We report on the fabrication of 3D buried micro-structures in fused silica glass using the selective chemical etching along femtosecond laser irradiated zones. Specifically, we have exploited a novel approach combining two different etching agents in successive steps. The widely used hydrofluoric acid solution, which provides fast volume removal, and potassium hydroxide solution, which exhibits high selectivity, are used to fabricate microfluidic structures. We demonstrate that this hybrid approach takes advantage of both of the individual etchants' special characteristics and facilitates prototyping and fabrication of complex geometries for microfluidic devices.
A scanning optical microcavity is exploited to achieve lens-free 3D tomography of microfluidic channels. The microcavity, powered by a low-coherence source, is realized by approaching a cleaved fiber to few tens of micrometers over the sample. The interference of scattered waves inside the cavity shapes the transverse field distribution by focusing the beam and overcoming the diffraction limit due to the optical-fiber numerical aperture. The focusing effect is also preserved in the inner layers of the sample, allowing optical 3D tomography. Analysis of microfluidic channels was demonstrated through this noninvasive technique. Although the experimental setup recalls the well-known fiber-optic Fourier-domain common-path optical coherence tomography, the proposed method has intrinsic characteristics that distinguish it from the former one.
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