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Journal of the Optical Society of America B

Phase-controlled optical trap potential in a closed-loop atomic system

Seyedeh Hamideh Kazemi, Mohammad Mahmoudi

Published Feb 25, 2016• 9 citations
Publisher

Abstract

The optical trap potential is studied in a four-level double-Λ closed-loop atomic system in the multiphoton resonance condition. We apply four femtosecond Gaussian laser beams to generate a closed-loop configuration. It is shown that the induced optical dipole force on the system dramatically depends on the relative phase of applied fields, and it switches from focusing to defocusing in the transverse direction, simply by properly changing the relative phase. Through special switching of the relative phase, the trapping potential splits into two positions, which has novel applications in trapping and manipulating the particles. Moreover, it is further demonstrated that trap depth can be manipulated by the relative phase. Finally, a maximum coherent population transfer is achieved for a special value of the relative phase of applied fields.

DOI
10.1364/josab.33.000479

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The optical trap potential is studied in a four-level double-Λ closed-loop atomic system in the multiphoton resonance condition. We apply four femtosecond Gaussian laser beams to generate a closed-loop configuration. It is shown that the induced optical dipole force on the system dramatically depends on the relative phase of applied fields, and it switches from focusing to defocusing in the transverse direction, simply by properly changing the relative phase. Through special switching of the relative phase, the trapping potential splits into two positions, which has novel applications in trapping and manipulating the particles. Moreover, it is further demonstrated that trap depth can be manipulated by the relative phase. Finally, a maximum coherent population transfer is achieved for a special value of the relative phase of applied fields.
We report coherent population transfer(CPT) in a four level atomic system, coupled by three chirped Gaussian femtosecond pulses. CPT is studied under two specific conditions beyond the RWA. It is observed that nearly complete population transfer to the states |3> and |4> can be achieved by maintaining proper resonance condition and judiciously choosing the laser parameters. In addition to this, the transverse optical dipole force on the four-level atomic system is numerically studied. The transverse force provides an acceleration to an atom which is eight order of magnitude higher than earth’s gravitational acceleration g. The force changes from a focusing force to a defocusing one as the initial population changes from the ground states to the excited states.
The optical properties of a four-level double-V-type closed-loop system is studied in the multiphoton resonance condition. It is shown that the phase-dependent electromagnetically induced transparency (EIT) window is established and by changing the relative phase of the applied fields, EIT switches to electromagnetically induced absorption. Moreover, the optical bistability (OB) behavior of the system is studied, and it is demonstrated that the cross-Kerr-effect-induced OB depends on the relative phase of the applied fields, and it can be controlled by either intensity or relative phase of laser fields. By applying a microwave field to drive the hyperfine transition between two lower states, the optical properties change and OB switches to optical multistability. This can be used for detection of longer wavelengths than visible light.
We report a scheme to achieve coherent population transfer in a four-level double-Λ closed-loop atomic system by utilizing the coherent beams with Laguerre–Gaussian profiles. It is demonstrated that a maximal population transfer is achieved for a special value of the relative phase of the applied fields. We then compare the results with a similar configuration, but for the laser pulses with the usual Gaussian profiles and conclude that using the Laguerre–Gaussian modes can result in almost complete population transfer. Besides, the creation of a superposition between two lower states can be achieved through properly changing the relative phase. The suggested scheme may hold great practicality for the realization of the population transfer as well as creation of the superposition between states in the quantum systems with multiple states.
Abstract We present a convenient and effective way to generate a novel phenomenon of trapping, named ‘trap split’, in a conventional four-level double-Λ atomic system, driven by four femtosecond Laguerre-Gaussian laser pulses. We find that trap split can always be achieved when atoms are trapped by such laser pulses, as compared to Gaussian ones. This feature is enabled by the interaction of the atomic system and the Laguerre-Gaussian laser pulses with zero intensity in the center. A further advantage of using Laguerre-Gaussian laser pulses is the insensitivity to fluctuation in the intensity of the lasers in such a way that the separation between the traps remains constant. Moreover, it is demonstrated that the suggested scheme with Laguerre-Gaussian laser pulses can form optical traps with spatial sizes that are not limited by the wavelength of the laser, and can, in principle, become smaller than the wavelength of light. This work would greatly facilitate the trapping and manipulating of particles and the generation of trap split. It may also suggest the possibility of extension into new research fields, such as micro-machining and biophysics.
In this research, the disentanglement conditions between the coated atom and its spontaneous emission in double- lambda quantum system was investigated. Initially, the entanglement increased due to the interaction between the laser fields and the four- level atom. This entanglement can be controlled by changing the intensity of the applied fields. By applying the two coupling fields, the system is entangled and the system would be disentangled with one coupling field and one probe field. The present research demonstrated that spontaneous emission by changing the population of atoms in the alignments is a good source of entanglement, but fields were manipulated in such a way that this entanglement was reduced by coherent trapping of the population for the atoms.
The third-order susceptibility is investigated in a five-level atomic system in which the laser beams couple the ground state to a four-level closed-loop system. It is found that under the condition of the multiphoton resonance, one can enhance the Kerr nonlinearity of such a medium by properly adjusting the amplitudes and phases of the applied fields. In this case, the linear and nonlinear absorption reduce considerably in a region with a positive group velocity. It is demonstrated that the third-order susceptibility is very sensitive to the relative phase of the applied fields. An analytical model is presented to elucidate such phase control of the Kerr nonlinearity. A comparison is also made between the Kerr-nonlinear indices for the five-, four-, and three-level systems. It is realized that the magnitude of the Kerr nonlinearity for the five-level system is larger than that of the three- and four-level counterparts. Finally, it is shown that effect of Doppler broadening can lead to an enhanced Kerr nonlinearity while maintaining linear and nonlinear absorption.