Narrow Results

The spin-galvanic effect and the inverse effect, which yeilds current induced spin polarization, in low dimensional semiconductor structures are reviewed. Both effect are caused by asymmetric spin relaxation in systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian.

Tuned narrow-band-sensing of microwave and terahertz radiation can be realized by subjecting an irradiated two-dimensional electron system to both a static and a time varying magnetic field, and detecting at the harmonics of the modulation.

Progress in the synthesis and engineering of semiconductor materials has led to improved device performances and functionalities. In particular, in the last decade, there has been considerable interest in the physics and applications of highly-mismatched alloys in which small and highly-electronegative isovalent N-atoms are incorporated onto the anion sublattice of a III-V compound semiconductor.¹ The most studied material is the GaAs_1-xN_x alloy. Our magnetotunnelling studies have shown that a small percentage of N (x < 1%) perturbs dramatically the electronic properties of the host GaAs crystal leading to a large increase of the electron effective mass and an unusual response of the energy-wavevector dispersions to hydrostatic pressure.^2–6 These effects differ from the smoother variation of the energy band gap and electron effective mass with alloy composition observed in other semiconductor compounds, such as In_yGa_1-yAs. The incorporation of N in GaAs gives rise to a qualitatively different type of alloy phenomenon: N-impurities and N-clusters tend to localize the extended Bloch states of GaAs at resonant energies in the conduction band (CB), thus fragmenting the energy-wavevector dispersion relations.

The possibility of tailoring the electronic properties of III-V compounds by N-incorporation has stimulated proposals for innovative devices in optoelectronics and high frequency (terahertz, THz) electronics.⁷ However, to date, the implementation of dilute nitrides in these technologies presents several challenges, including a degradation of the electron mobility. Also, despite a rapidly expanding body of work on the electronic properties of GaAs_1-xN_x, the range of N-concentrations over which this alloy behaves as a good conductor is not yet well established.

Our magnetotransport experiments have revealed how the incorporation of N in GaAs affects the electrical conductivity. Our studies in n-type GaAs_1-xN_x epilayers revealed a large increase of the resistivity, ρ, for x > 0.2%, which we have attributed to the emergence of defect states with deep (~ 0.3 eV) energy levels. Electron trapping onto these states was not observed at low x (x = 0.2%). In this ultra-dilute alloy regime and at low electric fields (F < 1 kV/cm) the electrical conductivity retains the characteristic features of transport through extended states, albeit with relatively low mobility (µ ~ 0.1 m²/Vs at RT) due to scattering of electrons by N-atoms.

We have focused our research on this ultra-dilute regime and exploited the admixing of the localized single N-impurity level with the extended conduction band states of GaAs to realize an unusual type of negative differential velocity (NDV) effect: at large F (> 1 kV/cm), electrons gain sufficient energy to approach the energy of the resonant N-level, where they become spatially localized.^7–10

This Resonant Electron Localization in Electric Field, to which we give the acronym RELIEF, leads to NDV and strongly non-linear current-voltage characteristics. We envisage that the RELIEF-effect could be observed in other III-N-V alloys, such as InP_1-xN_x and InAs_1-xN_x. In these compounds the nature of the resonant interaction between the N-level and the conduction band states of the host-crystal is still relatively unexplored. However, it is clear that the different energy positions of the N-level relative to the conduction band minimum of different materials could offer new degrees of freedom in the design of the electronic band structure and electron dynamics.

The RELIEF-effect may open up prospects for future applications in fast electronics. We have shown that the maximum response frequency, f_max, of a RELIEF-diode can be tuned by the applied electric field in the THz frequency range.⁷ This is of potential technological significance for the development of detectors/sources in the 0.6-1 THz region, which is not currently attainable using conventional Transferred Electron Devices and Quantum Cascade Lasers. Our recent studies of GaAs_1-xN_x have also shown a fast response of the current in the sub-THz frequency range.¹¹ Experiments involving diodes optimized for THz-operation coupled with a quantitative theoretical model of the THz dynamics will be now needed to assess the use of GaAs_1-xN_x and other III-N-V alloys in detectors/sources of THz radiation.

Note from Publisher: This article contains the abstract only.

We demonstrate that lightly-doped Si(P) displays extremely sharp absorption lines – the narrowest yet reported for any impurity in natural Si. The Zeeman splitting of many of these lines in magnetic fields <10 T has been studied previously by a number of groups. In this paper we focus on the behavior of metastable states associated with conduction-band Landau levels. The use of a rather heavily doped sample and strong magnetic fields, up to 18 T, assists in the observation of these.

We develop a semiclassical theory of the nondegenerate parametric amplification in a single miniband of superlattice. We present the formulas describing absorption and gain of signal and idler fields in superlattice and analyze the limiting cases of strong and weak dissipation. We show how the well-known Manley-Rowe relations arise in the tight-binding lattice in the weak dissipation limit. Our results can be applied to an amplification of THz signals in semiconductor superlattices and a control of nonlinear transport of cold atoms in optical lattices.

We demonstrate theoretically that terahertz (THz) fundamental band-gap between the electron mini-band in the InAs layer and the heavy-hole mini-band in the GaSb layer can be realized in InAs/GaSb-based type II superlattices (SLs). The THz band-gap can be tuned by varying the widths of the InAs/GaSb layers. The presence of such band-gap can result in a strong cut-off on optical absorption at THz frequencies. For typical sample structures, the THz cut-off of the optical absorption depends sensitively on temperature and a sharper cut-off can be observed at relatively high temperatures. This study is pertinent to the application of InAs/GaSb type II SLs as THz photodetectors.

We theoretically study the static and dynamic transport properties of Mott–Gurney diodes based on semiconducting single-walled zigzag carbon nanotubes (CNTs). The electric field and velocity distribution of the diode under dc voltage is obtained by solving the steady-state drift-diffusion equations, which involve the negative differential velocity. The current–voltage characteristic of CNT diode exhibits a distinctive positive differential resistance. The high-frequency impedance is calculated with the small-signal analysis method. A major feature of the proposed CNT diode is that the bias- and tube index-dependent impedance show several negative windows in terahertz frequency range despite the positivity of the dc differential resistance. This property makes the CNT-based Mott–Gurney diode a promising candidate for the generation and amplification of terahertz signals within the desired frequency region.

In this paper, by utilizing the variable conductivity with photo-injection in gallium arsenide (GaAs), we have designed an asymmetrical planar terahertz (THz) metamaterial, which is connected with two single-gap split ring resonator (SRR) by GaAs strip and demonstrated the resonant conversion of SRR within the THz range under appropriate optical pumping. As central trailing arm of the structure, GaAs is skillfully inserted between the two cross arms of the THz metamaterial and plays a key role in resonant conversion. Through modulation of its conductivity (σ_GaAs), the variable conductivity of GaAs can make one dual-gap SRR into two connective single-gap SRRs in physical structure, at the same time, the state conversion of two different resonances in the THz metamaterial has been achieved. The simulation results show that the resonant states of THz metamaterial can be switched from one LC and one dipole (state 1) to two LC and one new dipole (state 2) through the intermediate state with the increasing σ_GaAs. This structural design provides a new example to apply variable conductivity to achieve state conversion of resonance and can be extended to the additional application in THz devices.

A photoconductive fractal antenna significantly improves the performance of photomixing-based continuous wave (CW) terahertz (THz) systems. An analysis has been carried out for the generation of CW-THz radiation by photomixer photoconductive antenna technique. To increase the active area for generation and hence the THz radiation power we used interdigitated electrodes that are coupled with a fractal tree antenna. In this paper, both semiconductor and electromagnetic problems are considered. Here, photomixer devices with Thue-Morse fractal tree antennas in two configurations (narrow and wide) are discussed. This new approach gives better performance, especially in the increasing of THz output power of photomixer devices, when compared with the conventional structures. In addition, applying the interdigitated electrodes improved THz photocurrent, considerably. It produces THz radiation power several times higher than the photomixers with simple gap.

The terahertz absorption spectrum of the five aging explosive samples (PETN, RDX, HMX, LLM-105 and TATB) was measured and calculated by Terahertz time-domain spectroscopy system (THz-TDS) and air-biased coherent detection system (ZAP-ABCD), respectively. In this paper, compared with the unaging explosive, each aging explosive sample’s terahertz time-domain spectra were obtained and the terahertz absorption spectra were calculated by using Fourier transform and Lambert’s law. The results show that there are several terahertz absorption peaks which were called “fingerprint spectra” for different aging explosive samples in the range of 0.3–6.0 THz spectrum. Meanwhile, the results also show that the locations of the characteristic absorption peaks are not the same. Moreover, the unaging and aging explosive samples have obviously different terahertz absorption spectra.

In this paper, a hollow core photonic crystal fiber (PCF)-based THz chemicals sensor has been presented. Hexagonal shaped hollow core and symmetrical hexagonal air lattices have been used in the cladding section to construct the PCF geometry. The developed PCF-based chemical sensor yields high performance in ethanol, methanol, water and benzene detection in targeted liquid samples in the THz regime, which is nearly 99% at 3 THz. Additionally, it reveals very negligible losses in both polarization modes. In addition, it renders significant improvement in different sensing properties like effective area, effective refractive index, numerical aperture (NA), nonlinear coefficient, spot size and beam divergences because of strategic geometrical arrangements. The performance of the proposed PCF sensor is numerically investigated and designed by COMSOL software v.5.3a. Fabrication feasibility of developed geometry is also stated here.

Terahertz (THz) focusing metasurface plays a key role in micro-focusing and micro-imaging. Most of the existing focus sets only show the number and deflection of focuses. However, few researches are concerned about the adjustment of focus intensity. Herein we designed a complementary metasurface based on a C-ring and cross-shaped composite slot structures, and the effect of structural thickness on focusing intensity and focal length were studied. We find that the angle of anomalously transmitted beam can be scanned from $17.5^{\circ }$ to $26.5^{\circ }$ when the frequency changes from 1.2 THz to 1.8 THz, while the focal length of the focusing metasurface can be linearly changed from 1000 $\mu \mathrm{\text{m}}$ to 1650 $\mu \mathrm{\text{m}}$. With increase in the thickness of the C-ring area, more interestingly, the focus intensity decreased linearly from 8.0 dB (V/m) to 4.5 dB (V/m). However, the focus intensity showed no obvious change when the thickness of the cross-shaped area was increased from 0.2 $\mu \mathrm{\text{m}}$ to 5 $\mu \mathrm{\text{m}}$. Therefore, our work would enhance the application of THz in high-resolution imaging, new THz device, and flat lens with adjustable focus intensity.

The transmission properties of terahertz wave through subwavelength metallic slits have been investigated using a quasianalytical modal expansion method. The effects of slit widths, periodic length and incident angle on the transmission spectrum and scattering phase are given and dicussed. The results demonstrate that the transmittance increases with the increasing of the ratio of width to slit period. As the incident angle increases, the transmittance increases and shows more peaks.

We have analyzed the spatio-temporal patterns of current self-oscillation under DC bias and terahertz frequency-dependent nonlinear dynamics in semiconducting single-walled zigzag carbon nanotubes (CNTs). It is found that different transport states, including periodic and chaotic, appear with the influence of an external AC signal. The global transitions between periodic and chaotic states are clearly resolved from Poincaré bifurcation diagram. When the driving frequency is fixed at inverse golden mean ratio times natural frequency, the nonlinear CNT system exhibits a more complex transition behavior than at other driving frequencies.

We have calculated the fifth-order nonlinear optical response at experimentally relevant field strengths within the model of massless Dirac fermions by coupling the massless Dirac fermions to the time-dependent electric field quantum mechanically. It demonstrates that the fifth-order nonlinear optical response plays an important role in the contribution to the optical conductivity of pristine single-layer graphene in the low frequency part of the terahertz regime. The nonlinear effect can enhance the optical activity of single-layer graphene in the terahertz regime and significantly decreases the transmittance of graphene in the regime of frequencies from 0.1 to 0.5 THz. These properties of graphene may be used for photonic and optoelectronic device in the terahertz regime.

On the basis of transfer matrix method (TMM), the effective medium theories (EMT) which can be applied to the n-layer unit cell structures have been developed. By using this method, we have systematically investigated the influences of the structure parameters, operation frequency, and material gain on the dispersion properties and transmission of the metallic-dielectric stratified structure (MDSS) in the terahertz (THz) regime. The n-unit cell structures show much more freedom to study the MDSS. The results show that the dispersion relationship acquired from the EMT agrees well with those from the TMM when the value of k_x is small. With the increase of the frequency, the n-layer unit cell MDSS can show more than one dip, and the dip positions can be changed by altering the doping concentration of the InSb layer. As the material gain increase, the transmission increases, and the full-width-half-maximum of the transmission spectrum decrease. This proposed method can also be applied to other spectral regions, such as microwave and infrared regimes. The results are beneficial to improve the performance of MDSSs devices and have great potential for many applications in fields of sub-wavelength image and superlens design.

The characteristics of terahertz (THz) generation based on cascaded difference frequency generation (DFG) process in periodically inverted gallium arsenide (GaAs) and gallium phosphide (GaP) is calculated from coupled wave equations, in which the output enhancement factors are 5.4 and 3.9 in the two crystals, respectively, compared with DFG without cascading. The optimal interaction length, influence of crystal absorption, wave vector mismatch and pump intensity are analyzed. A short discussion on wavelength tuning is also given. The calculated optimal operating parameters and conclusions can provide good directions for the experimental design.

We propose the design of a broadband planar metamaterial absorber (MA) at terahertz frequencies. The unit cell of the MA is composed of four dual-band sub-cells with different dimensions in a coplanar. The four dual-band sub-cell structures resonate at several neighboring frequencies. The absorber consists of two metallic layers separated by a dielectric spacer. Simulation results show that the metamaterial absorption at normal incidence is above 90% in the frequency of 6.56–8.10 THz. This design provides an effective way to construct broadband absorber. The multiple-reflection theory was used to explain the absorption mechanism of our investigated structures. The coupling of adjacent four dual-band sub-cells can introduce additional capacitance to affect the performance of absorber.

The terahertz nonlinear optical response in Kronig–Penney (KP) type graphene superlattice is demonstrated. The single-, triple- and quintuple-frequencies of the fifth-order nonlinear responses are investigated for different frequencies and temperatures with the angle φ along the periodicity of the superlattice toward the external field tuning from 0 to π/2. The results show that the fifth-order nonlinear optical conductance of graphene superlattice is enhanced in the terahertz regime when φ = 0, i.e. an external field is applied along the periodicity of the superlattice. The fifth-order nonlinear optical conductances at φ = 0 for different frequencies and temperatures are calculated. The results show that the nonlinear optical conductance is enhanced in low frequency and low temperature. Our results suggest that KP type graphene superlattices are preferred structures for developing graphene-based nonlinear photonics and optoelectronics devices.

The asymmetrical nested metamaterial, composed of two split-ring resonators (SRRs) and two embedded gallium arsenide (GaAs) islands placed in the two SRRs, has been elaborately designed on quartz substrate. Its tunable and switchable resonances at terahertz (THz) frequencies are numerically demonstrated here based on different conductivities of GaAs, which can be transformed from semiconductor to metallic state through appropriate optical excitation. Without photoexcitation, our designed metamaterial has three resonance peaks in the range of monitored frequency range, and they are located at 0.813, 1.269 and 1.722 THz, respectively. As the conductivity of the two GaAs islands increases, different new resonances appear and constantly strengthen. Finally, four new resonant points are generated, at 0.432, 0.948, 1.578 and 1.875 THz, respectively. At the same time, the metamaterial structure is changed from the original nested mode to a new integral mode. Applying reversible changing conductivity of semiconductor to push the conversion of resonance, this asymmetrical nested design provides a new instance in application and development of additional THz devices.