Blister Formation Using Ultrafast Laser Pulses

Abstract

Ultrafast laser pulses are frequently used for machining and structuration of materials. They provide superior localization of energy deposition compared to nanosecond pulses and continuous-wave lasers. Additionally, the inherent high intensities of ultrafast laser pulses provide access to nonlinear absorption processes, which enable laser machining at scales below the diffraction limit. This thesis explores nonlinear absorption of ultrafast laser pulses in laser-induced blister formation. In this process, laser pulses are focussed through a transparent substrate and deposit energy beneath a film, giving rise to intact protrusions known as blisters. In the past, laser-induced blisters were used for indirect laser transfer of materials by imparting momentum to material placed on top of the film, and are often noticed as a pre-ablation phase of solid materials irradiated by pulsed lasers. We first demonstrate that nonlinear absorption of single tightly-focussed femtosecond pulses can create polymer blisters below the diffraction limit (400 nm full-width at half-maximum), a factor of 10 smaller than previous polymer blisters and a factor of 2 smaller than previous blisters in glass films. We model the energy deposition process and observe a linear relationship between the deposited energy and the resulting blister volume. We then characterize blister-patterned polymer films using a variety of microscopy and spectroscopy techniques. We provide direct confirmation that laser modification is confined entirely below the film surface and that the chemistry of the film surface is left unchanged. We demonstrate that blister patterning of polymer films adds purely morphological changes that increase hydrophobicity. We introduce a multilayer film for blister formation consisting of a reflective metal layer sandwiched between two layers of polymer, for application in laser transfer of materials. The metal layer ensures that a material to be transferred (which is placed on top of the multilayer stack, opposite the laser) is not exposed to the laser directly, preventing modification, while allowing high intensities to drive nonlinear absorption in the multilayer film. Lastly, we explore laser-induced blister formation as a method of microlens fabrication. We fabricate arrays of blisters in a polymer film and characterize the resulting focussing properties. We find that blisters act as meniscus lenses with positive focal lengths for sufficiently large blister curvatures. This thesis demonstrates new possibilities of laser-induced blister formation. Nanoscale laser-induced blisters using tightly-focussed ultrafast pulses could enable laser transfer of materials at sizes below the diffraction limit, and can structure films while preserving surface chemistry. Blister formation in multilayer films may be useful for material transfer below the diffraction limit, and could also be used in existing micrometer- and millimeter-scale transfer processes. Laser-induced blister formation further provides an on-demand single-pulse microlens fabrication method with no post-processing steps required.