Three-dimensional nanofabrication with elastomeric phase mask

This dissertation describes the optical techniques for fabricating three-dimensional (3D) nanostructures with diverse structural layouts. The approach, which we refer to as proximity field nanopatterning (PnP), uses conformable, elastomeric phase masks to pattern thick layers of transparent, photosensitive materials in a conformal contact mode geometry. Aspects of the optics, the materials, and the physical chemistry associated with this method are outlined in chapter 1. We also combine micro/nanomolding, or soft imprint techniques with PnP to form (i) fine (1 ??m) that are used with phase masks to provide access to large structure dimensions, and (iii) fine structures that are used together phase masks to achieve large, multilevel phase modulations, as shown in chapter 2. Chapter 3 exploits this exposure mode, which we refer to as maskless PnP, for fabricating silicon three dimensional photonic crystals using polymer templates defined by maskless PnP. The resulting crystals have face-center cubic symmetry and exhibit high structural quality over large areas, displaying geometries consistent with calculation. Spectroscopic measurements of transmission and reflection through the silicon and polymer structures reveal excellent optical properties, approaching properties predicted by simulations that assume ideal structures. Besides maskless PnP, multiple exposure steps with or without phase mask can also yield structures different to access by maskless PnP or normal PnP alone. In chapter 4, we demonstrate a dual-exposure, two-photon (2ph) PnP for producing woodpile polymer structures with high structural quality over large areas, and layouts that quantitatively match expectation based on optics simulations of the process. Depositing silicon into these polymer templates followed by removal of the polymer forms silicon woodpile photonic crystals for which calculations suggest sizeable photonic bandgaps over a wide range of structural fill fractions. Spectroscopic measurements of normal incidence reflection from both the silicon and polymer structures reveal good optical properties. In chapter 5, we demonstrate the fabrication of unusual classes of three dimensional (3D) nanostructures using two-photon PnP in both maskless and phase mask modes through elastomeric phase masks with five fold, Penrose quasicrystalline layouts. Confocal imaging, computational studies and 3D reconstructions reveal the essential aspects of the flow of light through these quasicrystal masks. The resulting nanostructures show interesting features, including quasicrystalline layouts in planes parallel to the sample surfaces, with completely aperiodic variations through their depths, consistent with the optics. Spectroscopic measurements of transmission and reflection provide additional insights. Chapter 6 uses the soft imprint technique developed for maskless PnP to generate light trapping structures on thin Si solar cells. Rigorous coupled wave analysis (RCWA) simulations and spectroscopic measurements of transmission, reflection, and absorption reveal insights for designed light trapping structures. Photovoltaic performance measurements on a 6 um Si solar cell showed energy conversion efficiency improvements over 80 % compared to bare Si. Spectral-resolved efficiency measurements reveal results consistent with simulations