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Japanese Journal of Applied Physics

Measurements of Time Delays in Superradiant Laser Emssions from Binary Dye Mixtures

Yeeping Yap, Teckyong Tou Teckyong Tou, Kuanhiang Kwek Kuanhiang Kwek

Published Nov 1, 1996• 4 citations
Publisher

Abstract

The concentration dependence of the time delays, in the range of 80–800 ps, between two-color superradiant laser emissions from three enthanol solutions of the binary dye mixtures, C460/DF, DF/R640, and C460/R640, which were excited using a 1 ns transversely excited atmospheric (TEA) N2 laser was measured by a fast streak camera. While these time delays confirmed that radiative laser energy transfer as the predominant mechanism for multicolor laser emission from a single dye-mixture solution, these emissions were not usually obtained simultaneously, even when a broadband optical resonator was used.

DOI
10.1143/jjap.35.5718

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The concentration dependence of the time delays, in the range of 80–800 ps, between two-color superradiant laser emissions from three enthanol solutions of the binary dye mixtures, C460/DF, DF/R640, and C460/R640, which were excited using a 1 ns transversely excited atmospheric (TEA) N2 laser was measured by a fast streak camera. While these time delays confirmed that radiative laser energy transfer as the predominant mechanism for multicolor laser emission from a single dye-mixture solution, these emissions were not usually obtained simultaneously, even when a broadband optical resonator was used.
The characteristics of the superradiance in organic dyes pumped by a N_2 laser have been studied. It is confirmed that the experimental results satisfy the theoretical characteristics of the superradiance in a two level system, though the dyes employed are not such a system. Furthermore, in the high concentration, the damping characteristics depended on the square root of the dye concentration which are regareded as the superradiant damping are observed. The characteristics of the double emission have been also studied.
Emission spectra of concentrated (~10 18 –10 19 cm –3 ) ethanol solutions of Rhodamine C excited by high-power (~10 25 –10 26 cm –2 s –1 ) laser radiation were measured. The emission spectrum consists of two narrow bands (with half-widths of ~200–300 cm –1 ). The long-wavelength band was interpreted as cooperative spontaneous emission, and the short-wavelength band was assigned to the amplified spontaneous emission. The ratio of their intensities depends on the dye concentration and the pump power density. The ratio of band intensities is assumed to be determined by the dephasing rate of excited molecules responsible for cooperative spontaneous emission.
RbXe bands are observed using pulsed dye laser excitation of atomic alkali transitions in Rb/Xe mixtures containing 193 and 465 Torr xenon. Bands observed in absorption (A) or emission (E) at 427.0 nm (E), 497.8 nm (A), 501.8 nm (A, E), 509.0 nm (A, E), 520.0 nm (A, E) and 535.0 nm (E) are assigned to molecular potentials associated with the 6 2 P 1/2 atomic rubidium states for the first, 6 2 S 1/2 for the next two and 4 2 D 3/2,5/2 for the remaining three. The time evolution of the emission bands depends on the population mechanism of the parent atomic state which involves superradiant transitions from the pumped excited state. Monitoring of the corresponding infrared emission allows a control over the population mechanisms and an evaluation of the lifetimes, collisional quenching and reaction rates of the excimer and parent states. This in turn allows a determination of the atomic parent states of the bands observed.
An image-converter streak camera with picosecond time resolution was used in a study of the kinetics of formation of pulses in a traveling-wave laser in which molecular dye solutions were subjected to two-photon optical pumping. An analytic expression was obtained for the lasing threshold of a nonresonator traveling-wave laser in the case of pulsed (τp τe) and quasicontinuous (τp > τe) pumping. Satisfactory agreement was obtained between the calculated and experimental lasing threshold. The kinetics of the compression of output pulses, determined as a function of the pump radiation intensity, was described by rate equations. At pumping rates up to 10 times the threshold a qualitative agreement between the experimental and calculated results was obtained employing a two-band energy model of the dye with a short (compared with the duration of the pump pulses τp) vibrational relaxation time τe. At rates of excitation well above the threshold the kinetics of compression and delay changed, and the output pulses were split into two components, which was interpreted (in the specific case of rhodamine 6G) using a four-band energy scheme. A practical conclusion of this investigation was in the form of recommendations on the use of the proposed approach as a simple method for generating high-intensity single picosecond superradiance pulses of duration τsr with frequency up-con version and a compression coefficient τp/τsr ~ 10.
The transport of resonance radiation through an optically thick vapor of Sr atoms is studied. A pulsed dye laser tuned to the 461-nm resonance line excites a narrow (\ensuremath{\sim}250-\ensuremath{\mu}m-diameter) column of Sr atoms along the axis of a cylindrical oven containing Sr vapor and Ar buffer gas. After a delay of \ensuremath{\le}80 ns, a second dye laser excites the atom from the first excited state (5s5p5s5p5s5p) to a higher excited state (5s7s5s7s5s7s). The fluorescence from this latter transition is monitored as the second laser is translated parallel to the first. Since the excited-state-excited-state fluorescence is not trapped, the result is a plot of density of atoms in the 5s5p5s5p5s5p state as a function of position from the originally excited volume. The results are discussed qualitatively.