Optical studies of super-collimation in photonic crystals

Abstract

Recent developments in material science and engineering have made possible the fabrication of photonic crystals for optical wavelengths. These periodic structures of alternating high-to-low index of refraction materials allow the observation of peculiar effects, in particular, the propagation of optical beams without spatial spreading. This effect, called super-collimation (also known as self-collimation), allows diffraction-free propagation of micron-sized beams over centimeter-scale distances. This linear effect is a natural result of the unique dispersive properties of photonic crystals. In this thesis, these dispersive properties are studied in a two-dimensional photonic crystal slab. Both qualitative and quantitative descriptions are presented. The beam propagation method was used to simulate the evolution of a Gaussian beam inside such structures. The wavelength dependence of the super-collimation effect was studied, and it was observed that the optimum wavelength for this device was around 1500 nm. A precise contact-mode near-field optical microscopy technique was used to obtain high-resolution images of the beam profile at different positions along the photonic crystal, and showed that a 2 [micro]m beam width was conserved over 3 mm. In addition, high-resolution confocal measurements confirmed the size of the beam after 5 mm of propagation.

(cont.) The figure of merit associated with the super-collimation effect is defined by the number of diffraction lengths over which the beam stays collimated. The diffraction length is the distance in which a beam will broaden to 2¹ʹ² of its initial width. Previous experimental studies showed figures of merit smaller than 6; the results of this experiment show figures of merit as high as 376, which correspond to more than 14200 lattice constants. Preliminary results were obtained with an 8 mm sample that could achieve a figure of merit of 601.