Designing of periodic nanostructures with optical interference for photonic applications

Abstract

Laser interference lithography is promising method to generate periodic nanostructures under the wavelength length scales. Interference of multiple coherent laser beams produces one-dimensional (1D), 2D and 3D periodic light intensity patterns by varying the number of interfered beams with different wave vectors, phases and polarizations. By transferring the interfered intensity variation to the photoresist polymer, nanopatterns with various morphologies are produced, which is useful for the photonic applications such as photonic crystal, plasmonic structures. Diffraction grating is simple optical device to split the incident wave into the several diffraction orders for the generation of interference patterns at Fresnel diffraction region. For the fabrication of diffraction grating, we used molding technique and self-assembly of colloidal particles. The order of diffraction beams are determined by the ratio between grating period and wavelength of incident light. High-order diffraction beams produce complex and hierarchical nanopatterns due to the multi-beam interference with different wave vectors. Interference patterns from diffraction grating have periodicity along the propagation direction of incident light as well as lateral direction. Periodic patterns from diffraction grating are used as the photonic crystals and surface-enhanced Raman scattering (SERS) applications. Periodic dielectric nanostructures whose refractive index is periodically arranged on the length scale of interacting waves have photonic band gap. The position of photonic bandgap can be tuned by modulating the lattice constant of nanosturctures as well as refractive index of materials. Small periodicity under 100 nm structures can be fabricated by vertical standing waves in polymer regions and it can be used as ‘hot-spot’ regions for highly localization of electromagnetic (EM) field after metal deposition. Because the intensity of Raman signal is proportional to the square of electric field intensity, the plasmonic nanostructures with high EM field enhancement is necessary for the detection of low concentration of molecules.