By switching between linear and circular polarization in the irradiation of supersaturated solutions of the amino acid glycine in water with intense nanosecond pulses of near-infrared laser light, we have obtained the gamma and alpha phases, respectively, through nonphotochemical light-induced nucleation (NPLIN). This is the first report of light polarization controlling crystal structure. The intensity dependence of NPLIN in aqueous urea is also reported.

Supersaturated aqueous solutions of glycine exposed to intense pulses of plane-polarized laser light at 1.06 μm unexpectedly crystallized into the polar γ-polymorph of glycine. Control solutions not exposed to the laser always produced crystals of α-glycine, the expected and most stable form. This result suggests a new approach to polymorph control and possibly a means to produce new polymorphs.

We have measured the intensity dependence of nonphotochemical laser-induced nucleation (NPLIN) in 11.9 M aqueous urea solutions at 532 and 1064 nm, using both linearly and circularly polarized light. In all cases, we observed a nonlinear intensity dependence of the nucleation efficiency with thresholds of approximately 0.02-0.06 GW/cm 2 . The threshold at the green wavelength is lower than the threshold at the near-infrared wavelength. Moreover, the green nucleation efficiency is greater than the near-infrared efficiency at the same intensity. We attribute these differences to the greater absorption coefficient of water at 1064 nm compared to that at 532 nm. At both 532 and 1064 nm, the threshold for NPLIN is lower, and the efficiency of NPLIN at a given intensity is higher for linear polarization than for circular polarization. These differences are consistent with an optical Kerr effect mechanism for NPLIN.