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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum superradiant amplification in rubidium vapors: gain assessment

Charles W. Ballmann, Andrew J. Traverso, Luqi Yuan, Vladislav V. Yakovlev

Published Feb 20, 2017• 0 citations
Publisher

Abstract

New approaches for generating coherent light are of great interest due to the increasing demand for unique sources tailored to specific applications. In fields like microscopy, remote sensing, and soft X-ray/XUV lasers this interest is especially pronounced, as novel laser sources would have a profound impact on those technologies. Here, we evaluate the possibility for generating coherent light by inducing collective coherence in the gain medium.

DOI
10.1117/12.2252836

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•Sep 30, 2014•0 citations
New approaches for generating coherent light are of great interest due to the increasing demand for unique sources tailored to specific applications. In fields like microscopy, remote sensing, and soft X-ray/XUV lasers this interest is especially pronounced, as novel laser sources would have a profound impact on those technologies. Here, we evaluate the possibility for generating coherent light by inducing collective coherence in the gain medium.
Lasers and masers typically require population inversion. But with phase coherent atoms (phasers), we get lasing without inversion (e.g. 10% of the atoms excited). However, in recent work we found that it is possible to get coherent light emitted with no atoms excited, via Quantum Amplification of Superradiant Emission of Radiation (QASER). In particular, we found that by utilizing collective superradiant emission, we can generate coherent light at high frequency in the UV or x-ray bands by driving the atomic system with lower frequency source. Here, we present a simple analysis based on near-resonant QASER operation and on a multi-photon Hamiltonian obtained by, e.g. a canonical transformation.
It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is “highly anomalous” and yields superradiant emission without population inversion. We here show that transient gain in a three-level system has common features with Dicke superradiance and can yield strong extreme ultraviolet lasing in, for example, He atoms (at 58 nm) or He-like ions such asB3+(at 6.1 nm).
New approaches for generating coherent light are of keen interest to the scientific community due to the increasing demand for unique sources tailored to specific applications. In fields like microscopy, remote sensing, and soft X-ray/XUV lasers this interest is especially pronounced as novel techniques would have a profound impact. Here, we present new methods for generating coherent light by inducing collective coherence in the gain medium. In particular, we demonstrate coherent emission produced through four-wave mixing on a dressed state, stimulated Raman by inducing coherence between two vibrational levels via 2-photon coupling, and finally, a remote atmospheric coherent source via strong-oscillatory superfluorescence in atomic oxygen. Four-wave mixing on a dressed state could be used to generate XUV coherent radiation or remote atmospheric sensing, as it produces both a forward and backward propagating field. We observe this in Rb vapor with emission occurring at detunings of up to 30 nm from the transition. Similarly, 2-photon enhanced stimulated Raman could be implemented for XUV generation as well, due to the asymmetric gain observed on the anti-Stokes emission relative to the Stokes in Methane. It is also a great platform for microscopy as it is background-free unlike other conventional methods like CARS and CSRS. Atmospheric lasing via oxygen is a prime candidate for nextgeneration remote sensing techniques, and here we study its emission mechanisms in depth demonstrating that superfluorescence serves as the main driving process for the observed 845 nm generation. This intense strong-oscillatory emission lends itself for use in coherent Raman techniques. Furthermore, a dual Raman-Brillouin microscope is also presented and demonstrated, which could prove to be a powerful new tool in biomedical and material characterizations applications.
Quantum coherence and interference effects in atomic and molecular physics has been extensively studied due to intriguing counterintuitive physics and potential important applications. Here we present one such application of using quantum coherence to generate and enhance gain in extreme ultra-violet(XUV)(@58.4nm in Helium) and infra-red(@794.76nm in Rubidium) regime of electromagnetic radiation. We show that using moderate external coherent drive, a substantial enhancement in the energy of the lasing pulse can be achieved under optimal conditions. We also discuss the role of coherence. The present paper is intended to be pedagogical on this subject of coherence-enhanced lasing.
This work is focused on quantum coherence and superradiant emission in a dense pencil-like multi-level medium where many novel effects appear such as transient lasing without inversion, coherence-brightened sky laser, and quantum amplification by superradiant emission of radiation. \n\nWe start from an interesting cascade model where quantum coherence effects can lead to surprising phenomena, gain without population inversion and gain suppression, under different parameters. We further show superradiant emission inside helium plasma. The population evolution shows the decay is significantly faster than collisional decoherence and spontaneous decay rates. This indicates superradiant coherent behavior of the atomic system inside the plasma.\n\nBased on these results, we demonstrate lasing without inversion on a time scale shorter than the decoherence time. The possibility of transient lasing without inversion holds promise for lasing in the extreme-ultraviolet/x-ray regime. We propose experiments to demonstrate this in helium or helium-like ions plasma. \n\nWe also study coherent emission from ambient air and demonstrate efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The emission process exhibits nonadiabatic quantum coherence, which is similar in nature to Dicke superradiance. This coherence-brightened backward light source in air provides possibility for atmospheric remote sensing through a phase-matched coherent Raman scattering process.\n\nFinally, we present a new kind of quantum amplifier, the QASER (quantum amplification by superradiant emission of radiation), based on collective superradiant emission which does not require initial population in the excited state. We show that parametric resonance between the driving field and collective superradiant oscillations of the atomic polarization can yield light amplification at high frequencies. The resulting superradiant amplifier is many orders of magnitude more efficient than nonlinear multiphoton excitation and holds promise as a new way to generate high-frequency coherent radiation.