The study of GaAs/AlAs terahertz resonant tunneling diodes demonstrates the competition of self-excitation and amplification processes incorporating the terahertz-range polaritons. The effects of the magnitude of the resonant peak current and low current in the valley of the current-voltage characteristic are demonstrated taking into account the general laws of detection in the mode of internal amplification in the region of negative differential conductivity.
The paper demonstrates results of our experimental study of THz emission from the plasma of a single-color laser filament for two laser wavelengths of 740 and 940 nm. Particular attention is paid to the study of a frequency-angular distribution of the THz emission within 0.1 – 3 THz spectral range. It is observed that different spectral components of THz radiation have different angular distribution, and an increase in the laser beam numerical aperture leads to a growth of the THz emission divergence, especially in its low-frequency range. The study revealed a significant effect of a laser pulse initial wavelength on the THz emission characteristics. Transformation of frequency-angular THz emission spectrum produced by a single-color (740 nm) laser filament plasma under an external electrostatic field of various strength is also experimentally studied. If there is no static electric field, THz emission is predominantly generated in the low-frequency spectral range around 0.1 THz and propagates within a hollow cone. When the electric field is applied, the transition from the hollow cone to a filled one is observed with the field strength rise, THz emission frequency being within of 0.3 - 0.5 THz. Higher frequency emission of ~1 THz fills the whole cone with the emission maximum along the laser filament axis. Furthermore, the angular distribution for the low-frequency THz emission depends significantly on the laser pulse energy in contrast to the case of no electric field. Namely, the laser pulse energy rise results in a decrease of the propagation angles for low-frequency THz emission and disappearance of the local minimum in the angular distribution on the propagation axis.
Emission of THz radiation from a single-color ultraviolet (248 nm) and infrared (744 nm) filament in air is studied experimentally and compared at similar pulse durations, focusing conditions and excess of peak pulse power over the critical power for self-focusing. An angular distribution of the terahertz emission for both ultraviolet and infrared pump is conical with the closed cone angle. In contrast, the terahertz radiation energy and spectrum differ significantly. The energy of terahertz emission from ultraviolet filament is 1-2 orders of magnitude lower than the terahertz yield from the infrared filament. The terahertz spectrum of ultraviolet filament is shifted to the low-frequency range and narrower as compared to the spectrum of terahertz emission from infrared filament. We explain qualitatively the difference in terahertz yields and spectra by lower intensity and plasma density in the ultraviolet filament. Similar behavior of THz spectra is observed when changing the IR filament parameters.
The paper presents the results of application of terahertz radiation for detection of traces of explosives on surfaces of objects in reflected light. The process of detection and identification of explosives is based on a recording of interferograms of reflected radiation in spectral range of 0.5 -2.5 THz with help of a Michelson interferometer. The reverse Fourier processing lets to obtain reflection or transmittance spectra and images of objects. Spectral ranges for imaging are chosen by an operator. An installation elaborated for this purpose is described. Specific features of reflection spectra of some organic substances are determined.
An operator of the permittivity can completely describe alone a microwave response of conductors with the spatial dispersion. An eigenvalue problem for the nonself-adjoint permittivity operator Ễa was considered generally to search the wave solutions for conductors and superconductors. An appearance of additional solutions (additional waves) due to the spatial dispersion can strongly influence the properties of nanoelectronic devices or novel superconducting materials in the form of anomalous losses for example, and should be accounted in simulation and modeling of micro- and nanoelectronic devices. It was concluded that the modulus |Ž| of the surface impedance is proportional to the degree of frequency ω2/3 for all normal conductor solutions except that for the superconductor. There was some criticism related to the idea that the electrodynamics of superconductors should be in principle reduced to those for conductors as the temperature approaches and beyond the critical temperature. We demonstrate that appropriately taken into account effects of the spatial dispersion can give the general frequency dependence of the surface impedance for the obtained solutions including that for the superconductor. It is shown that an incorporation of the spatial dispersion leads to an appearance of the Meissner effect in perfect conductors in the same manner as in superconductors.
The periodical-in-voltage features of the negative differential conductance (NDC) region in the current-voltage characteristics of a high-quality GaAs/AlAs terahertz resonant-tunneling diode have been detected. The found oscillations are considered taking account of the LO-phonon excitation stimulated by tunneling of electrons through the quantum active region in the resonance nanostructure where an undoped quantum well layer is sandwiched between two undoped barrier layers. Rearrangements in the I-V characteristics of the resonant-tunneling diode as a consequence of the topological transformation of a measurement circuit from the circuit with the series resistance Rs to the circuit with the shunt Rp have been experimentally studied and analyzed. The revealed substantial changes in the current-voltage characteristics of the resonant-tunneling diode are discussed schematically using Kirchhoff's voltage law.
Conducting media with the spatial dispersion may be described formally by a singly operator – an operator of a dielectric permittivity, which completely defines a microwave response of conductors with the spatial dispersion. So the eigenvalue problem for the permittivity operator of conductors and superconductors possessing a strong spatial dispersion at low temperatures is of a great importance since the corresponding solutions are the stable waves for the constitutive equation in a self-consistent microwave field. Here a wave problem is formulated to search the solutions, which correspond to the eigenvalues of a permittivity operator, similar to the relationship and the general solutions are obtained. A significant role of the spatial-type conjugated. Dispersion relationship and general solutions are obtained. A significant role of the spatial-type force resonances is considered. Conditions for the spatial resonances are derived. The obtained resonances include particular solutions corresponding to the related to a polarization, two of which correspond to waves with an amplitude increasing into the depth of a conductor, and two else describes solutions with unusual properties.
K. Romanov, N. Dyakonova, D. But, F. Teppe, W. Knap, M. Dyakonov, C. Drexler, P. Olbrich, J. Karch, M. Schafberger, S. Ganichev, Yu. Mityagin, O. Klimenko
We use two antenna model to develop a theory of the recently observed helicity-sensitive detection of terahertz radiation by FETs. The effect is due to the mixing of the ac signals produced in the channel by the two antennas. We obtain the helicity-dependent part of the photoresponse and its dependence on the antenna impedance, gate length, and gate voltage.
Development of physical principles of THz-wave amplification and oscillation is one of problems determining progress in modern solid state electronics towards high frequencies and ultrahigh performance. Novel perspectives are tied with use of resonant tunneling quantum effects, characterized by transient times less than 1 ps, comparable with fast response of superconducting devices. The information about these properties can be obtained from investigation of high-frequency oscillations or current-voltage switching phenomena in resonant-tunneling (RTD) nanostructures. In the paper the results of theoretical and experimental studies of high-frequency properties of RTD elements in subterahertz and terahertz frequency range are presented basing on developed theory of high-frequency response in RTD as well as on experimental high-frequency investigation data and current-voltage switching phenomena investigation results of effects correspondingly related to stationary current characteristics changes in single-quantum-well as well as in doublequantum- well resonant-tunneling diode nanostructures under external electromagnetic electrical field.
Multi quantum-well long-period structures are promising for a number of important applications including the far infrared intersubband-transition-based narrow-band radiation devices, microwave resonant-tunneling and self-sustained current oscillation generators, multilevel-logic element devices based on the recently found switching effect between the multistable current states, terahertz emission detectors. All devices have in common the operation dependence on resonant-tunneling rearrangement effects in the long-period structure. We present the results of optical investigation of
resonant-tunneling rearrangement processes in long-period GaAs/AlGaAs superlattice structures under application of vertical electric field by means of low-temperature photoluminescence (PL) technique in comparisons with the data of vertical transport measurements performed simultaneously on the same structures. The effect of appearance of the new PL peaks accompanied by suppression of the old ones with increasing bias voltage has been detected, resulting from the Stark shift phenomenon. PL intensity dependences on the applied voltage are presented for the first time which complement the measured current-voltage data. The transition effect from bound (exciton) to free (electron and hole) states in electric field is observed. It is shown that the optical research method can be more sensitive in some situations to provide the crucial information about the resonant-tunneling rearrangement effects even under condition when the ordinary current-voltage measurements do not reveal any features.
Photoluminescence technique is developed for characterization of resonant-tunneling diode structures formed of the GaAs/AlGaAs long-period superlattices in process of fabrication, which allows to estimate quality of the fabricated structure after the main stages of the technological process, including the MBE growth of multi-layer structure, lithography and annealing. The long-period multiquantum-well structures are promising for development of a new kind of solid-state intersubband-transition devices emitting the narrow band radiation in far infrared. This PL technique permits the corrections of the technology parameters to grow the structures with required properties and high homogeneity and can be used at room temperature as well as at low temperature.
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