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.
The response of metallic split ring resonators (SRRs) scales linearly with their dimensions. At higher frequencies, metals
do not behave like perfect conductors but display properties characterized by the Drude model. In this paper we compare
the responses of nano-sized gold-based SRRs at near infra-red wavelengths. Deposition of gold SRRs onto dielectric
substrates typically involves the use of an additional adhesion layer. We have employed the commonly used metal
titanium (Ti) to provide an adhesive layer for sticking gold SRRs to silicon substrates - and have investigated the effect
of this adhesion layer on the overall response of these gold SRRs. Both experimental and theoretical results show that
even a two nm thick titanium adhesion layer can shift the overall SRR response by 20 nm.
Depth and profile information of one or two-dimensional photonic crystals can be obtained through
measurements of reflective diffractive patters obtained from the structures and subsequent numerical analysis. The
technique is known as a scatterometry. The method is non-invasive and fast, and competitive to the alternatives of AFM,
SEM etc. In our paper we presented results of investigation 1D photonic crystal fabricated in GaN with period
Λ = 400 nm, fill factor ff = 50% and depth d = 400 nm. Using computer algorithm of Rigorous Coupled Wave Analysis
(RCWA) and measuring diffracted light we extracted the profile parameters of Λ = 420 nm, ff = 51%, d = 400 nm.
Possibility of application of our method for analysis 2D photonic crystals is discussed also.
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.
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.
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.
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.
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.