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Conference on Lasers and Electro-Optics

Experimental demonstration of Rabi Oscillations produced by adiabatic pulse due to initial atomic coherence

Zhenhuan Yi, Xingchen Zhao, Zhiguo Wang, Tao Peng, Anatoly A. Svidzinsky, Hichem Eleuch, Marlan O. Scully

Published Jan 1, 2018• 0 citations
Publisher

Abstract

We experimentally demonstrate that initial atomic coherence yields Rabi oscillations in a two-level system driven by an adiabatic pulse. This method can be used to achieve Quantum Amplification by Superradiant Emission of Radiation (QASER) in atomic vapors.
DOI
10.1364/cleo_qels.2018.fm4h.4

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If an electromagnetic pulse is detuned from atomic transition frequency by amount Δ>1/τ, where τ is the turn-on time of the pulse, then atomic population adiabatically follows the pulse intensity without causing Rabi oscillations. Here we show that, if initially, the atom has nonzero coherence, then the adiabatic pulse yields Rabi oscillations of atomic population ρaa(t), and we obtain analytical solutions for ρaa(t). Our findings can be useful for achieving generation of coherent light in the backward direction in the QASER scheme in which modulation of the coupling between light and atoms is produced by Rabi oscillations. Initial coherence can be created by sending a short resonant pulse into the medium followed by a long adiabatic pulse, which leads to the light amplification in the backward direction.
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.
A collection of NNN Rydberg atoms and a resonant millimeter-wave cavity are shown to exchange energy back and forth at a rate $2(\frac{d}{\ensuremath{\hbar}}){\mathcal{E}}_{0}\sqrt{N}$, where ddd is the electric dipole matrix element of the atomic transition and E0{\mathcal{E}}_{0}E0​ the "field per photon" in the cavity. This experiment is a demonstration of self-induced Rabi oscillations in a two-level atom system coupled to a single electromagnetic field mode and can also be considered as a very simple illustration of "ringings" in superradiant emission.
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 investigate the statistical properties of superradiant pulses emitted by a system of many atoms. By using a representation of the atomic density operator in terms of directed angular-momentum states (which are also known as atomic coherent states), we find that the statistical behavior of the atoms is formally equivalent to that of a harmonic oscillator subject to linear amplitude amplification and driven by Gaussian white noise. The emitted light pulses are then found to exhibit very large quantum fluctuations for atomic initial states corresponding to complete or nearly complete excitation. For all other atomic initial states the superradiant pulses show classical behavior.