Circular microchannels inside bulk PMMA generated by femtosecond laser using slit beam shaping

We report on laser generation of three dimensional microchannel architectures with fundamentally unlimited channel length by focused 514 nm femtosecond laser pulses inside PMMA bulk material. The three-dimensional channel layout is implemented by moving the specimen using three dimensional motorized stages, allowing freely chosen complex shaped channel architectures. Based on nonlinear absorption of high intensities around the focal volume, a material modification including a refractive index shift is triggered. After laser exposing, the channel is created by a gaseous degradation of exposed areas during an annealing process. Cross section and aspect ratio of thus generated microchannels are strongly influenced by the numerical aperture of the applied objective. To obtain a circular cross-sectional shape, high focusing objectives are necessary. In this report, we improve the existing setup by using an elliptical input beam transforming the intensity distribution of the focal area in propagation direction to a circular shape even for low and medium numerical aperture focusing objectives and therefore avoiding a small working distance as a limiting factor for processing depth. The elliptical input beam is obtained by insertion of a slit mask in front of the focusing objective. The ellipticity of the incident beam defines the aspect ratio of the generated microchannels. Furthermore, the size of internal microchannels is adjustable by varying laser and annealing process parameters.