Abstract
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.
Highlights
Species-specific remote sensing/spectroscopy in the sky has been a profound challenge in modern applied physics
We study the backward superfluorescent emissions (BSFEs) from two-photon excited sodium (Na) atomic vapor
Based on the simplified theoretical estimations, we find the total number of excited atoms, N, to be 1010 and the τR for the 1140 nm SF pulse to be τR ≈ 20 ps at an input power of 2 mW
Summary
Species-specific remote sensing/spectroscopy in the sky has been a profound challenge in modern applied physics. The conventional atmospheric light detection and ranging (LIDAR) techniques [1, 2] have essential tools for detecting traces of air impurity at long distances. A tremendous amount of research has been devoted to upgrading conventional LIDAR techniques. The femtosecond-LIDAR, based on commercially available highintensity femtosecond (fs) lasers, has been demonstrated [3]. A fs filament formation [4] in air enables propagation of fs pulses for distances of tens of kilometers in scale. LIDAR techniques rely on incoherent light-scattering processes which are the fundamental limitation for LIDARs performance.
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