Wavelength Dependence of the Non-Linear Transmission of Hitci Using the Z-Scan Technique
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Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
Figure 1. The electronic singlet energy level structure for HITCI. The absorptive and stimulated processes are indicated by the solid arrows and the relaxation processes are indicated by the broken arrows. The parameters considered in the model are: xv,the vibronic lifetime, tai, the lifetime for a transition between electronic level i to j, and O'k the absorption cross-section from electronic level k to 1.
SO,S1, S2 and S3 represent the ground, first, second and third excited energy levels respectively. The solid arrows represent absorption or stimulated emission processes, and dotted arrows represent the decay routes. In the model there is no discrimination between non-radiative and radiative decay routes. A system of eight coupled differential equations describe the energy transfer processes. Each equation represents the rate of change of population for one energy level. All factors that influence the rate of change of the population are considered. For example, the time dependent populations of all other energy levels, the Gaussian temporal and spatial intensity profile of the incident laser beam. The Gaussian focussing geometry of the experimental configuration are also rigorously included 8 . EXPERIMENTAL A Nd:YAG Q-switched laser, frequency tripled to 355 na, with a pulse energy of approximately 10 mJ, was used to pump an optical parametric oscillator. The output from the OPO could be frequency tuned from 460 to 630 na having output energies of up to 100 gJ for all the wavelengths considered. The temporal and spatial profiles are near Gaussian, having durations and radius of 4.52 ns (e- 2) and 0.96 mm (e- 2) respectively, at 530 run. A variable attenuator controlled the energy of the pulse. The beam was split into two; the weaker beam was used as a reference to monitor any energy fluctuations, while the stronger beam was used to excite the sample. The excitation beam was focussed by an achromatic lens having a focal length of 175 mm. The calculated focussed spot radius, for 530 nm, was 22 pim (e-2). After the beam passed through the sample it was detected by a calibrated pyroelectric detector. The transmitted light from the sample was detected in an open aperture configuration. This allowed the measurement of non-linear absorption without the complications of non-linear refraction. Losses due to surface reflections from the sample and lens were taken into account. The reference beam provided a precise calibration of the energy incident on the dye sample. This incident energy was fixed for the duration of the z-scan. The transmitted energy was measured and a transmission of the sample was calculated by averaging 100 pulses. The transmission was measured for a number of z positions through the focus. To be sure that no surface damage effects were contributing to the non-linear signal, it was necessary to translate the sample through the focus in one direction and then back in the reverse direction. If surface damage
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