Svetlana Koshevaya

Frequency multiplication of terahertz radiation in the crystals of strontium titanate paraelectric

In this article we have investigated frequency multiplication in the crystals of strontium titanate paraelectric at a temperature of 77 K. Frequency dispersion affects the process of harmonics generation. It has been shown that the efficiency of higher harmonics generation is high and it is equal to 30%. One can perform the selective extraction of certain harmonic by means of an optimal choice of the crystal length.

Modulation instability of transversely limited electromagnetic waves of terahertz range in strontium titanate paraelectric

Inthe framework of developed approximate phenomenological model we have performed the investigation of modulation instability of transversely limited electromagnetic waves of terahertz range in paraelectric crystals SrTiO3 at the temperatures of about 77 K. Cubic nonlinearity and frequency dispersion correspond to the existence of modulation instability of long input pulses. The results of numerical investigations of the modulation instability are presented. Transversal boundedness of the input pulses can stabilize the modulation instability. Modulation instability threshold is reduced in the presence of reflections from the crystal boundaries. In the case of development of the modulation instability it is possible to generate a sequence of short terahertz pulses at the output of the crystal. The focusing of input pulses reduces the threshold of modulation instability.

Amplification of optical phonons in narrow band semiconductors at low temperatures

Terahertz radiation generation by using the natural modes of solid body has been investigated. The numerical simulation of instability of terahertz range optical phonons in semiconductor structures with quantum wells during the drift of two-dimensional electron gas was performed. The main obstacle of implementing the specified instability is the heating of electron gas during its drift. That is why the investigations were performed for low temperatures T < 77 K, and the electron gas was assumed to be degenerate. Due to the existing closed cycle microcoolers and also the feedback implementation, it is possible to experimentally observe the generation of optical phonons. The numerical simulation involved the use of both kinematic and hydrodynamic approaches. It has been shown that the kinetic approach is more adequate, while the hydrodynamic one leads to overestimated values of instability increments.

Amplification and nonlinear interaction of space charge waves of microwave band in heterogeneous gallium nitride films

Theoretical study of amplification and nonlinear interaction of space charge waves caused by negative differential conductivity in gallium nitride n-GaN films placed over a semi-infinite substrate is presented. A general case of transversally heterogeneous films is considered. Hydrodynamic diffusion-drift equations for bulk electron concentration together with Poisson equation are used. Transversal heterogeneity leads to decreased electronic mobility near the film’s surface and to decreased space amplification increments of space charge waves. Heterogeneous alloying may compensate the influence of surfaces on space amplification increments. Amplification of space charge waves in n-GaN films of sub-micron thickness on higher frequencies f ≥ 100 GHz is possible in comparison to n-GaAs films. High output electric fields ∼10 kV/cm can be obtained in the short-wave range of the millimeter band.Nonlinear interaction of space charge waves in n-GaN films is also considered using the diffusion-drift equation. The possibility of generating second and third multiples of input signal as well as amplification of combination frequencies is proven.

Integrated silicon p–i–n structures with highly doped p++, n++ regions for modulation in terahertz frequency band

Modulators of terahertz range on the base of silicon integrated p-i-n-structures are investigated theoretically. The generalization of the Fletcher boundary conditions at the injecting contacts has been put forward for the case of highly doped p++, n++ regions, where both forbidden gap narrowing and dependence of coefficients of diffusion on doping concentration are taken into account. The problem of double injection into i-region has been simulated in a two-dimensional case. The investigations of modulation properties of integrated p-i-n-structures in the terahertz range have demonstrated a possibility to use these structures up to the frequencies 8 THz.

Generation of short light pulses under the stimulated Brillouin scattering in fibers with an optimal feedback

The generation of short light pulses (≤1 ns) in single mode fibers under pumping by wide laser pulses (of a microsecond duration) due to the backward stimulated Brillouin scattering (SBS) is numerically investigated. The influence of the acoustic diffraction is taken into account. The cases of acoustic waveguide and anti-waveguide fibers are considered. For an acoustic anti-waveguide fiber, a dependence of overlap integral S on the acoustic mode number n has a sharp peak in the region of n ∼ 100. Computer simulations have demonstrated the energy conversion of the pump wave into short pulses of the signal (Stokes) wave in the case of synchronous pumping. The optimal length of the fiber should be approximately equal to the half-length of the pump pulse. The bypass time of the Stokes pulse of the optimal circuit fiber and the feedback loop must be equal to the repetition period of the pump pulse. An importance of acoustic mode structure of the fiber for the process of forming pulse train in shown. We have found that the acoustic anti-waveguide fibers with a small core (a < 3 μm) can be preferable for obtaining the stable train of compressed pulses.

Self-Action of Intense Millimeter Waves in Waveguides with Integrated P-I-N Structures

The nonlinear interaction of high power millimeter (mm) electromagnetic waves with silicon integral p-i-n structures placed in a metal waveguide is theoretically investigated. The level of double injection of charge carriers due to detection of high intensity millimeter wave electric field in p-i-n structures is estimated. A mathematical model of the mutual influence of electromagnetic waves and injected charge carriers in the active region of p-i-n structures is formulated. A numerical solution of the nonlinear Helmholtz equation supplemented by proper boundary conditions on the active region boundary is obtained. The effect of high-power electromagnetic waves leads to an excessive injection of carriers into the active region of the semiconductor between p+-i, n+-i injection junctions and redistribution of the electric field in the structure. The reflection and transmission coefficients vary rapidly with the change in the input amplitude of the electromagnetic wave. This leads to bistability of these coefficients. The bistability is more pronounced in the low-frequency part of the mm range.

Design and simulation of a MEM pressure microgripper based on electrothermal microactuators

Design and simulation of a novel pressure microgripper based on Microelectromechanical, MEM technology, and composed by several electrothermal microactuators were carried out in order to increment the displacement and the cutoff force. The implementation of an element of press or gripping in the arrow of chevron actuator was implemented to supply stability in the manipulation of micro-objects. Each device of the microgripper and its fundamental equations will be described. The fundamental parameters to understand the operation and behaviour of the device are analyzed through sweeps of temperature (from 30 °C up to 100 °C) and voltage (from 0.25 V up to 5 V), showing the feasibility to operate the microgripper with electrical or thermal feeding. The design and simulation were development with Finite Element Method (FEM) in Ansys-Workbench 16.0. In this work, the fundamental parameters were calculated in Ansys-Workbench. It is shown, that structural modifications have great impact in the displacement and the cut-off force of the microgripper.

COMBINED METHOD FOR SIMULATING ELECTRON SPECTRUM OF δ-DOPED QUANTUM WELLS IN N-SI WITH MANY-BODY CORRECTIONS

The combined method to investigate the electron spectrum of single n-type δ-doped quantum wells in silicon is proposed. It is based on computing the electron potential energy by means of the Thomas-Fermi method at finite temperatures; then the obtained potential energy is applied to the iteration procedure with solving the Schrödinger equations for the electron spectrum and the Poisson one for the potential energy. The combined method demonstrates rapid convergence. It is shown that that the simple TF method gives a good approximation for the electron potential energy and for the total electron concentration within the well.

Superheterodyne amplification of electromagnetic waves of optical and terahertz bands in gallium nitride films

Superheterodyne amplification of electromagnetic waves of optical and terahertz bands in the case of three-wave interaction in n-GaN films with the space change wave of millimeter band amplified due to negative differential resistance is studied. It is shown that amplification of the space change wave in n-GaN films may be achieved on higher frequencies f ≤ 500 GHz than when using GaAs. The case of three-wave resonant interaction of two counter-propagating waves with the space charge wave is considered for the waveguide on based on GaN film on dielectric substrate. It is shown that gain of electromagnetic waves of optical band may reach 20–40 dB on the waveguide lengths of up to 100 μm.