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Laser Physics

The QASER revisited: insights gleaned from analytical solutions to simple models

Marlan O. Scully

Published Aug 15, 2014• 5 citations
Publisher

Abstract

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.

DOI
10.1088/1054-660x/24/9/094014

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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.
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.
We find that intense short pulses of XUV radiation can be produced by cooperative spontaneous emission from visible or IR laser pulses driving atoms or ions. The process depends on the generation and utilization of atomic coherence as is the case in lasing without inversion. However, the radiation process is not stimulated emission, but is rather cooperative spontaneous emission in the sense of Dicke. More precisely, the many atom mathematics of superradiance has much in common with coherent anti-Stokes Raman scattering. The generation of Raman coherence is considered from the perspectives of multi-photon excitation and breaking of adiabaticity. In particular, we find that the advent of an ultrashort (e.g. single cycle) pulse rewards rethinking of conventional (nanosecond) nonlinear optics with new insights.
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).