Diffraction Measurement of Grating Structure Induced by Irradiation of Femtosecond Laser Pulse in Acrylate Block Copolymers

Abstract

Diffraction measurement was carried out for the evaluation of grating structures induced by irradiation of near-infrared (NIR) femtosecond laser pulse in block copolymers of methyl methacrylate and (ethyl acrylate and butyl acrylate) {p-(MMA/EA-BA) block copolymer}. Line irradiation, performed by scanning these copolymer bulks with a laser spot, produced the aggregated deposits of sub-micron scale. Stripe-shaped grating structures of approximately 3–5 µm width and approximately 30–220 µm thickness were induced at an interval of 7 to 15 µm. The total number of gratings was between 10 and 50. The grating structure obtained was evaluated by means of diffraction measurement. Raman–Nath type and Bragg type transmission diffraction patterns with diffraction efficiencies of approximately 1–13% and Q values of approximately 0.1–9 were observed depending on the grating interval and thickness. Refractive index changes evaluated using the Bessel function were approximately 0.20-3.0×10-3. A Bragg type diffraction pattern was observed in the sample with a Q value of approximately 9 and a refractive index change of 0.30×10-3 was obtained by optimization using the Robust design (Taguchi Method). Relative grating intensity linearly increased with increasing photo-crosslinker content in the acrylate block copolymer. Multiple scanning of line irradiation increased the refractive index from 0.30×10-3 for a single scan to 1.1×10-3 for three scans.