Showing papers on "Photonic crystal published in 2019"

Robust topologically protected transport in photonic crystals at telecommunication wavelengths.

Abstract: Photonic topological insulators offer the possibility to eliminate backscattering losses and improve the efficiency of optical communication systems. Despite considerable efforts, a direct experimental demonstration of theoretically predicted robust, lossless energy transport in topological insulators operating at near-infrared frequencies is still missing. Here, we combine the properties of a planar silicon photonic crystal and the concept of topological protection to design, fabricate and characterize an optical topological insulator that exhibits the valley Hall effect. We show that the transmittances are the same for light propagation along a straight topological interface and one with four sharp turns. This result quantitatively demonstrates the suppression of backscattering due to the non-trivial topology of the structure. The photonic-crystal-based approach offers significant advantages compared with other realizations of photonic topological insulators, such as lower propagation losses, the presence of a band gap for light propagating in the crystal-slab plane, a larger operating bandwidth, a much smaller footprint, compatibility with complementary metal-oxide-semiconductor fabrication technology, and the fact that it allows for operation at telecommunications wavelengths.

Visualization of Higher-Order Topological Insulating Phases in Two-Dimensional Dielectric Photonic Crystals

Abstract: The studies of topological phases of matter have been developed from condensed matter physics to photonic systems, resulting in fascinating designs of robust photonic devices. Recently, higher-order topological insulators have been investigated as a novel topological phase of matter beyond the conventional bulk-boundary correspondence. Previous studies of higher-order topological insulators have been mainly focused on the topological multipole systems with negative coupling between lattice sites. Here we experimentally demonstrate that second-order topological insulating phases without negative coupling can be realized in two-dimensional dielectric photonic crystals. We visualize both one-dimensional topological edge states and zero-dimensional topological corner states by using the near-field scanning technique. Our findings open new research frontiers for photonic topological phases and provide a new mechanism for light manipulating in a hierarchical way.

Direct Observation of Corner States in Second-Order Topological Photonic Crystal Slabs.

Abstract: Recently, higher-order topological phases that do not obey the usual bulk-edge correspondence principle have been introduced in electronic insulators and brought into classical systems, featuring in-gap corner or hinge states. In this Letter, using near-field scanning measurements, we show the direct observation of corner states in second-order topological photonic crystal slabs consisting of periodic dielectric rods on a perfect electric conductor. Based on the generalized two-dimensional Su-Schrieffer-Heeger model, we show that the emergence of corner states roots in the nonzero edge dipolar polarization instead of the nonzero bulk quadrupole polarization. We demonstrate the topological transition of two-dimensional Zak phases of photonic crystal slabs by tuning intracell distances between two neighboring rods. We also directly observe in-gap one-dimensional edge states and zero-dimensional corner states in the microwave regime. Our work presents that the photonic crystal slab is a powerful platform to directly observe topological states and paves the way to study higher-order photonic topological insulators.

Topologically enabled ultrahigh-Q guided resonances robust to out-of-plane scattering.

Abstract: Because of their ability to confine light, optical resonators1–3 are of great importance to science and technology, but their performance is often limited by out-of-plane-scattering losses caused by inevitable fabrication imperfections4,5. Here we theoretically propose and experimentally demonstrate a class of guided resonances in photonic crystal slabs, in which out-of-plane-scattering losses are strongly suppressed by their topological nature. These resonances arise when multiple bound states in the continuum—each carrying a topological charge6—merge in momentum space and enhance the quality factors Q of all nearby resonances in the same band. Using such resonances in the telecommunication regime, we experimentally achieve quality factors as high as 4.9 × 105—12 times higher than those obtained with standard designs—and this enhancement remains robust for all of our samples. Our work paves the way for future explorations of topological photonics in systems with open boundary conditions and for their application to the improvement of optoelectronic devices in photonic integrated circuits. Bound states in the continuum are merged in momentum space by varying the periodicity of the photonic crystal lattice, giving high-quality-factor guided resonances that are robust to out-of-plane scattering.

Photonic crystal nanocavity based on a topological corner state

Abstract: Topological photonics has emerged as a novel approach to engineering the flow of light and provides unprecedented means for developing diverse photonic elements, including robust optical waveguides immune to structural imperfections. However, the development of nanoscale standing-wave cavities in topological photonics is rather slow, despite its importance when building densely integrated photonic integrated circuits. In this Letter, we report a photonic crystal nanocavity based on a topological corner state, supported at a 90-deg-angled rim of a two-dimensional photonic crystal. A combination of the bulk-edge and edge-corner correspondences guarantees the presence of the higher-order topological state in a hierarchical manner. We experimentally observe a corner mode that is tightly localized in space while supporting a high Q factor over 2,000, verifying its promise as a nanocavity. These results cast new light on the way to introduce nanocavities in topological photonics platforms.

Realization of a three-dimensional photonic topological insulator.

Abstract: Confining photons in a finite volume is highly desirable in modern photonic devices, such as waveguides, lasers and cavities. Decades ago, this motivated the study and application of photonic crystals, which have a photonic bandgap that forbids light propagation in all directions1–3. Recently, inspired by the discoveries of topological insulators4,5, the confinement of photons with topological protection has been demonstrated in two-dimensional (2D) photonic structures known as photonic topological insulators6–8, with promising applications in topological lasers9,10 and robust optical delay lines11. However, a fully three-dimensional (3D) topological photonic bandgap has not been achieved. Here we experimentally demonstrate a 3D photonic topological insulator with an extremely wide (more than 25 per cent bandwidth) 3D topological bandgap. The composite material (metallic patterns on printed circuit boards) consists of split-ring resonators (classical electromagnetic artificial atoms) with strong magneto-electric coupling and behaves like a ‘weak’ topological insulator (that is, with an even number of surface Dirac cones), or a stack of 2D quantum spin Hall insulators. Using direct field measurements, we map out both the gapped bulk band structure and the Dirac-like dispersion of the photonic surface states, and demonstrate robust photonic propagation along a non-planar surface. Our work extends the family of 3D topological insulators from fermions to bosons and paves the way for applications in topological photonic cavities, circuits and lasers in 3D geometries. A three-dimensional photonic topological insulator is presented, made of split-ring resonators with strong magneto-electric coupling, which has an extremely wide topological bandgap, forbidding light propagation.

A silicon-on-insulator slab for topological valley transport

Abstract: Backscattering suppression in silicon-on-insulator (SOI) is one of the central issues to reduce energy loss and signal distortion, enabling for capability improvement of modern information processing systems. Valley physics provides an intriguing way for robust information transfer and unidirectional coupling in topological nanophotonics. Here we realize topological transport in a SOI valley photonic crystal slab. Localized Berry curvature near zone corners guarantees the existence of valley-dependent edge states below light cone, maintaining in-plane robustness and light confinement simultaneously. Topologically robust transport at telecommunication is observed along two sharp-bend interfaces in subwavelength scale, showing flat-top high transmission of ~10% bandwidth. Topological photonic routing is achieved in a bearded-stack interface, due to unidirectional excitation of valley-chirality-locked edge state from the phase vortex of a nanoscale microdisk. These findings show the prototype of robustly integrated devices, and open a new door towards the observation of non-trivial states even in non-Hermitian systems. Backscattering is one of the major factors that limit the performance of integrated nanophotonics. Here, He et al. realize topologically protected, robust and unidirectional coupling as well as optical transport on a silicon-on-insulator platform by exploiting the valley degree of freedom.

Generating optical vortex beams by momentum-space polarization vortices centered at bound states in the continuum

Abstract: An optical vortex (OV) is a beam with spiral wave front and screw phase dislocation. This kind of beams is attracting rising interest in various fields. Here we theoretically proposed and experimentally realized a novel but easy approach to generate optical vortices. We leverage the inherent topological vortex structures of polarization around bound states in the continuum (BIC) in the momentum space of two dimensional periodic structures, e.g. photonic crystal slabs, to induce Pancharatnam-Berry phases to the beams. This new class of OV generators operates in the momentum space, meaning that there is no real-space center of structure. Thus, not only the fabrication but also the practical alignment would be greatly simplified. Any even order of OV, which is actually a quasi-non-diffractive high-order quasi-Bessel beam, at any desired working wavelength could be achieved in principle. The proposed approach expands the application of bound states in the continuum and topological photonics.

Structural color three-dimensional printing by shrinking photonic crystals.

Abstract: The coloration of some butterflies, Pachyrhynchus weevils, and many chameleons are notable examples of natural organisms employing photonic crystals to produce colorful patterns. Despite advances in nanotechnology, we still lack the ability to print arbitrary colors and shapes in all three dimensions at this microscopic length scale. Here, we introduce a heat-shrinking method to produce 3D-printed photonic crystals with a 5x reduction in lattice constants, achieving sub-100-nm features with a full range of colors. With these lattice structures as 3D color volumetric elements, we printed 3D microscopic scale objects, including the first multi-color microscopic model of the Eiffel Tower measuring only 39 µm tall with a color pixel size of 1.45 µm. The technology to print 3D structures in color at the microscopic scale promises the direct patterning and integration of spectrally selective devices, such as photonic crystal-based color filters, onto free-form optical elements and curved surfaces. The ability to print arbitrary colors and shapes in all three dimensions at microscopic length scales is still lacking. Here, the authors introduce a means to produce three-dimensionally-printed photonic crystals with a periodicity as small as 280 nm, achieving sub-100-nm features with a full range of colors.

Photonic crystals for nano-light in moir\'e graphene superlattices

Abstract: Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moire superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus obtained twisted bilayer graphene (TBG). We studied propagation of plasmon polaritons in TBG by infrared nano-imaging. We discovered that the atomic reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway towards controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials eliminating need for arduous top-down nanofabrication.

Circularly Polarized States Spawning from Bound States in the Continuum.

Abstract: Bound states in the continuum in periodic photonic systems like photonic crystal slabs are proved to be accompanied by vortex polarization singularities on the photonic bands in the momentum space. The winding structures of polarization states not only widen the field of topological physics but also show great potential that such systems could be applied in polarization manipulating. In this Letter, we report the phenomenon that by in-plane inversion (${C}_{2}$) symmetry breaking, pairs of circularly polarized states could spawn from the eliminated bound states in the continuum. Along with the appearance of the circularly polarized states as the two poles of the Poincar\'e sphere together with linearly polarized states covering the equator, full coverage on the Poincar\'e sphere could be realized. As an application, ellipticity modulation of linear polarization is demonstrated in the visible frequency range. This phenomenon provides a new degree of freedom in modulating polarization. The $C$ points could also find applications in light-matter interactions. Further studying and manipulating the reported polarization singularities may lead to novel phenomena and physics in radiation modulating and topological photonics.

Double-lattice photonic-crystal resonators enabling high-brightness semiconductor lasers with symmetric narrow-divergence beams.

Abstract: Achieving high brightness (where brightness is defined as optical power per unit area per unit solid angle) in semiconductor lasers is important for various applications, including direct-laser processing and light detection and ranging for next-generation smart production and mobility. Although the brightness of semiconductor lasers has been increased by the use of edge-emitting-type resonators, their brightness is still one order of magnitude smaller than that of gas and solid-state/fibre lasers, and they often suffer from large beam divergence with strong asymmetry and astigmatism. Here, we develop a so-called ‘double-lattice photonic crystal’, where we superimpose two photonic lattice groups separated by one-quarter wavelength in the x and y directions. Using this resonator, an output power of 10 W with a very narrow-divergence-angle (<0.3°) symmetric surface-emitted beam is achieved from a circular emission area of 500 μm diameter under pulsed conditions, which corresponds to a brightness of over 300 MW cm−2 sr−1. In addition, an output power up to ~7 W is obtained under continuous-wave conditions. Detailed analyses on the double-lattice structure indicate that the resonators have the potential to realize a brightness of up to 10 GW cm−2 sr−1, suggesting that compact, affordable semiconductor lasers will be able to rival existing gas and fibre/disk lasers. An optimized design for a broad-area surface-emitting photonic-crystal laser leads to high brightness of over 300 MW cm–2 sr–1 and an output power of 10 W under pulsed excitation.

Structural Color 3D Printing By Shrinking Photonic Crystals

Abstract: The rings, spots and stripes found on some butterflies, Pachyrhynchus weevils, and many chameleons are notable examples of natural organisms employing photonic crystals to produce colorful patterns. Despite advances in nanotechnology, we still lack the ability to print arbitrary colors and shapes in all three dimensions at this microscopic length scale. Commercial nanoscale 3D printers based on two-photon polymerization are incapable of patterning photonic crystal structures with the requisite ~300 nm lattice constant to achieve photonic stopbands/ bandgaps in the visible spectrum and generate colors. Here, we introduce a means to produce 3D-printed photonic crystals with a 5x reduction in lattice constants (periodicity as small as 280 nm), achieving sub-100-nm features with a full range of colors. The reliability of this process enables us to engineer the bandstructures of woodpile photonic crystals that match experiments, showing that observed colors can be attributed to either slow light modes or stopbands. With these lattice structures as 3D color volumetric elements (voxels), we printed 3D microscopic scale objects, including the first multi-color microscopic model of the Eiffel Tower measuring only 39-microns tall with a color pixel size of 1.45 microns. The technology to print 3D structures in color at the microscopic scale promises the direct patterning and integration of spectrally selective devices, such as photonic crystal-based color filters, onto free-form optical elements and curved surfaces.

Ultra-high sensitive 1D porous silicon photonic crystal sensor based on the coupling of Tamm/Fano resonances in the mid-infrared region

Abstract: Porous silicon one-dimensional photonic crystals (PSi-1DPCs) are capable of sensing solutions and liquids based on the smallest variation of the refractive indices. In the present work, we present a novel metal/PSi-1DPC as a liquid sensor based on Tamm/Fano resonances. The operating wavelength range is from 6.35 to 9.85 μm in the mid-infrared (MIR) spectral region. Different metals (Al, Ag, Au, and Pt) are attached to the top surface of the PSi-1DPCs structure to show Tamm/Fano resonances more clearly. To the best of our knowledge, it is the first time that Tamm/Fano resonances exhibit simultaneously in PSi-1DPCs within the same structure. The reflection spectra were calculated for the metal/PSi-1DPC structure by using the transfer matrix method (TMM) and the Bruggeman’s effective medium approximation (BEMA). The simulations show that the Tamm/Fano resonances are red-shifted towards the higher wavelengths with increasing the refractive index of the pores. The Ag/PSi-1DPC sensor showed the highest performance. Its sensitivity can be reached to the value 5018 nm/RIU with a high-quality factor of about 2149.27. We predict the proposed sensors can be easily fabricated and we expect them to show higher performance than other reported sensors of this type. Therefore, it will be of interest in the field of optical sensing in different fields.

Three-dimensional femtosecond laser nanolithography of crystals

Abstract: So far, nanostructuring of hard optical crystals has been exclusively limited to their surface, as stress-induced crack formation and propagation render high-precision volume processes ineffective1,2. Here, we show that the rate of nanopore chemical etching in the popular laser crystals yttrium aluminium garnet and sapphire can be enhanced by more than five orders of magnitude (from <0.6 nm h−1 to ~100 µm h−1) by the use of direct laser writing, before etching. The process makes it possible to produce arbitrary three-dimensional nanostructures with 100 nm feature sizes inside centimetre-scale laser crystals without brittle fracture. To showcase the potential of the technique we fabricate subwavelength diffraction gratings and nanostructured optical waveguides in yttrium aluminium garnet and millimetre-long nanopores in sapphire. The approach offers a pathway for transferring concepts from nanophotonics to the fields of solid-state lasers and crystal optics. Direct laser writing is shown to dramatically enhance the chemical etch rate of laser crystals yttrium aluminium garnet and sapphire, allowing nanostructuring.

Machine learning approach for computing optical properties of a photonic crystal fiber

Abstract: Photonic crystal fibers (PCFs) are the specialized optical waveguides that led to many interesting applications ranging from nonlinear optical signal processing to high-power fiber amplifiers. In this paper, machine learning techniques are used to compute various optical properties including effective index, effective mode area, dispersion and confinement loss for a solid-core PCF. These machine learning algorithms based on artificial neural networks are able to make accurate predictions of above-mentioned optical properties for usual parameter space of wavelength ranging from 0.5-1.8 µm, pitch from 0.8-2.0 µm, diameter by pitch from 0.6-0.9 and number of rings as 4 or 5 in a silica solid-core PCF. We demonstrate the use of simple and fast-training feed-forward artificial neural networks that predicts the output for unknown device parameters faster than conventional numerical simulation techniques. Computation runtimes required with neural networks (for training and testing) and Lumerical MODE solutions are also compared.

Highly Stretchable Photonic Crystal Hydrogels for a Sensitive Mechanochromic Sensor and Direct Ink Writing

Abstract: Photonic crystals, which are materials with periodic dielectric constants on the submicroscale, have been the focus of research for an extended period. Photonic soft materials have been extensively...

Enhanced sensitivity of cancer cell using one dimensional nano composite material coated photonic crystal

Abstract: We theoretically analyze the detection of a cancer cell in the one-dimensional photonic crystal by infiltrating different sample cells in the cavity layer. The defect modes appear in their transmission spectra only if the nanocomposite layers are included on either side of the cavity layer. This analysis is carried out by a dielectric constant and the transmittance peak of the cancer cell is compared with the normal cell. The transmittance peak shifts are analyzed with various filling factors for optimization purposes. Through the shifting spectrum, the sensitivity of cancer cell from the normal cell is obtained from a minimum of 42 nm/RIU to a maximum of 43 nm/RIU.

Beyond 30% Conversion Efficiency in Silicon Solar Cells: A Numerical Demonstration.

Abstract: We demonstrate through precise numerical simulations the possibility of flexible, thin-film solar cells, consisting of crystalline silicon, to achieve power conversion efficiency of 31%. Our optimized photonic crystal architecture consists of a 15 μm thick cell patterned with inverted micro-pyramids with lattice spacing comparable to the wavelength of near-infrared light, enabling strong wave-interference based light trapping and absorption. Unlike previous photonic crystal designs, photogenerated charge carrier flow is guided to a grid of interdigitated back contacts with optimized geometry to minimize Auger recombination losses due to lateral current flow. Front and back surface fields provided by optimized Gaussian doping profiles are shown to play a vital role in enhancing surface passivation. We carefully delineate the drop in power conversion efficiency when surface recombination velocities exceed 100 cm/s and the doping profiles deviate from prescribed values. These results are obtained by exact numerical simulation of Maxwell’s wave equations for light propagation throughout the cell architecture and a state-of-the-art model for charge carrier transport and Auger recombination.

Hierarchical Photonic Pigments via the Confined Self-Assembly of Bottlebrush Block Copolymers

Abstract: Hierarchical, structurally colored materials offer a wide variety of visual effects that cannot be achieved with standard pigments or dyes. However, their fabrication requires simultaneous control over multiple length-scales. Here we introduce a robust strategy for the fabrication of hierarchical photonic pigments via the confined self-assembly of bottlebrush block copolymers within emulsified microdroplets. The bottlebrush block copolymer self-assembles into highly ordered concentric lamellae, giving rise to a near perfect photonic multilayer in the solid state, with reflectivity up to 100%. The reflected color can be readily tuned across the whole visible spectrum by either altering the molecular weight or by blending the bottlebrush block copolymers. Furthermore, the developed photonic pigments are responsive, with a selective and reversible color change observed upon swelling in different solvents. Our system is particularly suited for the scalable production of photonic pigments, arising from their rapid self-assembly mechanism and size-independent color.

Plasmonic Surface Lattice Resonances: Theory and Computation

Abstract: Plasmonic surface lattice resonances (SLRs) are mixed light-matter states emergent in a system of periodically arranged metallic nanoparticles (NPs) under the constraint that the array spacing is able to support a standing wave of optical-frequency light. The properties of SLRs derive from two separate physical effects; the electromagnetic (plasmonic) response of metal NPs and the electromagnetic states (photonic cavity modes) associated with the array of NPs. Metal NPs, especially free-electron metals such as silver, gold, aluminum, and alkali metals, support optical-frequency electron density oscillations known as localized surface plasmons (LSPs). The high density of conduction-band electrons in these metals gives rise to plasmon excitations that strongly couple to light even for particles that are several orders of magnitude smaller than the wavelength of the excitation source. In this sense, LSPs have the remarkable ability to squeeze far-field light into intensely localized electric near-fields that can enhance the intensity of light by factors of ∼103 or more. Moreover, as a result of advances in the synthesis and fabrication of NPs, the intrinsic dependence of LSPs on the NP geometry, composition, and size can readily be exploited to design NPs with a wide range of optical properties. One drawback in using LSPs to enhance optical, electronic, or chemical processes is the losses introduced into the system by dephasing and Ohmic damping-an effect that must either be tolerated or mitigated. Plasmonic SLRs enable the mitigation of loss effects through the coupling of LSPs to diffractive states that arise from arrays satisfying Bragg scattering conditions, also known as Rayleigh anomalies. Bragg modes are well-known for arrays of dielectric NPs, where they funnel and trap incoming light into the plane of the lattice, defining a photonic cavity. The low losses and narrow linewidths associated with dielectric NPs produce Bragg modes that oscillate for ∼103-104 cycles before decaying. These modes are of great interest to the metamaterials community but have relatively weak electric fields associated with dielectric NPs and therefore are not used for applications where local field enhancements are needed. Plasmonic lattices, i.e., photonic crystals composed of metallic NPs, combine the characteristics of both LSPs and diffractive states, enabling both enhanced local fields and narrow-linewidth excitations, in many respects providing the best advantages of both materials. Thus, by control of the periodicity and global symmetry of the lattice in addition to the material composition and shape of the constituent NPs, SLRs can be designed to simultaneously survive for up to 103 cycles while maintaining the electric field enhancements near the NP surface that have made the use of LSPs ubiquitous in nanoscience. Modern fabrication methods allow for square-centimeter-scale patches of two-dimensional arrays that are composed of approximately one trillion NPs, making them effectively infinite at the nanoscale. Because of these advances, it is now possible to experimentally realize SLRs with properties that approach those predicted by idealized theoretical models. In this Account, we introduce the fundamental theory of both SLRs and SLR-mediated lasing, where the latter is one of the most important applications of plasmonic SLRs that has emerged to date. The focus of this Account is on theoretical concepts for describing plasmonic SLRs and computational methods used for their study, but throughout we emphasize physical insights provided by the theory that aid in making applications.

Efficient nonlinear beam shaping in three-dimensional lithium niobate nonlinear photonic crystals

Abstract: Nonlinear beam shaping refers to spatial reconfiguration of a light beam at a new frequency, which can be achieved by using nonlinear photonic crystals (NPCs). Direct nonlinear beam shaping has been achieved to convert second-harmonic waves into focusing spots, vortex beams, and diffraction-free beams. However, previous nonlinear beam shaping configurations in one-dimensional and two-dimensional (2D) NPCs generally suffer from low efficiency because of unfulfilled phase-matching condition. Here, we present efficient generations of second-harmonic vortex and Hermite-Gaussian beams in the recently-developed three-dimensional (3D) lithium niobate NPCs fabricated by using a femtosecond-laser-engineering technique. Since 3D χ(2) modulations can be designed to simultaneously fulfill the requirements of nonlinear wave-front shaping and quasi-phase-matching, the conversion efficiency is enhanced up to two orders of magnitude in a tens-of-microns-long 3D NPC in comparison to the 2D case. Efficient nonlinear beam shaping paves a way for its applications in optical communication, super-resolution imaging, high-dimensional entangled source, etc.

Lasing at topological edge states in a photonic crystal L3 nanocavity dimer array

Abstract: Topological photonics have provided new insights for the manipulation of light. Analogous to electrons in topological insulators, photons travelling through the surface of a topological photonic structure or the interface of two photonic structures with different topological phases are free from backscattering caused by structural imperfections or disorder. This exotic nature of the topological edge state (TES) is truly beneficial for nanophotonic devices that suffer from structural irregularities generated during device fabrication. Although various topological states and device concepts have been demonstrated in photonic systems, lasers based on a topological photonic crystal (PhC) cavity array with a wavelength-scale modal volume have not been explored. We investigated TESs in a PhC nanocavity array in the Su-Schrieffer-Heeger model. Upon optical excitation, the topological PhC cavity array realised using an InP-based multiple-quantum-well epilayer spontaneously exhibits lasing peaks at the topological edge and bulk states. TES characteristics, including the modal robustness caused by immunity to scattering, are confirmed from the emission spectra and near-field imaging and by theoretical simulations and calculations.

Doubly resonant χ (2) nonlinear photonic crystal cavity based on a bound state in the continuum

Abstract: Photonic nanostructures simultaneously maximizing spectral and spatial overlap between fundamental and second-harmonic confined modes are highly desirable for enhancing second-order nonlinear effects in nonlinear media. These conditions have thus far remained challenging to satisfy in photonic crystal cavities because of the difficulty in designing a band gap at the second-harmonic frequency. Here, we solve this issue by using instead a bound state in the continuum at that frequency, and we design a doubly resonant photonic crystal slab cavity with strongly improved figures of merit for nonlinear frequency conversion when compared to previous photonic crystal designs. Furthermore, we show that the far-field emission at both frequencies is highly collimated around normal incidence, which allows for simultaneously efficient pump excitation and collection of the generated nonlinear signal. Our results could lead to unprecedented conversion efficiencies in both parametric down-conversion and second-harmonic generation in an extremely compact architecture.

Biomimetic Chiral Photonic Crystals

Abstract: Although it is well known that the amazing iridescent colors of the cuticle of beetles reflect the intricate nanoscale organization of bio-fibers, artificial inorganic materials with comparable optical responses have not yet been synthesized from abiotic nanoscale building blocks. Such materials could find broad applications, including in circular polarizers, to generate circularly polarized luminescence, or in lasers. Herein, we describe a general method for the fabrication of biomimetic chiral photonic crystals by Langmuir-Schaefer assembly of colloidal inorganic nanowires. We not only reproduced the intricate helical structure and circularly polarized color reflection observed in beetles, but also achieved the highest chiroptical activity with a dissymmetry factor of -1.6 ever reported for chiral inorganic nanostructures. More importantly, the programmable structural control based on the precise interlayer arrangement endows us with unprecedented freedom to manipulate the optical activity of as-fabricated chiral photonic crystals.

Ultrahigh-Q photonic crystal nanocavities based on 4H silicon carbide

Abstract: Photonic nanocavities with high quality (Q) factors are essential components for integrated optical circuits. The use of crystalline silicon carbide (SiC) for such nanocavities enables the realization of devices with superior properties. We fabricate ultrahigh-Q SiC photonic crystal nanocavities by etching air holes into a 4H-SiC slab that is prepared without using hydrogen ion implantation, which usually causes higher absorption losses. In addition, compared to usual designs, a relatively thin slab is utilized to avoid losses through cross-polarized mode coupling induced by the tapered air holes. We obtain a heterostructure nanocavity with a high experimental Q factor of 6.3×105, which is 16 times larger than the highest Q among the previously reported values for nanocavities based on crystalline SiC. We also show that our nanocavity exhibits a high normalized second-harmonic conversion efficiency of 1900%/W.