators have already been cooled to cryogenic temperatures, no cooling of semiconductors has been observed to date, despite more than a decade of extensive research progress. This research for the first time investigates the dynamics of the excitations in the Urbach tail of a semiconductor on an ultrafast time scale, providing insight into how these excitations couple to the internal degrees of freedom of a solid. More specifically, temporal evolution of both carrier density and crystal lattice temperature is measured simultaneously by techniques of transient absorption spectroscopy and coherent anti-Stokes Raman spectroscopy. Highly sensitive experiments are required due to the small signals in the Urbach tail. Unprecedented sensitivity is achieved by employing highly configurable ultra-stable fiber-based femtosecond spectroscopy systems, pioneered by Prof. Leitenstorfer's group at the University of Konstanz, in Germany. Together with novel detection schemes, these high resolution experiments offer new insights into the dynamics of the Urbach tail states and the subsequent laser cooling cycle. Better understanding of the Urbach tail states will advance our knowledge of the principle interactions of low-energy excitations in condensed matter physics. New knowledge of the ultrafast dynamics of UT states can have implications on the telecommunication industry as well as possibly benefit the development of energy efficient technologies such as photovoltaic elements, high quantum efficiency light emitting diodes and ultra-low voltage transistors. Increased understanding of the laser cooling cycle will propel research towards miniaturized all-solid-state cryocoolers, with potential applications ranging from fast and locally-addressable cooling of electronic and space-borne components to brain imaging using novel medical devices with laser-cooled superconducting quantum interference detectors. Continued development of these technologies is essential to address the future of the growing health, communication and energy demands of our society. Finally, state-of-the-art experimental facilities and expertise in the group of Prof. Leitenstorfer at the University of Konstanz are essential for success of this research as well as for PI's professional development. This award will also help Dr. Seletskiy to broaden his network of potential collaborators as well as his scientific scope, proving invaluable for success of his future career.