Development of a Tetrahertz Spectrometer for Investigating Correlated-Electron Materials

L 8-w-L 9704032用现有的实验技术对材料在太赫兹频率下的光学响应进行比尔曼测量是最困难的。这是不幸的，因为MANWMM中有趣的物理现象导致了在太赫兹频率下强烈依赖于频率的电导率。该项目的目标是利用一种产生和检测宽带太赫兹辐射的新技术来构建一台太赫兹光谱仪，用于测量低温下相关材料的频率相关特性。该项目需要由两位首席调查员(P.I.S)提供的跨学科专业知识。太赫兹光谱仪基于超快激光领域的最新进展。利用飞秒脉冲激光在砷化镓晶体中光生载流子。注入载流子在亚皮秒尺度的耗尽电场中加速并接近漂移速度。来自这些载流子的辐射是一个相干皮秒电磁脉冲，它沿着泵浦脉冲的方向远离晶体传播。该电源的效率相当高，峰值功率为几兆瓦。利用普通光学元件，太赫兹光束可以从具有可变温度能力的低温恒温器内的样品反射，并且可以通过测量非线性ZnTe晶体中延时的飞秒探测脉冲产生的电光相位延迟来检测电场随时间的变化。利用这种技术，可以产生中心在0.5到1.0太赫兹之间的辐射，带宽为~1太赫兹。入射电场和反射电场的复傅里叶变换可以用来确定材料复介电函数的实部和虚部作为频率的函数。与传统方法相比，宽带太赫兹光谱仪在测量相关系统中重要频率的响应方面取得了重大进步。首先，在不引用Kramers-Kronig关系的情况下，目前很难同时测量太赫兹频率下介电函数的实部和虚部。在建立和测试了光谱仪后，将测量几个重费米子和混合价系统在1.5-300K的温度范围内的太赫兹响应。在这些系统中，强的电子关联导致低温下费米-液体基态的增强。使用微波和远红外光谱的测量已经在其中一些系统中观察到了窄的德鲁德响应，但传统技术的限制使得无法对各种材料上的响应进行完整的表征。一些新的系统，如小间隙近藤绝缘子和表现出非费液体行为的材料，也将被研究。在这两个系统中，电导率的特征预计都是太赫兹频率。其他系统，包括量子磁体、量子井和点结构、具有电荷和自旋密度波基态的化合物，以及无序诱导局域化很重要的材料，都是强关联在太赫兹频率下产生频率相关电导率的例子，未来计划对其中许多系统进行实验。最后，光谱仪的用途超出了所提出的测量范围。在开发了将样品的S温度降低到1K以下的能力后，我们可以研究一些重费米子材料在奇异超导基态下的电动力学响应。***

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. ***