Development of a Tetrahertz Spectrometer for Investigating Correlated-Electron Materials

开发用于研究相关电子材料的四赫兹光谱仪

• 批准号: 9704032
• 负责人: Ward Beyermann
• 金额: $ 25.4万
• 依托单位: University of California-Riverside
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1997
• 资助国家: 美国
• 起止时间: 1997-09-01 至 2000-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9704032&HistoricalAwards=false
• 关键词: Development; Tetrahertz; Spectrometer; Investigating; Correlated

l 8 -w- l 9704032 Beyermann Measurements of the optical response of materials at terahertz frequencies are the most difficult to perform with existi.pg e,xperimental techniques. This is unfortunate since interesting physical phenomena in manwsnms leads to a strongly frequency-dependent conductivity at terahertz frequencies. The objective of this project is to construct a terahertz spectrometer for measuring the frequency dependent properties of correlated materials at low temperatures using a new technique for generating and detecting broad-band terahertz radiation. The project requires interdisciplinary expertise that is provided by the two principal investigators (P.I.'s). The terahertz spectrometer is based on recent advances in the ultrafast laser community. A femtosecond pulsed laser is used to photogenerate carriers in a GaAs crystal. The injected carriers accelerate and approach drift velocity in the depletion electric field on a subpicosecond timescale. The radiation from these carriers is a coherent-picosecond-electromagnetic pulse which propagates away from the crystal in the direction of the pump pulse. The source is reasonably efficient with a peak power of several mW. Using ordinary optical components, the terahertz beam can be reflected from a sample, which is located inside a cryostat with a variabletemperature capability, and the electric field can be detected as a function of time by measuring the electro-optic phase retardation induced on a time-delayed femtosecond probe pulse in a nonlinear ZnTe crystal. With this technique, radiation can be produced that is centered between 0.5 and 1.0 THz with a bandwidth of ~1 THz. The complex Fourier transforms of the incident and reflected electric fields can be used to determine the real and imaginary parts of the complex dielectric function of the material as a function of frequency. The broad-band terahertz spectrometer represents a significant advancement over traditional methods in its ability to measure the response at frequencies important in correlated systems. For one thing, it is currently very difficult to measure both the real and imaginary parts of the dielectric function at terahertz frequencies without invoking the Kramers-Kronig relation. After building and testing the spectrometer, the terahertz response of several heavy-fermion and mixed-valent systems will be measured over a temperature range from 1.5 to 300 K. In these systems, strong electron correlations lead to an enhanced Fermi-liquid ground state at low temperatures. Measurements using microwave and far-infrared spectroscopy have observed a narrow Drude response in some of these systems, but the limitations of the conventional techniques prohibit a complete characterization of the response on a variety of materials. Some new systemse such as small-gap Kondo insulators and materials that display nonfermi-liquid behavior, will also be examined. Features in the conductivity are expected at terahertz frequencies in both these systems. Other systems, including quantum magnets, quantum well and dot structures, compounds with charge and spin-density-wave ground states, and materials where disorder-induced localization is important are all examples where strong correlations produce a frequency-dependent conductivity at terahertz frequencies, and experiments are planned for many of these systems in the future. Finally, the spectrometer's utility goes beyond the measurements that are being proposed. After developing capabilities to reduce the sample' s temperature below 1 K, we could examine the electrodynamic response in the exotic superconducting ground state of some heavy fermion materials. ***

材料在太赫兹频率下的光学响应的测量是用现有的实验技术进行的最困难的。这是不幸的，因为manwsnms中有趣的物理现象导致在太赫兹频率下强烈的频率依赖性电导率。本项目的目标是构建一个太赫兹光谱仪，用于测量相关材料在低温下的频率相关特性，使用一种新的技术来产生和检测宽带太赫兹辐射。该项目需要由两位主要研究者（P.I.）提供的跨学科专业知识。的）。 太赫兹光谱仪是基于超快激光社区的最新进展。利用飞秒脉冲激光在GaAs晶体中光生载流子。注入的载流子在耗尽电场中以亚皮秒的时间尺度加速并接近漂移速度。来自这些载流子的辐射是在泵浦脉冲的方向上远离晶体传播的相干皮秒电磁脉冲。该源的效率相当高，峰值功率为几mW。利用普通的光学元件，将太赫兹光束反射到低温恒温器中的样品上，通过测量延迟飞秒探测脉冲在非线性ZnTe晶体中产生的电光相位延迟，可以检测到电场随时间的变化.利用这种技术，可以产生中心在0.5和1.0 THz之间的辐射，带宽为~1 THz。入射和反射电场的复傅立叶变换可用于确定作为频率的函数的材料的复介电函数的真实的实部和虚部。 宽带太赫兹光谱仪在测量相关系统中重要频率响应的能力方面比传统方法有了重大进步。 首先，目前很难在太赫兹频率下测量介电函数的真实的和虚部，而不调用Kramers-Kronig关系。 在搭建并测试了该谱仪后，将在1.5 ~ 300 K的温度范围内测量几个重费米子和混合价体系的太赫兹响应。在这些系统中，强电子关联导致在低温下增强的费米液体基态。使用微波和远红外光谱的测量已经观察到在这些系统中的一些窄的Drude响应，但是常规技术的局限性禁止对各种材料的响应的完整表征。 一些新的系统，如小间隙近藤绝缘体和材料，显示非费米液体行为，也将被检查。在这两个系统中的太赫兹频率的电导率的功能，预计。其他系统，包括量子磁体，量子阱和点结构，具有电荷和自旋密度波基态的化合物，以及无序诱导局部化很重要的材料，都是强相关性在太赫兹频率下产生频率依赖电导率的例子，并且计划在未来对许多这些系统进行实验。最后，光谱仪的实用性超出了正在提出的测量。在开发出将样品温度降低到1 K以下的能力之后，我们可以检查一些重费米子材料的奇异超导基态中的电动力学响应。 ***