Precision Drilling With Lasers

Arrays of identical microscopic holes and other features in thin films are useful in photonics applications such as gratings, waveguides, and photonic crystals. Compared with forming those features via micromachining or synthesis methods, drilling them with ultrashort laser pulses would be convenient and quick. But that method is difficult to control. Pulse fluctuations limit feature reproducibility, and the laser beam can damage the substrate supporting the thin film. Laurent Mercadier, David M. Rayner, and Paul B. Corkum of Canada’s National Research Council, in Ottawa, demonstrate that those problems can be bypassed by irradiating the film from the back—through a transparent substrate. The team reports that they drilled sets of holes reproducibly without damaging the substrate by delivering 45-femtosecond pulses of 800-nm light through a silica support to an 8-nm-thick film of poly(allylamine hydrochloride) and poly(sodium 4-styrene-sulfonate) (Phys. Rev. Appl. 2014, DOI: 10.1103/physrevapplied.2.034001). The precision in drilling, verified with atomic force microscopy, results from interactions of the beam as it propagates through the substrate. Those processes reshape the beam’s spatial profile and curb its intensity, sidestepping problems caused by laser fluctuations.