Fluorescein-doped boric acid glass is a material characterized by an extremely low saturation intensity of-15 mWcm and a nonlinear susceptibility g"' as large as-1 esu. The saturated absorption of this material is shown both theoretically and experimentally to depend on the state of polarization of the saturating beam, even though the unsaturated absorption is polarization insensitive. Phase-conjugate reflectivities as large as 0.6% have been obtained through use of degenerate fourwave mixing in this material. These measured reflectivities are in good agreement with the predictions of a theory that includes the effects of excited-state absorption and grating washout. In addition, two-beam coupling due to the nonlinearity of saturable absorption has been demonstrated in this material. The magnitude of the coupling is maximized by inducing a frequency shift between the two beams of-0.1 Hz.

A new nonlinear-optical material based on lead-tin fluorophosphate glass doped with the organic dyes Acridine Orange and Acridine Yellow has been developed. These materials are saturable absorbers characterized by saturation intensities of-100 mW cm-2 , response times of-1 msec, and third-order nonlinear susceptibilities of-0.1 esu. The low melting temperature of the lead-tin fluorophosphate glass permits doping with many organic dyes without causing their decomposition. p, Tg, at, and nD are typically 4.4 g cm-3 , 50 0 C, 35 X 10-6°C-i, and 1.90, respectively. A more restricted region of composition space with a lead content from 3 to 6 cation % and a tin content from 40 to 60 cation % yields exceptional durability. In a water immersion test at 501C for 24 h, dissolution rates in these compositions can be less than 1 mg cm-2 day'. For comparison, soda-lime float (window) glass typically dissolves at a rate of 0.02 mg cm-2 day-'. The Knoop hardness of these glasses is in the range 90-120, as compared with a typical value of 330 for soda-lime glass and 20 for the polymer, polymethyl methacrylate.

The saturation properties of dye molecules that are rigidly held in a solid host are qualitatively different from those of molecules that are free to rotate. We have found that these unique saturation characteristics can be exploited to achieve nearly perfect vector phase conjugation for field strengths near the saturation intensity. We have studied these properties experimentally by using fluorescein-doped boric acid glass as the nonlinear-optical material. Organic dyes doped into solid matrices form an important and interesting class of optical materials. Indeed, some of the largest optical nonlinearities are those due to saturable absorption in organic molecules doped into various solids. 1,2 The nonlinear-optical properties of rigidly held molecules can differ substantially from the properties of isotropic absorbers such as atoms or of molecules that are free to rotate. For example, for a randomly oriented, rigidly held collection

We have demonstrated a passive method for recovering an optical image that has been degraded by being passed through a thin phase-aberrating medium. This method relies on a point source situated near the object of interest to sample the aberration impressed upon the wave front. Degenerate four-wave mixing in fluorescein-doped boric acid glass was used to reconstruct the wave front.

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