D. L. Hovhannisyan

Theoretical investigation and computational modeling of the difference frequency generation by interaction of few cycle laser pulses in a GaAs crystal

We present the results of theoretical studies of the generation process of difference frequency radiation arising via interaction of mutually orthogonal linearly polarized few-cycle laser pulses propagating in an isotropic nonlinear medium. Numerical time-integration by the finite-difference method of nonlinear Maxwell-equation systems has been performed. We consider the interaction of pulses having the central wavelengths of 1.98 and 1.55 µm, duration of 30 fs with the corresponding electric field amplitudes of 295 × 106 and 463 × 106 V m−1, propagating along the normal to the ⟨110⟩ plane in 854 µm thickness of GaAs crystal. The process of difference frequency pulse formation arising via spectral filtration of a supercontinuum formed in the spectra of pump pulses at the output of a nonlinear crystal is studied.

Bunched electron beam properties measurement by means of single-shot multibeam cross-correlation technique

A new single-shot multibeam cross-correlation technique is proposed for the measurement of a bunched electron beam temporal structure. As an optical laser pulse modulated by the Coulomb field of the electron bunch in an electro-optical crystal, it is proposed to use a non-uniform pulse modulated both in time and in space. In the proposed technique, measurement of the time profile is realized during a single laser shot. In this case, as a result of electro-optical modulation of a non-uniform femtosecond pulse, the information on the time profile of the Coulomb field is contained in only the spatially shifted spectral components of the modulated pulse.

Generation of difference-frequency radiation in GaAs in the field of a few-cycle laser pulse in the regime of weakly expressed chromatic dispersion

We derive a system of coupled modified Korteweg-de Vries equations in the approximation of unidirectional waves which describe the evolution of electric fields of mutually-orthogonal linearly polarized laser pulses with duration of several optical oscillations in a nonlinear crystal with quadratic nonlinearity in the regime of weakly expressed chromatic dispersion. An isotropic GaAs crystal is considered as a nonlinear crystal, and the direction of propagation of interacting pulses is assumed to coincide with the normal to the 〈110〉 plane of the crystal. The derived system of equations was solved numerically by the finite difference method. We obtained the dependence of the relative shift of the central wavelength of pumping pulse spectrum on the distance travelled. It is shown that the decrease in the crystal thickness down to values commensurate with the coherence length of difference-frequency radiation leads to an increase in conversion efficiency. In particular, it is shown that the efficiency of difference-frequency generation at 4.1 μm and 10 μm for the 30 fs duration of pumping at 1.98 μm and the 100 MV/m amplitude of the electric field is −51 dB and −14 dB, respectively.

Influence of the Voltage Applied to the Semiconductor Substrate on Birefringence of Nematic Liquid Crystal

The influence of longitudinal voltage, applied along semiconductor substrate of planar-oriented liquid-crystal cell, on the birefringence of nematic liquid crystal (NLC) has been studied. It is shown that with the fixed value of transverse voltage the application of longitudinal voltage creates an additional possibility to control the electro-optically induced phase shift of light passing through NLC. The dependence of phase shift vs. control voltages is obtained. The results of simulation of the longitudinal voltage influence on the process of NLC reorientation are given. The obtained results can be used for development and fabrication of liquid-crystal phase modulators.

Modeling of difference frequency radiation energy redistribution process in a spectrum of laser pulse of a few optical cycles propagating in a quasi-phase-matching GaAs crystal

We present the results of a numerical study of the generation process of difference frequency radiation (DFR) arising via the interaction of mutually orthogonal linearly- polarized few-cycle laser pulses propagating in a quasi-phase-matching (QPM) GaAs crystal. Considered the interaction of pulses having the central wavelengths of 1.98 µm, duration of 30 fs with the electric field amplitude 200 MV/m, propagating along the normal to the ⟨110⟩ plane. The period Λ of the QPM GaAs crystal in numerical simulations varied from 20.89 µm to 53.23 µm. It is shown that by changing the grating period of the QPM GaAs crystal from 23.02 µm to 37.29 µm it is possible to improve the efficiency of QPM generation of DFR in the 5.48 µm – 10.12 µm spectral range at least by 8 dB in comparison with the generation of DFR in bulk crystal GaAs in the same spectral range.

Some Peculiarities of the Recording of Polarization Diffraction Gratings by Doubling Method

We investigated the dependence of angular dispersion of replica on its distance from the master polarization diffraction grating (PDG). The distribution of field diffracted on master PDG was calculated using the method of finite-difference time-domain. According to the numerical model, within the transition region, conditioned by interference of secondary waves with the same polarization state, the angular dispersion of replica changes depending on the distance between master and replica. Outside the transition region, the angular dispersions of replica and master completely coincide. The experimental investigation was also carried out, the results of which are in good agreement with the calculated data.

Difference frequency generation by optical rectification of femtosecond laser pulse in GaAs crystal with a periodic domain structure

We study the influence of spatial boundedness of a femtosecond laser pulse (FLP) on the efficiency of generation of difference frequency radiation (DFR) in a GaAs crystal with a periodic domain structure. It is shown that at propagation of DFR a spectral-angular filtration occurs in the transverse distribution of the beam. An expression is obtained for the frequency-angular spectrum of DFR. For a pump pulse with the duration of 100 fs, the beam radius of 24μm, the energy of 30 nJ, and the wavelength of 1.98μm propagating in 1.716-mm long GaAs crystal with the domain structure period of 74.6μm the efficiency of generation of DFR at the wavelength of 14μm is calculated to be close to 2×10−6.

Numerical simulation of propagation of a femtosecond optical soliton in a single-mode fiber with allowance for the imaginary part of Raman response

We consider the effect of Raman inertial response of a medium on the stability of a first-order femtosecond soliton. Numerical solution to the high-order nonlinear Schrödinger equation, with the complex Raman term, describing propagation of a femtosecond optical soliton in a single-mode fiber, is obtained. It is shown that a soliton solution of the high-order nonlinear Schrödinger equation exists under certain conditions imposed on the equation coefficients. These conditions lead to limitations on the wavelength, fiber type, and the highest energy. Results of numerical solutions are in agreement with available experimental data.

Pulse compression of difference frequency radiation by liquid crystal phase transparent

The results of theoretical research and numerical simulation of the difference frequency radiation (DFR) pulse generation in 5–25 μm wavelength range in the field of the pumping femtosecond laser pulse are presented. The pulse has the following characteristics: the central wavelength is 2 μm, duration is 34 fs and electric field amplitude is 70.71 MV/m. The pulse propagates in the ZnTe/air periodic structure with the number of periods along the normal to the (110) plane of the ZnTe crystal equal to 13; and the efficiency of the DFR generation is 1.11 × 10–4. It is shown that the use of the liquid crystal phase transparent, placed in the focal plane of the frequencyspatial shaping system, allows one to realize the DFR pulse compression at which the maximum intensity is increased by a factor of six.

Recording of geometric phase elements based on liquid crystal polymers

A technique is developed for recording the centrally-symmetric liquid crystal phase plates with the anisotropic orientation of molecules providing a smooth change of the optical axis in a thin film of liquid crystal polymer. The technique enables one to record the elements with the cylindrically-symmetric and planar-symmetric distributions. Such structures can be used to develop the optical elements with the new functional possibilities. Using the developed techniques, the polarization-sensitive Fresnel lens is realized which is functioning either as a collecting or as a scattering lens for the light beams with the orthogonal circular polarizations.