NUV and NIR Femtosecond Laser Modification of PMMA

Substantial refractive index changes in clinical grade poly(methyl methacrylate) (PMMA) with both femtosecond NIR (800 nm) and NUV (387 nm) laser pulses are demonstrated, enabling volume gratings and waveguides in bulk PMMA. In the NIR, temporal pulse duration is shown to be critical, with modification efficiency increasing rapidly as pulse duration decreases below 200 fs. On the other hand, in the NUV, efficient index modification can be accomplished with longer, 200 fs pulses. The successful direct modification of ultra-pure, clinical grade PMMA demonstrates an alternative to processes that require doping to increase sensitivity. Grating diffraction efficiency and refractive index profile measurements infer a maximum positive refractive index change of nmax = 3x10 -3 in exposed regions. Chemical analysis of the modified structures in the NUV suggests direct polymer back bone cleavage and monomer production as photomodification pathway. Holographic writing at 387 nm with a high NA objective produced weak periodic features of period = 0.42 m. The combined results in the NIR and NUV demonstrate the importance of peak intensity for inducing non-linear absorption and suggest that three and two photon absorption are responsible for modification of pure PMMA at 800 nm and 387 nm, respectively.