Femtosecond Laser Patterned Templates and Imprinted Polymer Structures

Femtosecond laser machining is a direct-write lithography technique by which user-defined patterns are efficiently and rapidly generated at the surface or within the bulk of transparent materials. When femtosecond laser machining is performed with tightly focused amplified pulses in single-pulse mode, transparent substrates like fused silica can be surface patterned with high aspect ratio (>10:1) and deep (>10 μm) nanoholes. The main objective behind this dissertation is to develop single-pulse amplified femtosecond laser machining into a novel technique for the production of fused silica templates with user-defined patterns made of high aspect ratio nanoholes. The size of the nanoholes, both lateral and vertical, is controlled to a certain degree by controlling laser machining parameters or by chemical etching in a post-machining treatment.

Fused silica templates produced by this new technique, both as-machined and chemically etched, are shown to be useful for imprinting polymer structures by a simple replication procedure using polymer thin films or solutions. In particular, a solution-based replication procedure, termed solution casting, is developed to imprint polymer structures from fused silica templates. Polymer structures in the form of nanowires, nanocones, and micropillars are successfully imprinted from various polymer types. Imprinted polymer structures are easily functionalized by subsequent surface treatment processes like cryogenic sputter coating and vapor deposition. A novel low-temperature chemical vapor deposition process is developed to coat polymer nanowires with silica to produce silica nanoneedles. Silica nanoneedles thus produced are shown to be useful as synthetic cell culture substrates to study the behavior of NIH 3T3 fibroblasts.

In the final part of this dissertation, a report is given on more in-depth collaborative experiments to study the role of optical aberrations as part of the mechanism for producing high aspect ratio nanoholes by single-pulse amplified femtosecond laser machining. The results indicate that (i) precise optical alignment of the focusing lens is needed to avoid coma, which significantly deteriorates the ability to produce nanoholes, and (ii) 10-micron deep nanoholes can be produced by focusing a beam without spherical aberration but even deeper nanoholes are formed when the beam is focused with undercorrected spherical aberration.