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. ***
