April 2020

Abstracts of the QSIT Lunch Seminar, Thursday, April 2, 2020

Scheduled Zoom meeting

Magneto-capacitance measurement of electron-hole bilayer systems

Simon Parolo - Advanced Semiconductor Quantum Materials Group (Wegscheider group), ETH Zurich

Closely spaced 2D electron and 2D hole systems are of great interest because the coulomb interaction may lead to exciton formation which might condense in a superfluid or become an excitonic insulator.
We use MBE to grow GaAs-based heterostructures which contain both a p-doped and a n-doped layer, enclosing an undoped GaAs region. An AlGaAs barrier is placed in the centre of that GaAs region. Thus, the device resembles a p-i-n diode with a central barrier. The magneto capacitance is probed by superimposition of AC- and DC-voltage. Under forward bias the 2D systems form as the bias exceeds the GaAs gap energy resulting in a sudden increase of the capacitance.
By lowering the barrier width, we observe new and unexpected behaviour in the capacitance as well as in the tunnelling.

Topological charge of finite-size photonic crystal modes

Zhixin Wang - Quantum Optoelectronics Group (Faist group), ETH Zurich

Topological charges are the winding numbers of polarization vectors around the vortex centers of far-field radiation. In this work, the topological charge of photonic crystal modes is theoretically analyzed using an envelope function approach. A group of modes is discovered with unique polarization properties dictated by their non-trivial envelope functions. Experimentally, lasing operation on such mode is demonstrated in an electrically pumped mid-infrared photonic crystal surface-emitting laser with high slope efficiency. The topological charge is directly observed from the polarization properties of single-mode laser emission.