The amplified spontaneous emission in dye solutions excited with intense picosecond light pulses is studied theoretically. A multi-level model is applied to take into account the effects of the various dye parameters such as ground-state absorption, stimulated emission, excited-state absorption, reabsorption and relaxations from populated excited levels. The influence of the pump laser duration and intensity and of the dye concentration and sample length is investigated. Optimum situations are derived for the generation of intense picosecond light pulses at new frequencies by amplified spontaneous emission. On the other hand, conditions are found where amplification of spontaneous emission may be neglected. The analysis allows the determination of unknown dye parameters by comparing the calculations with the experiment. IntroductionAmplification of spontaneous emission (superfluorescence) is readily observed in organic dye solutions when the sample is excited with intense light pulses [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The spontaneous fluorescence light is amplified by stimulated emission. When picosecond light pulses are used to excite the dye solutions, the fluorescence signal is shortened in time, increased in intensity, narrowed in spectral extension, and concentrated into a small divergence angle [1-3, 7,8,12,14]. The effect of amplified spontaneous emission allows the generation of picosecond light pulses in new frequency regions. The conventional fluorescence studies are carried out with weak excitation light sources and the amplification effects are negligibly small; but the amplification of spontaneous emission alters the spectroscopic behaviour substantially when iritense pump pulses are used [15,16].The purpose of this paper is to study, theoretically, the influence of various parameters on the fluorescence behaviour of dyes which are excited with intense picosecond light pulses. A model is used that allows the study of the influence of pump pulse absorption, stimulated emission, reabsorption and excited-state absorption as well as of relaxation phenomena in excited states. The effects of the geometrical arrangement, the pump pulse parameters and of the dye concentration are discussed. The analysis allows the determination of unknown dye parameters, such as relaxation rates, by comparing the calculations with the experiment. Optimum situations are found for the generation of picosecond light pulses at new frequencies. Conditions are derived where amplification of spontaneous emission is negligible and conventional fluorescence spectroscopy leads to correct results.

The following quantitative investigations with picosecond pump pulses were made as a function of intensity: (1) fluorescence in the forward direction; (2) duration of the fluorescence; (3) and (4) spectral peak and width of the emission; (5) energy transmission of the pump pulse. Large energy amplification (factor 300) and drastic reduction of the duration of the fluorescence (factor 150) were observed. Comparison with model calculations allows the determination of various molecular absorption and relaxation parameters.During the past decade, amplified spontaneous emission of fluorescent molecules has been studied under a variety of experimental conditions [1]. While stimulated emission is the prerequisite for any laser, the same physical process might be quite disturbing when spectroscopic parameters (e.g. the spontaneous emission lifetime) are measured at high excitation intensities. Qualitative investigations of amplified spontaneous emission of organic dyes have been reported by a number of authors using picosecond light pulses as pump source [2][3][4][5][6].In this letter we present quantitative measurements of five physical phenomena studied as a function of input intensity with molecular concentration as the main parameter. We investigated; (1) the fluorescence emission in the forward direction; (2) the reduced duration of the fluorescence; (3) the wavelength shift of the peak of the emission, (4) the spectral width of the fluorescence and (5) the energy transmission of the picosecond pump pulse. The purpose of these investigations is two-fold: First, we wanted to see to which extent the fluorescence emission changes for high input intensities; in particular we were interested in the shortest possible pulse duration of the fluorescence * Present address: Universität Regensburg, Germany emission. Second, we determined important molecular absorption and relaxation parameters by fitting model calculations to our various experimental results.Experimentally we worked with a mode-locked Ndglass laser [7]. A single pulse is selected from the beginning of the pulse train with an electro-optical shutter. The second harmonic (? L = 18 910 cm -1 ) of the laser pulse is generated in a KDP crystal. The duration and width of the green pulse is Af L ^ 4 ps and Av « 5 cm -1 , respectively. The peak intensity is determined by measuring the energy transmission through a two-photon absorbing rutile crystal [8]. Rhodamine 6G and rhodamine B dissolved in ethanol are investigated. The dye cells are tilted to avoid amplification of reflected fluorescence light. Different detection systems were applied after the sample depending upon the specific investigation. The divergence of the amplified spontaneous emission was analysed with apertures of different size. The fluorescence energy within a solid angle A£2 was measured with a photomultiplier. The fluorescence duration was recorded with a streak camera (3ps resolution). The spectral distribution of the fluorescence light, the spectral narrowing, and the spectral shift were studi...

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The nonlinear absorption of single picosecond light pulses (A « 1.06 ßtn) in CdS is investigated at very high light intensities. Three-photon absorption and subsequent excited-state absorption of the generated electrons and holes explain the rapid decrease of transmission with increasing intensity. A three-photon absorption cross-section of -(2 ± 0.5) X 10~8 0 cm 6 $' and an average excited state absorption cross-section of o ex = (7 ± 3) X 10" 18 cm 2 was determined.

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