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Microoptics Conference

Electromagnetically-induced focusing controlled by a microwave field

Onkar N. Verma, Sourabh Roy

Published Nov 1, 2017• 1 citation
Publisher

Abstract

We have theoretically predicted the effect of a microwave field on cross-focusing of an optical weak probe beam by a copropagating non-uniform control beam in an electromagnetically-induced transparency medium. Our numerical simulation shows probe beam focusing about 30% of its initial width in absence of microwave, whereas it focuses 70% in presence of microwave field.

DOI
10.23919/moc.2017.8244531

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We have theoretically predicted the effect of a microwave field on cross-focusing of an optical weak probe beam by a copropagating non-uniform control beam in an electromagnetically-induced transparency medium. Our numerical simulation shows probe beam focusing about 30% of its initial width in absence of microwave, whereas it focuses 70% in presence of microwave field.
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.
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).
The theoretical basis for electromagnetically-induced focusing (EIF), which is caused by spatial variations in the coupling laser strength in an electromagnetically-induced transparency (EIT) experiment, is studied in detail. Using a numerical model it is shown that radial changes in both absorption and refractive index are important in predicting the probe beam's propagation conditions. Detailed calculations of the focusing and defocusing during EIF under various conditions are presented and compared with appropriate experiments. Diffractionlike patterns are predicted for, and observed on, a probe beam after propagation through a smaller EIT aperture. \textcopyright{} 1996 The American Physical Society.
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 study the lensing effect experienced by a weak probe field under conditions of electromagnetically induced transparency (EIT) involving a Rydberg state. A Gaussian coupling beam tightly focused on a laser-cooled atomic cloud produces an inhomogeneity in the coupling Rabi frequency along the transverse direction and makes the EIT area acting like a gradient-index medium. We image the probe beam at the position where it exits the atomic cloud and observe that a red-detuned probe light is strongly focused with a greatly enhanced intensity whereas a blue-detuned one is defocused with a reduced intensity. Our experimental results agree very well with the numerical solutions of Maxwell-Bloch equations.