In 2005 I started with experiments which were designed to expand the spectral range available for precise ellipsometric measurements into the THz frequency domain. This is a very exciting and challenging new research area which is about to emerge and will enable the quantitative determination of the dielectric optical response of matter in a spectral domain from 0.1 THz to 3 THz – the spectral range which is often termed the final frontier in spectroscopy.
The dramatic improvement of light sources and – to some extend – detectors operating in this spectral region led to the advent of time- and frequency-domain spectrometer systems. These systems now have sufficiently high signal-to-noise ratios to allow nvestigations of opaque and transparent materials in reflection and transmission geometries, respectively. Presently, however, THz spectroscopy mostly relies on simple (un-)polarized reflection measurements which do not permit the reliable determination and the quantitative analysis of the material’s dielectric properties under investigation. THz ellipsometry will fill this gap and provide crucial information for the long awaited breakthrough in optical THz technologies which is at present obstructed because of the lack of reliable THz material dielectric properties.
Presently, I explore new avenues to establish THz ellipsometry using electron beam based table-top THz sources at University Nebraska-Lincoln in the workgroup of Prof. Dr. M. Schubert. This research will provide pioneering access to accurate optical constants of bulk materials and thin films in the THz range. The goal is to employ ellipsometry for applied research for the precise and accurate determination of optical constants required for the identification, characterization, and classification including arbitrary anisotropy of THz optical coatings and stealth paint, hazardous materials, and semiconductor layer and nano-structures for example. I further envision THz ellipsometry to become an important characterization technique to address fundamental questions in the basic research areas of nanostructured metamaterials and electron correlated systems in the THz frequency domain.
THz ellipsometry also bears an immense potential for the investigation of complex and biological molecules. It is known that vibrational modes associated with protein tertiary structure lie in the far-infrared or terahertz frequency range. The polarization-sensitive investigation of these vibrations will provide valuable information in order to understand the structure-function relationship of biomolecules and thereby address one of the problems of the modern biophysics – the analysis of the protein functioning.
Hole-channel conductivity in epitaxial graphene determined by terahertz optical-Hall effect and midinfrared ellipsometry, T. Hofmann, A. Boosalis, P. Kühne, C. M. Herzinger, J. A. Woollam, D. K. Gaskill, J. L. Tedesco, and M. Schubert, Appl. Phys. Lett. 98, 041906 (2011), Vir. J. Nan. Sci. & Tech., Volume 23 , Issue 5
1. T. Hofmann, C. M. Herzinger, A. Boosalis, T. E. Tiwald, J. A. Woollam, and M. Schubert, “Variable-wavelength frequency-domain THz ellipsometry”, Rev. Sci. Instrum. 81, 023101 (2010).
2. T. Hofmann, C. M. Herzinger, T. E. Tiwald, J. A. Woollam, and M. Schubert, “Hole diffusion profile in a p-p+ Silicon homojunction determined by terahertz and mid-infrared spectroscopic ellipsometry”, Appl. Phys. Lett. 95, 032102 (2009).
3. T. Hofmann, C. M. Herzinger, J. A. Woollam, and M. Schubert, “Materials Characterization
using THz Ellipsometry”, Mat. Res. Soc. Symp. Proc. 1163E, 1163-K08-04 (2009).
4. V. Darakchieva, T. Hofmann, M. Schubert, B. E. Sernelius, B. Monemar, P. O. A. Persson, F. Giuliani, E. Alves, H. Lu, and W. J. Schaff, “Free electron behavior in InN: on the role of dislocations and surface electron accumulation”, Appl. Phys. Lett. 94, 022109 (2009).
5. T. Hofmann, C. M. Herzinger, C. Krahmer, K. Streubel, and M. Schubert, “The optical Hall effect”, phys. stat. sol. (a) 205, 779 (2008).
6. T. Hofmann, U. Schade, W. Eberhardt, C. M. Herzinger, P. Esquinazi, and M. Schubert, “Terahertz magnetooptic generalized ellipsometry using synchrotron and black-body radiation”, Rev. Sci. Instrum. 77, 063902 (2006).
7. T. Hofmann, M. Schubert, G. Leibiger, and V. Gottschalch, “Electron effective mass and phonon modes in GaAs incorporating boron and indium”, Appl. Phys. Lett. 90, 182110 (2007).
8. T. Hofmann, U. Schade, K. C. Agarwal, B. Daniel, C. Klingshirn, M. Hetterich, C. M. Herzinger, and M. Schubert, “Conduction-band electron effective mass in Zn0.87Mn0.13Se measured by terahertz and far-infrared magneto-optic ellipsometry”, Appl. Phys. Lett. 88, 043105 (2006).
9. T. Hofmann, T. Chavdarov, V. Darakchieva, H. Lu, W. J. Schaff, and M. Schubert “Anisotropy of the Gamma-point effective mass and mobility in hexagonal InN”, phys. stat. sol. (c) 3, 1854 (2006).
10. M. Schubert, T. Hofmann, and J. Sik, “Long-wavelength interface modes in semiconductor layer structures”, Phys. Rev. B 71, 035324 (2005).