Terahertz Ellipsometry Using Electron-Beam Based Sources
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1108-A08-04
Terahertz Ellipsometry Using Electron-Beam Based Sources T. Hofmann1, C. M. Herzinger2, U. Schade3, M. Mross4, J. A. Woollam2, and M. Schubert1 1 University of Nebraska-Lincoln, Lincoln, NE, USA. 2 J.A. Woollam Co., Lincoln, NE, USA. 3 BESSY mbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany 4 Vermont Photonics Technologies Co., Bellows Falls, VT, USA. ABSTRACT The precise determination of materials' optical constants in the THz frequency domain is an important new challenge in basic research and is crucial for novel technological applications. Spectroscopic ellipsometry is known as a vital tool for the determination of the materials’ dielectric function including its anisotropy. However, ellipsometric measurements at very long wavelengths are difficult due to the lack of reliable sources of sufficient intensity and brilliance. Here we report on our recent advances to use ellipsometry in combination with different electron beam based sources in order to in investigate condensed matter samples in the frequency range from 0.1 to 8 THz. We successfully employ terahertz radiation emitted from two different tunable desktop sources (Smith-Purcell-effect source and a backward wave oscillator) in a polarizer-sample-analyzer ellipsometer scheme. We discuss and present THz range physical material properties due to bound and unbound charge resonances in semiconducting materials. This research will provide important understanding of optical properties for novel materials, inspire new designs, and accelerate development of optical Terahertz devices.
INTRODUCTION In the past few years the advances in optical terahertz (THz) technology have increased the demand for reliable determination of accurate optical constants for materials in the spectral region between 1000 - 100 µm (300 GHz - 3 THz), which is the least explored long wavelength range of the electromagnetic spectrum [1,2]. Spectroscopic ellipsometry is a versatile and precise technique for the determination of the complex optical constants of thin film and bulk materials [3-9]. Ellipsometry determines the polarization state of a light beam, which carries intrinsic material (polarizabilities) and geometry (layer thickness, major polarizability axes directions, etc.) information if determined after the interaction of the light beam with a sample of sufficiently smooth interfaces. Over the last decades spectroscopic ellipsometry has been established as the standard technique for accurate measurement of indices of refraction n and extinction coefficients k for isotropic and anisotropic materials [3-9]. However, only very few reports address successful ellipsometry investigations in the THz spectral range [10-12]. One of the major difficulties for THz spectroscopic ellipsometry instrumentation is the lack of easy-to-access, powerful, reliable, continuous wave light sources suitable for polarization-dependent measurements. In general, ellipsometry requires polarization and angle-of-incidence analysis, which incorporates both near crossed-polarizer and near Brew
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