Femtosecond to Nanosecond Characterization of Optical Limiting Mechanisms in Power Limiting Liquids and Solids
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increasing laser fluence[I]. OL materials can thus strongly attenuate intense optical beams while keeping high transmittance at low-intensity ambient light levels (providing passive protection). OL materials are desirable for protecting optical sensors or human eyes from the high fluence output of pulsed lasers. Although there are several mechanisms which lead to OL (including two-photon absorption, photorefractive effects, and nonlinear scattering), the best OL performance in the visible spectral range has been observed using reverse saturable absorption (RSA)[1]. RSA occurs when the exited states formed through optical pumping have a larger absorption crosssection than the ground state. While fullerenes such as C60 show RSA with reasonably strong OL in the visible regime[2,3], the strongest RSA effects have been observed for metallo-porphyrins [4] and metallophthalocyanines[5,6]. Most OL studies have reported measurements in solution. However, for practical optical devices, solid-state limiters are strongly preferable. In recent years, we have developed techniques for doping OL dyes into various optical glasses using sol-gel techniques, via both predoping and postdoping[7,8,9]. In this paper, we report our recent advances in making solid-state blends which show both strong OL effects, as well as very high damage thresholds. In order to increase the damage threshold (and hence the dynamic range of OL devices), "tandem" limiter[4] and gradient (or "bottleneck") limiter geometries[10] have been suggested. To facilitate ease of manufacturing, we chose to study dual tandem limiters. Dual tandem solution cells with different cell thickness and separation, and at different positions relative to the focal point, were studied to provide benchmark performance for our solid-state designs. Our best result in solution has been an attenuation of approximately 500x, with a linear transmittance of 50%, and a damage threshold of approximately 0.5 mJ. Our best result for a solid-state dual tandem limiter is 400x attenuation, 437 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
with linear transmission of 65%, and damage thresholds as high as 3 mJ. unprecedented performance and dynamic range in the solid-state.
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EXPERIMENTAL Two experimental configurations were used for this work. Femtosecond transient absorption (TA) spectroscopy was used to determine the spectral characteristics and dynamics of excited-state absorption leading to RSA. Our setup utilized an amplified Ti:sapphire laser to generate broadband, 150 fs pulses at 1 kHz repetition rate; TA was measured using a standard pump/probe geometry. Chirp-free transient absorption spectra were measured in the range from 450-800 nm, as described elsewhere[l 1]. Nanosecond fluence-dependent transmittance measurements were performed to quantify the OL performance. In this setup, a 6 ns Nd:YAG laser/optical parametric oscillator was used to generate tunable light 400-2000 nm. All OL measurements reported here used a wavelength of 532 nrn, which
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