Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances, is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape

Cyclotron resonance in HgTe/CdTe-based heterostructures in high magnetic fields

Cyclotron resonance study of HgTe/CdTe-based quantum wells with both inverted and normal band structures in quantizing magnetic fields was performed. In semimetallic HgTe quantum wells with inverted band structure, a hole cyclotron resonance line was observed for the first time. In the samples with normal band structure, interband transitions were observed with wide line width due to quantum well width fluctuations. In all samples, impurity-related magnetoabsorption lines were revealed. The obtained results were interpreted within the Kane 8·8 model, the valence band offset of CdTe and HgTe, and the Kane parameter E(P) being adjusted

Temperature-dependent low-frequency vibrational spectra of sodium magnesium chlorophyllin

Terahertz time-domain spectroscopy has been used to investigate the vibrational spectra of polycrystalline sodium magnesium chlorophyllin - one of the natural derivatives of chlorophyll - over the temperature range 88 K–298 K. A number of well-resolved absorption peaks were observed in the frequency range 0.2–2.5 THz, which are interpreted as originating from mixed character of intramolecular and intermolecular vibration modes. As the temperature is increased, the observed absorption features resolve into broader peaks. The peak centered at 1.83 THz shifts towards higher frequencies, indicating that for this feature, significant intermolecular anharmonicity exist

Realistic picture of helical edge states in HgTe quantum wells

International audienceWe propose a minimal effective two-dimensional Hamiltonian for HgTe/CdHgTe quantum wells (QWs) describing the side maxima of the first valence subband. By using the Hamiltonian, we explore the picture of helical edge states in tensile and compressively strained HgTe QWs. We show that both dispersion and probability density of the edge states can differ significantly from those predicted by the Bernevig-Hughes-Zhang (BHZ) model. Our results pave the way towards further theoretical investigations of HgTe-based quantum spin Hall insulators with direct and indirect band gaps beyond the BHZ model

Quantum spin Hall insulator with a large bandgap, Dirac fermions, and bilayer graphene analog

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Hybridization of topological surface states with a flat band

International audienceWe address the problem of hybridization between topological surface states and a non-topological flat bulk band. Our model, being a mixture of three-dimensional Bernevig–Hughes–Zhang and two-dimensional pseudospin-1 Hamiltonian, allows explicit treatment of the topological surface state evolution by continuously changing the hybridization between the inverted bands and an additional ‘parasitic’ flat band in the bulk. We show that the hybridization with a flat band lying below the edge of the conduction band converts the initial Dirac-like surface states into a branch below and one above the flat band. Our results univocally demonstrate that the upper branch of the topological surface states is formed by Dyakonov–Khaetskii surface states, known for HgTe since the 1980s. Additionally we explore an evolution of the surface states and the arising of Fermi arcs in Dirac semimetals when the flat band crosses the conduction band

Disorder-induced topological phase transition in HgCdTe crystals

International audienceUsing the self-consistent Born approximation, we study a topological phase transition appearing in bulk HgCdTe crystals induced uncorrelated disorder due to both randomly distributed impurities and fluctuations in Cd composition. By following the density-of-states evolution, we clearly demonstrate the topological phase transition, which can be understood in terms of the disorder-renormalized mass of Kane fermions. We find that the presence of a heavy-hole band in HgCdTe crystals leads to the topological phase transition at much lower disorder strength than is expected for conventional three-dimensional topological insulators. Our theoretical results can also be applied to other narrow-gap zinc-blende semiconductors such as InAs, InSb, and their ternary alloys InAsSb

Focal-plane micro-bolometer arrays for 0.5 THz spatial room-temperature imaging

The possibility of using commercial classical near-infrared (similar to 30 THz) focal-plane micro-bolometer arrays in the sub-terahertz frequency range is studied. Real-time room-temperature pixilated detector imaging of a 0.5 THz (lambda = 600 mu m) spatial beam pro. le emitted from a backward-wave oscillator is presented. Images, with a signal-to-noise ratio of 5 are demonstrated, leading to an estimate of the noise equivalent power of 11 nW . Hz(-1/2)

Coherent and Tunable Terahertz Emission from Nano-metric Field Effect Transistor at Room Temperature

We report on reflective electro-optic sampling measurements of TeraHertz emission from nanometer-gate-length InGaAs-based high electron mobility transistors. The room temperature coherent gate-voltage tunable emission is demonstrated. Our results shows that properly exciting nanotransistors can pave the way for new class of coherent and easily tunable THz sources. (C) 2010 Optical Society of Americ

Softening of spin resonance at low temperature in p-doped Cd1-xMnxTe quantum wells

International audienceTime-domain spin resonances in p-doped CdMnTe quantum wells have been studied via time-resolved magneto-optical Kerr rotation. The resonances related to quantum well electrons Mn and electrons excited in the substrate are identified. An additional spin resonance which shows an unusual temperature dependence is detected in one of the samples. The corresponding frequency tends to zero when approaching the Curie temperature. This "soft" spin precession mode has been studied as a function of magnetic field, temperature, magnetic field orientation, and optical excitation density. Its origin is discussed in the framework of the existing collective spin excitation model