Partial Photonic Bandgap Research Articles

Radioluminescent Photonic Bandgap Hydrogels: Mechanochromic Tunable Emissions.

Fully organic, radioluminescent crystalline colloidal arrays (CCAs) with covalently incorporated emitters were synthesized by using up to three organic fluorophores that were Förster resonance energy transfer (FRET) pairs with each other. The emitters were covalently incorporated into monodisperse poly(styrene-co-propargyl acrylate) nanoparticles in various combinations, resulting in blue-, green-, and red-emitting CCAs when excited with an X-ray source. The negatively charged surfaces of the monodisperse nanoparticles caused self-assembly into a crystal-like structure, which resulted in a partial photonic bandgap (i.e., rejection wavelength) within the near-visible and visible light spectrum. When the rejection wavelength of the CCA overlapped its radioluminescence, the spontaneous emission was inhibited and the emission intensity decreased. A poly(ethylene glycol) methacrylate-based hydrogel network was used to encapsulate the CCAs and stabilize their crystal-like structure. Within the hydrogel, coupling the photonic bandgap with the radioluminescence of the CCA films led to robust optical systems with tunable emissions. These fully organic, hydrogel-stabilized, radioluminescent CCAs possess mechanochromic tunable optical characteristics with future applications as potentially less toxic X-ray bioimaging materials.

3D Tomographic Analysis of the Order-Disorder Interplay in the Pachyrhynchus congestus mirabilis Weevil.

The bright colors of Pachyrhynchus weevils originate from complex dielectric nanostructures within their elytral scales. In contrast to previous work exhibiting highly ordered single‐network diamond‐type photonic crystals, here, it is shown by combining optical microscopy and spectroscopy measurements with 3D focused ion beam (FIB) tomography that the blue scales of P. congestus mirabilis differ from that of an ordered diamond structure. Through the use of FIB tomography on elytral scales filled with platinum (Pt) by electron beam‐assisted deposition, it is revealed that the red scales of this weevil possess a periodic diamond structure, while the network morphology of the blue scales exhibit diamond morphology only on the single scattering unit level with disorder on longer length scales. Full wave simulations performed on the reconstructed volumes indicate that this local order is sufficient to open a partial photonic bandgap even at low dielectric constant contrast between chitin and air in the absence of long‐range or translational order. The observation of disordered and ordered photonic crystals within a single organism opens up interesting questions on the cellular origin of coloration and studies on bio‐inspired replication of angle‐independent colors.

IP-Dip-based woodpile structures for VIS and NIR spectral range: complex PBG analysis

In this paper, we present a manual for the preparation of fully functional woodpile structures with partial photonic band gap (PBG) for applications in the visible (VIS) spectral range using an attractive polymer IP-Dip that is novel in photonic applications. In an experimental preparation of polymer-based woodpile structures using the IP-Dip polymer with partial PBG in VIS spectral range, a single-step laser lithography technique based on direct laser writing (DLW) was used. The woodpile structure preparation is based on a complex theoretical analysis of dispersion diagrams for the woodpile structure with fcc symmetry and IP-Dip polymer. We found partial PBGs in the Γ - X direction and dependence of PBG on the filling factor. Using a conventional DLW lithography system, we prepared a series of low-periodic woodpile structures with PBG in NIR and VIS spectral range attacking the yellow-green spectral range, which can be easily applied on different photonic components.

Photonic crystal structures: Beam deflector and beam router

We investigate the optical characteristic, transverse magnetic (TM) and transverse electric (TE) band of two-dimensional (2D) square lattice photonic crystal structure, which is composed of cylindrical air regions positioned at the corners of the square shaped dielectric rods. We obtain the wide photonic bandwidths between TM1–TM2 and TM3–TM4 bands. According to the results, we demonstrate the band gaps close to each other in the TM and TE frequencies for proposed structures. The resulting photonic gaps are formed to be about 8% at the higher frequencies of TE modes (TE4–TE5) and TM modes (TM7–TM8 and TM9–TM10). In addition, we examine isotropically generated structures for light guiding properties and observe that the light is directed in a particular route without using any deflection. We also investigate the self-collimation effect with the designed structure. The obtained results reveal the influences of the radius of cylindrical air holes and the angle between these air holes on absolute and partial photonic band gaps. Moreover, we observe the TM and TE band gaps that overlap. It is thought that the obtained band overlap will provide an easy way to produce the photonic crystals in practical applications like photonic insensitive waveguide. It is also believed that these results can provide the photonic crystal structures to work as a beam deflecting and beam router in integrated optical circuit applications.

Photonic hyperuniform networks obtained by silicon double inversion of polymer templates

Hyperuniform disordered networks belong to a peculiar class of structured materials predicted to possess partial and complete photonic bandgaps for relatively moderate refractive index contrasts. The practical realization of such photonic designer materials is challenging however, as it requires control over a multi-step fabcrication process on optical length scales. Here we report the direct-laser writing of hyperuniform polymeric templates followed by a silicon double inversion procedure leading to high quality network structures made of polycrystalline silicon. We observe a pronounced gap in the shortwave infrared centered at a wavelength of $\lambda_{\text{Gap}}\simeq $ 2.5 $\mu$m, in nearly quantitative agreement with numerical simulations. In the experiments the typical structural length scale of the seed pattern can be varied between 2 $\mu$m and 1.54 $\mu$m leading to a blue-shift of the gap accompanied by an increase of the silicon volume filling fraction.

Evidence of near-infrared partial photonic bandgap in polymeric rod-connected diamond structures.

We present the simulation, fabrication, and optical characterization of low-index polymeric rod-connected diamond (RCD) structures. Such complex three-dimensional photonic crystal structures are created via direct laser writing by two-photon polymerization. To our knowledge, this is the first measurement at near-infrared wavelengths, showing partial photonic bandgaps for this structure. We characterize structures in transmission and reflection using angular resolved Fourier image spectroscopy to visualize the band structure. Comparison of the numerical simulations of such structures with the experimentally measured data show good agreement for both P- and S-polarizations.

The Modeling, Fabrication, and Optical Characterization of Silicon and Polymer-Based Photonic Crystals for Biosensing Applications

Two-dimensional, nano-scale photonic crystals (PhCs) in silicon and biocompatible polymer materials, such as: Polydimethylsiloxane (PDMS) and epoxy, are potential core structures in ultra-sensitive biosensors enhancing fluorescence emission in the near IR and visible range. A triangular PhC lattice (r/a = 0.33) of silicon pillars suspended in toluene was designed to enhance emission in the near-IR range. We present here a 27-fold enhancement of PbS-Quantum-Dot emission at 1100 nm. Moving to more biocompatible materials, we also present frequency-domain modeling results demonstrating partial photonic bandgaps for triangular PhC lattices in PDMS and epoxy. The existence of these bandgaps suggests that PhCs in polymer materials could potentially enhance visible-range fluorescence emission and become co-integrated with other on-chip components, such as microfluidic channels and optical waveguides, to produce cost-effective biosensors.

Assembly of Optical-Scale Dumbbells into Dense Photonic Crystals

We describe the self-assembly of nonspherical particles into crystals with novel structure and optical properties combining a partial photonic band gap with birefringence that can be modulated by an external field or quenched by solvent evaporation. Specifically, we study symmetric optical-scale polymer dumbbells with an aspect ratio of 1.58. Hard particles with this geometry have been predicted to crystallize in equilibrium at high concentrations. However, unlike spherical particles, which readily crystallize in the bulk, previous experiments have shown that these dumbbells crystallize only under strong confinement. Here, we demonstrate the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the color and birefringence quickly go away. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, we find that if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick. Analysis of the crystal structure indicates that the dumbbells have a packing fraction of 0.7862, higher than the densest known packings of spheres and ellipsoids. We perform numerical experiments to more generally demonstrate the importance of controlling the orientation of anisotropic particles during a concentration quench to achieve long-range order.

Photonic Polyethylene from Self-Assembled Mesophases of Polydisperse Olefin Block Copolymers

We have discovered a synthetic process capable of producing olefin block copolymers (OBCs) with controlled block length polydispersity. Certain compositions of these OBCs self-assemble in the melt to form ordered mesophases. The morphologies and dielectric contrast of the semicrystalline and amorphous blocks produce transparent films exhibiting a partial photonic band gap for frequencies in the visible spectrum. The domain spacings are not only much larger than expected for monodisperse block copolymers of similar molecular weight, but they also exceed the predictions of recent theories for polydisperse block copolymers. An extension to Strong Segregation Theory demonstrates that many molecules have a weak preference for segregation to the interface versus the center of a domain. Minor perturbations can then produce highly swollen but relatively stable domains.

Enhanced emission from fcc fluorescent photonic crystals

We fabricated a high quality fcc photonic crystal composed of ruthenium tris-bipyridine ${[\text{Ru}{(\text{bpy})}_{3}]}^{2+}{({\text{PF}}_{6}^{\ensuremath{-}})}_{2}$. Solid-angle-resolved spectroscopy was performed over 76% of $2\ensuremath{\pi}$ sr, with $\ensuremath{\sim}2\ifmmode^\circ\else\textdegree\fi{}$ resolution. Anisotropic emission patterns are explained with an analytic model of two plane waves coupled by diffraction from a Bragg plane. Two distinct types of emissive enhancements are observed, one due to modes coupling confined directions into leaky directions and the other due to maxima in the optical mode density adjacent to partial photonic band gaps.

Electrical tuning of photonic crystals infilled with liquid crystals

In this paper we perform a complete study of electrical tuning in liquid crystal-infilled two-dimensional (2D) photonic crystals (PCs). The nematic liquid crystal (NLC) is characterized by a full range of bulk and surface elastic parameters. An essentially DC tuning field is applied in the axial direction. By minimizing the total (elastic plus electromagnetic) free energy, the configuration of the NLC directors, as a function of radial distance, is obtained. Three possible configurations are considered: escaped radial, planar radial, and axial. It is found that, in general, the escaped radial configuration is the preferred one. However, for sufficiently large applied fields, a phase transition occurs to the axial configuration. For example, in the case of the NLC 5CB, this transition is realized at about 14V/μm provided that the cylinder radius is greater than about 50nm. The configuration of the NLC directors determines the dielectric tensor as function of radial distance and this, in turn, leads to the eigenvalue equation for the PC. We present two such equations: one exact and the other approximate. The exact eigenvalue equation is based on the full anisotropy of the dielectric tensor and does not result in the usual separation of normal modes in a 2D PC. The approximate eigenvalue equation is derived from the average (over the cylinder cross-section) dielectric tensor and leads to modes that are polarized in the directions either parallel (E-mode) or perpendicular (H-mode) to the cylinders. Our calculations of the photonic band structure, by both methods, show that the approximate calculation works very well for the 5CB NLC cylinders in a silicon oxide (silica) host. This allows us to introduce the terminology quasi-E and quasi-H polarizations. We show how the partial photonic band gap in the [100] direction for these polarizations can be tuned and even completely closed. This behavior could be applied to the design of versatile, tunable polarization filters.

Fabrication of silica/zinc oxide core–shell colloidal photonic crystals

We have fabricated three-dimensional ZnO photonic crystals by the self-assembly of SiO2/ZnO core–shell microspheres of diameter 180–220 nm by means of a vertical deposition method. The organized crystals adopt a uniform close-packed hexagonal structure with long-range order and a high filling fraction. They exhibit strong partial photonic band gaps at 565–688 nm in the transmission spectrum and a broadened band edge emission at 383 nm in the photoluminescence spectrum. For comparison with these ZnO photonic crystals, we have also prepared pure silica crystals of the same dimensions. The photonic band gaps of the former have been found to be shifted to a longer wavelength by about 200 nm compared to those of the latter as a result of the higher refractive index of ZnO. This approach provides an efficient, economical alternative means of obtaining other, more complicated photonic crystals.

Thermal radiation in photonic crystals

We analyze the properties of thermal radiation in photonic crystals and show that the spectral energy density, the spectral intensity, and the spectral hemispherical power are only limited by the total number of available photonic states and their propagation characteristics. In addition, we show that the central quantity that determines these thermal radiation characteristics is the area of the isofrequency surfaces and not the photonic density of states as it is generally assumed. Through the presence of partial or complete photonic band gaps and the associated spectral and angular redistribution of photonic states, it is possible to have spectral regions over which the thermal radiation intensity is enhanced relative to the free space blackbody limit and propagation directions along which thermal photon focusing effects appear.

Optical properties of porous silicon Thue‐Morse structures

AbstractDielectric aperiodic Thue‐Morse structures up to 128 layers have been realized by the porous silicon technology. Normal incidence reflectivity measurements have been performed to investigate the photonic properties of the devices. A partial photonic band gap region, centered at 1100 nm and 70 nm wide has been observed for the S6 and S7 Thue‐Morse structures. The S6 multilayer has been studied as sensor device on exposure to several chemical substances. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modulation of photonic band structures with dielectric anisotropy

By the multiple scattering method and the extended Mie theory, we have calculated the photonic band structure of the photonic crystals consisting of the dielectric spheres with uniaxial/biaxial anisotropy. The results demonstrate that for fcc lattice structure there exist two partial photonic band gaps which does not appear in the isotropic case. Among them, the lower one, lying between the second and the third bands, exists in one third of the first Brillouin zone, while the upper one, opening between the fourth and fifth bands, can appear simultaneously in the rest two thirds of the first Brillouin zone. The effects of anisotropy on the band structures are studied as well, which suggests the biaxial anisotropy are much more flexible than the uniaxial anisotropy in modulating the band structures.

Experimental and Finite-Difference Time-Domain Technique Characterization of Transverse In-Line Photonic Crystal Fiber

We characterize a microstructured photonic crystal fiber in the transverse direction, observing photonic bandgap effects in the transmission spectra. This is modeled using band structure and finite-difference time-domain techniques and reasonable agreement is found, confirming the observation of higher order partial photonic bandgaps. A tapered transverse bandgap fiber is used to create a reduced loss device utilizing the fundamental gap. This technique may be used to monitor the draw process for bandgap fibers, or fibers used in this way may be utilized as microphotonic elements.

Self-assembly under gravity sedimentation of large size silica particles

Highly monodispersed colloidal silica particles in sizes ranging from 700 to 1300 nm were synthesized by a two-step method. The transmission electron microscope images showed that the mean standard deviation of these particles is less than 5%. Silica particle size range for self-assembly in the gravitational sedimentation was broadened through adjusting the viscosity of media in silica suspensions. The synthetic opals with particle diameters ranging from 700 to 1300 nm were obtained,and their optical characteristics were examined. Scanning electron microscope images showed that the synthetic opals have a face-centered-cubic structure and have partial photonic band-gaps. This provides the foundation for the study of complete photonic crystals with wavelengths within near-infrared and infrared regions and for the fabrication of other macroporous materials.

Coloration strategies in peacock feathers.

We report the mechanism of color production in peacock feathers. We find that the cortex in differently colored barbules, which contains a 2D photonic-crystal structure, is responsible for coloration. Simulations reveal that the photonic-crystal structure possesses a partial photonic bandgap along the direction normal to the cortex surface, for frequencies within which light is strongly reflected. Coloration strategies in peacock feathers are very ingenious and simple: controlling the lattice constant and the number of periods in the photonic-crystal structure. Varying the lattice constant produces diversified colors. The reduction of the number of periods brings additional colors, causing mixed coloration.

The Sea Mouse and the Photonic Crystal

Sea mice (Aphroditidae, Polychaeta) are furry-looking marine worms that forage on, or in, the sea-bed. They are distinguished by an amazing iridescence along the lower sides of the body, associated with both hairs and spines known as chaetae. We show by electron microscopic and optical analysis that the iridescence of spines is caused by a highly regular, sub-micron scale structure resembling that being developed for photonic crystals, and that in fact the sea mouse exploits a partial photonic band gap to achieve its remarkable coloration effects. Manuscript received: 27 October 2000. Final version: 24 July 2001.