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Journal of Contemporary Physics (Armenian Academy of Sciences)

Ultrashort pulse generation in the medium with induced optical anisotropy

M. H. Hovhannisyan

Published Apr 24, 2014• 0 citations
Publisher

Abstract

Abilities of the method of accumulation of coherent unstable response of a medium for amplification of the rotated component of the probe wave’s polarization in the atomic medium of three-level atoms are studied. Probe wave is examined in frames of the perturbation theory and its frequency is considered near to one photon resonance from the ground to the first excited level. The intense monochromatic pump field connects two excited energy levels and propagates through the medium without a change. The broadening of spectral lines caused by the relaxation processes and the Doppler broadening are included into the calculations. It is shown that by multiple consistent instantaneous change of input probe field’s phase from 0 to π and back, it is possible to obtain, in the tail part of the rotated component of the probe field, a separate narrowed light pulse along with the increase in intensity by more than an order of magnitude with respect to the input field.

DOI
10.3103/s1068337214030049

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Abilities of the method of accumulation of coherent unstable response of a medium for amplification of the rotated component of the probe wave’s polarization in the atomic medium of three-level atoms are studied. Probe wave is examined in frames of the perturbation theory and its frequency is considered near to one photon resonance from the ground to the first excited level. The intense monochromatic pump field connects two excited energy levels and propagates through the medium without a change. The broadening of spectral lines caused by the relaxation processes and the Doppler broadening are included into the calculations. It is shown that by multiple consistent instantaneous change of input probe field’s phase from 0 to π and back, it is possible to obtain, in the tail part of the rotated component of the probe field, a separate narrowed light pulse along with the increase in intensity by more than an order of magnitude with respect to the input field.
We study possibilities of the method of accumulation of the coherent-nonstationary response of the medium for amplifying the polarization rotation of probe radiation in conditions of a cascade two-photon resonance. The probe field couples the ground level nS 1/2 of an alkali metal atom with the excited doublet nP 1/2,3/2, while the monochromatic pump field couples this doublet with a higher excited state n’S 1/2. We obtain analytic expression for the rotated component of the probe field polarization. It is shown that consistent switching by π the phase of the input probe field may create in the tail portion of its rotated component a separate light pulse with more than an order of magnitude higher intensity, which is a desired result for polarization laser spectroscopy.
A theoretical investigation of the change in polarization of ultrashort pulses of light near the two-photon resonance is carried out. A strong wave of circular polarization in the medium induces optical anisotropy which affects the change in weak probe-wave polarization. It is shown, that owing to an interaction transient, the probe-wave polarization becomes elliptical whose parameters change depending on time and position. This leads, in turn, to oscillation dependence of the rotated component of the probe field amplitude on the pumping energy, and the length and density of the resonant medium.
The propagation of strongly polarized electromagnetic waves through a resonance medium, with arbitrary angular momenta of the levels, is investigated in the adiabatic-following approximation. Exact wavefunctions of the two-level atomic system with degenerate levels in the field of a circularly (linearly) polarized wave are constructed and the picture of the shift of energy level splittings is discussed. Exact formulae for the nonlinear refractive index of the medium in the field of the circularly (linearly) polarized wave are found. The propagation of counterpropagating polarized waves—an intense pump wave and a probe light signal—through the resonance medium is studied. The studies are carried out in the scope of a two-level (one-photon resonance) and a three-level (one- and two-photon resonances) medium. Formulae are found for the induced rotation of the plane of polarization for the probing signal that are exact in terms of the intensity of the pump wave. The spectral dependence of the angle of rotation is studied, and it is shown that a series of Stark-shifted resonance poles of atomic absorption and a three-photon scattering process arise.
The process of propagation of short probe pulses of electromagnetically induced transparency has been analyzed theoretically in the case of elliptically polarized control radiation. As a model of a resonant medium, a Λ scheme of quantum transitions between the 3P0^{3}{{P}_{0}}3P0​ , 3P10^{3}P_{1}^{0}3P10​ , and 3P2^{3}{{P}_{2}}3P2​ degenerate levels of the 208Pb isotope has been used. The situation in which the probe radiation is rather weak compared to the control radiation has been examined. In this case, the field of the probe pulse can be represented as a sum of the fields of two elliptically polarized pulses that propagate in the medium independently of one another without changing their polarization state, which makes it possible to interpret them as nonstationary normal modes. Numerical simulation has shown that the structure of normal modes depends on the ratio of the width of the spectrum of the input probe pulse to the Doppler frequency broadening of the quantum transition that is resonant to the probe field. If this ratio is small, each mode in the medium is a single pulse similar to the input probe pulse. Under these conditions, the propagation of each normal mode in the medium can be fairly well characterized by the group velocity depending on the intensity and the state of polarization of the control radiation. As this ratio increases, normal modes in the medium initially acquire the shape of a regular decaying train of pulses, and then their structure becomes chaotic. The described evolution is accompanied by an increase in the energy absorption of the probe radiation by the medium and by a significant deterioration in the applicability of the notion of group velocity for describing the propagation process of normal modes.
The main regularities of induced change of light polarization in the field of powerful ultrashort pulses in the resonant gaseous medium are investigated. Frequency- tunable ultrashort pulses allows to observe the polarization plane rotation at large values of detuning, in the different schemes of single-photon and two-photon interaction. On the other hand, the high intensity of these pulses allows to investigate the nonresonant interaction in the crystalline and solid transparent media. Our experiments carried out permit to propose the polarization technique for forming the given parameters pulses or train of pulses. In the observing medium placed between crossed polarizers, at counter propagation, powerful pump and linearly polarized probe pulses interact, because of which the probe pulse polarization changes. The polarization rotation angle as a time function is determined by the intensity of ultrashort pulse, the parameters of medium and frequencies of interacting waves. At the definite choice of the experimental conditions, requiring a probe wave after a crossed polarizer we can obtain the pulse which duration is determined by the length of medium, where interaction occurs. Using a nonresonant transparent media we have the possibility of forming the pulses of given duration tunable in a wide spectral region.