A. S. Nikoghosyan

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.

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.

Efficient ultrashort light pulse conversion in GHz-THz pulses in ZnTe, GaAs, and DAST crystals

The potentiality to increase light pulse conversion efficiency into pulses of the millimeter and submillimeter (GHz - THz) range using the waveguide partially filled with a nonlinear crystal is suggested. This technique has been theoretically substantiated and experimentally studied. Phase matching is defined by the degree of partial filling. The calculated data as well as the experimental results of the difference frequency generation in LiNbO3, ZnTe, GaAs and DAST crystals are presented. DAST refractive index as well as its tn(delta) have been measured in the 70 - 900 GHz frequency range.

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.

Terahertz radiation generation in waveguide partially filled with nonlinear crystal

We report on the effective generation pulse of THz radiation via optical rectification of femtosecond laser pulses in a nonlinear crystal. By placing the same nonlinear crystal in the free space and then inside the waveguide the phase-matching and consequently time and spectral features of the emitted THz field are changed. The rise of conversion efficiency in the order of magnitude has been obtained in waveguide structures partially filled with LiNbO3 crystal.

Terahertz generation in waveguide partially loaded with nonlinear crystal

We report on the studies of kinetics of broad-band generation via optical rectification into terahertz domain using a sample of LiNbO3 partially filling a hollow metallic waveguide to increase the coherence length, enabling the observation of transition between phase-mismatched and phase-matched mechanisms of terahertz frequency generation.

Applications of terahertz spectral imaging-diagnostics in dentistry

We present a review on terahertz diagnostics in dentistry. Time domain and spectral domain imaging studies are compared. Imaging built on mapping time-domain dependencies delivers the information on mechanisms of light transport in tissue: ballistic, quasi-ballistic and diffusive photon transfer. Imaging utilizing transform in frequency-domain provides a wealth of data on specific characterization of object: spectral and chemical content, tissue structure and composition and various kind of tissue abnormalities.

Laser driven terahertz dielectric wedge antenna placed in free space or in hollow metallic waveguide

Results on effective generation of terahertz (THz) radiation in band 0.1–2.5 THz via optical rectification of femtosecond laser pulses in LiNbO3 plate tapered at the end are presented. It is shown that placing the LiNbO3 crystal in the free space and in the waveguide both the phase-matching and time and spectral features of the emitted THz field are changed.

Theoretical investigation and modeling of mid-infrared pulse generation in a GaAs crystal using 30 fs pulses from fiber-optical laser

The results of the THz radiation generation via interaction of 30 fs fiber-optical lasers pulses with the wavelengths of 1.98 and 1.55 μm in GaAs are presented. Numerical time-integration by the finite-difference method of nonlinear Maxwell-equation systems has been performed. The process of mid-infrared pulse generation obtained from spectral filtration of a supercontinuum formed in the pump-pulse spectrum at the output of GaAs is studied.

THz generation in lithium niobate crystal with tapered end placed within a hollow waveguide

The results of the THz pulse generation via optical rectification of pico- and femtosecond laser pulses in nonlinear crystal are presented. Detection of THz pulse are performed at room temperature by both Schottky diode and ZnTe crystal. The quantum efficiency of energy conversion up to 28 % of maximum value is obtained with Schottky diode.