Chapter 11 THz Generalized Ellipsometry Characterization of Highly-Ordered Three-Dimensional Nanostructures Tino Hofmann, Daniel Schmidt and Mathias Schubert Abstract Routine generalized ellipsometry measurements at terahertz (THz) frequencies have become available only very recently, and we present and discuss the application to highly-ordered three-dimensional nanostructure thin films. Such nanostructure thin films are obtained from glancing angle deposition, and consist of slanted columnar structures with high spatial coherence. The slanted columnar thin films reveal strong birefringence due to electronic coupling and screening phenom- ena. Despite their extreme smallness compared with the THz wavelength equivalent, slanted columnar nanostructure thin films can be used as sensors for dielectric fluids in transmission or reflection geometries, where measurements can be made through the back side of THz-transparent substrates. We describe an anisotropic biaxial effective medium dielectric function approach which comprises structural, geomet- rical and constituent fraction information, and which enables quantitative analysis of THz generalized ellipsometry measurements. We further describe a frequency- domain generalized ellipsometer setup which incorporates backward wave oscillator sources. 11.1 Introduction Ellipsometric investigations with electromagnetic radiation at THz frequencies offer unique access to high-frequency electrical properties that are of great interest for T. Hofmann (B ) · D. Schmidt · M. Schubert Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, NE68588-0511, USA e-mail: thofmann@engr.unl.edu D. Schmidt e-mail: schmidt@huskers.unl.edu M. Schubert e-mail: schubert@engr.unl.edu M. Losurdo and K. Hingerl (eds.), Ellipsometry at the Nanoscale, 411 DOI: 10.1007/978-3-642-33956-1_11, © Springer-Verlag Berlin Heidelberg 2013