2D Photonic Crystals Research Articles

All-optical 3D blue phase photonic crystal switch with photosensitive dopants

Blue phase (BP) liquid crystals (LC) have lately become the focus of extensive research due to their peculiar properties and structure. BPs exhibit a highly organized 3D structure with a lattice period in the hundreds of nm. Owing to such structure, BPs are regarded as 3D photonic crystals. The unique properties of this complex LC phase are achieved by the self-assembly of the LC molecules into periodic cubic structures, producing bright selective Bragg reflections. Novel applications involving 3D photonic crystals would certainly benefit from enhanced ground-breaking functionalities. However, the use of BPs as 3D has been traditionally curtailed by the BP crystals trend to grow as random polycrystals, making it difficult to develop practical BP-based photonic devices. The possibility of generating mm-sized BP monocrystals was recently demonstrated. However, besides increasing the scarce number of 3D photonic structural materials, their applications as 3D photonic crystals do not show apparent advantages over other solid materials or metamaterials. Having a tunable BP monocrystal, where crystals could be switched, modulating simultaneously some of their properties as 3D photonic crystals, they would constitute a new family of materials with superior performance to other existing materials, opening up a plethora of new applications. In this work, an all-optical switchable 3D photonic crystal based on BPs doped with tailored photoactive molecules is demonstrated. Two switching modes have been achieved, one where the BP reversibly transitions between two BP phases, BPI and BPII, (two different cubic crystal systems) while maintaining the monocrystallinity of the whole system. The second mode, again reversible, switches between BPI and isotropic state. None of these modes are related to the regular thermal transitions between LC phases; switching is triggered by light pulses of different wavelengths. This all-optical approach allows for a seamless fast remotely controlled optical switch between two 3D photonic crystals in different cubic crystal systems and between a photonic crystal and an isotropic matrix. Applications of switchable BPs for adaptive optics systems or photonic integrated circuits would make great advances using 3D photonic crystal switches. All-optical photonic systems such as these hold great promise for the development of tunable and efficient photonic devices such as dynamic optical filters and sensors, as they enable light-driven modulation and sensing applications with unprecedented versatility.

High-efficiency fabrication of uniform photonic crystals by a vertical deposition method from multiple glass slides of simple moulds and their application in isomer recognitions

Vertical deposition method is a simple, cheap and common method for fabricating photonic crystals (PCs). However, a majority of PCs are obtained based on the traditional vertical deposition method from one glass slide, the method is low efficiency. Herein, it is proposed to efficiently fabricate uniform PCs by an optimized vertical deposition method from double or multiple glass slides of simple moulds. Amounts of SiO2 spheres, distances of two glass slides of moulds, preparation temperatures and different diameters of SiO2 spheres are investigated for obtaining uniform PCs. And different types of PCs (the bottom-middle-top 3D PCs, bilayer 2D PCs and amorphous PCs) are also fabricated based on the optimized vertical deposition method. The above method is a simple, cheap and high-efficiency way for mass production of uniform PCs. Meanwhile, the chemicals with similar refractive indices (isomers and cis-trans isomers) can be easily distinguished under high humidity based on dynamic reflection spectra of the PCs. For example, the difference in the refractive indices of m-xylene and p-xylene is only 0.002, the difference of desorption balance time of PCs is 263 s. The PCs have potential applications in visual detections, isomer recognitions and monitoring of volatilization rates of trace liquids.

Multiple topological states within a common bandgap of two non-trivial photonic crystals.

Topological photonic crystals (PCs) provide an effective method for controlling how light propagates and concentrates through their topological states. However, it remains unclear whether topological states can be obtained by combining two different two-dimensional (2D) PCs with topological non-trivial states. In this Letter, two types of 2D Penrose-square (P-S) PCs are proposed. These PCs can generate topological edge states (TESs) and topological corner states (TCSs) within the low-frequency part of the bandgap. Moreover, by combining these two non-trivial PCs, a total of two groups of TESs and four groups of TCSs can be generated in both the high-frequency and low-frequency parts of the common bandgap. To the best of our knowledge, the two proposed P-S PCs offer a new platform for investigating topological photonics and related devices, providing novel approaches and perspectives for generating topological states in 2D PCs.

Interactive Exploration of Vivid Material Iridescence using Bragg Mirrors

AbstractMany animals, plants or gems exhibit iridescent material appearance in nature. These are due to specific geometric structures at scales comparable to visible wavelengths, yielding so‐called structural colors. The most vivid examples are due to photonic crystals, where a same structure is repeated in one, two or three dimensions, augmenting the magnitude and complexity of interference effects. In this paper, we study the appearance of 1D photonic crystals (repetitive pairs of thin films), also called Bragg mirrors. Previous work has considered the effect of multiple thin films using the classical transfer matrix approach, which increases in complexity when the number of repetitions increases. Our first contribution is to introduce a more efficient closed‐form reflectance formula [Yeh88] for Bragg mirror reflectance to the Graphics community, as well as an approximation that lends itself to efficient spectral integration for RGB rendering. We then explore the appearance of stacks made of rough Bragg layers. Here our contribution is to show that they may lead to a ballistic transmission, significantly speeding up position‐free rendering and leading to an efficient single‐reflection BRDF model.

Conical quasi-phase-matching second harmonic generation in a 2D photonic crystal with hybrid Ewald geometric method

The direction variation of the fundamental wave in the same nonlinear photonic crystal would cause different pattern of harmonics generation. In a 2D/3D crystal with dense reciprocal lattice vectors, there will be large numbers of conical harmonic beams evolving with direction change of the fundamental wave. By rearranging the Ewald sphere and superposing it into the Ewald shell, we have a hybrid Ewald construction. It becomes a simple but useful geometric method to comprehensively depict the distribution of these quasi-phase-matching second harmonics and their conical form evolution. It presents conical second harmonic beams by their related reciprocal lattice vectors and simplifies the beams’ distribution according to spatial arrangement of those reciprocal lattice vectors. It finds that the conical beams will create, annihilate, or get enhanced in specific order when fundamental waves change incident directions. We applied the method on a periodically poled 2D LiTaO3 crystal and all observed phenomena, meet the method’s predictions. In our experiment, we observed that the conical beams distorted along the optic axis of the sample due to anisotropy, which was generally overlooked by earlier researches. The eccentricities of their ring projections suggest a potential auxiliary approach for crystal dispersion measurement.

Configurable dual-topological-interface-states induced reflection in hybrid multilayers consisting of a Ge2Sb2Te5 film

Active control of induced reflection is crucial for many potential applications ranging from slowing light to biosensing devices. However, most previous approaches require patterned nanostructures to achieve controllable induced reflection, which hinders their further applications due to complicated architectures. Herein, we propose a lithography-free multilayered structure to achieve the induced reflection through the coupling of dual-topological-interface-states. The multilayers consist of two one-dimensional (1D) photonic crystals (PCs) and an Ag film separated by a Spacer, topological edge state (TES) and topological Tamm state (TTS) can be excited simultaneously and their coupling induces the reflection window. The coupled-oscillator model is proposed to mimic the coupling between the TES and TTS, and the analytical results are in good agreement with finite element method (FEM). In addition, the TES-TTS induced reflection is robust to the variation of structural parameters. By integrating an ultra-thin phase-change film of Ge2Sb2Te5 (GST) into the multilayers, the induced reflection can be switched through the phase transition of the GST film. The multipole decomposition reveals that the vanished reflection window is arising from the disappearance of TTS associated with the toroidal dipole (TD) mode.

Three-Dimensional Photonic Crystals Based on Porous Anodic Aluminum Oxide.

Photonic crystals (PCs) consisting of a periodic arrangement of holes in dielectric media have found success in light manipulation and sensing. Among them, three-dimensional (3D) PCs are in high demand due to their unique properties originating from multiple photonic band gaps (PBGs) and even full ones. Here, 3D PCs based on porous anodic aluminum oxide (AAO) were fabricated for the first time. Our approach involves prepatterning of the aluminum surface by a focused ion beam to form a hexagonal array of pore nuclei. Subsequent anodization in 1 M H3PO3 using a sine wave profile of voltage provides AAO with a defect-free in-plane porous structure and out-of-plane porosity modulation. The ability to tune the position, width, and depth of the PBGs is demonstrated. The combination of the flexibility of the proposed approach with the unique properties of AAO extends the range of practical applications of 3D PCs far beyond the current achievements.

Ultra-high Q resonances based on zero group-velocity modes accompanied by bound states in the continuum in 2D photonic crystal slabs

Optical resonators made of 2D photonic crystal (PhC) slabs provide efficient ways to manipulate light at the nanoscale through small group-velocity modes with low radiation losses. The resonant modes in periodic photonic lattices are predominantly limited by nonleaky guided modes at the boundary of the Brillouin zone below the light cone. Here, we propose a mechanism for ultra-high Q resonators based on the bound states in the continuum (BICs) above the light cone that have zero-group velocity (ZGV) at an arbitrary Bloch wavevector. By means of the mode expansion method, the construction and evolution of avoided crossings and Friedrich-Wintgen BICs are theoretically investigated at the same time. By tuning geometric parameters of the PhC slab, the coalescence of eigenfrequencies for a pair of BIC and ZGV modes is achieved, indicating that the waveguide modes are confined longitudinally by small group-velocity propagation and transversely by BICs. Using this mechanism, we engineer ultra-high Q nanoscale resonators that can significantly suppress the radiative losses, despite the operating frequencies above the light cone and the momenta at the generic k point. Our work suggests that the designed devices possess potential applications in low-threshold lasers and enhanced nonlinear effects.

Surface potential-adjusted surface states in 3D topological photonic crystals

Surface potential in a topological matter could unprecedentedly localize the waves. However, this surface potential is yet to be exploited in topological photonic systems. Here, we demonstrate that photonic surface states can be induced and controlled by the surface potential in a dielectric double gyroid (DG) photonic crystal. The basis translation in a unit cell enables tuning of the surface potential, which in turn regulates the degree of wave localization. The gradual modulation of DG photonic crystals enables the generation of a pseudomagnetic field. Overall, this study shows the interplay between surface potential and pseudomagnetic field regarding the surface states. The physical consequences outlined herein not only widen the scope of surface states in 3D photonic crystals but also highlight the importance of surface treatments in a photonic system.

On the sensitivity of defect modes outside the first photonic bandgap in optical sensors based on defected 1D photonic crystals

In this paper, we investigated the possibility of using of defect modes (DMs) in the second photonic bandgap (PBG) in defective one dimensional (1D) photonic crystals (PCs) for creating optical sensors. The dependencies of the relative sensitivities on the defect layer (DL) thickness for the first and second PBGs at optimized PC parameters are obtained and compared and the advantage of the first PBG over the second PBG in all ranges of the DL thickness is shown. However, as the order of the defect mode (DM) and optical contrast of the structure increase, this relative advantage becomes less prominent. The behavior of the DMs and their relative sensitivity outside the PBG when DL thickness changes are considered. The dependences of the relative sensitivity on different parameters of the PC for DM in the first and second PBGs are also compared. This work is mainly theoretical and aims at finding general patterns that can be generalized to any specific examples and parameters of 1D PC with DL for any practical realizable sensors.

Recent Advances in 1D Photonic Crystals: Diverse Morphologies and Distinctive Structural Colors for Multifaceted Applications

Abstract1D photonic crystals (1DPCs) with hierarchically structured lamellar periodic frameworks that enable precise control of light‐matter interactions and contribute robust structural colors represent a groundbreaking advancement in optical materials. The microstructural characteristics and the constituent material properties of these photonic materials play pivotal roles in determining their optical performance and functionality. In recent years, a diverse array of novel 1D photonic structures crafted from various materials emerged, showcasing their tremendous potential in advanced applications. This article provides an in‐depth review of the recent developments in 1DPCs, emphasizing their morphological designs, fabrication strategies, and optical applications. In detail, 1DPCs featuring distinct geometrical morphologies, including lamellar, helical, fibrous, spherical, and nanochained structures are systematically introduced, highlighting the unique optical properties arising from their periodic microstructures. Then, various fabrication methods, involving some innovative techniques utilizing standing‐wave optics, UV dual photopolymerization, and inkjet printing, are succinctly summarized for constructing different photonic structures by using diverse building materials. Subsequently, some typical application examples of 1D photonic materials are listed and discussed involving visual sensing, intelligent displays, anti‐counterfeiting technology, photonic pigments, and optical devices. Finally, the passage addresses the current challenges and presents a forward‐looking perspective on the future fabrication and application of 1DPCs.

Topological Flexible 3D Microwave Perfect Absorber

AbstractIn this work, an approach to achieve perfect absorption and microwave absorption enhancement is proposed by combining traditional wave‐absorbing materials with metamaterials. Specifically, an absorbing basement membrane (BM) is integrated with 3D woodpile (WP) photonic crystals, both of which are composed of flexible silicon materials as matrix and carbonyl iron powder (CIP) as absorbent. Direct ink writing (DIW) is used to fabricate the 3D photonic crystals with either single or coupled concentrations of absorbent. Experiments show that the 10‐dB bandwidth of the integrated absorber can reach 10 GHz with a wide elevation angle of incidence. The absorber composed of BM and WP with coupled concentrations of absorbent leads to perfect absorption with a maximum absorption of over 100 dB at an elevation angle of 30°. By changing the concentration of absorbent in BM, the frequency of perfect absorption can be engineered in the range of 6–16 GHz. Such absorption singularity reveals an experimentally observable topological charge that confirms the robustness of the singularity of perfect absorption. The topologically protected flexible 3D microwave absorber offers exceptional performance for versatile scenarios and enriches the studies on topology arising from scattering singularity.

Moisture–resistant and ppb–level trace chemical warfare agent optical gas sensor based on ZIF–67 3D photonic crystals with structural engineering

The rapid and trace detection of sulfur mustard (SM), a classical hazardous chemical warfare agents (CWAs) causing irreversible health damages and fatal, is an urgently serious issue for security safeguard but remains challenges especially in real-time changing atmospheric humidity. Herein, a zeolite imidazolate framework–67 (ZIF–67) three–dimensional (3D) photonic crystal optical sensor has been designed and proposed to detect trace SM simulants 2–chloroethyl ethyl sulfide (CEES). Compared to traditional semiconductors, the optical sensor can work at room temperature and shows significant moisture resistance. At real-time relative humidity (RH) of 35%, the sensor exhibits excellent selectivity among seven interfering gases due to the co-interactions of metal sites Co atoms of ZIF–67 via Co···Cl and Co···S with CEES (C4H9ClS), the ultralow detection limit (16.5 ppb), ultrafast response (0.5 s) for CEES can benefit from the ordered structure of ZIF-67 3D photonic crystal and its complete photonic bandgap. Most importantly, at higher RH 50%–95%, the sensor shows excellent moisture resistance and still keeps excellent selectivity, ultralow detection limit (19.1 ppb), ultrafast response (0.5 s), long–term stability and reusability for CEES even at RH 95%, warranting its utility as a moisture–resistant trace chemical warfare agent optical sensor.

Deterministic placement and effective-mass pinning of topological polariton bound states in the continuum

The exciton-polaritons derived from the light-matter interaction of an optical bound state in the continuum (BIC) with the strong excitonic resonance in a transition metal dichalcogenide (TMD) monolayer can inherit ultra-long radiative lifetimes and significant nonlinearities up to room temperature. Yet such realization can be challenging with conventional approaches to the photonic cavity design, typically due to poorly-resolved Rabi splittings at room temperature and an unstable energy positioning of the BIC state. We show and experimentally validate a strategy to dramatically improve the state-of-the-art on both points, by embedding a tungsten disulfide (WS2) monolayer deep within a Bloch-surface-wave stack, where the photonic mode is moulded by a 1D photonic crystal with a compound periodicity. In particular, we introduce a deterministic placement principle to the design of the PhC, allowing to stabilize the energy positioning of a topologically-protected BIC polariton eigenstate, with an effective mass which we can robustly pre-assign at choice as either positive or negative. This is in stark contrast to typical waveguide realizations of polariton BICs: only negative polariton effective masses can be commonly achieved, while sudden jumps to a weaker-interacting positive-effective-mass BIC are at the same time possible upon small perturbations, in fact hijacking the advantage from a topological protection when present.

Transformation of topological states in three-dimensional photonic crystal under magnetic field modulation

The realization and modulation of high-order topological states in three-dimensional (3D) photonic crystals (PCs) hold great significance for practical applications in optical communication, optical information processing, and optical computing. However, they have encountered technological challenges due to the difficulty in achieving a complete bandgap in 3D PCs. To address this issue, we introduce a design for a 3D PC based on the Su–Schrieffer–Heeger (SSH) model that exhibits distinctive topological surface states, topological hinge states, and topological corner states. Furthermore, these three types of topological states can be interconverted through the modulation of magnetic field because the topological states are closed and extremely sensitive to the structure, which are modulated by the magnetic field. This intriguing capability holds potential applications in the manipulation of optical flow, optical signal storage, and magnetic field sensors.

Hydrogen Bond-Mediated Self-Shielded Moisture-Responsive Structural Color for Time-Temperature Indicating.

Effective monitoring of the time-temperature history of biological reagents, chemical drugs, and perishable foods during cold chain storage is crucial for ensuring their quality and efficacy. Time-temperature indicators (TTIs) are developed to assess the cumulative impact of time and temperature on product quality. However, current indicators face challenges related to complex wrapping procedures, narrow tracking ranges, susceptibility to photobleaching, and pre-use instability, hampering widespread use. Herein, the first moisture-responsive 1D photonic crystal (1DPC) TTIs featuring robust structural colors, customizable time-temperature ranges, and reliable renewability are demonstrated. The indicators exhibit distinct color-changing responsiveness toward water vapor, which remains observable after prolonged storage at low temperatures. Significantly, the moisture responsiveness gradually diminishes at elevated temperatures over time due to ambient water-induced hydrogen bond formation, effectively shielding the indicator from external stimuli. This property enables the naked-eye inspection of product efficacy during cold chain storage. Additionally, the endowed flexibility of the TTI facilitates its easy attachment to targets, functioning as a convenient indicator label. Remarkably, the indicator can be stably stored for an extended period at room temperature before use, thereby showcasing substantial market potential.

Bio‐Inspired Camouflage Skin with Photonic Crystal Structure and Size‐Confinement Effect

AbstractCamouflage skin holds immense potential for a variety of advanced applications, such as artificial intelligence, soft robotics, and flexible/wearable electronics. Currently, most existing materials for constructing camouflage skin are mainly based on inorganic materials with an inverse opal scaffold, which significantly limits their further development because of the poor mechanical properties and environmental instability. Inspired by rana slvaticas, a new type of camouflage skin based on one‐dimentional (1D) photonic crystal (PC) gel is designed and synthesized, which combines the advantages of 1D PC structures with 3D flexible gels in a single skin device. Taking advantage of the unique structures, the resulting camouflage skins demonstrated attractive properties, including excellent mechanical performance, self‐adaptive camouflage capabilities, and long‐term stability in ambient conditions. Specifically, the camouflage skin can quickly recognize and match the background by modulating the optical signals of external stimuli. The synergistic effect of the glycerol, the dodecyl glycerol itaconate (DGI), and the nano‐space structures altogether contribute to the outstanding long‐term environmental stability of the camouflage skin. Overall, by combining the bio‐inspired concept and molecular engineering strategy, camouflage skin based on PC gels is synthesized, which greatly enriched the application and development in the smart skins field.

A Flexible and Free‐Standing Photonic Crystal‐Hydrogel Film for Enhanced Fluorescence from Nitrogen Doped Carbon Dots

AbstractEnhancement of fluorescence emission in a flexible and free‐standing fluorescing film is a key requirement for sensing platforms with augmented sensitivity. In this context, a study on the enhanced fluorescence emission from nitrogen‐doped carbon dots (NCDs) embedded on a flexible free‐standing photonic crystal hydrogel film prepared via photopolymerization, is presented. The composite structure utilizes the photonic crystal enhanced fluorescence (PCEF) technique and incorporates a self‐assembled 3D photonic crystal (3D PC) onto a photopolymerizable hydrogel matrix containing the NCDs. The 3D PC template is prepared through vertical deposition self‐assembly using monodisperse polystyrene (PS) spheres. By leveraging the enhanced excitation and extraction effect in a PC‐fluorophore composite, the authors’ structure exhibits a fourfold enhancement in fluorescence emission compared to the control sample. The relative positioning of the photonic stopband with the excitation and emission spectra of the fluorophore (NCDs) is thoroughly studied by selecting four different 3D PCs with varied lattice parameters. In addition, the hydrogel matrix offers improved photostability and enhanced lifetime for the NCDs. This structure can be further modified into an efficient sensor through appropriate functionalization.

“Fluid nonlinear coefficient sensor” designed on 2D photonic crystal

“Fluid nonlinear coefficient sensor” designed on 2D photonic crystal

Direct Determination of Photonic Stopband Topological Character: A Framework Based on Dispersion Measurements

Ascertainment of photonic stopband absolute topological character requires information regarding the Bloch eigenfunction spatial distribution. Consequently, the experimental investigations predominantly restrict themselves to the bulk‐boundary correspondence principle and the ensuing emergence of topological surface state. Although capable of establishing the equivalence/inequivalence of bandgaps, the determination of their absolute topological identity remains out of its purview. The alternate method of reflection phase‐based identification also provides only contentious improvements owing to the measurement complexities pertaining to the interferometric setups. To circumvent these limitations, the Kramers–Kronig amplitude‐phase causality considerations are resorted to and an experimentally conducive method is proposed for bandgap topological character determination directly from the parametric reflectance measurements. Particularly, it is demonstrated that in case of 1D photonic crystals, polarization‐resolved dispersion measurements suffice in qualitatively determining bandgaps’ absolute topological identities. By invoking the translational invariance of the investigated samples, a parameter “differential effective mass” is also defined, that encapsulates bandgaps’ topological identities and engenders an experimentally discernible bandgap classifier.