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Applied Physics Letters

Ultrafast laser control of backward superfluorescence towards standoff sensing

Gombojav O. Ariunbold, Vladimir A. Sautenkov, Yuri V. Rostovtsev, Marlan O. Scully

Published Jan 13, 2014• 24 citations
Publisher

Abstract

We study infrared backward cooperative emission in a rubidium vapor induced by ultrafast two-photon optical excitations. The laser coherent control of the backward emission is demonstrated by using a pair of 100 fs pulses with a variable time delay. The temporal variation (quantum beat) of the backward beam intensity due to interference of atomic transitions in the rubidium atomic level system 5S-5P-5D is produced and controlled. Based on the obtained experimental results, we discuss possible applications of the developed approach for creation of an effective “guide star” in the sodium atomic layer in the upper atmosphere (mesosphere).

DOI
10.1063/1.4861934

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A dense sample of atomic rubidium vapor excited by a short (4 ps) transform-limited laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed omnidirectional infrared superfluorescence pulse (coherent flash) on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in prompt directional yoked UV emission and coherent population transfer back to the ground 5S state. This emission completes the process of parametric four-wave mixing, selectively involving only those atoms on the 6P level whose 5D-6P emission satisfies the phase-matching condition. The unique feature of this four-wave mixing radiation is that it occurs after the termination of the pump pulse, as evidenced by direct measurement of emission timing. The multidirectional character of superfluorescent emission is confirmed by the observation of non-collinear parametric four-wave mixing when exciting the sample by two simultaneous overlapping pulses, for a wide range of beam angles. Also observed and investigated is superfluorescent transient diffraction that appears when the sample is excited by a sequence of three laser pulses.

•Jan 1, 1998•2 citations
We study infrared backward cooperative emission in a rubidium vapor induced by ultrafast two-photon optical excitations. The laser coherent control of the backward emission is demonstrated by using a pair of 100 fs pulses with a variable time delay. The temporal variation (quantum beat) of the backward beam intensity due to interference of atomic transitions in the rubidium atomic level system 5S-5P-5D is produced and controlled. Based on the obtained experimental results, we discuss possible applications of the developed approach for creation of an effective “guide star” in the sodium atomic layer in the upper atmosphere (mesosphere).
We study backward cooperative emissions from a dense sodium atomic vapor. Ultrashort pulses produced from a conventional amplified femtosecond laser system with an optical parametric amplifier are used to excite sodium atoms resonantly on the two-photon 3S –4S transition. Backward superfluorescent emissions (BSFEs), both on the 4S –3P and 4S –3P transitions, are observed. The picosecond temporal characteristics of the BSFE are observed using an ultrafast streak camera. The power laws for the dependencies of the average time delay and the intensity of the BSFEs on input power are analyzed in the sense of cooperative emission from nonidentical atomic species. As a result, an absolute (rather than relative) time delay and its fluctuations (free of any possible external noise) are determined experimentally. The possibility of a backward swept-gain superfluorescence as an artificial laser guide star in the sodium layer in the mesosphere is also discussed.
We demonstrate temporal coherent control in two-photon transitions. A pair of broadband (Δλ∼20nm), ultrashort (Δt∼100fs), collinear pulses with a variable time delay excites the rubidium atoms into the 5D state from the 5S ground state where the two-photon transitions are enhanced by the intermediate level 5P. The excited atoms radiate 5μm (5D–6P) and 420nm (6P–5S) light. As a result of tuning the wavelength of the input laser, a superfluorescence at 420nm exhibits different temporal behaviors. A switching from a beating at the frequency given by the difference between the sequential atomic transitions that involve the 5P3/2 intermediate level, to a quantum beating due to two different two-photon excitation paths, 5S→5P1/2→5D and 5S→5P3/2→5D is observed. Based on the simple atom-field interaction theory, an analytic solution, which qualitatively elucidates experimental results, is obtained.
Technological progress in commercializing ultrafast lasers and detectors has allowed realization of cooperative processes on an ultrashort time scale, which demand a re-evaluation of the conventional cooperative phenomena with a new insight. Ultrafast cooperative phenomena in coherently prepared media and various applications of superfluorescence and coherent Raman scattering are studied in this dissertation. In particular, a simple theoretical testimony on analogy between a cooperative emission and coherent Raman scattering is presented by offering an opportunity to perform parallel research on these two processes from a unified point of view.\n\nOn one hand, the superfluorescent pulse with a time duration of a few tens of picoseconds (ps) from alkali metal vapor is observed for the first time, even though cooperative phenomena in atomic vapor have been extensively studied for more than five decades. A dense rubidium vapor pumped by ultrashort (100 femtosecond, fs) pulses allows a realization of the ultrafast superfluorescence while a time-resolved study of superfluorescence is accomplished by using a streak camera with 2 ps time resolution. Experimental research on quantum nature of cooperative emissions has been “frozen” over the years (three decades) possibly because of the technical difficulties. Quantum fluctuations of superfluorescence development are explored experimentally by taking advantage of the ultra fast streak camera. Presumable applications of the superfluorescent pulse in e.g., a remote sensing, and an ultraviolet upconversion of the input infrared laser pulse are presented. The quantum interference due to different excitation pathways is revealed by the temporal coherent control technique while observing interferometric signals from alkali metal vapors. \n\nOn the other hand, a new spectroscopic technique based on ultrafast coherent Raman scattering is developed. The key advantage of the presented technique is to suppress the non-resonant background noise which usually obscures possible applications of the other conventional coherent Raman techniques in practice. A reduction of the background noise is achieved by shaping and delaying the third pulse which probes the coherence of the medium (i.e., an enhancement of specific vibrations of the target molecules in unison) firstly prepared by two broadband pulses. We demonstrate a robustness and superiority of signal-to-noise ratio of the developed technique by identifying as few as 10000 bacterial spores at a single laser shot level.\n\nFinally, several comparative studies between cooperative and uncooperative processes are presented. A picosecond cooperative phenomenon in a three-photon resonant medium induced by a single as well as two-color ultrashort pulses is investigated. A time-resolved study shows that a picosecond cooperative effect is crucial in the well-established fields of resonant-enhanced multiphoton ionizations and harmonic generations. We also present a quantitative analysis for spontaneous versus broadband coherent Raman scattering on pyridine molecules. The spontaneous Raman signal is enhanced by 5 orders as a result of cooperative phenomena.
We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the 5D5D5D state from the 5S5S5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the 6P6P6P state and then to the 5S5S5S state. The SF emission at 420 nm on the 6P6P6P--5S5S5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.
We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).
A dense sample of atomic rubidium vapor excited by a short (4 ps) transform-limited laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed omnidirectional infrared superfluorescence pulse (coherent flash) on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in prompt directional yoked UV emission and coherent population transfer back to the ground 5S state. This emission completes the process of parametric four-wave mixing, selectively involving only those atoms on the 6P level whose 5D-6P emission satisfies the phase-matching condition. The unique feature of this four-wave mixing radiation is that it occurs after the termination of the pump pulse, as evidenced by direct measurement of emission timing. The multidirectional character of superfluorescent emission is confirmed by the observation of non-collinear parametric four-wave mixing when exciting the sample by two simultaneous overlapping pulses, for a wide range of beam angles. Also observed and investigated is superfluorescent transient diffraction that appears when the sample is excited by a sequence of three laser pulses.