2-d photonic crystals - periodic arrays in photoresist for further processing

Abstract

Nanostructured materials with ordered arrays of holes or rods are practical realisation of photonic band-gap concept. In this article we present useful method for fabricating periodic arrays of random in shape and size openings. This method is based on exposing thin photoresist film with two interfering laser beams. Formerly experimental exposing setup with pulsed Nd-YAG laser was utilised to fabricate diffraction gratings in positive photoresist films, which was a base for grating coupler device. An idea of making use of the same setup as mentioned above with two-step exposure of photoresist was first proposed in 1975 [3], although no further concept of periodic arrays employment was given then. We show two examples of obtained periodic structures in exposure setup where the sample was rotated through an angle α (inclination angle) about an axis normal to the substrate surface approximate to 750 and 900. In both cases two sets of one-dimensional grating with period of 1.26 μm were used. Two different shapes of openings in photoresist were a result of test exposure. Nearly circular openings with diameter of 780 nm were achieved. The periodicity of resulting array, grid pattern, shape of the openings and the size of the openings can be varied by varying different exposure parameters which can lead to fabricating two - dimensional photonic crystal [4].
Autorzy: Rafał DYLEWICZ, Jarosław MYŚLIWIEC, Sergiusz PATELA, Andrzej MINIEWICZ

1. INTRODUCTION TO PHOTONIC CRYSTALS

The periodic arrangement of ions on a lattice gives rise to the energy band structure in semiconductors. Energy bands control the motion of charge carriers through the crystal. Similarly, in a photonic crystal, the periodic arrangement of refractive index variation, controls how photons are able to move through the crystal. Photons react to the refractive index contrast in an analogous manner to the way electrons react when confronted with a periodic potential of ions. Each results in a range of allowed energies and a band structure characterised by an energy gap or photonic band gap. The optical band gap is formed when the wavelength is comparable to the photonic crystal period. The simplest form of a photonic crystal is a one-dimensionally periodic structure, such as a multilayer film – electromagnetic wave propagation in such systems was first studied by Lord Rayleigh in 1887. 1D-periodic systems appear in applications from reflective coatings to distributed feedback (DFB) lasers. The possibility of two and three-dimensional periodic crystals with corresponding 2D and 3D band gaps was suggested by Eli Yablonovitch [1] and Sajeev John [2] in 1987, and such structures have since seen growing interest by a number of research groups around the world. With applications including LED's, optical fiber, nanoscopic lasers, ultrawhite pigment, radio frequency antennas and reflectors, and photonic integrated circuits.