Photonic Gap Research Articles

Gap modes of one-dimensional photonic crystal surface waves

The finite-difference time-domain method is employed for the analysis of coupling of the surface modes of two truncated one-dimensional photonic crystals separated by a gap. The wave vector, field distributions, and existence conditions of the coupled surface modes are investigated. The wave vector of symmetric gap modes increases with decreasing gap width, while that of antisymmetric modes decreases-exactly opposite of the situation for surface plasmons on metallic half-spaces separated by a dielectric gap. Photonic crystal gap modes could easily and effectively be used as nondissipating gap-mode waveguides.

Optical properties of real photonic crystals: anomalous diffuse transmission

Unavoidable structural disorder in photonic crystals causes multiple scattering of light, resulting in extinction of coherent beams and generation of diffuse light. We demonstrate experimentally that the diffusely transmitted intensity is distributed over exit angles in a strikingly non-Lambertian manner, depending strongly on frequency. The angular redistribution of diffuse light reveals both photonic gaps and the diffuse extrapolation length, as confirmed by a quantitative diffusion theory that includes photonic band structures. Total transmission corrected for internal reflection shows that extinction increases slower with frequency than Rayleigh's law predicts. Hence disorder affects the high-frequency photonic bandgap of fcc crystals less severely than expected previously.

Unusual modes and photonic gaps in a Vicsek waveguide network

We propose a simple model of a waveguide network designed following the growth rule of a Vicsek fractal. We show, within the framework of real space renormalization group (RSRG) method, that such a design may lead to the appearance of unusual electromagnetic modes. Such modes exhibit an extended character in RSRG sense. However, they lead to a power law decay in the end-to-end transmission of light across such a network model as the size of the network increases. This, to our mind, may lead to an observation of power law localization of light in a fractal waveguide network. The general occurrence of photonic band gaps and their change as a function of the parameters of the system are also discussed.

Two-photon lithography of nanorods in SU-8 photoresist

Studies on two-photon lithography in negative SU-8 photoresistdemonstrate the possibility of obtaining mechanically stable, stress-free,extended nanorods having lateral sizes of about 30 nm (corresponding toλ/25 resolution). The high resolution achievable with the given combination of materials andfabrication techniques demonstrates its potential for the fabrication of large-scalenanostructures, such as photonic crystals with photonic stop gaps at visible wavelengths.

Omnidirectional Gaps of One-Dimensional Photonic CrystalsContaining Single-Negative Materials

We show that a one-dimensional photonic crystal containing two kinds ofsingle-negative materials can possess an omnidirectional gap whoseedges are insensitive to incident angle and polarization, owing to theunusual field configuration in the structure. In contrast to anomnidirectional gap based on a photonic gap that corresponds to zeroaveraged refractive index, it can be made very wide by varying theratio of the thicknesses of two media. Moreover, it is independent ofpolarization under special parameters. These properties may provide amechanism to design a broadband omnidirectional reflector with fixedbandwidth.

Ultrafast all-optical switching in two-dimensional organic photonic crystal

An all-optical switching with high switch efficiency is realized based on two-dimensional nonlinear photonic crystal made of polystyrene. The prism-film coupling method is used to couple energy of probe light into photonic crystal waveguide. High transmittance contrast of more than 60% is realized for the probe light. Time response of the optical switching is around l0 ps. The dynamical shifts of photonic gap induced by pump light are measured and analyzed in detail. The photonic gap shifts l0 nm under the excitation of 16.7GW∕cm2 pump intensity, which is in agreement with the theoretical predictions.

Increasing the Conversion Efficiency of Dye-Sensitized TiO2 Photoelectrochemical Cells by Coupling to Photonic Crystals

The mechanism of enhancing the light harvesting efficiency of dye-sensitized TiO(2) solar cells by coupling TiO(2) inverse opals or disordered scattering layers to conventional nanocrystalline TiO(2) films has been investigated. Monochromatic incident photon-to-current conversion efficiency (IPCE) at dye-sensitized TiO(2) inverse opals of varying stop band wavelengths and at disordered titania films was compared to the IPCE at bilayers of these structures coupled to nanocrystalline TiO(2) films and to the IPCE at nanocrystalline TiO(2) electrodes. The results showed that the bilayer architecture, rather than enhanced light harvesting within the inverse opal structures, is responsible for the bulk of the gain in IPCE. Several mechanisms of light interaction in these structures, including localization of heavy photons near the edges of a photonic gap, Bragg diffraction in the periodic lattice, and multiple scattering events at disordered regions in the photonic crystal or at disordered films, lead ultimately to enhanced backscattering. This largely accounts for the enhanced light conversion efficiency in the red spectral range (600-750 nm), where the sensitizer is a poor absorber.

Acrylate‐Based Photopolymer for Two‐Photon Microfabrication and Photonic Applications

AbstractThis paper provides a photopolymerizing material suitable for stereolithography of complex submicrometer‐sized three‐dimensional (3D) structural elements to a broad scientific public. Here, we present the formulation of a polymer (LN1 resin) that allows further research in the field of nanofabrication and ‐technology as it surpasses current material limitations. The polymer consists of multifunctional acrylate oligomers as binder, polyfunctional monomers, and a photoinitiator (PI). The chemistry to form 3D structures is based on photopolymerization of the acrylate system initiated by free‐radical species that are triggered by two‐photon absorption of a PI. Important parameters of photocuring, such as the effects of PI concentration, temperature, and light intensity, were studied using photocalorimetry. The thermal stability of the material was tested using thermal gravimetric analysis, providing key information for electronic and photonic applications. Photonic‐crystal structures generated from this resin exhibiting photonic stop gaps in near‐infrared‐ and telecommunication‐wavelength regions are presented.

Electrical tuning of three-dimensional photonic crystals using polymer dispersed liquid crystals

Electrically tunable three-dimensional photonic crystals with a tunable wavelength range of over 70nm of stop gaps between 3 and 4μm have been generated in a liquid crystal-polymer composite. The photonic crystals were fabricated by femtosecond-laser direct writing of void channels in an inverse woodpile configuration with 20 layers providing an extinction of infrared light transmission of 70% in the stacking direction. Stable structures could be manufactured up to a liquid crystal concentration of 24%. Applying a direct voltage of several hundred volts in the stacking direction of the photonic crystal changes the alignment of the liquid crystal directors and hence the average refractive index of the structure. This mechanism permits the direct tuning of the photonic stop gap.

Non-close packed colloidal crystals

Solid colloidal crystals (also known as opals) self assemble mostly in close packed structures. They can show full gaps for high values of the refractive index contrast. However, they are not the best topologies for photonic band gap applications. Recently, we have shown that non-close packed structures can improve photonic gap performance. Here we will discuss the possibilities of this new approach to the quest of new colloidal-based methods to achieve much better optical gaps in colloidal lattices. We will show that non-close packed opals would present a full gap when the connectivity of spheres in the planes perpendicular to the (111) direction is suppressed.

Unification of Gap Soliton Classes

We introduce a generalized nonlinear Schrodinger equation valid throughout the photonic gap region for a dielectric nonlinear system with a periodic linear background. The equation is derived using a transfer matrix approach and a local Bloch wave description characterized by a universal dimensionless band-structure parameter �. This single equation provides a unified interpretation and novel insight into three classes of gap solitons. The analytical solutions compare

Photonic gaps of amorphous photonic structures

Photonic gaps of amorphous photonic structures

Adiabatically driven electron dynamics in a resonant photonic band gap: Optical switching of a Bragg periodic semiconductor

The adiabatic driving of the resonant electron dynamics in a one-dimensional resonant photonic band gap is proposed as an optical mechanism for nonlinear ultrafast switching. Pulsed excitation inside the photonic gap results in an ultrafast suppression and recovery of the gap. This behavior results from the adiabatic carrier dynamics due to rapid radiative damping inside the band gap.

Fabrication of three-dimensional photonic crystal with alignment based on electron beam lithography

We demonstrate the fabrication of a three-dimensional woodpile photonic crystal in the near-infrared using a layer-by-layer approach involving electron beam lithography and spin on glass planarization. The alignment accuracy between the first and the fifth layer is within 10% of the lattice spacing as measured from cross section scanning-electron-microscopy images. Optical reflectivity measurements reveal peaks consistent with the photonic gap frequency. The method offers a way of rapid prototyping full three-dimensional photonic band gap devices with considerable flexibility of materials choice. Moreover, lattice structure that can operate at wavelengths into the visible can be fabricated using this approach.

Fabrication of a three-dimensional terahertz photonic crystal using monosized spherical particles

Three-dimensional artificial crystals with periodicity corresponding to terahertz wave lengths were fabricated by self-assembling monosized metal spherical particles. The metal crystals were weakly sintered to utilize them as templates. The metal templates were inverted to air spheres crystal embedded in dielectric resin though infiltration and etching. The resulting resin inverted crystals clearly presented the photonic stop gaps within terahertz wave region and the frequencies of the gaps were confirmed to agree well with calculation by plane wave expansion method.

Complex-shaped three-dimensional microstructures and photonic crystals generated in a polysiloxane polymer by two-photon microstereolithography

Two-photon photopolymerization of inorganic–organic hybrid materials permits the generation of complex-shaped three-dimensional microstructures at submicrometer resolution of structural elements. Due to their favorable optical, chemical and thermal properties these materials are particularly useful for photonic microdevice fabrication. Focussing ultrashort pulsed visible light into a modified commercially available polysiloxane polymer a Sydney Opera House design and a series of woodpile-type photonic crystals were fabricated. Fourier transform infrared spectroscopy revealed photonic stop gaps in the stacking direction at wavelengths varying from 6 to 4 μm upon reduction of the woodpile rod size. The structures allowed for the observation of higher-order stop gaps.

High external efficiency in a monomode full-photonic-crystal laserunder continuous wave electrical injection

We demonstrate continuous wave with an external efficiency over 0.15W∕A and monomode operation of a full photonic crystal (PhC) laser. The optical confinement and a wavelength selectivity better than 25dB are all ensured by a unique PhC. We take advantage of the feedback on the fundamental mode which exists within the photonic gap for some specific designs of the PhC guide. These designs are calculated using the plane wave expansion method.

Photonic gaps in cholesteric elastomers under deformation.

Cholesteric liquid crystal elastomers have interesting and potentially very useful photonic properties. In an ideal monodomain configuration of these materials, one finds a Bragg reflection of light in a narrow wavelength range and a particular circular polarization. This is due to the periodic structure of the material along one dimension. In many practical cases, the cholesteric rubber possesses a sufficient degree of quenched disorder, which makes the selective reflection broadband. We investigate experimentally the problem of how the transmittance of light is affected by mechanical deformation of the elastomer, and the relation to changes in liquid crystalline structure. We explore a series of samples which have been synthesized with photonic stop gaps across the visible range. This allows us to compare results with detailed theoretical predictions regarding the evolution of stop gaps in cholesteric elastomers.

Effects of symmetry reduction in two-dimensional square and triangular lattices

We investigate the absolute photonic band gap (PBG) formation in two-dimensional (2-D) photonic crystals designed using symmetry reduction approach. The lattice symmetry, shape and orientation of dielectric scatterers affect the photonic gap parameters. We use symmetry reduction, achieved either by including additional rods into the lattice unit cell or by reorienting noncircular scatterers to engineer the photonic band gaps in 2-D square and triangular structures. The case of air rods drilled into silicon background is considered. We show that for square structures symmetry reduction can be an effective way to enlarge the absolute PBG, but for triangular lattices any modification of the crystal structure considerably reduces the absolute PBG width. We also discuss the practical technological feasibility of the proposed structures.

Properties of one-dimensional photonic crystals containing single-negative materials

The transmission properties of a one-dimensional photonic crystal containing two kinds of single-negative (permittivity- or permeability-negative) media are studied theoretically. We show that this structure can possess a type of photonic gap with zero effective phase (phi(eff) ). The zero-phi(eff) gap distinguishes itself from a Bragg gap in that it is invariant with a change of scale length and is insensitive to thickness fluctuation. In contrast to a photonic gap corresponding to zero averaged refractive index, the zero-phi(eff) gap can be made very wide by varying the ratio of the thicknesses of two media. An equivalent transmission-line model is utilized to explain the properties. A photonic quantum-well structure based on zero-phi(eff) gaps is proposed as a multiple channeled filter that is compact and robust against disorder.