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

国际研究奖学金计划使美国科学家和工程师能够在国外进行9到24个月的研究。该项目的奖项提供了联合研究的机会，并提供了使用国外独特或互补的设施、专业知识和实验条件的机会。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计划共同资助。该奖项将资助丹尼斯·V·塞莱茨基博士与德国康斯坦茨大学的阿尔弗雷德·莱滕斯托弗教授合作的为期24个月的研究奖学金。光制冷需要激光激发低于其平均发光能量的固体，自然地在其吸收光谱的尾部(乌尔巴赫尾部)。在这个过程中，根据能量守恒的要求，每发射一个高能光子，晶格振动就会湮灭一个量子。如果这个过程的量子效率很高，激光就会冷却固体。虽然稀土掺杂的绝缘体已经冷却到低温，但到目前为止还没有观察到半导体冷却，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Urbach尾部的激发动力学，为了解这些激发如何与固体的内部自由度耦合提供了洞察力。更具体地说，利用瞬时吸收光谱和相干反斯托克斯拉曼光谱技术同时测量了载流子密度和晶格温度的时间演化。由于Urbach尾部的小信号，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，获得了前所未有的灵敏度，该系统由德国康斯坦茨大学的Leitenstorfer教授的团队首创。结合新的探测方案，这些高分辨率实验为了解厄尔巴赫尾态和随后的激光冷却循环的动力学提供了新的见解。更好地理解Urbach尾态将促进我们对凝聚态物理中低能激发的主要相互作用的认识。有关UT态超快动力学的新知识可能会对电信行业产生影响，并可能有助于光伏元件、高量子效率发光二极管和超低压晶体管等节能技术的发展。对激光冷却循环的了解的增加将推动对小型化全固态制冷器的研究，其潜在应用范围从电子和航天部件的快速和局部寻址冷却，到使用带有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、通信和能源需求的未来至关重要。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于这项研究的成功以及PI的专业发展都是必不可少的。这一奖项还将帮助塞莱茨基博士扩大他的潜在合作者网络以及他的科学视野，事实证明，这对他未来的职业生涯的成功是非常宝贵的。

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.