Saturable absorbers with concentration-dependent absorption recovery time

Abstract

Abstract. The ground-state bleaching of highly concentrated rhodamine 6G solutions in methanol is studied with intense picosecond light pulses. The ground-state recovery time changes with concentration from about 3.9 ns at low concentration (10~5 mol/dm 3 ) to about 1 ps at high concentration (0.6 mol/dm 3 ). The shortening of the absorption recovery time is determined by the concentration dependent quenching of the S 1 -state population due to unbound dimers and by the intensity dependent longitudinal and transverse amplified spontaneous emission. 42.65, 42.70, 42.20 The bleaching of absorbing media by intense light pulses is a general phenomenon. The only requirement for reduction of absorption with increasing light signal is that the ground-state absorption cross-section is larger than the excited-state absorption cross-section [1][2][3][4][5]. For laser pulse durations At L short compared to the absorption recovery time z A the bleaching behavior is determined by the input pulse energy density e [6]. At a pulse energy density s s = hv L /<j L the ground-state population is reduced to approximately half its initial value (exact value depends on absorption dynamics [6]). s s is called the saturation energy density, v L is the laser frequency and o L is the ground-state absorption cross-section. For laser pulse durations long compared to the absorption recovery time the light transmission is intensity dependent. At a pulse intensity of h = hv L /(T L T A , the saturation intensity, the groundstate population is reduced to approximately half its initial value [1][2][3][4][5][6]. The application of saturable absorbers as modelocking dyes requires absorption recovery times short compared to the resonator round-trip time. In passively mode-locked Nd lasers and ruby lasers (small stimulated emission cross-section, no gain dependent pulse shortening) absorption recovery times of some picoseconds are necessary for picosecond pulse generation [7]. Shortening of picosecond pulses to the subpicosecond region may be achieved by passing picosecond pulses through saturable absorbers of some picosecond absorption recovery time [8][9][10]. The radiative S t -S 0 lifetime of dyes is generally between 1 and 10 ns. Picosecond absorption recovery times are due to fast radiationless S x -S 0 relaxation by internal conversion. At low concentrations fast internal conversion is found for molecules with flexible structure in low viscous solutions [11][12][13][14]. For infrared dyes the internal conversion is fast because of the small S 0 -S l energy gap [15][16][17]. At high concentrations the Si -So lifetime is shortened by selfquenching [11,12,[18][19][20][21]. Molecular aggregates (dimers, trimers, etc. and molecules in near distance) with fast internal conversion rate are formed. Molecular diffusion and Förster-type energy transfer transport the excitation to the quenching centers and a fast absorption recovery occurs [11,12,22]. In this paper the concentration dependent absorption recovery time of rhodamine 6G in methanol is s...

Experimental and theoretical investigation of tunable picosecond pulse generation in longitudinally pumped dye laser generators and amplifiers

“…The shift of the emission to longer wavelengths lowers the stimulated emission cross-section and reduces the energy conversion efficiency rj ASE . For highly concentrated dyes the penetration depth may become shorter than the pump beam diameter, and stimulated emission in the transverse direction may be preferred [32]. Transverse amplified spontaneous emission is facilitated by focusing the pump pulse with a cylindrical lens to the sample [5][6][7][8][9].…”

Fluorescence behaviour of highly concentrated rhodamine 6G solutions

“…Closely spaced pair fluorescence was resolved at high concentrations. The ground-state absorption recovery time versus concentration was measured in [17] and found to be approximately equal to the fluorescence lifetime. The short fluorescence lifetimes (e.g.…”

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…”