Optical fabrication of micro-patterned photopolymer surfaces for application in sensing

In the study of surface relief based diffractive sensors, surface morphology is a key parameter. There are many approaches to patterning of surfaces on the micro and nanoscale, such as lithography, self-assembly, ultra-thin alumina mask nano-surface patterning, and Scanning probe techniques. While these techniques allow for fabrication of structures with very high resolution, the development of an alternative technique that can deliver complex, easily customisable surface structures at a low cost is well justified. Two optical techniques for patterning of photopolymers of different chemical compositions will be presented and compared. They allow the fabrication of surface structures on the micron scale directly into a photopolymer layer by single step recording of the whole pattern. The first approach utilizes holographic recording with three laser beams with different states of polarization and adjusted intensities. A semi-automated optical system utilising this approach currently allows for the creation of surface relief cross-gratings (SRCG) of unit cell size ranging from 8 x 8 μm² to as small as 2 x 2 μm², but smaller features are theoretically possible. Control over surface morphology and surface roughness by optical patterning is experimentally confirmed. The second approach involves the use of a digital micro-mirror array spatial light modulator. This approach allows for the creation of surface relief with features as small as 3 μm², with higher achievable image complexity, due to the control over individual micro-mirrors. Surfaces fabricated in photopolymer materials utilising both approaches were characterised via atomic force microscopy, phase contrast microscopy, fast Fourier transform analysis of collected images, and diffraction efficiency measurements. Both approaches are compared in view of their application in diffractive optical sensors.