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Japanese Journal of Applied Physics

Microwave field controlled electromagnetically induced focusing

Onkar N. Verma, Sourabh Roy

Published Jul 6, 2018• 3 citations
Publisher

Abstract

Abstract We theoretically show the effect of a microwave field on the induced focusing of an optical weak probe beam by a copropagating nonuniform control beam in an electromagnetically induced transparency medium. Our numerical simulation shows that, in the presence of a microwave field, probe beam focusing is phase-sensitive and the focused beam size is reduced by 70% of its initial width. In contrast, in the absence of a microwave field, the reduction in the size of the transmitted probe beam is only about 30%. This approach is also successfully applied to the focusing of the diffraction-limited structure of the probe beam.

DOI
10.7567/jjap.57.08pf01

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Abstract We theoretically show the effect of a microwave field on the induced focusing of an optical weak probe beam by a copropagating nonuniform control beam in an electromagnetically induced transparency medium. Our numerical simulation shows that, in the presence of a microwave field, probe beam focusing is phase-sensitive and the focused beam size is reduced by 70% of its initial width. In contrast, in the absence of a microwave field, the reduction in the size of the transmitted probe beam is only about 30%. This approach is also successfully applied to the focusing of the diffraction-limited structure of the probe beam.
In atomic systems, the spatially nonuniform distribution of a coupling field leads to the focusing of a probe beam producing a recipe for electromagnetically induced focusing (EIF). A diffraction-like pattern for the output probe beam can arise, and the probe radiation experiences focusing and defocusing across an electromagnetically induced transparency window. This phenomenon has critical implications for experiments on atomic coherence effects. Here, we study the EIF in a four-level closed-loop atomic system and show that full width at half-maximum (FWHM) of focused output probe intensity in the focal plane can be controlled by both intensity profile and relative phase of applied fields. We also demonstrate that the FWHM of the focused output probe intensity in focal length is much smaller than that of the input probe intensity, and in addition, through a special set of parameters, the minimum value of the FWHM can be obtained. Moreover, the FWHM can be made small by increasing Rabi frequency of the Gaussian signal field. Furthermore, the Gaussian probe intensity profile switches to a doughnut-like one just by changing the relative phase of applied fields. Finally, we apply a Laguerre–Gaussian signal field and find that the characteristics of the output probe field depend on the intensity profile of the signal field. Our results can be used to design a lens-like device with a controllable focal length and focusing strength that would pave the way toward all-optical switching devices.
We investigate theoretically a scheme for all-optical generation of structured light beams with and without a microwave field in an electromagnetically induced transparency (EIT) medium composed of rubidium atoms. In a conventional EIT system, two optical fields (namely, a strong control field and a weak probe field) drive two electric-dipole-allowed transitions of an atom to form a three-level system in a $\mathrm{\ensuremath{\Lambda}}$ configuration. At EIT resonance, we found that a Laguerre-Gaussian (LG) control beam induces a radial modulation of probe's absorption profile in such way that a Gaussian probe beam is reshaped into a hollow-Gaussian beam following the position-selective absorption. The transmitted probe beam has a doughnut-shaped intensity structure similar to that LG beam whereas the phase profile is still a spherical wave. Moreover, the presence of a microwave field between two ground states of three-level system leads to a closed-loop system, and probe response is modified with an additional term proportional to the amplitudes of Rabi frequencies of control and microwave fields, and relative phase of three fields. This term acts as driving field which is responsible for the generation of a vortex probe beam having a helical phase profile. We have further shown that the amplitude and helical phase of generated vortex beam can be manipulated by changing the relative phase of applied fields and temperature of the vapor cell. The present study may provide a useful tool for creation of structured light beams with and without orbital angular momentum.
We describe the temporal and spatial dynamics of propagating electromagnetically induced transparency pulses in an optically thick medium. Results include pulse velocities as slow as c/165c/165c/165 with 55% transmission, strong-probe-field effects, and the observation of near diffraction-limited transmitted beam quality.
We describe cross focusing in a four-level atomic medium under electromagnetically induced transparency. We show that due to the giant Kerr nonlinearity experienced by the atoms, cross focusing between weak signal and probe fields (both fields with intensities below line saturation level) is possible. By applying different intensity masks to the signal field, different lenses (cylindrical, Fresnel, Gaussian) can be induced in the atomic sample. Focusing of the probe beam is analyzed in terms of the excitation parameters (signal Rabi frequency and detuning, as well as optical depth of the atomic medium).