Fabrication of Polymer Photonic Crystals by Two-Photon Nanolithography
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Fabrication of Polymer Photonic Crystals by Two-Photon Nanolithography Tae-Woo Lee, Oleg Mitrofanov, Christopher A. White and Julia W. P. Hsu Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974 ABSTRACT We use a two-photon laser-scanning microscope to fabricate two-dimensional (2D) photonic crystal structures in commercially available SU-8 polymer films, and successfully demonstrate making nanostructures beyond the diffraction limit with high aspect ratios. By varying the laser beam power, scanning speed, focal depth, line spacing and scanning angles, we obtain 2D photonic crystals with circular, elliptical, rectangular, or diamond-shape unit cells in a hexagonal or square lattice. An aspect ratio as high as 6.9 with 250 nm line width was achieved. In addition, we can controllably place defects of specific patterns, e.g. lines, dots, and Y-splitters, in the otherwise perfect photonic crystal. We also combine two-photon nanolithography with conventional UV photolithography to make 2D photonic crystals between waveguides. The combination of these two lithography methods was done on a single polymer film, suggesting potential for easy fabrication of complex photonic devices. INTRODUCTION Photonic crystals are candidates for performing light manipulating in restricted spaces in future photonic devices. They require feature sizes smaller than the diffraction limit with high index contrast. Nanostructures that meet these requirements have high-aspect-ratio features and are difficult to make using conventional photolithography based on one photon process. Photopolymerization based two-photon absorption provides a solution [1,2]. The absorption in the two-photon initiated processes occurs only in the vicinity of the focal point in all three dimensions because of the quadratic dependence on excitation intensity [3,4]. In this work, we use two-photon nanolithography to make 2D photonic crystals, and demonstrate sub-diffractionlimit features with high aspect ratios as well as controlled placement of defects in the structures. EXPERIMENTAL DETAILS Our two-photon microscope setup includes a 130 fs, 76 MHz, 710 nm mode-locked Ti:sapphire laser as the light source and a modified Nikon microscope with a piezo scanner to
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move sample with respect to the laser beam [5]. The polymer material was SU-8 (Micro Chem Corp.), a negative photoresist commonly used for high-aspect-ratio structures. The surface of the glass substrate was treated by oxygen plasma (0.13 Torr) for 2 min at 50 W to improve adhesion between the polymerized SU-8 and the glass. After spinning the SU-8 films on the substrate, the film was prebaked at 65oC for 30 min. The laser beam was focused with a Nikon 100x objective (0.95 NA) at the interface of glass and SU-8 to make p
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