We have found the possibility to probe the magnetic field-driven topological phase transition in HgTe-based transistors by measuring their Terahertz photoconductivity response. At the critical magnetic field to which zero-mode Landau levels cross, we have observed a pronounced photoconductivity peak independent on incident frequency and carrier concentration. Our results pave the way towards terahertz topological field effect transistors.

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.

We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in a temperature range from 4.2 up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau levels, which split from the bottom conduction and upper valence sub-bands and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such a phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells.

It has recently been shown that electronic states in bulk gapless HgCdTe offer another realization of pseudo-relativistic three-dimensional particles in condensed matter systems. These single valley relativistic states, massless Kane fermions, cannot be described by any other relativistic particles. Furthermore, the HgCdTe band structure can be continuously tailored by modifying cadmium content or temperature. At critical concentration or temperature, the bandgap collapses as the system undergoes a semimetal-to-semiconductor topological phase transition between the inverted and normal alignments. Here, using far-infrared magneto-spectroscopy we explore the continuous evolution of band structure of bulk HgCdTe as temperature is tuned across the topological phase transition. We demonstrate that the rest mass of Kane fermions changes sign at critical temperature, whereas their velocity remains constant. The velocity universal value of (1.07±0.05)x1E6 m/s remains valid in a broad range of temperatures and Cd concentrations, indicating a striking universality of the pseudo-relativistic description of the Kane fermions in HgCdTe.

We report on sub-terahertz photoconductivity under the magnetic field of a two dimensional topologicalinsulator based on HgTe quantum wells. We perform a detailed visualization of Landau levelsby means of photoconductivity measured at different gate voltages. This technique allows oneto determine a critical magnetic field, corresponding to topological phase transition from invertedto normal band structure, even in almost gapless samples. The comparison with realistic calculationsof Landau levels reveals a smaller role of bulk inversion asymmetry in HgTe quantum wellsthan it was assumed previously.

We evaluate the optical performance of the InP heterojunction bipolar transistors (DHBTs) designed for 100 Gbit/s circuit applications as a room temperature detector operating above 1 THz. They can operate far above the frequencies at which they have gain and can still rectify THz current and voltage.

Phenomena of the radiation coupling to the field effect transistors based terahertz detectors is studied. We show that in the case of flat metallic antennas important part of radiation, instead coupling to the transistors, is coupled to the substrate leading to losses. Experimental and theoretical investigations of the responsivity versus substrate thickness are performed. They clearly show how to minimize the losses by the detector substrate thinning.