The research in our group focuses on characterization of linear and non-linear optical properties of new materials, their heterostructures as well as artificial nanostructures, such as isolating or conducting nanodots, -gratings, -wires, -”ziczac”s, -spirals, etc. We investigate phenomena related to electromagnetic interaction with bound and unbound charge carrier excitation modes (phonons, plasmons, excitons, magnons, …) in spatially confined structures subjected to external slowly-varying magnetic and electric fields. Materials with semiconducting, superconducting, ferroelectric, ferromagnetic and eventually with multiferroic, i.e., with multiple collective polarization mode properties are of potential interest. Current research subjects include (i) polarization-exchange-coupling in wurtzite-perovskite heterointerfaces, (ii) free-charge-carrier properties in metal-oxide and –nitride semiconductors, (iii) spin-momentum coupling influence on electronic properties in diluted magnetic semiconductors, (iv) charge-correlation phenomena in quantum-regime two-dimensional carrier systems, and (v) “optical permeability” in conducting chiral nanowires. We put strong emphasize on development of electromagnetic constitutive model description for the material systems and physical phenomena studied, as well as on important material parameters derived thereupon. Materials preparation and structural characterization is done in collaboration with partners within the NCMN, and within national and international laboratories.
Coworkers in our group perform research in optical characterization and modeling of the frequency-, wavevector- and polarization-dependent electric and magnetic polarization response. The spectroscopic techniques comprise generalized ellipsometry from the terahertz region into the deep ultra violet, luminescence, as well as polarization-dependent Rayleigh, and Raman scattering, in-situ and ex-situ. We currently extend our existing spectroscopy techniques by setting up a Terahertz Magnetooptic Ellipsometer and we plan to design a new UV Raman system. We will seek additional access to Terahertz synchrotron light sources. Required structural and electrical measurements are performed in collaboration with NCMN partners. Since the scientific topics cover a broad range of subjects, networking with NCMN collaborators is essential.
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
Free electron behavior in InN: on the role of dislocations and surface electron accumulation, 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, Appl. Phys. Lett. 94, 022109 (2009).
Hole diffusion profile in a p-p+ Silicon homojunction determined by terahertz and mid-infrared spectroscopic ellipsometry, T. Hofmann, C.M. Herzinger, T.E. Tiwald, J.A. Woollam, and M. Schubert Appl. Phys. Lett. 95, (2009).
Annealing effects on the optical properties of semiconducting boron carbide, R.B. Billa, T. Hofmann, M. Schubert, and B.W. Robertson, J. Appl. Phys. 105, (2009).
"Monoclinic optical constants, birefringence, and dichroism of slanted titanium nanocolumns determined by generalized ellipsometry", D. Schmidt, B. Booso, T. Hofmann, E. Schubert, A. Sarangen, and M. Schubert, Appl. Phys. Lett. 94, 011914 (2009).
"Terahertz Ellipsometry Using Electron-Beam Based Sources", T. Hofmann, C. M. Herzinger, U. Schade, M. Mross, J. A. Woollam, and M. Schubert, Mat. Res. Soc. Symp. 1108, A08-04 (2009).
"Generalized ellipsometry for monoclinic absorbing materials: determination of optical constants of Cr columnar thin films", D. Schmidt, B. Booso, T. Hofmann, E. Schubert, A. Sarangan, and M. Schubert, Opt. Lett. 34, 992 (2009).
"Optical, structural, and magnetic properties of cobalt nanostructure thin films" ,D. Schmidt, A. C. Kjerstad, T. Hofmann, R. Skomski, E. Schubert, and M. Schubert, J. Appl. Phys. 105, 113508 (2009).
"Resistive hysteresis and interface charge coupling in BaTiO3-ZnO heterostructures", V. M. Voora, T. Hofmann, M. Brandt, M. Lorenz, M. Grundmann, N. Ashkenov, and M. Schubert, Appl. Phys. Lett. 94, 142904 (2009).
"Polaron and Phonon properties in proton intercalated amorphous tungsten oxide thin films," M. F. Saenger, T. Höíng, B. W. Robertson, R. B. Billa, T. Hofmann, E. Schubert, and M. Schubert, Phys. Rev. B 75, 245205 (2008).
"Generalized Ellipsometry Determination of Non-reciprocity in Chiral Silicon Sculptured Thin Films", D. Schmidt, E. Schubert, and M. Schubert, phys. stat. sol. (a) 205, 748 (2008).
?The optical Hall effect?, T. Hofmann, C.M. Herzinger, C. Krahmer, K. Streubel, and M. Schubert, phys. stat. sol. (a) 205, 779 (2008).
"Optical Hall-effect in hexagonal InN", T. Hofmann, V. Darakchieva, B. Monemar, H. Lu, W. J. Schaff, and M. Schubert, J. Electron. Mater 37, 611-615 (2008).
"Kramers-Kronig-consistent optical functions of anisotropic crystals: generalized spectroscopic ellipsometry on pentacene", M. Dressel, B. Gompf, D. Faltermeier, A. K. Tripathi, J. Pflaum, and M. Schubert, Opt. Express 16, 19770 (2008).
"Interface-charge-coupled polarization response of Pt-BaTiO3-ZnO-Pt heterojunctions: A physical model approach", V. M. Voora, T. Hofmann, M. Brandt, M. Lorenz, M. Grundmann, N. Ashkenov, and M. Schubert, J. Electron. Mater 37, 1029-1034 (2008).
“Anisotropic strain and phonon deformation potentials in GaN,” V. Darakchieva, T. Paskova, M. Schubert, P. P. Paskov, D. Hommel, M. Heuken, J. Off, B. A. Haskell, P. T. Fini, J. S. Speck, S. Nakamura, Phys. Rev. B 75, 195217 (2007).
“MOVPE growth investigations of doping and ordering in AlGaAs and GaInP with reflectance anisotropy spectroscopy,” C. Krahmer, M. Philippens, M. Schubert, K. Streubel, J. Cryst. Growth 289, 18 (2007).
“Ion beam assisted growth of sculptured thin films: Structure alignment and optical fingerprints,” E. Schubert, F. Frost, H. Neumann, B. Rauschenbach, B. Fuhrmann, F. Heyroth, J. Rivory, B. Gallas, M. Schubert, Advances in Solid State Physics, Volume 46, 2007 (ISSN: 1617-5034, Springer, Berlin/Heidelberg).
“Strong increase of the electron effective mass in GaAs incorporating boron and indium,” T. Hofmann, G. Leibiger, N. Ashkenov, V. Gottschalch, M. Schubert, Appl. Phys. Lett. 90, in press (2007)
“Dielectric constants and phonon modes of amorphous hafnium aluminate deposited by metal organic chemical vapor deposition,” C. Bundesmann, O. Buiu, S. Hall, M. Schubert, Appl. Phys. Lett. 90, in press (2007).
“Dielectric anisotropy and phonon modes of ordered indirect-gap Al0:52In0:48P studied by far-infrared ellipsometry,” T. Hofmann, V. Gottschalch, M. Schubert, Appl. Phys. Lett. 90, in press (2007).
“Another Century of Ellipsometry,” M. Schubert, Annalen der Physik 15, 480-497 (2006).
“Conduction-band electron effective mass in Zn0.87Mn0.13Se measured by terahertz and far-infrared magneto-optic ellipsometry,” T. Hofmann, U. Schade, K. C. Agarwal, B. Daniel, C. Klingshirn, M. Hetterich, C. M. Herzinger, M. Schubert, Appl. Phys. Lett. 88, 043105 (2006).
“Terahertz magnetooptic generalized ellipsometry using synchrotron and black-body radiation,” T. Hofmann, U. Schade, W. Eberhardt, C. M. Herzinger, P. Esquinazi, M. Schubert, Rev. Sci. Inst. 77, 063902 (2006).
- Carsten Bundesmann, PhD, 2005, Currently at University Leipzig.
- Beri N. Mbenkum, M.Sc., 2004, Currently at MPI Stuttgart.
- Class v. Middendorf, M.Sc. 2004, Currently at University Heidelberg.
- Tino Hofmann, PhD, 2003, Currently at University of Nebraska.
- Nurdin Ashkenov, M.Sc., 2002, Currently at OPTEG.
- Alexander Kasic, PhD, 2001, Currently at Infineon Technologies.