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Physica Scripta

Optical Ramsey fringes observed during temporal evolution of Zeeman coherences in Rb buffer gas cell

Stanko N. Nikolić, Milan Radonjić, Nemanja Lučić, Aleksandar J. Krmpot, B. M. Jelenkovic

Published Sep 1, 2014• 1 citation
Publisher

Abstract

We experimentally studied the temporal evolution of Zeeman electromagnetically induced transparency (EIT) resonances induced by the laser resonant to hyperfine transition of 87Rb in a rubidium buffer gas cell. We simultaneously modulated the laser beam intensity and polarization to achieve the repeated interaction of the laser beam with coherently prepared atoms. Our cell was placed in a homogenous magnetic field to obtain the Larmor precession of the phase of coherences. The weak laser beam was used to probe the atoms at the end of the Ramsey sequence. We measured the transparency of the probe pulse at different magnetic fields for a given excitation pulse and the period of free evolution of Zeeman coherences in the dark. From these data, we reconstructed the temporal evolution of EIT resonances. The Ramsey fringes that appeared on the EIT curves at the beginning of the second probing pulse disappeared at later moments due to various decay processes.

DOI
10.1088/0031-8949/2014/t162/014038

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We experimentally studied the temporal evolution of Zeeman electromagnetically induced transparency (EIT) resonances induced by the laser resonant to hyperfine transition of 87Rb in a rubidium buffer gas cell. We simultaneously modulated the laser beam intensity and polarization to achieve the repeated interaction of the laser beam with coherently prepared atoms. Our cell was placed in a homogenous magnetic field to obtain the Larmor precession of the phase of coherences. The weak laser beam was used to probe the atoms at the end of the Ramsey sequence. We measured the transparency of the probe pulse at different magnetic fields for a given excitation pulse and the period of free evolution of Zeeman coherences in the dark. From these data, we reconstructed the temporal evolution of EIT resonances. The Ramsey fringes that appeared on the EIT curves at the beginning of the second probing pulse disappeared at later moments due to various decay processes.
Abstract We investigate, experimentally and theoretically, time development of Zeeman electromagnetically induced transparency (EIT) during propagation of two time separated polarization laser pulses, preparatory and probe, through Rb vapour. The pulses were produced by modifying laser intensity and degree of elliptical polarization. The frequency of the single laser beam is locked to the hyperfine transition of the D 1 line in 87 Rb. Transients in the intensity of component of the transmitted light are measured or calculated at different values of the external magnetic field, during both preparatory and probe pulse. Zeeman EIT resonances at particular time instants of the pulse propagation are reconstructed by appropriate sampling of the transients. We observe how laser intensity, Ramsey sequence and the Rb cell temperature affect the time dependence of EIT line shapes, amplitudes and linewidths. We show that at early times of the probe pulse propagation, several Ramsey fringes are present in EIT resonances, while at later moments a single narrow peak prevails. Time development of EIT amplitudes are determined by the transmitted intensity of the component during the pulse propagation.
We experimentally studied the effects of laser beam diameter on electromagnetically induced transparency (EIT) due to Zeeman coherences induced by a laser resonant with the hyperfine transition Fg = 2 → Fe = 1 of 87Rb in a rubidium buffer gas cell. We use two laser beams of Gaussian intensity radial profile for laser beam diameters of 6.5 and 1.3 mm, laser intensities in the range of 0.1–35 mW cm−2 and cell temperatures between 60 and 82 °C. The results show that the amplitude of the normalized EIT resonance has a maximum at a laser intensity which depends on laser beam diameter and cell temperature. The laser intensity corresponding to the maximum EIT amplitude is higher for a smaller laser beam and higher cell temperature. The linewidth of Zeeman EIT resonance varies nearly linearly with laser intensity, almost independent of cell temperature and laser beam diameter.
Experimental and theoretical investigations show the strong effect of the pump beam, spatially separated from the probe beam, on the probe's 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.
We report the experimental observation of transient dynamics in the weak probe coherence under electromagnetically induced transparency (EIT) in hot Rb atoms with a buffer gas. By modulating the control beam in a non-adiabatic fashion, we have examined the corresponding changes in probe transparency. Upon switching the control beam on, the system evolves through multiple dynamical processes before reaching steady-state transparency. We have decoded each of those dynamical processes region by region and notably observed two-photon damped Rabi oscillations, Raman gain, and direct dark state rotation during this evolution. For the theoretical insight, we have numerically solved the time-dependent density matrix elements for a Λ system, incorporating the buffer gas effects phenomenologically into these equations. Our findings provide a detailed understanding of transient and steady-state dynamics in EIT systems, highlighting the interplay between coherent and incoherent processes and the role of the buffer gas in modulating the evolution. The buffer medium extended relevant timescales, thus enhancing the coherence and stability of the atomic system, which are key factors for practical implementations in precision metrology and quantum optics.
Electromagnetically induced transparency (EIT) due to Zeeman coherences in the Rb buffer gas cell is studied for different laser beam profiles, laser beam radii and intensities from 0.1 to 10 mW cm −2 . EIT line shapes can be approximated by the Lorentzian for wide Gaussian laser beam (6.5 mm in diameter) if laser intensity is weak and for a Π laser beam profile of the same diameter. Line shapes of EIT become non-Lorentzian for the Gaussian laser beam if it is narrow (1.3 mm in diameter) or if it has a higher intensity. EIT amplitudes and linewidths, for both laser beam profiles of the same diameter, have very similar behaviour regarding laser intensity and Rb cell temperature. EIT amplitudes are maximal at a certain laser beam intensity and this intensity is higher for narrower laser beams. The EIT linewidth estimated at zero laser intensity is about 50 nT or 0.7 kHz, which refers to 1.5 ms relaxation times of Zeeman coherences in 87 Rb atoms in our buffer gas cell. Blocking of the centre of the wide Gaussian laser beam in front of the photo detector yields Lorentzian profiles with a much better contrast to the linewidth ratio for EIT at higher intensities, above ∼2 mW cm −2 .
We investigate the evolution of the atomic coherence between Zeeman sublevels with $\ensuremath{\Delta}{m}_{F}=2$ in the region between pump and probe laser beams, in a Rb vapor cell using the Hanle configuration. The transmission of the probe beam, placed inside the hollow pump beam, shows the narrowing of the Hanle resonance (at external magnetic field B=0B=0B=0) by Ramsey fringes whose separation from the central peak depends on the distance between the beams. The temporal evolution of the pump induced alignment is evident from the changes of the probe transmission spectra with increasing the angle between linear polarizations of each beam. Experimental results are in agreement with the results of the model which calculates probe fluorescence by solving time-dependent optical Bloch equations, from the time when atom enters the pump beam to the time when it leaves the probe beam. The results for total excited state populations were averaged over atomic velocities distribution.