Photonic Crystal Nanostructures Research Articles

Shrinkage of microstructures produced by two-photon polymerization of Zr-based hybrid photosensitive materials

An investigation of the shrinking behaviour of a zirconium-based sol-gel composite micro-structured by two-photon polymerization is presented and a simple, straightforward methodology allowing the evaluation of shrinkage is suggested. It is shown that volume reduction is directly related to the average laser power (irradiation dose) used for the microfabrication and becomes a critical issue near the polymerization threshold. It is demonstrated that this shrinkage can be employed beneficially to improve the structural resolution. This is demonstrated by the presence of stopbands in the photonic crystal nanostructures fabricated with controlled volume reduction. Well above the polymerization threshold, the studied material exhibits remarkably low shrinkage. Therefore, no additional effort for the pre-compensation of distortion and for the improvement of structural stability is required.

Resonant scattering states in 2D nanostructured waveguides: a boundary wall approach

We determine resonant scattering states of two-dimensional photonic crystal nanostructures with defects. To do so, we use the boundary-wall method originally introduced to obtain the scattering eigenstates of one electron moving in a medium with arbitrary boundaries. We investigate geometries including beam bending and interferometer-like waveguides, as well as waveguides connected by resonant cavities. We are able to identify the electromagnetic modes that, due to the special resonance condition attained in the vicinity of defects, provide optimal transmission of an incoming plane wave. Based on the generality of the boundary wall technique and its numerical simplicity and efficiency to identify resonant scattering modes, we briefly discuss further possible applications of this method in analysing the performance of general photonic band-gap devices.

Optimization of photonic nanostructures

A Geometry Projection Method (GPM) for the optimization of photonic crystal nanostructures is used to improve the performance of two different photonic devices. The GPM is a level set based approach for defining a differentiable expression describing the material distribution of a structure. It gives direct control over areas of intermediate dielectric constant and implicit control over feature size, both key concerns for the design of nano-scale devices. The GPM, in conjunction with the finite element and adjoint methods, is used here to solve two different non-linear optimization problems. The diffraction from a two-dimensional photonic crystal waveguide termination is reduced, and the reflectivity of a three-dimensional photonic crystal laser cavity structure is increased. Significant improvements in both cases demonstrate that the GPM is an effective tool for optimization of photonic structures.

Air-suspended two-dimensional polymer photonic crystal slab waveguides fabricated by nanoimprint lithography

Air-suspended membrane-type photonic crystals were fabricated in polystyrene. The two-dimensional photonic crystal slab nanostructures were replicated by a thermal nanoimprint process, and then air-suspended membrane structures were generated by a selective sacrificial layer etch process. The optical transmission showed clear band gap characteristics at 1550nm wavelength, which is in good agreement with the theoretical calculations.

Living photonic crystals: Butterfly scales — Nanostructure and optical properties

The photonic crystal type nanostructures in the scales of male individuals of two butterfly species: Cyanophrys remus and Albulina metallica were investigated by electron microscopy and reflectance measurements. While the colors of C. remus arise from structures with rigorous long range (dorsal) or short range (ventral) three dimensional (3D) order, the colors of A. metallica are produced by quasi-ordered, layered structures. Surprisingly, the most efficient photonic band gap reflector is the quasi-ordered structure giving the shiny, yellowish green color of the ventral hind wings of A. metallica. All four investigated structures are based on a moderate refractive index contrast between chitin (n=1.58) and air, the various structures achieve a wide range of biological functions.

Nanopatterning of photonic crystals with a photocurable silica–titania organic–inorganic hybrid material by a UV-based nanoimprint technique

UV patternable high refractive index inorganic–organic hybrid materials prepared by sol–gel process can be nanoimprinted in order to get photonic crystal nanopatterns. Non-hydrolytic sol–gel process can make a UV curable and relatively high refractive index hybrid material, which can be cured without a significant shrinkage for nanoimprinting. Sol–gel process of heterogeneously functionalized silicon alkoxides can make the organically modified inorganic Si–O–Ti network very well. The UV-based nanoimprint technique utilizes transparent templates and UV curable high refractive index materials to allow pattern replication at room temperature and very low pressure. UV-based nanoimprint technique is an efficient way to fabricate this sort of polymeric photonic crystal nanostructures.