Amplified spontaneous emission in rhodamine dyes: Generation of picosecond light pulses and determination of excited state absorption and relaxation

Abstract

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...

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

“…In the experiments the pump pulses traverses convergently the generator cell. Divergent pump pulse geometries give larger divergence of the amplified spontaneous emission signal [8][9][10]. The small divergence is thought to be due to the build-up of spatial coherence (interference of amplifying photons with fluorescing molecules) in the process of amplified spontaneous emission [30,31].…”

“…The output pulses are amplified in three longitudinally pumped dye cells, then spectrally filtered with a grating spectrometer and finally reamplified in a fourth dye amplifier in order to generate intense frequency tunable picosecond light pulses. 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].…”

Efficient phase-matched third harmonic light generation in hexafluoroisopropanol solutions of a pyrimidonecarbocyanine dye

“…In case of fast depopulation of level 3 (T,,->0) it sets an upper limit of the S{ state population N2 [11][12][13][14], Excited-state absorption of the amplified spontaneous emission signal reduces the amplification of fluorescence light…”

Orientation of transition dipole moments of rhodamine 6G determined by excited state absorption

“…For rhodamine 6G it is r F ~ 4.2 ns (solvent ethanol) [14] and the critical concentration for conventional fluorescence polarization spectroscopy is c 0 ^ 3 X 10~3M [15].…”