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Journal of Applied Physics

Amplified spontaneous emission and superradiant pulse from the flash-pumped atomic iodine system

Gyu Ug Kim, Sang Soo Lee, Jae Won HAHN

Published Jan 15, 1988• 3 citations
Publisher

Abstract

A superradiant and amplified spontaneous transition pulse is obtained from an iodine photodissociation laser amplifier which uses i-C3F7I as an active molecule and is pumped with a flashlamp (full width at half maximum=3 μs). We have observed a sharp leading pulse of superradiance whose rise time is less than 50 ns, and the amplified spontaneous emission follows the superradiance during the pumping flash. By measuring the population inversion density in the active medium of the amplifier, we have obtained the threshold condition for the superradiance to take place. Also, the buildup time and peak intensity of the pulse are measured as a function of the flashlamp input energy. By placing a reflecting mirror at a varying distance (from 173 to 673 cm) behind the amplifier, we have found a periodic intensity fluctuation in the superradiance; the period (Δt) depends linearly on the distance (d), i.e., Δt=2d/c, where c is the light velocity.

DOI
10.1063/1.340290

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A superradiant and amplified spontaneous transition pulse is obtained from an iodine photodissociation laser amplifier which uses i-C3F7I as an active molecule and is pumped with a flashlamp (full width at half maximum=3 μs). We have observed a sharp leading pulse of superradiance whose rise time is less than 50 ns, and the amplified spontaneous emission follows the superradiance during the pumping flash. By measuring the population inversion density in the active medium of the amplifier, we have obtained the threshold condition for the superradiance to take place. Also, the buildup time and peak intensity of the pulse are measured as a function of the flashlamp input energy. By placing a reflecting mirror at a varying distance (from 173 to 673 cm) behind the amplifier, we have found a periodic intensity fluctuation in the superradiance; the period (Δt) depends linearly on the distance (d), i.e., Δt=2d/c, where c is the light velocity.
Recently, Jaroszynski and King found superradiance(SR) occurring in photodissociatively created systems using NaI and n-C3F7I moleculesl) , and Hahn and Lee have performe2)d the amplified spontaneous emission(ASE) experiment in an iodine photodissociation laser(IPL) amplifier . In general, to observe SR experimentally, a very short intense pumping pulse of which FWHM is about a few tens of nanoseconds, is used for making a totally inverted system. But in this work, the amplifier is pumped with conventional flashlamps of FWHM=3us, yet we have detected a output pulse which is a superposition of ASE and SR. The temporal behaviour of the pulse is investigated in detail, and the experimental results are compared with theoretical results.
Pulse propagations in an amplifier in the case of the photodissociation iodine laser are investigated with the use of semiclassical treatments, especially when the input pulse fed into the amplifier has a width comparable to or shorter than the transverse relaxation time of the amplifier medium. In our semiclassical equations for pulse propagation in an iodine amplifier, the populations of all hyperfine levels [2P1/2(F= 3, 2),2P3/2(F = 4, 3, 2, 1)], the polarizations of optically allowed transitions (\DeltaF = 0, \pm 1), and the Doppler detuning effects were taken into consideration. Based on the numerical integrations of the semiclassical equations, the propagated pulse shape and the population distributions of the hyperfine levels are discussed. From our calculations, it has been made clear that bandwidth limitations of pulses in the range of pulsewidths of 0.1 ns and below can be avoided by making use of the nonlinear amplification that results from the coherent interaction between the propagated pulse field and the amplifier medium.
We report the observation of near-infrared Dicke superradiant pulses. The active medium (atomic Na vapor) is optically pumped in the 5S125{S}_{\frac{1}{2}}5S21​​ state and superradiates on transitions cascading down from this level at 3.41, 2.21, and 9.10 \ensuremath{\mu}m. The superradiant character of the emission is checked by studying variations of pulse heights and delays versus initial population inversion density.
The transition from the amplification of spontaneous emission to superfluorescence in a three-level and swept-gain medium excited by an x-ray free-electron-laser pulse is theoretically investigated. Given the specific timescale of an x-ray free-electron-laser pulse, we investigate the swept pumping process in detail, and our results show that the temporal structure of an x-ray free-electron-laser pulse plays a more critical role than its peak intensity does for producing population inversion. The typical watershed of two radiant regions depends on the optical depth of the gain medium for a given coherence time, namely, the particle number density and medium length are equally important. However, we find that medium length plays a more important role than particle density for making the forward-backward asymmetry. The transient gain length and the total medium length are identified as two important factors to observe length-induced backward transition. The present results suggest an application of parametric controls over a single-pass-amplified light source.
We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 1s(2) → 1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ are observed, corresponding to a photon number conversion efficiency of up to 10%. We observe the peak intensity to scale as ρ(2) and the emitted pulse width and delay to scale as ρ(-1), where ρ is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75,000 times faster than the spontaneous 1s3p → 1s2s decay rate. To our knowledge, this is the first observation of superfluorescence following pumping in the extreme ultraviolet wavelength region, and extension of the technique to the generation of extreme ultraviolet and x-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.
Macroscopic fluctuation of superradiance most sensitively appears as the fluctuation of the polarized photon number. This fluctuation is not much affected by unexpected variation of the exciting pulse area. In the superradiance limit the relative fiuctuation of the polarized photon number is equal to 1/3\sqrt{1/3}1/3​, and it decreases rapidly as the atomic number density is reduced below a threshold density. Such a characteristic change is attributed to a transition from the superradiant emission to the amplified spontaneous emission.