Inhibition and Enhancement of Quantized, Interference‐Driven, Ultrafast‐Laser Cleaving, and Intrafilm Ejection with Angle and Polarization Control

Abstract

interactions, electronic phenomena, and mechanical responses. With a preference, for top-down patterning, optical lithography or direct laser writing offers flexible interaction controls to structure multiple types of thin to thick film materials.In the case of transparent films, femtosecond lasers drive nonlinear optical interactions that further localize modification into the third dimension. For example, for focusing into the volume of photoresist, multilevel diffractive optic elements have been constructed that, in turn, enabled large volume formation of 3D photonic crystal templates. [6,7] In non-photosensitive film, the laser interaction can be selectively guided into a buried interface to drive full film mechanical ejection. This can permit the patterning of conductive coatings on display glass [8] through to new means of biological or electronic printing such as laser-induced forward transfer. [9][10][11] At reduced exposure, thin-film delamination enables the formation of interface micro-and nanochannels, and blisters in silica-silicon systems. [12,13] Thin-film optical interference has offered a promising means for confining nonlinear interactions, for example, to enable step-wise ablation of organic films. [14] However, directing energetic laser processes to physically structure internally in transparent film has been more challenging. By moving to dielectric materials, Kumar et al. introduced a controllable quantized intrafilm modification in sub-micrometer thick Si 3 N 4 -coated silicon. [15,16] By confining nonlinear interactions to fringe maxima, spaced by λ/2n film through thin-film interference, periodic stacks of plasma disks were formed above critical density that enable a novel intrafilm cleaving geometry. Expanding plasma disks in the film can override interface interactions to open nanocavities and lead to blister formation or quantized segment ejection for SiO x and Si 3 N 4 thin films on silicon substrates, for thicknesses in the range of 400 nm [15] to 3.5 µm [17] at multiple laser wavelengths. [18] However, this method is limited to a narrow range of thin-film and substrate combinations, wherein sufficient refractive index contrast is necessary to generate strong interference fringes. Fringe visibility is a key underlying factor that has not been previously examined in facilitating quantized processing of thin films. In particular, The combination of optical interference and ultrafast laser interaction within microscale transparent films offers novel high resolution means for nonlinear confinement of dissipated energy to facilitate 3D nanostructuring. This approach relies on the formation of nanoscale (≈40 nm) plasma disks stacked on halfwavelength spacings, λ/2n film (film refractive index, n film ), opening directions for intrafilm cleaving and nanostructuring of free-standing blisters or embedded nanocavities with controllable surface topography. Given a limited number of film-substrate systems suitable for generating high contrast interference fringes, this paper introduces angle and...