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Vinh University Journal of Science

Fast and slow light propagations in a four-level cascade atomic medium

Nguyen Van Phu, Ho Hai Quang, NGUYEN Thi Ngan, DINH Van Diep, LUONG Nguyen Tuyet Nghi, NGUYEN Kim Thuan, Luong Thi Yen Nga

Published Sep 15, 2022• 1 citation
Publisher

Abstract

Under weak probe field, rotating wave and electric dipole approximations, we have derived analytic expressions for absorption and dispersion coefficients, group index and group velocity in a four-level cascade atomic medium interacting with the probe and pump laser fields. The model is applied to 85Rb atomic system to study the control of the fast and slow light propagation of the probe beam according to the intensity and frequency of the pump laser beam. With this excitation model, we find two transparent domains and thus the light group velocity is also controlled at these two frequency domains. In particular, the amplitude and the sign of the group index are easily changed by adjusting the intensity or frequency of the pump laser. That is, the probe beam propagation can be changed between fast and slow light modes by adjusting the intensity or frequency of the pump laser. Our analytical results can be useful for related experimental studies.

DOI
10.56824/vujs.2022nt28

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We investigate the propagation of a weak probe laser field in a medium of warm Rb atoms, controlled with two strong resonant pump fields tuned to the D1 and D2 optical transitions to form an N-scheme arrangement. We have shown theoretically that four-wave mixing has a profound effect on the probe-field group velocity and absorption, allowing the probe-field propagation to be continuously tuned from superluminal to slow-light regimes with amplification. We have also identified the experimental conditions for observation of such tunable slow-to-fast light regime (continuously through the point of zero group index) with positive probe-field gain, and demonstrated that the spectral range corresponding to the zero group index can be tuned by controlling the power of one of the pump laser.
We propose a model to manipulate group velocity of a multi-frequency probe light in an electromagnetically induced transparency medium consisting of five-level cascade-type atoms associated with a giant self-Kerr nonlinearity. An analytic expression of group index for the probe light is derived as a function of parameters of the probe and coupling fields, atomic density, and lifetimes of excited atomic states. In the presence of the self-Kerr, both probe and controlling fields can be used as knobs to manipulate the probe light between the subluminal and superluminal propagation modes in three separated frequency regions. The theoretical model agrees with experimental observation, and it is helpful to find the optimized parameters and related applications. Furthermore, by using such a cascade excitation scheme, it could be possible to choose the uppermost excited electronic states having long lifetimes, as Rydberg states, to slow the light down to a few mm/s.
A four-level atomic system interacting with two strong drive fields and a weak probe field has been studied. We demonstrate that the group velocity of probe field can be controlled to switch from subluminal to superluminal or even negative values around the probe resonance without any significant absorption. The basic idea is to create a gain with slow light on the probe beam resonance with one drive field while the other drive creates a dip in the gain profile of probe leading to superluminal and negative group velocities. A simple switching from slow light to negative group velocity light can be obtained by changing the relative intensities of drive fields. We also describe a control to tune the position of slow light frequency continuously simply by adjusting the relative intensities of both the driving fields.
Abstract The optical response of a magnetic-degenerated four-level atom system to the two left and right circular polarization components of the probe field is represented at room temperature. The absorption spectrum and group index for the two polarization components of the probe field are controlled according to the static magnetic field and the coupling field under electromagnetically induced transparency condition. By varying the strength of the static magnetic field, the optical response of the atomic medium can be changed from transparency to absorption or vice versa and hence the amplitude of group index also changes from positive extreme to negative extreme or vice versa. The same phenomenon also occurs when changing the coupling field intensity. In addition, temperature also significantly influence on the optical response of the atomic medium, which changes not only the amplitude but also the sign of the group index as the temperature increases. Our analytical results can be useful for experimental observation and related applications of light group index/velocity at room temperature.
Abstract We report a theoretical investigation of the dispersion resulting from electromagnetically induced transparency (EIT) and the associated group refractive index profiles n g of a four-level inverted Y-type system formed by the interaction of three optical fields (probe, pump and control) with 87Rb atoms. The density matrix equations are derived from the semi-classical Liouville’s equation and solved both numerically and analytically to study the coherent nonlinear optical properties of the medium. We first present the EIT, dispersion and corresponding group index profiles n g under the switch-on/off and on/off-resonance conditions of the pump and control lasers. In presence of both pump and control lasers, an enhancement of the EIT window, a sharp EIT spike and related steeper dispersion slopes are obtained at the line centre of the probe frequency detuning. The group index profiles with the variation of the strengths of individual applied optical fields are studied. The effect of the ground state decoherence rates on the group index profile is examined in detail. It is found that the manipulation of n g values and the corresponding group velocities ν g of the probe light can be easily controlled from subluminal to superluminal values or vice versa by changing the strengths of the applied fields and the ground state decoherence rates. Besides, the EIT-based ‘optical switching’ phenomenon in the medium is explained by studying the variation of the group index with the pump and control Rabi frequencies.