Freimann Assistant Professor
Department of Physics
"Optical properties of topological insulators: state-of-the-art and future experiments"
Topological insulators are semiconductors that host Dirac surface electrons. Those Dirac electrons allow
efficient spin-charge conversion and strong coupling to polarized light. Most topological insulators are
narrow gap semiconductors and host a large amount of atomic defects. Their doping level is therefore
difficult to control, and surface conduction is impeded by bulk electrical carriers. I will present results
showing how this issue can be overcome using pseudo-binary alloy compositional tuning using  Pb1-
xSnxSe – a topological crystalline insulator. Magnetooptical measurements performed on this system
reveal the presence of Dirac surface states. A superlattice structure is used to enhance optical absorption
from these Dirac states up to more than 70%.  A systematic energy gap tuning is also demonstrated in
this system using temperature and composition as tuning knobs. Experiments aimed at tuning the optical
properties of these materials using pressure, strain and magnetic fields are envisaged. Furthermore, the
coupling of the Dirac energy levels to polarized light beams will be studied. Potential applications of the
tailored optical properties of the Dirac states in Pb1-xSnxSe include THz and IR sources and interband
detectors with inherent sensitivity to polarization.
 B. A. Assaf, T. Phuphachong, E. Kampert, V. V. Volobuev, P. S. Mandal, J. Sánchez-Barriga, O.
Rader, G. Bauer, G. Springholz, L. A. De Vaulchier, and Y. Guldner, Phys. Rev. Lett. 119, 106602
 G. Krizman, B. A. Assaf, T. Phuphachong, G. Bauer, G. Springholz, G. Bastard, R. Ferreira, L. A. de
Vaulchier, and Y. Guldner, Phys. Rev. B 98, 075303 (2018).