Diffractive optics near-field laser lithography for fabrication of 3-dimensional periodic nanostructures

The main objective of the present research work is to fabricate three dimensional photonic nanostructures in photo-sensitive polymers using a novel diffractive optical element (DOE) based lithography technique. A diffractive optical element is a promising alternative device for 3D fabrication where one DOE creates multiple laser beams in various diffraction orders that are inherently phase-locked and stable for reproducible creation of 3D near-field diffraction patterns from a single laser beam. These near-field patterns are captured inside a photosensitive material like photoresist to fabricate 3D photonic crystal templates. We have demonstrated fabrication of a wide range of 3D structures having different crystal symmetries and different relative crystal axis ratios. The present work has provided 3D photonic crystal nanostructures with uniform optical and structural properties over large sample area (∼3-4 mm diameter) and through large 15-50 mum thickness with large number of layers (> 40) having period 550 nm - 650 nm and feature sizes between 200 nm and 300 nm. The short exposure time and small number of process steps shows promise for scaling to very large volume fabrication, dramatically improving the throughput, quality and structural uniformity of 3D periodic nanostructures, especially over that provided by tedious and costly semiconductor processing technology. The diffractive optics lithography is a parallel processing method that is easily scalable to generate centimeter-scale 3D nanostructures having large number of layers in several seconds. Due to low refractive index contrasts these polymer templates possess partial stopgaps along several crystallographic directions which can be practically used in several device or sensor applications where complete bandgap is not necessary. The potential usefulness of these partial stopbands for refractive index sensing of liquids has been demonstrated. These low refractive index polymer structures have been inverted with amorphous silica to convert a "soft" polymer structure to a robust "hard" structure. Further, few preliminary tests were done in fabricating 3D nanostructures into micro-fluidic channels for potential chromatography applications. The practical merits of this 3D fabrication technique will enable new practical manufacturing methods for optical and MEMS applications of 3D micro and nano structures.