Sergiusz Patela

In-Plane Optical Anisotropy of GaN Refractive Index in Visible Light Region

The optical properties of metal-organic chemical vapor deposition gallium nitride layers were measured with the use of a grating-assisted optical coupler. The application of a waveguide-based grating structure for in-coupling of radiated modes was the basis of the work presented here. A set of six grating couplers, providing different propagation angles Phi with respect to the a-axis of GaN, was fabricated in a multimode GaN planar waveguide layer grown directly on c-axis (0001) sapphire. Measurements of the refractive index were carried out for laser wavelength lambda = 632.8 nm and both transverse-electric and transverse-magnetic polarized light. It was found that the refractive indices were dependent not only on the polarization state but also on the propagation direction of the excited optical mode in the a-b plane. The 60deg periodic in-plane anisotropy of GaN optical properties in the visible spectrum was clearly observed with maximum refractive index difference Deltan = 0.018 for both polarizations.

Photonic crystal microcavity in GaN-on-sapphire slab waveguide for sensor applications

Gallium nitride is an important material for the contemporary optoelectronics. Large electric band gap, high temperature resistivity and environmental resistance make GaN interesting also for sensor applications. However, asymmetric structure of GaN-on-sapphire slab waveguide, grown as a conventional epitaxial heterostructure, poses a problem with achieving high quality (Q) factor resonators. In this paper, issues related to an asymmetric structure of a waveguide and theoretical possibilities to achieve high Q-factor resonator in the GaN planar structures are discussed. Three dimensional (3-D) finite-difference time-domain (FDTD) modeling tools were used. It is shown that the highest Q-factor value of ~ 23 000 is obtained for a symmetrical membrane in L9 (nine points-defect cavity) micro-cavity based on GaN planar waveguide. In reference to the simulation results, we also discuss the technological issues, i.e. fabrication of photonic crystal patterns in GaN layers. New approach presented here included deep RIE etching with use of only single masking layer and conductive polymer usage in e-beam pattering. Possible applications of the micro-resonators for sensor applications are discussed.

Sol-gel-based optical waveguides on LTCC substrates

The aim of this work is to test the possibility of sol-gel optical waveguides fabrication on ceramic substrates. The silica sol-gel film is chosen for an optical substrate and cladding and a glaze layer as a material for a waveguide core. The presented paper contains results of examination of the optimal chemical composition and process parameters of sol-gel material and the glaze layer. The refractive indexes and thicknesses of the films have been measured. The technology allows to make waveguide in the grooves made in LTCC substrate. The grooves have 137 mum depth and width variable from 100 mum to 1500 mum. The sol-gel/glaze combination have been used to make stripe waveguides in the grooves.

SiO2 - TiO2 Thin Film for Integrated Optics Fabricated by the Sol-Gel Technique

In this paper the fabrication process and optical measurements of sol-gel SiO2-TiO2 thin films are presented. The first part is devoted to the basics of sol-gel technique, where fabrication of silica-titania films is shortly described. Investigated layers were obtained from three different precursors: tetraethyl orthosilicate (TEOS), tetrabutoxytitanate (TNBT) and titanium isopropoxide (TIPO), mixed in various amounts in order to tailor the refractive index of a layer. Sol-gel films were coated by dip-coating method on silicon substrate with 1 mum SiO2 optical substrate in the first case, and on bare silicon substrate in the second case. Thickness and refractive index of deposited layers was measured at 632.8 nm wavelength by use of ellipsometry. Refractive index dependence of SiO2-TiO2 composition in sol-gel thin film is presented

Grating coupler fabricated in photoresist for optical coupling into thin planar waveguides

In this article practical realisation of grating coupler fabricated in photoresist, integrated with the optical planar waveguides made of different materials and technologies, is presented. Particularly the silica-titania thin fiIm (SiO2:TiO2), prepared by sol-gel technology, were employed for coupling a laser light into optical waveguide experiment. Diffraction grating, which is a base for grating coupler device, with 640 nm period was made in positive photoresist Shipley SPR 700, using holographic setup with operating wavelength of 355 nm from pulsed Nd-YAG laser. The gratings were checked with a 632.8 nm He-Ne laser beam; waveguide mode excitation was observed, both as a guided light streak and the series of M-lines. M-link spectrum indicated that three TE modes were excited in the composite waveguide, The method was also successfully applied to S3N4 and TiO2 thin films obtained by magnetron sputtering process. The concept of investigation of planar 2-D photonic crystal is described also.

Grating couplers for integrated optoelectronics: fabrication and characterization

Problem of efficient light coupling into planar waveguide structures was always a stumbling block for the designers of integrated optical circuits. In this article methods of light coupling into the planar waveguides are described. Comparison of two main approaches -- prism and grating coupling -- is given. Examples of grating coupler technology are presented also.

Investigation of optical properties of silicon oxynitride films deposited by RF PECVD method

Abstract In this study, the authors deposited silicon oxynitride films by Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF PECVD) method. The research explores the relationship between the deposition process parameters and the optical properties of the deposited SiOxNy films. The optical constants of SiOxNy films were measured and calculated by spectroscopic ellipsometry method. Additionally, the authors investigated the possibility of controlling the deposited film composition by the flow ratio of different gaseous precursors: ammonia (NH3), diluted silane (2%SiH4/98%N2), nitrous oxide (N2O) and nitrogen (N2). The gas mixture introduced to the working chamber during the growth of the film has the influence on the Si–O and Si–N bonds formation and the ratio between these bonds determines the refractive index of the deposited film.

Numerical investigation of negative refraction effect in two-dimensional photonic crystal fabricated in gallium nitride

We investigated the conditions for obtaining negative refraction in photonic crystals fabricated in gallium nitride (GaN). Negative refraction at the interface between air and two-dimensional photonic crystals was numerically analyzed using Plane Wave Expansion Method (PWE) and Finite Difference Time Domain Method (FDTD). To describe the origin of negative refraction we have carried out simulations of equifrequency surfaces (EFS) of each photonic crystal structure. The presence of negative effective refractive index has been shown in two dimensional photonic crystal with hexagonal lattice of air holes etched in GaN. We have also observed all-angle negative refraction without existence of negative refractive index for 2D photonic crystal with square lattice of air holes. Additionally, the superlensing effect as a potential application of negative refraction in two dimensional photonic crystal was simulated.

Optimization of 2D slab photonic crystal geometry for gas sensing

The two-dimensional (2D) slab photonic crystal (PhC) geometry is optimized for the application of the PhC as a near-infrared spectroscopic gas sensors active element. Sensor operation in the slow-light regime is assumed. The 2D theoretical model is based on the finite-difference time-domain (FDTD) propagation of the electromagnetic field. Linear-defect 2D photonic crystal structures are considered. Optimization is performed according to the following criteria: maximum light coupling efficiency, maximum light extraction efficiency, and maximum light-gas overlap. In the simulations, output signal collection is performed by means of either an output strip waveguide. Antireflection sections suited for individual PhC geometry types, are applied to prevent excessive reflections resulting from the high group velocity of light propagating in the slow-light regime within the PhC. Configurations with and without antireflection sections are compared. Triangular-lattice 2D PhC types are: line-defect crystals and line-defect crystals featuring an additional row of holes that improve the light-gas overlap.

Application of spectrophotometry and ellipsometry for determination of optical parameters of optical coating thin films

In the present work usefulness of spectrophotometry and ellipsometry for the determination of different parameters of optical thin films have been discussed. The principles of their operation and some exemplary results for TiO2 thin films deposited by magnetron sputtering are presented.