Tunable one-dimensional photonic crystal slabs

Abstract

A 1D photonic crystal slab based on preferential etching of commercially available silicon-on-insulator wafers is presented. Compared to dry etching, anisotropic wet etching is more tolerant to errors as it is self-stopping on crystallographic {111} planes and it produces a more precise geometry with symmetries and homothetic properties, with surface roughness close to 1 nm. The resulting grooves are infiltrated by low viscosity liquid crystal having large positive optical anisotropy. The use of slanted grooves provides advantages: first of all the complete filling of slanted grooves is simplified when compared to vertical walls structures. Furthermore alignment is significantly facilitated. Indeed the liquid crystal molecules tend to align with their long axis along the submicron grooves. Therefore by forcing reorientation out of a rest position, the liquid crystal presents a choice of refractive indices to the propagating optical field. The liquid crystal behavior is simulated by a finite element method, and coupled to a finite difference time domain method. We investigate different photonic crystal configurations. Large tunability of bandgap edge for TE polarization is demonstrated when switching the liquid crystal with an applied voltage. We have also studied the use of the same device geometry as a very compact microfluidic refractometric sensor.

Modelling, design and analysis of liquid crystal waveguides in preferentially etched silicon grooves

“…As the LC permittivity varies in a wide range (from 7 to 20 for the LC E7), neglecting this coupling mechanism results in a significant overestimate of the reorientations [29,32]. First, the local electric field reorienting the LC molecules varies from point to point while reorientation occurs.…”

Tunable optical properties of silicon-on-insulator photonic crystal slab structures

“…The bottom and top bases equal to 370 nm and 50 nm, respectively, producing a structure with a photonic crystal period of D = 420 nm. The grooves are etched by anisotropic preferential etching, using an alkaline solution as reported in [18,19]. The refractive indices of Si and SiO 2 for the wavelengths here studied (1.3 µm < λ < 2.3 µm) are considered to be 3.5 and 1.45, respectively.…”