Femtosecond Laser Nanofabrication of Metal Structures Through Multiphoton Photoreduction

Abstract

Metal mirco- and nano-structures play important role in various areas such as catalysts or in plasmonics field. A recent application with growing interest that also incorporates metal nanostructures is metamaterials. Metamaterials are artificial materials that have unique structures engineered to have extraordinary electric and magnetic response. To generate these structures, most fabrication techniques can allow mass production but are non-controllable or are limited in two dimensions. Electron beam and nanoimprint lithography and focused ion beam milling are well known for making two dimensional arrays of structures. However these techniques suffer from high cost, low throughput and are only limited to two dimensions.

Here we present a facile ultrafast laser technique for the direct-writing of silver structures of tunable dimensions (hundreds of nanometers to micrometers). By utilizing nonlinear optical interactions between chemical precursors and femtosecond pulses, we can limit the metal-ion photo-reduction process to a focused spot smaller than that of the diffraction-limit. This creates metal nanostructures in a focal volume that can be scanned rapidly in three dimensions by means of a computer-controlled translation stage to produce complex patterns. We study the chemistry that effects the photo induced metal growth. By varying types of solvent, the concentration ratio between metal ion precursors and a polymer capping agent, as well as laser pulse parameters, we demonstrate our control over the morphology of the resulting metal structures. We show that this process is scalable over macroscopic volumes and suggest the fabricated structures can be used for metamaterials or surfaced enhanced Raman spectroscopy applications.