Limiting Mechanisms in a Photoconducting Liquid Phthalocyanine
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C 19 9 7
Materials Research Society
charge generation would lead to an optical breakdown. Optical breakdown can be useful mechanism for limiting at high fluences. The plasma formed and the associated scattering centers can absorb and scatter the light. It is reported to be the dominant mechanism in a carbon black suspension (CBS) limiter.5 In the CBS limiter, the plasma was reported to be initiated by thermally produced carriers formed by laser heating of carbon particles. It seemed possible that photo produced charge carriers could also lead to an optical breakdown at similar fluences. Optical breakdown and the subsequent plasma formation could be a useful high fluence limiting mechanism in RSA limiters. RSA limiters have the lowest limiting thresholds, but the effect does saturate for a simple four level system. Optical breakdown can provide an additional mechanism for high fluence limiting and could thereby substantially enhance the dynamic range of RSA limiters. EXPERIMENTAL APPARATUS Approximately 4 ftm thick liquid crystal cells were cleaned, dried and filled with PbPc(P-PEO) 4 by capillary action. These cells have an approximately 5 mm clear area with conducting ITO coatings on each window. The exact path length of each cell was determined by interferometry. The apparatus was an f/5 optical limiter. Briefly, the sample was irradiated with a spatially filtered beam from a doubled NdIYAG laser pumped dye laser or the doubled YAG laser itself. The NdIYAG laser was seeded to produce a temporally smooth 6 nsec pulse. The dye laser pulse width was approximately 4 nsec. Two different sets of focussing optics were used. For limiting studies, the beam was expanded and the central 10% was used. A multiplet lens, apertured to f/5, focussed the light to a spot (Airy disk) with a measured beam radius of 2.8±0.2 Jim (I=(1/e 2)Io) at 532 nm. Some experiments were performed using an f/30 lens with the sample placed away from the focus. Here, the measured beam diameter at the sample was 0.5 ±0.02 mm. In each case, the sample was mounted on a translation stage so that at higher incident energies, the sample could be translated to provide a fresh spot for each experiment. The voltage produced across the sample was stored with a Tektronics 2440 oscilloscope. Light generated in the sample was studied in the f/5 limiter by placing an f/0.8 collecting lens behind the sample. The light was passed through a line filter and focussed onto a fast photodiode detector. The line filter had an OD = 8 at 532 nm so that only emission at wavelengths other than the pump wavelength was detected.
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RESULTS Figure 1 presents a plot of the incident energy vs. the transmitted energy for an f/5 optical limiter using a 3.67 Am sample of PbPc(P-PEO) 4 as the nonlinear medium. At 532 nm, the transmission of the sample was 0.65. The transmission of the PbPc(P-PEO) 4 sample alone was 0.75. The relatively high losses in the cell were due to the ITO coatings on the windows. The limiting thresh-
old, where Tfr,= 0.5, was 22E2 nJ. The initial slope o
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