Transient light-induced refractive index change made by laser microfabrication in nitroaniline-doped PMMA film

We report the observation of high light-induced change in refractive index (recognizable by observation in conventional microscope) in PMMA film doped with an optically non-linear dye 2-nitroaniline (NO2(C6H4)NH2 abbreviated as 2NA). The optically altered micrometer-sized regions were fabricated by single-shot radiation of 120 fs laser pulses into doped PMMA film using high numerical aperture 1.3 and high magnification x100 objective lens. The doping of films can be achieved in a wide range of 2NA concentrations (up to 40 wt%) without precipitation. This allows to control a storage time of an optically altered region up to one month by the adjusting the energy of the femtosecond (fs) recording pulse at 800 nm. Typical recording energy was 10-80 nJ/pulse at the point of irradiation. Total recovery of transmission of the PMMA2NA film was confirmed by optical transmission measurements in a microscope. The light induced damage threshold (LIDT) (for permanent damage) was increased more than by four times (up to 40 nJ/pulse) when 2NA doping were ca.1e wt%. While the LIDT for transient damage was decreased by 1.5-2 times. Total optical recovery was observed single exponential with decay time of ca. 0.5-1 minute for moderate irradiation intensities (0.1xLIDT of permanent damage). The damage induced with at the higher intensities lasts up to a month, but the recovery was not total (residual transmission changes were observable). The phenomenon can be applied for the optical memory, photonic crystal, and micro-mechanical applications. The underlying mechanism of the phenomenon is discussed in terms of anelastic (alpha) and (beta) -relaxation (polymer backbone and side chain relaxation, respectively).