Rafal Dylewicz

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.

Holographic lithography for grating coupler fabrication in gallium nitride grown on sapphire substrate

The paper reports on holographic lithography for fabrication of grating coupler in gallium nitride grown on sapphire substrate. The impact of the speckle effect is especially significant when further etching process is taken into consideration

Defects generation in photonic crystal pattern by electron beam induced deposition technique

Two-dimensional (planar) photonic crystal waveguides give a possibility to propagate a light beam at narrow angles with small or no energy losses. Line and point defects introduced into the lattice modify the photonic structure of the crystal, which further leads to the possibility of designing more advanced integrated optical structures, such as strip waveguides, splitters or emitters. In our research we adopted Electron Beam Induced Deposition technique to produce the point and the line defects in a photolithographic pattern of a photonic crystal. First, we produced a pattern of holes in a positive photoresist film by two-beam interference lithography1. Then we utilised EBID technique to fill the selected holes, by adopting SEM Hitachi S 570 device. As a process precursor we used diluted vapour of trimethylpentaphenyltrisiloxane, which is the dominant constituent of diffusion pump oil2. Focused electron beam locally decomposes precursor molecules, which leads to solid material deposition. Composition of deposited structure is a mixture of amorphous carbon and some polymers. By the beam scanning in a line mode, the line of carbon can be deposited. Such a line defect in photoresist can act as a protecting mask during the further etching process. This controllable and high-resolution method can be used to fabricate W1, W2 and W3 types of channel waveguides. The best EBID resolution obtained in the selected setup gives lines with width of 15-25 nm.

About holographic lithography for grating coupler fabrication in gallium nitride grown by MOVPE on sapphire substrate

The use of the holographic lithography method for sub-nano pattering of photoresist layer deposited on bare sapphire substrate as well as on GaN grown by metaloorganic vapour phase epitaxy on Al2O3 is reported. Positive photoresist Shipley SPR700 was first diluted with photoresist thinner and then spin-coated on prepared substrates to obtain layers of final thickness of 227nm. Thin photoresist layer was exposed in the holographic setup with wavelength of 355nm to produce the surface relief grating. After development SEM observations reveled well-defined valleys and ridges of diffraction grating in SPR700 deposited on gallium nitride layer whereas the whole structure on sapphire was strongly affected by the speckles created by reflection from the unpolished back surface of the sapphire substrate. Latter, we confirmed with transmission spectroscopy, that even small amount of light transmitted through the substrate, which is back reflected by the unpolished back-surface of sapphire, canstrongly disturb nano-sized features in photoresist.

Two-dimensional photonic crystals: fabrication of the periodic arrays by visible light holographic technique

Application of photonic crystals in the future photonic integrated circuits (PICs) is one of the most interesting issues in modern photonics. With the photonic-crystal-based PICs it will be possible, at last, to realize compact in size, multicomponent optical integrated circuits. Nanostructured materials with ordered arrays of holes or rods are practical realization of photonic band-gap concept. In this paper we present a fabrication method for periodic arrays with openings of arbitrary shape and size. The method is based on exposition of thin photoresist film with two interfering laser beams 3rd harmonics of Nd-YAG laser source and positive photoresist has been used. Two examples of periodic structures are described here: obtained with 75° and 90° substrate rotation. In both cases the starting structure was one-dimensional diffraction grating with period of 1.26 μm. Two different patterns has been obtained in photoresist as a result of the test exposures. In one case nearly circular openings with diameter of 780 nm has been obtained. Periodicity of the resulting array, grid pattern, shape and size of the openings can be varied by adjustment of exposure parameters which can lead to fabrication of two-dimensional photonic crystal.

Applications of GaN-based materials in modern optoelectronics

In this paper new designs of modern optoelectronics devices based on GaN-type materials are presented. First, fundamental properties of gallium nitrides are presented, with special attention paid to its optical characteristics. Then examples of devices fabricated at Wroclaw University of Technology are shown, namely MSM detectors based on AlGaN. A short literature overview of devices based on gallium nitride compounds is also given. Presented applications include DFB lasers, structure, LED, optical waveguides, photonic crystals and light modulators made of GaN compounds.

Characterization of Ar+ based Ion Beam Etching of GaN

In this paper the characterization of dry etching process of gallium nitride epitaxial layers with use of ion beam etching technique (IBE) is presented. Dry etching based patterning of GaN in experimental setup employing Ar+ ions as a milling medium is shortly described in the first part of the article. The parameters of the process, on which the overall etching efficiency i.e. gallium nitride etch rate and etching selectivity depends, were changed successively. Slow-rate etching mode for different Ar+ ion energies (400divide800 eV) and Ar+ ion beam angles of incidence (0degdivide60deg) was achieved in this experiment. The usability of IBE etching technique for structuring and optical devices fabrication in GaN is discussed, where the inclination of sidewalls and optical roughness of etched surface were taken as determining factors

Photonic crystal waveguide - fabrication of the periodic arrays by visible light holographic technique coupled with electron beam-induced deposition

In recent years a lot of interest has been directed towards waveguide-based two dimensional photonic crystals. The reason behind this interest is a possibility of employing them in future photonic integrated circuits (PIC). With the photonic-crystal-based PIC it will be possible, for the first time, to realize compact in size, multicomponent optical integrated circuits. In this article we present a simple fabrication method for large arrays of holes of arbitrary shape and size. Later, a method of local modification of the structure (defect creation) is also given. Fabrication of the periodic array in photoresist is made through the multiple exposition of thin photoresist film with two interfering laser beams (holographic method). Exposition has been done with a pulsed Nd-YAG laser. The setup has been adapted from the existing one, previously used to fabricate diffraction gratings in positive photoresist films (the gratings served as grating couplers for planar waveguides). An idea of periodic pattern fabrication with the setup similar to ours was first proposed in 1975, but no further consideration for any application was given then. In this paper we present two examples of periodic structures obtained with double expose, with substrates rotated by the angles of 75/spl deg/ and 90/spl deg/. The one dimensional period of the structures was equal to 1.26 /spl mu/m. Two different shapes of holes were obtained in photoresist. In the second exposition setup (90/spl deg/) nearly circular openings with diameter of 780 nm were achieved. The commonly known problem with fabrication of photonic crystals by the holographic method is difficulty in defect creation. Here, we propose the use of three-dimensional additive nanolithography with electron beam-induced deposition (EBID) for this task. Both a point defect (local resonator) and a line defect (channel waveguide) have been fabricated and documented.