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Among the many methods to control condensed matter in all its phases, electromagnetic radiation has been the most flexible stimuli. The way in which these electromagnetic waves are used to maneuver the material degrees of freedom, has made tremendous strides at every step when new regions of the electromagnetic spectrum have been made accessible through the progress of appropriate sources and detectors. Optical radiation in the form of ultra-short laser pulses has helped in the rapid development of physics into new frontiers; they also became main candidate to produce table top terahertz (THz) sources. THz region (1-10 THz) is technologically important as it provides direct access to numerous low energy excitations such as molecular rotations, lattice vibrations, spin waves, internal excitations of bound electron-hole pairs and Cooper pairs through both resonant and non-resonant excitation. For all these applications, one needs to control/manipulate THz pulses dynamically.
In the first part of my talk, I will present our first demonstration [1] of all-optically created transient wire grid polarizer that allows us to manipulate the polarization of THz waveforms with sub-cycle switch-on times. To achieve this, we start with a homogeneous semiconducting substrate into which the photonic structure is imprinted all-optically, producing spatial modulation of the refractive index. The polarization-modulated pulses are potentially interesting for controlling elementary motions such as the vibration of crystal lattices, the rotation of molecules and the precession of spins. Second part of my talk will be on terahertz magneto-optical spectroscopy to investigate the cyclotron resonance frequency in a high mobility 2DHG, revealing a nonlinear dependence [2] on the applied magnetic field. This is shown to be due to the complex non-parabolic valence band structure of the 2DHG, as verified by multiband Landau level calculations. We also find that impurity scattering dominates cyclotron resonance decay in the 2DHG, in contrast with the dominance of super-radiant damping in two-dimensional electron gases. We have also carried out the same experiment on a two dimensional electron gas (2DEG) and found the variation of cyclotron frequency versus applied magnetic field, to be linear as expected. Our results shed light on the properties of 2DHGs, motivating further studies of these unique 2D nanosystems.
[1] N. Kamaraju, A. Rubano, L. Jian, S. Saha, T. Venkatesan, J. Nötzold, R. Campen, M. Wolf and T. Kampfrath, Nature Light: Science & Applications, 3 (2), e155 (2014).
[2] “THz Magneto-optical Spectroscopy of two dimensional hole gas”, N. Kamaraju, W. Pan, U. Ekenberg, D. Gvozdic, S. Boubanga-Tombet, P. C. Upadhya, J. Reno, A. J. Taylor, R. P. Prasankumar, Appl. Phys. Lett. 106, 031902 (2015). |