Fused-silica cantilevers reveal femtosecond-pulse volume changes

At certain fluences below the ablation threshold, ultrafast-laser pulses produce changes in fused silica including the formation of self-organized patterns consisting of “nanogratings.” Suspecting localized densification coupled to stress accumulation and relaxation in the nanograting regions, Audrey Champoin and Yves Bellouard of the Eindhoven University of Technology (Eindhoven, the Netherlands) have developed an experimental technique to test volume changes in fused silica produced by exposure to femtosecond-laser pulses.

In the technique, a number of fused-silica cantilevers are oriented parallel to each other side by side and fixed at one end to an anchor point. One cantilever is not illuminated and serves as the control; the others are illuminated on one side by scanning the laser beam in a pattern, focusing just below the surface. The result is that any volume changes produced by the laser action cause a beam to deflect through an angle, providing a quantitative measure of local volume change. Light from a laser emitting 380 fs pulses at 1030 nm and an 860 kHz repetition rate was focused through an objective with a 0.40 numerical aperture and scanned across the cantilever surfaces, with the light polarization either transverse or parallel to the scanning direction. The deflections (50 to 90 μm) showed a volume expansion with exposure and a possible weak dependence on polarization; relative estimated expansion was 0.03%. Principal stresses were on the order of 300 MPa. Contact Yves Bellouard at [email protected].