Investigation of quantum materials using tunable pump-probe Raman scattering

摘要：在超导体和拓扑绝缘体等量子材料中，量子物理是大多数重要性质的基础。解开这些材料带来的谜题是21世纪的主要科学挑战之一，与新一代的电子、能源和医疗技术直接相关。这个项目致力于观察这些材料在被短激光脉冲击中后的时间演变。研究人员使用一种新的实验技术——泵浦探测拉曼光谱，每0.00000000000001秒拍摄一帧，以记录量子材料是如何随时间演变的。在这些时间间隔内，光只传播了一毫米的一小部分，所以可以用慢动作检测到非常快的过程。微弱的激光脉冲可能会把一些电子踢出它们的轨道。它们会撞击原子和其他电子，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。观察它如何回到原始状态揭示了其他隐藏的机制。这些知识用于测试基本理论，并有助于利用量子材料在电子学，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）先前提供的资金建立的。它是两名研究生博士论文的一部分，并有助于本科教育。技术摘要：该项目专注于量子物理基础上的材料的突破性测量。总的目标是了解这些材料在远离热平衡状态时的行为：被研究的样品被短激光脉冲驱动出平衡状态，另一个延时激光脉冲作为拉曼光谱的激发激光探测其弛豫。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）先前提供的资金建立的。这个实验室的核心激光系统是世界上最先进的激光系统之一。它被用于泵浦-探针角度分辨光发射光谱和泵浦-探针拉曼光谱。这个项目代表了这一努力的拉曼部分。在松弛过程中跟踪所有拉曼主动激发。这些包括声子、磁振子和一些跨越高达5eV间隙的电子激发。拉曼散射可以直接探测激发态的能量、寿命和居群（如声子占据数）。时间分辨拉曼散射的测量对象是电荷密度波（CDW）系统、不同类型的超导体、莫特绝缘体和新型自旋轨道材料。它们展示了各种各样的、往往仍然是令人非常感兴趣的神秘的物理现象，首席研究员已经用其他相关技术对它们进行了研究。这是两位研究生博士论文的一部分，对本科教育也有贡献。

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.