A theoretical investigation is made of optical excitation of an intrinsic direct-gap semiconductor in two limiting cases of slow and fast electron-electron relaxation. The dependences of the density and average energy of hot electrons on the pump energy are found. An equation describing the behavior of the electron temperature under optical excitation conditions is derived allowing for electron-phonon collisions in the case of fast electron-electron relaxation. An analytic solution obtained for the interaction with piezoelectric phonons shows that there is an optimal duration of a photoexcitation pulse which ensures the maximum temperature of hot electrons.
New spectroscopic schemes studying noncentrosymmetrical media (nonracemic solutions of chiral molecules) are based on investigation of frequency dispersion of the fourth-order dipole susceptibility tensor (chi) ijklm(4D)((omega) a;(omega) 1,(omega) 1,(omega) 1,-(omega) 2). This paper contains the accurate quantum-mechanical calculations of the resonance part of hyper-polarizability tensor (gamma) '(4D) and its connection with Raman ((alpha) '(2) and hyper-Raman ((beta) '(3)) scattering tensors. In the case it was suggested pump frequencies' combination (omega) 1 - (omega) 2 scanning one of the vibrational resonance of the molecule: (omega) 1 - (omega) 2 approximately equals (Omega) . Tensor (gamma) '(4D) was averaged over arbitrary orientations of noncentrosymmetrical molecules in the solution by applying irreducible tensors operators' technique giving dipole susceptibility tensor (chi) '(4D) equals NL4<(gamma) '(4D)>. Symmetry of this tensor coincides with the model calculations. As it was expected, tensor (chi) ijklm(4D)((omega) a;(omega) 1,(omega) 1,(omega) 1,-(omega) 2) includes only 6 non zero components equal by their absolute values. For centrosymmetrical molecules in isotropic medium tensor (chi) '(4D) is equal to zero. Similar computations were made in the hyper-Raman resonance case 2(omega) 1 - (omega) 2 approximately equals (Omega) .
A nonconductive isotropic chiral medium is described by using classical electromagnetic theroy. The phenomenon of nonlinear optical activity of a linearly polarized aignal light under the action of a circularly polarized pump light is investigated. The expressions of nonlinear optical rotational angle are given. The calculated results using the expressions are in accordance to the experimental results.
We theoretically investigate in the aberrationless approximation the self-action of the elliptically polarized Gaussian pulse during its propagation in a thin dish with a nematic liquid crystal in the isotropic phase. Quadrature formulas are obtained to describe the time history of the intensity, the elliptisity degree and the rotation angle of the polarization ellipse of the output radiation at the different points of the beam cross-section. They are expressed in terms of the parameters describing two, essential near the temperature of the isotropic-nematic phase transition, mechanisms of the spatial nonlocality of the nonlinear medium optical response, and in terms of the other parameters, which describe the nematic liquid crystal and the elliptically polarized incident pulse. The former mechanism is specified by the medium heating due to light absorption; the latter is determined by the fluctuations of the nematic liquid crystal order parameter tensor near the temperature of the isotropic-nematic phase transition. It is shown that for some values of temperature and of the nematic liquid crystal parameters the elliptical polarization of the incident pulse, which is constant at the entry of the thin dish, transforms into the linear or another different elliptical one at the exit and keeps this new state up to the pulse tail. The dependence of the ellipticity degree on time becomes significantly nonmonotonic and changes its sign in some cases. The nonlocality of the nonlinear medium optical response weakens these transformations.
A theoretical investigation is reported of the propagation of light waves in the course of frequency doubling as a result of a fourth-order dipole nonlinearity of an isotropic noncentrosymmetric weakly nonlinear and weakly absorbing medium characterised by a slight spatial dispersion in the case of noncollinear interaction between the waves. Calculations are carried out by expansion into normal waves, taking account of the self-interaction of a strong wave and of its influence on a weak one. The equations describing propagation of the second-harmonic wave in the noncollinear interaction configuration are derived by the method of slowly varying amplitudes. It is shown that an interference field is formed as a result of the interaction between the strong and weak waves. Consequently, the amplitude of the second-harmonic wave acquires a complex spatial distribution along transverse coordinates and this distribution depends on the wave mismatch. An analysis of the angular spectrum of the second-harmonic wave is given.
The results are given of a theoretical investigation of the interaction between two ultrashort pulses J1 and J2 of frequencies ω31 and ω21 with a three-level amplifying medium. It is assumed that the signal J1 of frequency ω31 is a steady-state π pulse and the signal J2 is weak so that its influence on the level populations can be ignored. The case when the signal J2 has a steady-state amplitude profile and an exponential gain is considered. The analysis is made ignoring the group delay effects and also allowing for the mismatch of the group velocities of the signals. The results obtained can be used in studies of the amplification of ultrashort pulses in molecular media such as organic dyes or CO2 molecules.
Characteristics of the interaction of a laser pulse with a molecular medium are analyzed theoretically in the case when vibrational relaxation takes place in the ground and excited electronic states. The diffusion approximation is used to describe the relaxation transitions. The same approximation is employed in an analysis of the establishment of the limiting steady-state energy of a pulse in a molecular laser amplifier with linear nonresonance losses.
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