Stepped mask interference laser exposure (SMILE)

We present a proof-of-concept of a novel fabrication setup for polarization converters – stepped mask interference laser exposure (SMILE). The SMILE setup enables the cost-effective fabrication of polarization converters by means of standard laser interference lithography and dry etching processes. Key advantages of the proposed system include a large clear aperture diameter, limited only by the size of the lithography system, and a short fabrication time. Lloyd's mirror interference lithography setup is modified by adding a mask with a circular sector opening. The mask and a substrate have independent rotation drives enabling the fabrication of segmented diffraction gratings with different grating line orientations. Each segment acts as a half-wave plate converting incident linear into radial polarization. Radial polarization is particularly beneficial in high-power laser applications, e.g. in material processing, as it provides higher cutting efficiency. Using SMILE we fabricated a segmented diffraction grating with a pitch below 500nm and a clear aperture diameter of 30mm. With an exposure time of 18s per segment, the resulting total processing time was below 5 minutes.