D. Liu, Z. Kuang, W. Perrie, E. Fearon, S. P. Edwardson, G. Dearden, and K. G. Watkins
Liquid crystal Spatial Light Modulator (SLM) can suffer irreparable damage, when exposed at wavelengths shorter than 400 nm combined with high peak intensity, due to photodegradation of the liquid crystal or the polymer alignment layer. By placing a thin BBO nonlinear crystal immediately after an SLM addressed with Computer Generated Holograms (CGHs), the first order diffracted NIR components at 775 nm can be converted to parallel second harmonic NUV beams at 387 nm, avoiding the potential damage while simultaneously reducing the order of non-linear absorption for refractive index modification. This procedure requires attention to phase matching of multiple beams and opens up parallel processing at UV wavelengths. Multiple NUV femtosecond beam direct writing of volume Bragg gratings inside poly(methyl methacrylate) and fused silica is demonstrated. First order diffraction efficiency over 70% is observed. By changing CGH, grating parameters such as period and thickness can be easily adjusted. This technique provides good flexibility and shows great potentials in rapid fabrication of volume gratings. The limitations of this technique are also discussed.