Recent results on probing of topological phases in Montpellier

We report on room temperature electrically-induced terahertz emission from interdigitated GaN quantum well structures. The emission spectrum has been analysed in a Michelson interferometer using a 4K-Si bolometer as a terahertz detector. A resonant peak at the frequency of around 3 THz was observed in emission spectra. A threshold behaviour of the resonance with respect to applied voltage takes place. By using the proposed analytical model the measured/observed experimentally resonant behaviour of emission spectra is interpreted as a result of ungated stream-plasma instability in the channel.

We report on sub-Terahertz photoconductivity of bulk HgxCd1-xTe crystals withcomposition close to semiconductor-to-semimetal topological transition. Low magnetic fieldShubnikov-de Hass like oscillations were observed with a period which does not depend onthe incident frequency. Moreover, anomalous B-periodic oscillations were also observed athigher magnetic fields.

Terahertz power dependence of the photoresponse of field effect transistors, operating at frequencies from 0.1 to 3 THz for incident radiation power density up to 100 kW/cm2 was studied InGaAs high electron mobility transistors. The observed signal saturation behavior is explained by analogy with current saturation in standard direct currents output characteristics. The theoretical model of terahertz field effect transistor photoresponse was developed shows a good description match with experimental data. Our experimental results show that dynamic range of field effect transistors based terahertz detectors is very high and can extend from mW/cm2 up to kW/cm2.

The channel of the field effect transistor can operate as a cavity for plasma waves. For the electrons with high enough mobility, the plasma waves can propagate in field effect transistors (FETs) channel leading to resonant Terahertz (THz) emission or detection. In the low mobility case, plasma oscillations are overdamped, but a plasma density perturbation can be induced by incoming THz radiation. This perturbation can lead to efficient broadband THz detection. We present an overview of the main experimental results concerning the plasma oscillations in field effect transistors for the generation and the detection of terahertz radiations.

Surface states in semiconducting materials are usually fragile with respect to disorder and perturbations. However, there are systems in which surface states become robust because of the non-trivial topology of the band structure. A well-known example is the integer quantum Hall effect in which magnetic field splits conduction band of 2D carriers into discrete energy levels (Landau levels) inducing specific edge channels of quantized conductance. But recently, another type of topological phase was identified in materials with energy band inversion and strong spin orbit interaction. It is called the topological insulator phase. The most interesting property of 2D topological insulators is the presence of spin polarized counter-propagating edge channels of quantized conductance, protected from back scattering. The topologically nontrivial state was first realized in HgTe-based quantum wells.