We report on optical reflectivity experiments performed on Cd3As2 over a broad range of photonenergies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that weare probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.

We report on THz emission from HgCdTe bulk films. Our experiments clearly show magnetically tunable THz emission that can be attributed to electron radiative recombination between the Landau levels (cyclotron emission). Energy band structure theoretical calculations using Kane Hamiltonian are performed. They allow identification of observed resonances as related to transitions between two lowest conduction band Landau levels.

We report on the first THz magnetospectroscopy of double HgTe quantum well in magnetic fields up to 16 T. Very recently, specific metal phase, which holds some properties of natural bilayer graphene, was predicted in this system. In this work, we focus on the double HgTe QW with parameters, corresponding to such `bilayer graphene' phase. In our experiments we clearly observe THz optical transitions from double-degenerated zero-mode Landau level. A good agreement between experimental and theoretical transition energies confirms previous prediction of `bilayer graphene' phase in double HgTe quantum wells.

Recent developments of THz detector arrays based on plasma wave field effect transistors are presented. By simultaneous development of the transistor arrays with their read-out circuits and diffractive 3D printed optics we demonstrate systems for imaging in 300 GHz atmospheric window with cameras or fast linear scanner. The first high speed THz postal scanner developed for real time fast A4 envelopes security screening is presented.

HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the “single-valley” analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electricallytunable band gap. In this “bilayer graphene” phase, electric field opens the band gap and drives thesystem into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators.

We present theoretical investigations of pressure- and temperature-driven phase transitions in HgTequantumwells grown on a CdTe buffer. Using the eight-band k·p Hamiltonian we calculate evolution of energy-band structure at different quantum well widths with hydrostatic pressure up to 20 kbars and temperature ranging up to 300 K. In particular, we show that, in addition to temperature, tuning of hydrostatic pressure allows us to drive transitions between semimetal, band insulator, and topological insulator phases. Our realistic band-structure calculations reveal that the band inversion under hydrostatic pressure and temperature may be accompanied by nonlocal overlapping between conduction and valence bands. The pressure and temperature phase diagrams are presented.