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Laser Physics

Temporal cloak via Doppler broadening

M S Abdul Jabar, Bakht Amin Bacha, Iftikhar Ahmad

Published May 22, 2015• 13 citations
Publisher

Abstract

In this article we propose a new scheme for a temporal cloak based on a five level experimental rubidium or cesium atomic system. In the thought experiment, a cloaking time gap is created between the enhanced subluminal and superluminal pulse caused by Doppler broadening in the medium. A cloaking time in microseconds is attained and no distortion is observed in the pulse.

DOI
10.1088/1054-660x/25/6/065405

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In this article we propose a new scheme for a temporal cloak based on a five level experimental rubidium or cesium atomic system. In the thought experiment, a cloaking time gap is created between the enhanced subluminal and superluminal pulse caused by Doppler broadening in the medium. A cloaking time in microseconds is attained and no distortion is observed in the pulse.
We demonstrate the possibility of creating a time gap in the slow light based on spectral hole burning in a four-level Doppler broadened sodium atomic system. A time gap is also observed between the slow and the fast light in the hole burning region and near the burnt hole region, respectively. A cloaking time gap is attained in microseconds and no distortion is observed in the transmitted pulse. The width of the time gap is observed to vary with the inverse Doppler effect in this system. Our results may provide a way to create multiple time gaps for a temporal cloak.
We discuss a different scheme to achieve temporal cloaking in warm atomic vapors. Instead of creating a temporal-spatial window in a patched broadband short optical pulse using static differential dispersion of an optical fiber, we create a temporal-spatial window by directly controlling the propagation velocities of a pair of correlated narrow-band long optical pulses. This method eliminates the phase noise introduced by the broadband short-pulse swapping technique. When the event medium has a fast relaxation rate this leads to a temporal-cloaking scheme similar to that reported in fibers. When the event-medium relaxation cannot be neglected, we discuss a matched-mode scheme that can eliminate the phase change caused by the event field. The advantages of the dynamically and actively controlled atomic system over previously reported work are discussed.
Restricted to picosecond scale time gaps, the previous reported temporal cloaking schemes are hardly possible for practical applications. In this paper, we report a nanosecond scale temporal cloaking regime based on the fast/slow light effect in unbalanced fiber Mach-Zehnder interferometers. By exerting a radio frequency modulation onto the probe light, we broke through the gap time limit. A time gap up to 5.3ns was created and experimentally observed. The modulation signal in the time gap was concealed efficiently. Realizing nanosecond time gaps is critical for potential applications of temporal cloaking, such as secure communication and signal processing. This solution may push this fascinating idea into reality
We propose a new scheme for the coherent control of birefringent light pulses propagation in a four-level atomic medium. We modify the splitting of a light pulse by controlling the electric and magnetic responses. The Doppler broadening effect is also noted on the propagation of the birefringent pulses. The dispersions of the birefringence beams are oppositely manipulated for delay and advancement of time at a Doppler width of 10 γ . A time gap is created between the birefringence beams, which protects from hacking of information. The time gap is then closed to restore the pulse into the original form by a reverse manipulation of the dispersion of the birefringence beams, i.e., introducing another medium whose transfer function is the complex conjugate of that of the original medium. The results are useful for secure communication technology.
Considered to only exist in fairy tales in the past, invisibility cloaks have been successively converted into reality no matter in the spatial domain or the temporal domain. Inspired by the spatial cloaking, time gaps are utilized to hide temporal events. However, a sophisticated synchronization for cloaking is indispensable in these cloaking techniques, therefore leaving inconvenience for the realization of the temporal cloak. Here, by exploiting the temporal Talbot effect, we propose a brand new scheme and concept to achieve a temporal cloak without any synchronization for the cloaking process, under which the intensity-modulated event is directly turned into invisibility in intensity through temporal averaging effect induced by the Talbot effect. We successfully realize the temporal cloak for periodic and pseudo-random signals. We also find that the higher order temporal Talbot effect is beneficial to the cloaking performance. Due to the transformed information on the phase, the data could be recovered from the intensity-cloaked waveform, leading to a non-full-field cloak. This method and its concept render a distinct perspective for the temporal cloak, extend temporal cloak to the pulsed-wave, and promote the development of confidential communication.