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

In this work we presented the results of numerical analysis of higher order nonlinear Schrodinger equation (NLSE) performed by the method of lines (MOL). In the higher order NLSE we take into account the imaginary Raman term. It is studied the impact of the dissipative Raman term on the femtosecond first order soliton stability. According to the results of numerical simulations, when real part of Raman term is equal to zero in the wavelength range, where second and third order dispersion negative, there exist analytic solitary-wave solutions. Our numerical simulations closely follow the published analytical data.

We extend the study of Kapitza-Dirac diffraction in Raman-Nath regime removing the limitations put on resonance detuning. Discussing the case of extended Raman-Nath regime we develop an approximation for the case of strong standing waves which will correspond to the well known tight-binding approximation for periodic structures. Due to internal structure of the diffracting particles the system eigenvalues and eigenfunctions show important differences from the generally known cases.

Kapitza-Dirac scattering of a two-level atom is studied in Raman - Nath approximation without any restriction on the value of resonance detuning. The subject of interest, atom scattering amplitude, is obtained in form of a definite integral and is generalised in case of quantum superposition of initial momentums. It is shown that the discrete Gaussian form of the latter yields promising diffraction spectrum for the atom interferometry.

Intense, well-controlled regular light pulse trains start to play a crucial role in many fields of physics. We theoretically demonstrate a very simple and robust technique for generating such periodic ultrashort pulses from a continuous probe wave which propagates in a dispersive thermal gas media.

The coherent response accumulation method is used to assess the ability to amplify the polarization rotated component of a probe beam about two-photon resonance conditions. Atomic gas of alkaline metal atoms is chosen as a medium of interest. Probe wave connects the atomic ground state nS1/2with the excited doublet nP1/2,3/2, which in its turn is coupled to the higher excited level n'S1/2by an intense pump field. The relaxation processes and the Doppler effect are also included into the theory. It is shown that sequentially changing the input probe field's phase from 0 till and vice-versa, one can get about 2 times narrowing and more than 14 times amplification of intensity for the final part of rotated component of the probe wave.

Collagen is the major component of skin, tendon, cartilage, cornea, and, as a main structural protein it is the key determinant of thermo-mechanical properties of collagen-rich tissues in mammals. Thermal damage of chicken dermis and tendon, bovine leg tendon, and other collagen contained tissues were investigated with the use of second harmonic generation (SHG) and two-photon excited auto-fluorescence microscopy and spectroscopy. Samples were heating in a temperature-controlled water bath in the temperature range 18-90° C. SHG time-lapse imaging and analysis of intensity decay showed that the collagen thermal destruction depended on both temperature and heating time, and can be modeled by the Arrhenius equation. Temporal decay of SHG signal from the chicken dermis was single exponential during isothermal treatment at temperatures above 60º C that allowed to determine activation energy and frequency factor of skin collagen denaturation. Furthermore, two-exponential decay and partially reversible change in SHG intensity were registered during the tendon thermal treatment. A simple laser system and procedure is proposed for a real-time monitoring of collagen fiber thermal modification within a microscopic volume of 1 nl.

Optical-based microscopy plays an important role in various scientific fields such as physics, chemistry and biology. Second harmonic generation (SHG) microscopy has become one of the indispensable tools for biomedical imaging for the last decade because the signal generated from SHG is sensitive to the objective structure and this amazing non-invasive method can also directly observe the objective without using extra fluorescent labels, especially for collagen molecules. As the most abundant protein in animals, collagen is responsible for a number of important structural and functional roles in vertebrates. For certain diseases, it has been shown that collagen fiber diameter has a significant variation and thus as a vital symptom for diagnosis. Moreover, collagen diameter is also a key parameter for fibrogenesis studying. Therefore, the determination of collagen fiber diameter is important for studying biophysical processes and identifying bioengineering applications. In this study, we investigated various collagen fibril thicknesses and the corresponding forward (FSHG) and backward (BSHG) second harmonic signal intensity variation. Our result exhibits that SHG intensity can quantify describe the relative collagen fibril thickness alteration, which also indicates the coherent effect difference between FSHG and BSHG. This approach demonstrates the capability of SHG imaging in providing collagen mechanical information and that may be applied in the evaluation of advancing collagen issues in vivo.

The problem of Kapitza-Dirac diffraction is solved in the Raman-Nath approximation without limitations on resonance detuning. A formula for the scattering amplitude in a definite integral form is obtained. It shows that in the case of initial superposition state of discrete Gaussian form the scattering spectrum has a new regularity, more usable for atomic optics.