Yu. H. Avetisyan

Optical terahertz wave generation in a planar GaAs waveguide.

We report generation of terahertz (THz) radiation in a planar 61-microm-thick GaAs waveguide with a TM0 propagation mode, achieved by phase-matched difference frequency mixing. The THz output was centered near 2 THz and had 1 microW average power. As a pump source we utilized both the signal and the idler outputs of a near-degenerate type II synchronously pumped optical parametric oscillator operating near 2 microm with the average powers of 250 and 750 mW, correspondingly.

Analysis of plasmon terahertz waveguide formed by a dielectric ridge on semiconductor surface

We study, by the method of the effective index of refraction (EIR), the characteristics of plasmon terahertz waveguide formed by a quartz ridge above the surface of indium antimonide. EIR, propagation length, field distribution, and the condition of single-mode propagation are calculated for the fundamental TM00 mode at different sizes of the quartz ridge. It is shown that, as distinct from usual plasmon waveguides, high degree of field localization and long propagation length can be ensured simultaneously by optimum choice of ridge sizes. Comparison of field distributions calculated by the EIR technique and with COMSOL Multiphysics software indicates a good agreement, especially for not very small sizes of the ridge.

Efficient generation of terahertz pulses in lithium niobate crystal with integrated matching prism

We present results of experimental studies of Cherenkov-type generation of a terahertz (THz)-pulse during propagation of a femtosecond laser pulse in LiNbO3 crystal with a Si-prism output coupler. Application of the Si-prism allows reducing essentially the damping of THz radiation because of shortening of the path of THz pulse in the nonlinear crystal. Results of experimental studies agree sufficiently well with theoretical calculations.

Investigation of parameters of terahertz pulses generated in single-domain LiNbO3 crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.

Analysis of generation of terahertz radiation by nonlinear mixing of laser frequencies in GaAs crystal

Generation of terahertz (THz) radiation by nonlinear mixing of laser frequencies in GaAs crystal is considered for the case of free and guided propagation of THz waves. In the first case the strong diffraction of the THz radiation leads to the deviation from the known square-law growth of the generator power with increasing crystal length, In the second case the spatial divergence of the exciting laser beam results in the appearance of a maximum in the generated power dependence on the radius of the nonlinear waveguide. According to estimations, the optimal radius of the laser beam is 18 μm for the waveguide length of 6 mm and the maximal generated THz power is ∼27 W at the laser beam power of 10 kW.

Analysis of THz-wave difference-frequency generation in two- dimensional domain-inverted structure

We present theoretical and experimental results of surface-emitted THz-wave difference frequency generation (DFG) in 2 dimensional (D) periodically poled lithium niobate (PPLN) crystal. The two orthogonal periodic structures compensate the phase mismatch in two mutually perpendicular directions of the optical and THz-wave propagation. The tunable 1.5 - 1.8 THz wave generation with impulse power of 0.1 mW and repetition rate 1 MHz is obtained. The opportunity of the power enhancement using an interference of THz fields generated by the both forward and backward propagating optical waves is investigated. The power and radiation pattern of generated of THz-wave are calculated in far-field approximation. Analysis of THz-wave DFG in 2D PPLN and its correlation with experimental data is presented.

Radiometric millimeter-wave imager with frequency scanning antenna

Millimeter wave passive radiometric imager based on frequency scanning antenna is suggested. As a frequency scanning antenna, the periodically perturbed dielectric image waveguide is considered. In frequency range 30÷38 GHz, the scan of the main lob over 40° has been realized. Parallel multibeam forming is realized by means of the set of narrow bandwidth resonant modulators, each of them controlled by the own modulating frequency.

Approach for THz-wave difference frequency generation in surface emitting geometry

Summary form only given. Difference frequency generation (DFG) is a convenient technique to produce coherent THz radiation, although typically the efficiency of frequency conversion is low due to large damping of THz-wave in nonlinear crystal. In addition, it is impossible to increase the efficiency of THz-wave DFG by using high concentrations of laser radiation in an optical waveguide, as the thickness of optical waveguides is much less than the wavelengths of emitted wave, and therefore the output. THz-beam will have a very high divergence. These problems can be overcome by using the surface-emitting geometry. However offered earlier methods of surface-emitted THz-wave DFG are useless for organic nonlinear materials. One of best examples is 4-dimethylamino-N-methyl-4-stilbazolium-to-sylate (DAST) having both a great nonlinear coefficient and large THz-wave absorption. Recently reproducible methods for preparation of DAST optical waveguides have been established. Thus it is interesting to develop a scheme for surface-emitted DFG in DAST waveguide. In present report we offer a method of surface-emitted DFG based on the applying of metal grating (with a specific period) on the surface of a waveguide cover.