Investigation of quantum materials using tunable pump-probe Raman scattering

非技术摘要：在量子材料中，如超导体和拓扑绝缘体，量子物理是最重要的性质的基础。解开这些材料带来的谜题是21世纪的主要科学挑战之一，与新一代电子、能源和医疗技术直接相关。该项目致力于观察这些材料在被短激光脉冲击中后如何及时演变。利用一种新的实验技术，泵浦-探测拉曼光谱，研究人员制作了量子材料如何在每0.000000000001秒的时间内演变的电影。在这些时间间隔内，光的传播只有几分之一毫米，因此可以在慢动作中检测到非常快的过程。微弱的激光脉冲可以把一些电子踢出它们的轨道。这些将击中原子和其他电子，揭示电子和原子晶格之间的相互作用。如果激光脉冲很强，材料可以短暂地进入不同的状态，例如超导体将失去超导性。观察它是如何回到原始状态的，揭示了隐藏的机制。这些知识用于测试基础理论，并有助于利用量子材料在电子，光子学和能源方面的实际应用。这项工作是在科罗拉多大学博尔德分校的超快激光光谱实验室进行的，该实验室是由美国国家科学基金会（NSF）以前提供的资金建造的。它是博士学位的一部分。技术摘要：本项目的重点是突破性的材料测量，其中量子物理学是最重要的物理性质的基础。总的目标是了解这些材料在远离热平衡时的行为：被研究的样品被短激光脉冲驱动出平衡，另一个延时激光脉冲通过充当拉曼光谱的激发激光来探测其弛豫。这项工作是在科罗拉多大学博尔德分校新的超快激光光谱实验室进行的，该实验室是由美国国家科学基金会（NSF）以前提供的资金建造的。该实验室核心的激光系统是世界上同类系统中最先进的。它被用于泵浦-探测角分辨光电子能谱和泵浦-探测拉曼光谱。这个项目代表了这项工作的拉曼组成部分。所有的拉曼活性激发在弛豫过程中被跟踪。这些包括声子，磁振子，和一些跨越高达5eV的间隙的电子激发。拉曼散射可以直接探测激发态的能量、寿命和布居（例如声子占据数）。时间分辨的拉曼散射测量是在电荷密度波（CDW）系统，不同类型的超导体，莫特绝缘体，和新的自旋轨道材料。它们展示了各种各样的，往往仍然是神秘的物理现象，引起了极大的兴趣，主要研究人员已经通过其他相关技术对它们进行了研究。这是博士学位的一部分。两名研究生的论文，并有助于本科教育。

Non-Technical Abstract: In quantum materials such as superconductors and topological insulators quantum physics underlies most important properties. Unraveling the puzzles posed by these materials is one of the main scientific challenges of the 21st century, with direct relevance to new generations of electronic, energy, and medical technologies. This project is devoted to observing how these materials evolve in time after being hit by a short laser pulse. Using a new experimental technique, pump-probe Raman spectroscopy, the researchers make movies of how quantum materials evolve in time taking frames every 0.00000000000001 sec. Light travels a fraction of a millimeter during these time intervals, so very fast processes can be detected in slow motion. A weak laser pulse may kick some electrons out of their orbits. These will hit atoms and other electrons revealing interactions between electrons and the atomic lattice. If the laser pulse is strong, the material can briefly go into a different state, e.g. a superconductor will lose superconductivity. Observing how it comes back to the original state reveals otherwise hidden mechanisms. This knowledge is used to test fundamental theories and helps to harness quantum materials for practical applications in electronics, photonics, and energy. The work is carried out at the ultrafast laser spectroscopy laboratory at the University of Colorado (CU)-Boulder built with the funds provided previously by the National Science Foundation (NSF). It is be a part of a Ph.D. thesis of two graduate students and contributes to undergraduate education.Technical Abstract: This project focuses on groundbreaking measurements of materials where quantum physics underlies most important physical properties. The general goal is to understand how these materials behave away from thermal equilibrium: The investigated sample is driven out of equilibrium by a short laser pulse, and another time-delayed laser pulse probes its relaxation by serving as the excitation laser for Raman spectroscopy. The work is carried out at the new ultrafast laser spectroscopy laboratory at the University of Colorado (CU)-Boulder built with the funds provided previously by National Science Foundation (NSF). The laser system at the heart of this laboratory is one of the most advanced of its kind in the world. It is utilized for pump-probe angle-resolved photoemission spectroscopy and pump-probe Raman spectroscopy. This project represents the Raman component of this effort. All Raman active excitations are tracked during relaxation. These include phonons, magnons, and some electronic excitations across gaps of up to 5eV. Raman scattering can directly probe the energies, lifetimes, and population of excited states (e.g. phonon occupation numbers). The time-resolved Raman scattering measurements are on charge density wave (CDW) systems, different types of superconductors, Mott insulators, and novel spin-orbit materials. They display diverse and often still enigmatic physical phenomena of great interest and the principal investigator already investigated them by other related techniques. It is a part of a Ph.D. thesis of two graduate students and contributes to undergraduate education.