Nonlinear Mechanisms in Carbon-Black Suspension in a Limiting Geometry
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Mat. Res. Soc. Symp. Proc. Vol. 479 0 1997 Materials Research Society
at X = 440 nm to 41OmJ/cm 2 at X = 680 nm, where the limiting threshold is defined as the intersection point of lines corresponding to the linear and nonlinear part of a limiting curve.
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Incident Fluence (J/cmz) Figure 1. Spectral dependence of limiting response of CBS.
This spectral dependence of limiting threshold may be explained by the increased absorption of carbon particles at short wavelength leading to an increased temperature of the particles. Although the microscopic nonlinear mechanisms responsible for limiting are still not clearly evidenced in CBS, the temperature of the particle is a decisive parameter on the threshold in the two main mechanisms previously observed in this materials [1], [3]. These mechanisms are on one hand microplasma formations leading to enhanced scattering an absorption, on the other hand vapor bubble formation around the intensely heated carbon particles. In order to determine the respective influence of plasma and bubbles near the limiting threshold and at higher incident fluence, we have performed a collinear pump-probe experiment coupled with time resolved measurements of plasma emission. 4. PUMP PROBE MEASUREMENTS The pump probe set-up is shown in Figure 2. The pump beam was a Q-switched injected Nd:Yag laser emitting temporally gaussian pulses of duration "ias -= 3 ns (FWHM) and wavelength X= 1.064 pim. It passed through a waveplate/polariser pair which controlled the incident energy on the sample. The incident pulse energy was measured on a pyroelectric detector and the temporal profile using a fast photocathode (rise time = 270 ps). The pump beam was focused in the sample using a 200 mm focal length achromatic doublet and generated the nonlinearity in the CBS cell. The mesured beam waist was w(l/&) = 22 pim. This nonlinearity was read by a continuous He-Ne laser emitting at X = 632.8 rnm. This continuous probe was inserted on the optical path of the pump beam using a polarizing cube, and extracted by the same way. A fast photomultplier (rise time 0.6 ns) connected to a digital oscilloscope was used to analyse the probe signal. Time resolved measurements of plasma emission were simultaneously performed using the same type of photomultiplier in the sensitivity range (300-800 nm ) of the detector.
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Figure 2. Pump-Probeset up
Probe intensity variations were observed in two different configurations, the first one is an open aperture configuration where the whole beam is collected by the detector, the set-up is thus sensitive to nonlinear scattering and absorption. The second one is a small aperture configuration, the set-up is thus also sensitive to refraction. Probe variation can be measured during the pulse duration and on longer timescale, which corresponds to relaxation in the sample. Figu
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