Broadband Excited State Dynamics of Common Reverse Saturable Absorbing Dye Media.
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Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
after passing through the sample.
--- ....-P- 1 SPATIAL FILTER
CONTINUUM GENERATION CELL
-2 TELESCOPE LASER SYSTEM
Figure 1. The experimental set-up. The continuum probe beam is generated by focusing through a cell of path-length 5cm containing a flowing water/KCl solution. At an input energy of 150AJ the continuum extends from approximately 400nm to 800nm. Two nonlinear optical processes contribute to the spectral broadening: self phase modulation (SPM)[6] and four wave Raman mixing[6]. The salt solution is used to promote the more stable SPM effect at the expense of the Raman contribution. The continuum probe beam still contains a fraction of the original energy at 590nm; filter F1 is used to remove the residual component. An edge filter (RG610) is used to obtain a "red" probe beam (610-800nm) whereas a band pass filter (OG10) gives a "blue" probe beam (400-570nm). No data has been obtained for the region 570-610 except at the fundamental wavelength of 588nm. It should be noted that dichroic lenses are used after the continuum has been generated. The beam generated in the water cell also has a contribution from stimulated Raman scattering, which appears as a bright ring around the main beam. This is removed by using a spatial filter. Using a series of line filters it has been found that the continuum probe beam has a uniform spatial wavelength distribution. The probe beam is focused into the sample using a 10cm focal length lens, to l/e 2 diameter of 150pm, and passes through the same region as the excite beam. Since the probe beam is smaller than the excite beam it samples a uniform distribution of excited molecules. The beams cross at an angle of 50 inside the fluid sample, which is contained in a cell of pathlength 2mm. Since the continuum generation process in the water cell is initiated by self-focusing of the beam, small fluctuations in the spatial or temporal profile can result in large changes in the spectral profile of the continuum beam. To determine the transmission of a sample, the spectrum of the continuum beam is measured before it enters and after it passes through the sample, for each laser pulse. The signal is normalised by removing the sample. The continuum is split into monitor and probe beams at beam splitter BS2. The beams are respectively focused into two fibre optics and the spectra are recorded using a dual array optical multichannel analyzer. The beam which passes through the sample is much smaller than the fibre input to ensure that nonlinear refractive effects which may change the size of the beam do not contribute to the signal. Filter F2 is used to block scattered light at 588nm generated when the excite beam hits the sample.
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Typically 500 pulses are recorded at each delay position between the excite and probe beams and the ordinary transmission of the sample is determined by allowing the probe beam to arrive at the sample before the excite beam. The background signal, which may contain a contribution fr
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