Terahertz time-domain spectroscopy and its applications.

摘要

Mode-locked lasers can now readily produce optical pulses of femtosecond duration. In this thesis, results are presented on new methods by which these laser pulses can be exploited to measure and control electric fields on an ultrafast time scale. In addition to the development of these techniques, the resulting capability is used for time-domain far-infrared spectroscopy measurements.; With respect to approaches for the detection of electric fields on the femtosecond time scale, two new detection methods based on nonlinear optical interactions with femtosecond laser pulses have been developed. In one scheme, electro-optic sampling of ultrafast electrical transients is extended to permit the capture of a complete electric-field waveform with a single femtosecond laser pulse. This is achieved by using a multichannel detection scheme combined with an experimental geometry in which the femtosecond probe pulse experiences time skew across its wave front relative to the electric field being measured. This method eliminates the need for sampling and the associated scanning of a delay line. A further development involves application of the electric field-induced optical second-harmonic generation. This process is shown to permit the characterization of the magnitude and direction of electric fields in centrosymmetric materials, such as silicon, with high sensitivity, spatial and temporal resolution. With respect to the generation of ultrashort electromagnetic transients, results are presented on the enhancement of the emission efficiency of optically gated photoconductive emitters under application of an external magnetic field. The basic mechanism of the large observed enhancements is clarified within the context of a simple model incorporating a Lorentz-Drude description of carrier dynamics together with an appropriate treatment of the radiation process for the photo-induced current transients.; One important application of these capabilities is the time-domain spectroscopy in the terahertz (THz) or far-infrared spectral region. In this approach one produces a controlled electric-field waveform and measures directly in the time domain the changes in this waveform induced by the sample. The method provides access to a spectral region that is difficult to reach by conventional means. It also offers unique capabilities for probing the non-equilibrium properties of rapidly changing systems. These capabilities have permitted the study of the nature and transport of photo-generated electrons in a condensed-phase system consisting of liquid hexane. A second application illustrates the utility of controlled single-cycle THz pulses for the elucidation of fundamental questions about electromagnetic waves and their interactions. In particular, the properties of circularly polarized electromagnetic pulses with a duration approaching that of one optical cycle are investigated experimentally. The results are found to agree well with a theoretical treatment that predicts that the orthogonal components of the electric field of circularly polarized radiation satisfy a Hilbert transform relation. This new analysis provides a completely general description of polarized electromagnetic radiation, one which extends beyond the conventional quasi-monochromatic limit.