Abstract
When a drop of liquid falls onto a screen, e.g. a cell phone, the pixels lying underneath appear magnified. This lensing effect is a combination of the curvature and refractive index of the liquid droplet. Here, the spontaneous formation of such lenses is exploited to overcome the diffraction limit of a conventional laser direct-writing system. In particular, micro-droplets are first laser-printed at user-defined locations on a surface and they are later used as lenses to focus the same laser beam. Under conditions described herein, nanopatterns can be obtained with a reduction in spot size primarily limited by the refractive index of the liquid. This all-optics approach is demonstrated by writing arbitrary patterns with a feature size around 280 nm, about one fourth of the processing wavelength.
Highlights
Optical methods are unrivaled as patterning tools thanks to the possibility to operate in ambient conditions, integration in direct-writing systems and ease of use
We investigate the optimal conditions of Droplet-assisted laser processing (DALP) and demonstrate the feasibility of this approach by creating nanopatterns on a polymeric surface with a feature size about one fourth of the processing wavelength
The basis of our approach can be found in the lensing effect that a drop of liquid produces on a surface. This phenomenon – clearly observed when a liquid falls on a screen, with the underlying pixels appearing magnified - can be explained by the curvature and refractive index of the liquid (n > 1), causing each drop to act as a plano-convex lens
Summary
Optical methods are unrivaled as patterning tools thanks to the possibility to operate in ambient conditions, integration in direct-writing systems and ease of use. Sub-wavelength feature sizes can be fabricated by exploiting non-linarites in the interaction of light with a particular material, as in multiphoton absorption[8,9,10] or photopolymerization inspired by reversible saturable optical fluorescence transition (RESOLFT) microscopy[11,12] Since these methods are based on the particular photophysics of the material to be patterned[13], resolution enhancement is material-dependent and optimal results are typically constraint to a narrow range of available photoresists. One can consider the droplet to act as the liquid version of SILs without the constraints involved in its fabrication and placement – droplets present a flawless surface, and can be printed at desired locations on a surface Thereby, this approach produces a straightforward increase in the focusing capabilities of a system by a factor that depends to first order on the liquid refractive index. We investigate the optimal conditions of DALP and demonstrate the feasibility of this approach by creating nanopatterns on a polymeric surface with a feature size about one fourth of the processing wavelength
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