Theoretical investigation of amplified spontaneous emission with picosecond light pulses in dye solutions

Abstract

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.

A versatile dye-laser generator-amplifier system for intense, tunable picosecond pulse generation

“…The overall duration is Jt F~9 ps (FWHM). The generation of pulse durations shorter than the pump pulse duration is due to the high amplification of fluorescence light [14][15][16][17][18][19] and is supported by the short fluorescence lifetime of the dye. The amplifiers Al to A3 broaden the pulse duration from 9 ps after the generator to about 20 ps after the third amplifier (Fig.…”

“…In longitudinally pumped systems the amplification of spontaneous emission is limited by excitedstate absorption of the pump laser light [19]. In transversally excited dye cells the duration of the amplified spontaneous emission signal is restricted by the transit time of light through the pumped region, and two pulses are emitted in opposite directions.…”

“…N is the total number density of dye molecules, cr ex L is the excited-state absorption cross-section at the pump laser frequency. The amplification of spontaneous emission is given by the differential equation [19] energy is used to pump longitudinally a series of amplifier cells to increase the output signal of the generator cell to a high energy level.…”

“…The arrangements applying amplified spontaneous emission (ASE) may be grouped into longitudinally pumped [8][9][10], transversally pumped [11][12][13], and travelling-wave transversally pumped systems [14][15][16][17][18]. In longitudinally pumped systems the amplification of spontaneous emission is limited by excitedstate absorption of the pump laser light [19]. In transversally excited dye cells the duration of the amplified spontaneous emission signal is restricted by the transit time of light through the pumped region, and two pulses are emitted in opposite directions.…”

S1-S0 relaxation time of saturable absorber DDI

“…A shortening of T by amplified spontaneous emission is negligible under our experimental conditions and is not included in the calculations (eq. (26)) of [16] …”