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Journal of Contemporary Physics (Armenian Academy of Sciences)

Increase of probe beam light-induced polarization rotation via accumulation of coherent oscillations

M. H. Hovhannisyan, A. Zh. Muradyan

Published Aug 24, 2011• 1 citation
Publisher

Abstract

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.

DOI
10.3103/s1068337211050069

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Optics and Spectroscopy•Sep 1, 2017•1 citation
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.
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.
The nonstationary pump-probe interaction between short laser pulses propagating in a resonant optically dense coherent medium is considered. Special attention is paid to the case where the density of two-level particles is high enough that a considerable part of the energy of relatively weak external laser fields can be coherently absorbed and reemitted by the medium. Thus, the field of the medium reaction plays a key role in the interaction processes, which leads to collective behavior of an atomic ensemble in the strongly coupled light-matter system. Such behavior results in fast excitation interchanges between the field and a medium in the form of optical ringing, which is analogous to polariton beating in solid-state optics. This collective oscillating response, which can be treated as successive beats between light wave packets of different group velocities, is shown to significantly affect the propagation and amplification of the probe field under its nonlinear interaction with a nearly copropagating pump pulse. Depending on the probe-pump time delay, the probe transmission spectra show the appearance of either a specific doublet or coherent dip. The widths of these features are determined by the density-dependent field-matter coupling coefficient and increase during the propagation. Besides that, the widths of the coherent features, which appear close to resonance in the broadband probe spectrum, exceed the absorption-line width, since under the strong-coupling regime, the frequency of optical ringing exceeds the rate of incoherent relaxation. Contrary to stationary strong-field effects, the density- and coordinate-dependent transmission spectra of the probe manifest the importance of collective oscillations and cannot be obtained in the framework of a single-atom model.
Experimental and numerical investigation of single-beam and pump-probe interaction with a resonantly absorbing dense extended medium under strong and weak field-matter coupling is presented. Significant probe beam amplification and conical emission were observed. Under relatively weak pumping and high medium density, when the condition of strong coupling between field and resonant matter is fulfilled, the probe amplification spectrum has a form of spectral doublet. Stronger pumping leads to the appearance of a single peak of the probe beam amplification at the transition frequency. The greater probe intensity results in an asymmetrical transmission spectrum with amplification at the blue wing of the absorption line and attenuation at the red one. Under high medium density, a broadband of amplification appears. The theoretical model is based on the solution of the Maxwell-Bloch equations for a two-level system. Different types of probe transmission spectra obtained are attributed to complex dynamics of a coherent medium response to broadband polychromatic radiation of a multimode dye laser.
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 effect of separation of linearly polarized short probe pulses of electromagnetically induced transparency in the field of linearly polarized coupling radiation is modeled numerically. It is shown that the input-probe pulses polarized parallel or perpendicular to the input-probe field propagate in the medium without changing the state of their polarization. If the input-probe radiation is weak compared to the coupling radiation, then the probe field inside the medium is the sum of two independently propagating linearly polarized normal modes, which are excited by the projections of the input-probe pulse onto the direction of polarization of the coupling radiation and onto the perpendicular direction, respectively. The normal modes have the same phase velocities, but different velocities of their real envelopes. This circumstance leads to the rotation of the plane of polarization of the total probe field at short distances and to its separation into two pulses with mutually perpendicular directions of polarization at long distances. At a high intensity of the probe radiation, the input-probe pulse decays into pulses the planes of polarization of which are not mutually perpendicular. Under these conditions, it is impossible to represent the probe radiation as a sum of normal modes. The modeling is performed in the scheme of degenerate quantum transitions between states of levels 3 P 0, 3 P 1 0, and 3 P 2 of isotope 208Pb taking into account the Doppler broadening of spectral lines.