Z. D. Grujić

Enhancement of electromagnetically induced absorption with elliptically polarized light - laser intensity dependent coherence effect

Using the (2)S(1/2)F(g) = 2 --> (2)P(3/2)F(e) = 3 transition in (87)Rb vapor at room temperature, we study effect of the laser light polarization on the electromagnetically induced absorption (EIA). This work extends the recent study of the behavior of the EIA as a function of the laser ellipticity (Brazhnikov et. al., JETP Lett. 83, 64, 2006). We have shown that such behavior strongly depends on the laser power. For the low laser power EIA amplitude has maximum for linearly polarized light, while for high laser power elliptically polarized light of ellipticity 15-20 degrees generates maximum of the EIA amplitude. EIA width varies slowly with the laser ellipticity at lower laser power, and much stronger at higher laser power. Through our theoretical model we attributed observed results to combined effect of the laser ellipticity and power on the population of ground state Zeeman sublevels.

On non-vanishing amplitude of Hanle electromagnetically induced absorption in Rb.

Amplitude and linewidts of the Hanle EIA, obtained from transmission of the laser locked to closed F(g) ? F(e) = F(g) +1 transitions in (85)Rb and(87)Rb, have maximum values at few mW/cm2. Amplitude of the EIA reaches steady value different from zero for higher laser intensities, even for laser intensities of 40 mW/cm(2). Theoretical model of EIA, for the same atomic system as in the experiment, show that the laser intensity, at which maximum of amplitudes and widths occur, depends on the laser detuning. For smaller laser detuning of a few tens of MHz, EIA has a maximum and then vanishes at higher laser intensities. For larger laser detuning of the order of hundreds MHz (but still in the range of Doppler broadening) amplitude of the EIA has very broad maximum and remains above zero for intensities above 40 mW/cm(2). Such theoretical results indicate that Hanle absorption peak remains in the experimental results, regardless of the laser intensities, due to Doppler effect.

Ramsey effects in coherent resonances at closed transition Fg = 2 → Fe = 3 of 87Rb

Experimental and theoretical investigations show the strong effect of the pump beam, spatially separated from the probe beam, on the probes electromagnetically induced absorption (EIA) and nonlinear magneto-optical rotation (NMOR). Linearly polarized pump and probe laser beams are locked to the Fg = 2 → Fe = 3 transition of the 87Rb D2 line and pass a vacuum Rb gas cell coaxially. We show that the observed narrowing of EIA and NMOR resonances is due to the Ramsey effect. Linewidths of the resonances decrease when the size of the dark region between pump and probe lasers increases. Variation of the angle between pump and probe linear polarizations strongly influences the phases of atomic coherences generated by the pump beam and consequently the line-shapes of the probe EIA and NMOR resonances. Complete change of the resonance sign is possible if the phases of the ground state coherences, Δmg = 2, are altered by π. The central EIA fringe becomes less pronounced if the probe intensity increases, due to the larger probe contribution to atomic evolution. Ramsey-like interference is a manifestation of the evolution of ground state Zeeman coherences, required for EIA, in the dark region in the presence of a small magnetic field.

Numerical simulation of Raman resonance due to the Ramsey interference induced by thermal motion of atoms

We use the optical Bloch equations to calculate spontaneous emission and Zeeman coherences of the probe laser beam in coaxial pump–probe configuration in rubidium (Rb) vapor. To properly model the open Fg=2→Fe=1 transition, the solution of the time-dependent optical Bloch equations was necessary. Also, due to the nature of a room temperature Rb vapor, we performed averaging of the total population of the excited Fe=1 state over different atomic velocities according to the Maxwell–Boltzmann distribution and over different directions of propagation of atoms through pump and probe laser beams. Time-dependent diagonal and nondiagonal density matrix elements for atoms are presented and discussed.

Nonlinear magneto optical rotation narrowing due to Ramsey effect induced by thermal motion of atoms

Nonlinear magneto-optical rotation (NMOR) of linearly polarized light, resonant to the Fg = 2 → Fe = 1 atomic transition of D1 line of 87Rb is investigated experimentally, by applying spatially separated laser fields in the Rb vacuum cell. In our experiment, the probe laser beam propagates through the center of the pump laser beam, which has the shape of a hollow cylinder. We have measured the rotation of the probe laser beam polarization with respect to the incident polarization, as a function of the external magnetic field, parallel to the laser beam propagation. We show substantial narrowing of the resonance if the pump beam, with the same linear polarization, is present. The opposite direction of the probe rotation was obtained when the pump laser beam polarization is orthogonal to the probe laser beam polarization.

Absorption of elliptically polarized light in closed transitions of Rb vapor

We have studied experimentally and theoretically Hanle EIA on the closed Fg = 3 goes to Fe = 4 D2 transition of 85Rb. Atoms were excited by the laser light with different polarization, from linear to circular. Results of the theoretical calculations of the laser transmission, based on optical Bloch equations for density matrix elements for the same atomic system, show important effects of magnetic field transverse to the laser light. We were not able to control transverse magnetic field in the experiment but the theoretical results are still applicable to our experimental data due the presence of laboratory stray magnetic field in the vicinity of the Rb cell, occurring because of imperfect magnetic shielding.

Non-linear magneto-optical polarization rotation in dark and bright coherent states

We have measured non-linear Faraday effects for dark and bright long-lived Zeeman coherences in the D2 resonance line of 85Rb, for linearly polarized input laser light. Zeeman coherence in the bright state show enhanced nonlinear effects, larger values for the angle of polarization rotation and higher rates of the polarization rotation, similar to recent results for higher refractive index and dispersion in the bright state of Cs 1. Dark and bright states also show different dependence of both polarization rotation angle and of polarization rate on the laser intensity, while the resonance width for both states varies similarly with the laser intensity. We show small enhancement of such non-linear effects when the laser beam diameter increases from 1 mm to 5 mm. Ellipticity of transmitted laser beam vary strongly for amplitudes of the external magnetic field smaller then 50 mG, but the maximum value of the ellipticity remains below &pgr;/300.