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有機色素における超放射

真三 武藤, 島 浩一, Keiko Hosaka, Chiaki Ito

Published Jan 1, 1976• 0 citations
Publisher

Abstract

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.

DOI
10.34429/00002014

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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.
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 dependences of cooperative emission, superfluorescence, and lasing spectra on the wavelength of pumping radiation were studied for highly concentrated ethanol solutions of organic dye molecules. The relative intensity of cooperative emission was found to decrease, while the intensities of superfluorescence and lasing were found to increase with the energy of exciting photons. The increase in the dephasing rate of quantum states is supposed to be primarily responsible for the change in proportion between these intensities. On increasing the dephasing rate, with all other factors being the same, the concentration of the excited molecules with the phased quantum states decreases, while that with the dephased states increases. The ratio between the cooperative emission and dephasing rates determines the proportion between the intensities of cooperative emission, superfluorescence, and lasing. The onset of lasing at high concentrations (above >10 19 cm −3 ) in the case of pumping by high-energy photons occurs due to the high dephasing rates of quantum states resulting from a more rapid redistribution of vibrational energy.
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.
The dependence of the emission of a concentrated (∼10 19 cm −3 solution of Rhodamine C on the power density Φ of exciting laser radiation was studied. The emission intensity for the power density of exciting radiation above ∼10 25 cm −2 s −1 was found to have a nearly quadratic dependence on the power density Φ, and this emission was interpreted as the cooperative spontaneous emission of a Dicke type. For Φ≲10 25 cm −2 s −1 , the emission intensity increased with increasing Φ according to the exponential law. This emission was interpreted as the amplified spontaneous emission. The spectra of cooperative emission depended on the pump radiation power only weakly. The absence of lasing in dye solutions at high concentrations, which is a well-known phenomenon, was shown to be caused by the development of the cooperative spontaneous emission and not by the concentration quenching, and the former process is more rapid than the latter.
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.