Photonic Band Gap Engineering Research Articles

Co-assembled MoS2-TiO2 Inverse Opal Photocatalysts for Visible Light-Activated Pharmaceutical Photodegradation.

Heterostructured photocatalytic materials in the form of photonic crystals have been attracting attention for their unique light harvesting ability that can be ideally combined with judicious compositional modifications toward the development of visible light-activated (VLA) photonic catalysts, though practical environmental applications, such as the degradation of pharmaceutical emerging contaminants, have been rarely reported. Herein, heterostructured MoS2-TiO2 inverse opal films are introduced as highly active immobilized photocatalysts for the VLA degradation of tetracycline and ciprofloxacin broad-spectrum antibiotics as well as salicylic acid. A single-step co-assembly method was implemented for the challenging incorporation of MoS2 nanosheets into the nanocrystalline inverse opal walls. Compositional tuning and photonic band gap engineering of the MoS2-TiO2 photonic films showed that integration of low amounts of MoS2 nanosheets in the inverse opal framework maintains intact the periodic macropore structure and enhances the available surface area, resulting in efficient VLA antibiotic degradation far beyond the performance of benchmark TiO2 films. The combination of broadband MoS2 visible light absorption and photonic-assisted light trapping together with the enhanced charge separation that enables the generation of reactive oxygen species via firm interfacial coupling between MoS2 nanosheets and TiO2 nanoparticles is concluded as a competent approach for pharmaceutical abatement in water bodies.

Visible and infrared dual-band anti-counterfeiting with self-assembled photonic heterostructures.

Self-assembled photonic structures have greatly expanded the paradigm of optical materials due to their ease of access, the richness of results offered and the strong interaction with light. Among them, photonic heterostructure shows unprecedent advances in exploring novel optical responses that only can be realized by interfaces or multiple components. In this work, we realize visible and infrared dual-band anti-counterfeiting using metamaterial (MM) - photonic crystal (PhC) heterostructures for the first time. Sedimentation of TiO2 nanoparticles in horizontal mode and polystyrene (PS) microspheres in vertical mode self-assembles a van der Waals interface, connecting TiO2 MM to PS PhC. Difference of characteristic length scales between two components support photonic bandgap engineering in the visible band, and creates a concrete interface at mid-infrared to prevent interference. Consequently, the encoded TiO2 MM is hidden by structurally colored PS PhC and visualized either by adding refractive index matching liquid or by thermal imaging. The well-defined compatibility of optical modes and facility in interface treatments further paves the way for multifunctional photonic heterostructures.

Loss in hollow-core optical fibers: mechanisms, scaling rules, and limits

Over the past few years, progress in hollow-core optical fiber technology has reduced the attenuation of these fibers to levels comparable to those of all-solid silica-core single-mode fibers. The sustained pace of progress in the field has sparked renewed interest in the technology and created the expectation that it will one day enable realization of the most transparent light-propagating waveguides ever produced, across all spectral regions of interest. In this work we review and analyze the various physical mechanisms that drive attenuation in hollow-core optical fibers. We consider both the somewhat legacy hollow-core photonic bandgap technology as well as the more recent antiresonant hollow-core fibers. As both fiber types exploit different guidance mechanisms from that of conventional solid-core fibers to confine light to the central core, their attenuation is also dominated by a different set of physical processes, which we analyze here in detail. First, we discuss intrinsic loss mechanisms in perfect and idealized fibers. These include leakage loss, absorption, and scattering within the gas filling the core or from the glass microstructure surrounding it, and roughness scattering from the air–glass interfaces within the fibers. The latter contribution is analyzed rigorously, clarifying inaccuracies in the literature that often led to the use of inadequate scaling rules. We then explore the extrinsic contributions to loss and discuss the effect of random microbends as well as that of other perturbations and non-uniformities that may result from imperfections in the fabrication process. These effects impact the loss of the fiber predominantly by scattering light from the fundamental mode into lossier higher-order modes and cladding modes. Although these contributions have often been neglected, their role becomes increasingly important in the context of producing, one day, hollow-core fibers with sub-0.1-dB/km loss and a pure single-mode guidance. Finally, we present general scaling rules for all the loss mechanisms mentioned previously and combine them to examine the performance of recently reported fibers. We lay some general guidelines for the design of low-loss hollow-core fibers operating at different spectral regions and conclude the paper with a brief outlook on the future of this potentially transformative technology.

Three-phase co-assembly of compositionally tunable WO3/TiO2 inverse opal photoelectrodes

Heterostructured WO3/TiO2 photonic crystal films in the form of three-dimensional macroporous inverse opals were developed by single-step, three-phase co-assembly of colloidal templates with water soluble precursors enabling simultaneous growth of both metal oxides into a firmly interconnected periodic pore framework whose skeletal walls consisted of uniformly distributed nanoscale type II WO3-TiO2 heterojunctions. A marked size- and composition-dependence of the binary WO3/TiO2 inverse opal properties was evidenced by controlling the W/Ti molar ratio and macropore diameter that allowed Fermi level and photonic band gap engineering according to optical and photoelectron spectroscopies supplemented by electrochemical measurements. Compositional tuning along with the reduction of WO3 nanocrystallite size and the concomitant W5+ defect growth with increasing TiO2 phase content were explored for the optimization of photocurrent generation by WO3/TiO2 inverse opal photoanodes combining reduced charge carrier recombination and optimal slow light trapping. One step co-assembly of mixed metal oxides is concluded as a promising route for the development of heterostructured inverse opal networks with tailored electronic properties and improved solar light harvesting for photo-induced applications.

Photonic bandgap engineering in hybrid one-dimensional photonic crystals containing all-dielectric elliptical metamaterials.

Metamaterials with negative permittivities or/and permeabilities greatly enrich photonic bandgap (PBG) engineering in one-dimensional (1-D) photonic crystals (PhCs). Nevertheless, their inevitable optical losses strongly destroy the crucial prohibition characteristic of PBGs, which makes such engineered PBGs not utilizable in some relevant physical processes and optical/optoelectronic devices. Herein, we bridge a link between 1-D PhCs and all-dielectric loss-free metamaterials and propose a hybrid 1-D PhC containing all-dielectric elliptical metamaterials to engineer angle-dependence of PBGs. Associating the Bragg scattering theory with the iso-frequency curve analysis, an analytical model is established to precisely describe the angle-dependence of PBG. Based on the analytical model, two types of special PBGs, i.e., angle-insensitive and angle-sensitive PBGs, are designed. By further introducing defects into the designed 1-D PhCs, angle-dependence of defect modes can also be flexibly controlled. Our protocol opens a viable route to precisely engineering PBGs and promotes the development of PBG-based physics and applications.

Photonic bandgap engineering in (VO2) n /(WSe2) n photonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal–insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO2) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO2 as a strongly correlated phase transition material and tungsten diselenide (WSe2) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO2 with WSe2 in a photonic superlattices structure through the near and mid-infrared (NIR–MIR) thermochromic phase transition regions. By increasing the thickness of the VO2 layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO2 thickness. For the monoclinic case of VO2, the number of the forbidden bands increase with the number of layers of WSe2. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR–MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study’s findings may help understand vanadium oxide’s role in tunable photonic superlattice for infrared switchable devices and optical filters.

Photonic bandgap engineering using second-order supersymmetry

First-order supersymmetry (SUSY) adapted from quantum physics to optics manipulates the transverse refractive index of guided-wave structures using a nodeless ground state to obtain intended modal content. Second-order SUSY can be implemented using excited states as a seed function, even with the presence of nodes. We apply second-order SUSY to the coupled-mode equations by recasting them as the Dirac equation. This enables the engineering of non-uniform surface corrugation of waveguide gratings and coupling potential, which encapsulates the Bragg interaction between counterpropagating modes. We show that the added bound states appear as transmission resonances inside the bandgap of the finite grating. The probability density of each state provides the longitudinal modal energy distribution in the waveguide grating. The smooth modal energy distribution of the states obtained by SUSY can mitigate longitudinal spatial hole burning in high power laser operation. We demonstrate that degenerate second-order SUSY allows the insertion of two states, which can coalesce into Friedrich-Wintgen type bound states in the continuum (BIC) for one-dimensional grating. We show that the eigenfunctions of BIC states are doubly degenerate with opposite parity, and the corresponding transmission resonances have phase changes of 2π across these states. One-dimensional BIC states can find application as robust high-speed all-optical temporal integrators by lifting restrictions on the length of various sections in the phase-shifted grating.

Ferroelectric dielectric and optical properties of layered PbZr x Ti 1-x O3 films derived from precursor solutions containing polyvinylpyrrolidone polymer additive

PbZr x Ti 1-x O3 (PZT) films were fabricated on FTO substrates by using precursor solutions containing polyvinylpyrrolidone additive and spinning-coating/annealing process. The correlation of the formation, ferroelectric, dielectric and optical reflection features of the layered PZT films with Zr/Ti atomic ration had been studied. It was found that each PZT film was fully crystallized and displayed a polycrystalline phase. The PZT films with Zr content ∼ 0.4–0.6 showed clearly distinguishable layered structures, and exhibited excellent ferroelectric and dielectric properties, together with good optical performance as dielectric mirrors. These PZT films possessed high reflectivity bands, the band widths were all over 50 nm and the peak reflectivities were more than 70%. They also had dielectric constants >550 Fm−1 at 10 KHz, remnant polarizations >24 μC/cm2 at 100 V polarizing voltage, rendering their potential application in photonic band gap engineering and integrated optoelectronics.

Graphene Quantum Dot-TiO2 Photonic Crystal Films for Photocatalytic Applications.

Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO2 photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO2 interfacial electron transfer. TiO2 inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO2 photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO2 films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films.

Multi-channel photonic bandgap engineering in hyperbolic graded index materials embedded one-dimensional photonic crystals

Engineering of multi-channel photonic band gap sensing consequences has been demonstrated in hyperbolic graded index materials embedded one-dimensional (1-D) photonic crystal (PC) in the frequency 150–850 THz region. The multi-channel photonic band gap sensing properties have been investigated by taking into account the reflection and photonic band gap (PBG) spectra of the proposed PC structures. For quarter-wave stacking, we obtain single optical reflection band for band region 646.8 – 434.3 THz with the constituted normal layer refractive index 1.5. Band regions and bandwidths of the single PBG channel can be modulated by changing the refractive index of the constituted normal layer and grading parameter of the hyperbolic graded layer. The number of photonic bands increases with increasing the layer thickness of the GPC structures and leads to work as multi-channel PBG sensors. The operation frequency of the multi-channel PBG sensors can also be tuned by changing the constituted normal layer and grading parameter of the hyperbolic graded layer. These properties lead to design the tunable multi-channel optical sensors/filters engineering. Moreover, the demonstration of the reflection phase shift, group velocity, group delay, and electric field distributions shows the effect of hyperbolic graded index materials on the propagation of light in 1-D PCs. With the engineering of tunable PBGs and structure controllability, hyperbolic graded index materials embedded 1-D PCs provide a promising way to fabricate tunable optical reflectors and multi-channel optical sensors/filters for future optical devices.

Integration of Semiconducting Carbon Nanotubes Within a Silicon Photonic Molecule

Integration of nanomaterials within optical nanocavities provides a unique potential for flexible control of light emitters properties by photonic band gap engineering and cavity Purcell effects. Here, we propose a one-dimensional heterostructure nanocavity exhibiting both non-coupled and coupled cavity modes, i.e. simultaneously acting as a single cavity and as a photonic molecule. The main cavity resonances are engineered to yield a wide spectral separation and for the first time to match the emission wavelengths of two different kinds of semiconducting single wall carbon nanotubes (s-SWNTs). By probing the photoluminescence (PL) from s-SWNTs coupled with the nano cavity modes, coupling of the s-SWNTs PL simultaneously into the several cavity modes is demonstrated. For modes governed by the photonic molecule behavior, the wavelength splitting of the two coupled modes is dominated by the cavity barrier width. The excitation of the bonding (B) and anti-bonding (AB) cavity modes then yields PL resonant enhancement that can be tuned by the pumping position and polarization filter. These results demonstrate the potential of the proposed multimode photonic molecule to tailor light-nanomaterial interactions on chip, paving the way for the development of tunable hybrid photonic circuits relying on nanoemitters in cavities for light generation purposes.

Tamm plasmon photonic crystals: From bandgap engineering to defect cavity

Photonic bandgap engineering plays a key role in modern photonics since it allows for an ultimate control of photon propagation in periodic dielectric or metallic media. Tamm plasmon structures are a particularly attractive platform since the electromagnetic field can be completely controlled by patterning the thin metal layer. Up to now, only macroscopic patterning (larger than the operation wavelength) has been experimentally demonstrated, leading to 3D confinement of light but suffering from a lack of fine control of the dispersion properties of Tamm plasmons. Here, we report for the first time the opening of a bandgap in Tamm plasmon structures via subwavelength-periodic patterning of the metallic layer. By adopting a “double period” design, we experimentally put into evidence a photonic bandgap for the TE polarization up to 150 nm wide in the telecom wavelength range. Moreover, such a design offers a great flexibility to tailor on-demand, and independently, the bandgap size from 30 nm to 150 nm and its spectral position within a range of 50 nm. Finally, by implementing a defect cavity within the Tamm plasmon photonic crystal, a 1.6 μm cavity supporting a single highly confined Tamm mode is experimentally demonstrated. All experimental results are in perfect agreement with numerical calculations. Our results demonstrate the possibility to engineer novel band dispersion with surface modes of hybrid metallic/dielectric structures, thus opening the way to applications in topological photonics, metamaterials, and parity-time symmetry physics.

Holographic fabrication of octagon graded photonic supercrystal and potential applications in topological photonics.

Novel optical properties in graded photonic super-crystals can be further explored if new types of graded photonic super-crystals are fabricated. In this paper, we report holographic fabrication of graded photonic super-crystal with eight graded lattice clusters surrounding the central non-gradient lattices through pixel-by-pixel phase engineering in a spatial light modulator. The prospect of applications of octagon graded photonic super-crystal in topological photonics is discussed through photonic band gap engineering and coupled ring resonators.

Synthesis and Photocatalytic Properties of Tantalum Nitride with an Inverse Opal Structure

Certain oxynitrides (LaTiO2N and TaON) and nitrides (Ta3N5) demonstrate potential as visible-light-driven photocatalysts1. These materials are typically synthesized from oxide precursors by thermal ammonolysis at temperatures up to 950 oC. Accordingly, these oxynitrides and nitrides intrinsically possess an abundance of anion vacancies or defects which act as electron-hole pair recombination centers, thereby limiting photocatalytic performance. In this work, we introduce photonic band gap engineering as a strategy for suppressing electron-hole pair recombination in tantalum nitride thin films, thereby achieving improved photocatalytic performance. Briefly, a series of inverse opal (IO) Ta3N5 thin films with different macropore sizes were fabricated by the colloidal crystal template approach. This kind of approach has been successfully made on TiO2 photocatalysts2. Three batches of monodisperse polymethylmethacrylate (PMMA) colloids of diameters ranging from 230 to 430 nm were first synthesized by the surfactant-free emulsion polymerization of methyl methacrylate. PMMA colloidal crystal thin films were then grown on silica glass slides using a flow-controlled vertical deposition technique. As the meniscus of the suspension moved down the glass slide by pumping up a peristaltic pump, colloidal crystal layer of several microns were deposited. Several drops of TaCl5 solution were placed softly on the edge of the colloidal crystal and immersed into the crystal by capillary phenomena. Subsequently, the interstitial voids in the colloidal crystal films (synthetic opals) were filled with a tantalum hydroxide sol. Ammonolysis of the resulting composite films under an ammonia stream of 200 ml/min at 700ºC yielded IO Ta3N5 thin films, shown in Fig.1. The IO Ta3N5 films possess a 3-dimensionally ordered macroporous structure with periodicity on the scale of visible-light wavelengths, resulting in pseudo photonic band gaps at visible wavelengths. By matching the red-edge of the PBG location with the absorption edge of Ta3N5, electron-hole pair recombination is suppressed and the photocatalytic activity dramatically improved. In this experiment, strong reflection due to [111] photonic bandgap was clearly observed at 543nm for the IO amorphous Ta2O5 film and at 581nm for the IO Ta3N5 film, respectively. Appearance of the IO Ta3N5 film was reddish orange in color but it gave a strong yellow reflection in a certain angle, seen in Fig. 2. Since the optical bandgap of Ta3N5 was estimated to be 2.1eV (=590nm), the IO tantalum nitrides with both optical and bandgaps properly matched could be fabricated. Photocatalystic H2 evolution activity of the IO Ta3N5 under UV or visible ray irradiation will be appeared at the presentation, togather with a comparison with that of Ta3N5 powder. References Tsuyoshi Takata, Chengsi Pan and Kazunari Domen; Sci. Technol. Adv. Mater., 16 (2015) 033506 (18pp).Waterhouse et al., Sci. Rep., 3 (2013) 2849 (5pp). Figure 1

Influence of upconversion luminescence modification on near infrared luminescence and cooperative energy transfer in the YbPO4:Er3+, Nb3+/Er3+inverse opals excited at 980 or 808 nm

The photoluminescence modification of rare earth ions base on the photonic band gap engineering was extensively reported. In this work, the effect of up-converting luminescence suppression of Er3+ on the near infrared luminescence and cooperative energy transfer from Er3+ to Yb3+ was investigated in the YbPO4 inverse opals single-doped with Er3+ and co-doped with Nd3+ and Er3+. For the YbPO4:Er3+ inverse opals, when the photonic band gap is overlapped with the green or red up-converting luminescence of Er3+, the green or red up-converting luminescence of Er3+ was suppressed in the YbPO4:Er3+ inverse opals upon the 980 nm excitation, respectively. It is significative that the enhancement of near infrared luminescence of Er3+ was observed, which is attributed to the electron redistribution in the excited energy levels caused by the upconversion luminescence suppression in the YbPO4:Er3+ inverse opals. For the YbPO4:Nd3+,Er3+ inverse opals, the green upconversion and near infrared luminescence of Er3+ and the near infrared luminescence of Yb3+ were observed excited at 808 nm. The near infrared luminescence enhancement of Yb3+ was obtained due to the cooperative energy transfer enhancement from Er3+ to Yb3+ caused by the green upconversion luminescence suppression. The result is helpful to learn the relationship between upconversion and infrared luminescence in rare earth ions doped materials.

Self‐Collimation in Photonic Crystals: Applications and Opportunities

AbstractA comprehensive review considering recent advances in self‐collimation and its applications in optical integration is covered in the current article. Self‐collimation is compared to the conventional technique of photonic bandgap engineering to control the light propagation in photonic crystal‐based structures. It is fully discussed how the self‐collimation phenomenon can be tailored to be independent of the incident angle and polarization. This adds substantial flexibility to the structure to overcome light coupling challenges and simultaneously aids in the omission of bulk and challenging elements, including polarizers and lenses from optical integrated circuits. Additionally, designed structures have the potential to be rescaled to operate in any desired frequency range thanks to the scalability rule in the field of electromagnetics. Moreover, it is shown that one can boost the coupling efficiency by applying an anti‐reflection property to the structure, which provides not only efficient index matching but also the matching between external waves with uniform amplitude and Bloch waves with periodic amplitude.

Electrical and optical properties of a kind of ferroelectric oxide films comprising of PbZr0.4Ti0.6O3 stacks

A type of ferroelectric oxide films, consisting of three PbZr0.4Ti0.6O3 stacks with different periodic thicknesses, has been designed and fabricated on F-doped transparent conductive tin oxide substrates by using one single precursor solution and spinning-coating process. These films exhibit superior ferroelectric, dielectric, and optical performance. Each PbZr0.4Ti0.6O3 multilayer has a high reflectivity band with ∼110 nm photonic band width and average reflectivity of >80%, a dielectric constant of 530 and dielectric tunability of ∼28% at 1 MHz, a remnant polarization of 36 μC/cm2, and a polarization loss of <5% after 109 polarization switching cycles, rendering their perspective application in photonic band-gap engineering, microwave tunable devices, and integrated optoelectronics.

Photonic and Omnidirectional Band Gap Engineering in One-Dimensional Photonic Crystals Consisting of Linearly Graded Index Material

Engineering of photonic and omnidirectional band gaps has been demonstrated theoretically in one-dimensional photonic crystals (1-D-PCs) containing linearly graded and constant refractive index materials. The reflection spectra, photonic band gap spectra, reflection phase shift, and electric field distribution have been obtained by transfer matrix method in the 200–800 THz region. It has been found that the number of photonic bands increases with layers thickness. The omnidirectional band gap in a quarter-wave stacking arrangement of such PC has also been obtained. Results show that different constituted homogeneous layers can change the omnidirectional band gap remarkably. Moreover, we have demonstrated that photonic bandwidths can also be controlled by the contrast of initial and final refractive index of the graded layers.

Photonic and omnidirectional band gap engineering in stack of exponential graded index material and negative index material

We have investigated the photonic band gaps (PBG) and omnidirectional band gaps in one-dimensional photonic crystals made up of alternate layer of exponential graded index material and negative index material. We have considered the influence of material properties, geometrical parameters and material composition on the PBG and omnidirectional band gap. Results show that the parameters of exponential graded index material and negative index material can change the photonic and omnidirectional band structures remarkably. Number and bandwidth of PBG increases with increasing the negative index material layer thicknesses while thicknesses of graded index layer only have an effect on the bandwidth of PBGs. The bandwidth of PBG also depends on grading profile parameter of exponential graded index layers and bandwidth can be tuned with increase the value of grading profile parameter. This work can facilitate the design of filters and reflectors, and provide the basic understanding of the influence of graded index materials and metamaterials on the PBG properties.

Transmission properties of defect modes in one-dimensional photonic crystals containing gradient refractive index defects

The gradient refractive index (GRIN) defect layer is introduced into one-dimensional photonic crystal (1D PC) to develop the optical filters needed. The transmission properties of 1D PC with GRIN defect layer were investigated based on the finite difference time domain (FDTD) method. Defect mode was obtained according to the GRIN distribution functions. With a GRIN structure in the defect layer, simulation results show that the position and width of the photonic band gap can be effectively modulated by varying the thickness and GRIN distribution functions of defect layer and the parameters of symmetrical layers. The new study provides a certain reference for photonic band gap engineering and designing the photonic-based devices.