MULTIPLE NUV BEAM INTERNAL STRUCTURING OF MATERIALS USING A SPATIAL LIGHT MODULATOR Paper M301 Dun Liu 1, 2 , Zheng Kuang 2 , Walter Perrie 2 , Eamonn Fearon 2 , Stuart Edwardson 2 , Geoff Dearden 2 , and Ken Watkins 2 1 School of Mechanical Engineering, Hubei University of Technology, Wuhan, 430068, China 2 Laser Group, Department of Engineering, University of Liverpool, L69 3GQ, UK Abstract 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. Introduction When NIR femtosecond (fs) laser pulses are tightly focussed inside optical materials for refractive index (RI) structuring, pulse energy (E p ), scan speed (s), scan direction [1], effective NA [2], pulse duration ( τ) [3, 4] and wavelength (λ) [3] are important parameters. Translation of the focus within the bulk material allows the creation 3D refractive index modification. The use of a Spatial Light Modulator (SLM), addressed with Computer Generated Holograms (CGHs) can greatly accelerate 3D modification by generating many, uniform intensity arbitrary parallel beams, recently used, for example to create thick, efficient volume gratings inside poly(methyl methacrylate) (PMMA) at 775 nm [5]. By using an SLM to dynamically correct wavefront aberrations at the air/dielectric interface during the writing process in fused silica, real time parallel 3D generation of longitudinally written waveguide couplers was demonstrated [6]. Pulse duration τ > 100 fs in the NIR, the contribution of impact ionisation towards plasma density increases relative to multi-photon ionisation, reducing the damage threshold of optical breakdown. [3, 4]. With pure PMMA at τ = 180 fs, the advantage of NUV wavelength at 387 nm over the fundamental NIR wavelength at 775 nm was demonstrated by reducing the order of the non-linear absorption from three to two photon [3]. Filamentation, in which self- focussing is balanced with defocussing due to plasma formation, is useful in extending the modification depth at low Numerical Aperture (NA) [7], essential for thick grating production. Presently, no SLM based on nematic liquid crystals operates in the UV at 355 nm due to photodegradation of the liquid crystal or the polymer alignment layer with such high photon energy. Thus, exposure at wavelengths  400 nm with high peak intensity risks irreparable damage and possible device failure. By placing a thin beta barium borate (BBO) non-linear crystal immediately after an SLM addressed with CGHs, the first order diffracted NIR components at 775 nm here are converted to parallel NUV beams at 387 nm, avoiding this potential problem while simultaneously reducing the order of non-linear absorption for Δn structuring. Experimental Fig. 1 shows a schematic of the experimental setup. The output from a Clark-MXR CPA 2010 system, (775 nm, 170 fs, 1 kHz, 1 mJ), is attenuated then expanded to ~ 7 mm diameter on to an SLM (Holoeye, LC- R2500) which has a 45 twisted nematic liquid crystal layer. A thin BBO crystal (8 × 8 × 0.7 mm 3 , cut angle 730