Preparation of multi-colored binary silica supraballs and color fine-tuning based on color mixing.

Structural colors are highly valued for their eco-friendliness and long-term color stability, deriving from the interaction of structural units with incident light. However, traditional methods for adjusting structural colors typically involve altering the size of structural units, a labor-intensive process necessitating specific diameters for each desired color. Moreover, colors exhibited by photonic crystal materials are monochromatic colors with a narrow wavelength range, failing to exhibit polychromatic colors. This restricts their practical applications, as they do not accurately represent the actual color of objects themselves. Hence, this study focuses on fabricating binary supraballs can display polychromatic colors. These supraballs consist of two types of structural units with distinct diameter differences. By adjusting the mass ratio between these units within the supraballs, fine color tuning is achievable. Utilizing three different diameters of silica nanospheres, this method enables the fabrication of supraballs with a diverse range of colors spanning nearly the entire visible spectrum. The adjustable colors of these binary supraballs not only enhance their ability to replicate the colors of objects, but also reduce the significant workload involved in preparing the original structural units. The synthesized supraballs are in powder form, directly applicable as coatings, inks, and other materials.

Defected photonic crystal as propylene glycol THz sensor using parity-time symmetry.

Detecting unsafe levels of chemical gases and vapors is essential in improving and maintaining a healthy environment for all to enjoy. Propylene glycol is a colorless, synthetic gas commonly used in medications, fragrances, and cosmetics. It causes side effects such as headaches, lightheadedness, nausea, and fainting. So, monitoring of propylene glycol is critically vital. This study uses a defected photonic crystal as a propylene glycol THz sensor. Due to the high absorption of propylene glycol, the intensity of the resonant confined mode is very small. As a result, the performance of the designed sensor seems unsatisfactory. We will use parity-time symmetry for the first time in THz to magnify the resonant confined mode to detect propylene glycol. The effect of microcavity thickness, incident angle, and gain/loss factor will be studied. The optimized sensor recorded distinguished results compared to other studies for the detection of propylene glycol.

A Highly Sensitive D-Shaped PCF-SPR Sensor for Refractive Index and Temperature Detection.

A novel highly sensitive D-shaped photonic crystal fiber-based surface plasmon resonance (PCF-SPR) sensor for dual parameters of refractive index and temperature detecting is proposed. A PCF cladding polishing provides a D-shape design with a gold (Au) film coating for refractive index (RI) sensing (Core 1) and a composite film of silver (Ag) and polydimethylsiloxane (PDMS) for temperature sensing (Core 2). Comsol Multiphysics 5.5 is used to design and simulate the proposed sensor by the finite element method (FEM). The proposed sensor numerically provides results with maximum wavelength sensitivities (WSs) of 51,200 and 56,700 nm/RIU for Core 1 and 2 as RI sensing while amplitude sensitivities are -98.9 and -147.6 RIU-1 with spectral resolution of 1.95 × 10-6 and 1.76 × 10-6 RIU, respectively. Notably, wavelength sensitivity of 17.4 nm/°C is obtained between -20 and -10 °C with resolution of 5.74 × 10-3 °C for Core 2 as temperature sensing. This sensor can efficiently work in the analyte and temperature ranges of 1.33-1.43 RI and -20-100 °C. Due to its high sensitivity and wide detection ranges, both in T and RI sensing, it is a promising candidate for a variety of applications, including chemical, medical, and environmental detection.

Nanomechanical Crystalline AlN Resonators with High Quality Factors for Quantum Optoelectromechanics.

High-quality factor (Qm) mechanical resonators are crucial for applications where low noise and long coherence time are required, as mirror suspensions, quantum cavity optomechanical devices, or nanomechanical sensors. Tensile strain in the material enables the use of dissipation dilution and strain engineering techniques, which increase the mechanical quality factor. These techniques have been employed for high-Qm mechanical resonators made from amorphous materials and, recently, from crystalline materials such as InGaP, SiC, and Si. A strained crystalline film exhibiting substantial piezoelectricity expands the capability of high-Qm nanomechanical resonators to directly utilize electronic degrees of freedom. In this work, nanomechanical resonators with Qm up to 2.9 × 107 made from tensile-strained 290 nm-thick AlN are realized. AlN is an epitaxially-grown crystalline material offering strong piezoelectricity. Nanomechanical resonators that exploit dissipation dilution and strain engineering to reach a Qm × fm-product approaching 1013 Hz at room temperature are demonstrated. A novel resonator geometry is realized, triangline, whose shape follows the Al-N bonds and offers a central pad patterned with a photonic crystal. This allows to reach an optical reflectivity above 80% for efficient coupling to out-of-plane light. The presented results pave the way for quantum optoelectromechanical devices at room temperature based on tensile-strained AlN.

Design and Simulation of High-Performance D-Type Dual-Mode PCF-SPR Refractive Index Sensor Coated with Au-TiO2 Layer.

A novel surface plasmon resonance (SPR) refractive index (RI) sensor based on the D-type dual-mode photonic crystal fiber (PCF) is proposed. The sensor employs a side-polished few-mode PCF that facilitates the transmission of the fundamental and second-order modes, with an integrated microfluidic channel positioned directly above the fiber core. This design minimizes the distance to the analyte and maximizes the interaction between the optical field and the analyte, thereby enhancing the SPR effect and resonance loss for improved sensing performance. Au-TiO2 dual-layer material was coated on the surface of a microfluidic channel to enhance the penetration depth of the core evanescent field and tune the resonance wavelength to the near-infrared band, meeting the special needs of chemical and biomedical detection fields. The finite element method was utilized to systematically investigate the coupling characteristics between various modes and surface plasmon polariton (SPP) modes, as well as the impact of structural parameters on the sensor performance. The results indicate that the LP11b_y mode exhibits greater wavelength sensitivity than the HE11_y mode, with a maximum sensitivity of 33,000 nm/RIU and an average sensitivity of 8272.7 nm/RIU in the RI sensing range of 1.25-1.36, which is higher than the maximum sensitivity of 16,000 nm/RIU and average sensitivity of 5666.7 nm/RIU for the HE11b_y mode. It is believed that the proposed PCF-SPR sensor features both high sensitivity and high resolution, which will become a critical device for wide RI detection in mid-infrared fields.

Smart Window with Reversible and Instantaneous Photoluminescence based on Microsphere Structure.

A smart window that dynamically regulates light transmittance is crucial for modern life end-users and promising for on-demand optical devices. The advent of three-dimensional (3D) photonic crystal microspheres has enriched the functions of a smart window. However, the smart window formed by polymer microspheres encounters poor mechanical strength and microstructural defects. Herein, to solve this limitation, we report the microsphere-based smart window composed of tightly packed cross-linked polymer microspheres (as a precursor) containing organic photochromic dyes, followed by compression under a high elastic state. When excited under an ultraviolet supply, our smart window showed a rapid and reversible fluorescent photoluminescence without fatigue (50 cycles). Moreover, the bulk devices with a microsphere cross-linked network structure enable excellent mechanical strength (hardness reached 0.158 GPa) and visible-light transparency. Interestingly, a QR code can be recognized under visible light exposure but not under ultraviolet light exposure because of photoluminescence of the smart window. Our method generally provided a paradigm for various amorphous polymers, which can be regarded as a simple and effective approach to build a versatile strategy to introduce an ideal marketplace with economic and community benefits.

Highly sensitive SERS detection of melamine based on 3D Ag@porous silicon photonic crystal.

The stability, reproducibility and engineering of SERS substrate faces a great challenge in melamine SERS assay. In this work, a simple, highly sensitive, stable and cost-efficient SERS detection platform for melamine was established based on its Raman fingerprints spectrum. The Ag@ porous silicon photonic crystal (Ag@PPC) was prepared as the 3D SERS substrate by electrochemical etching and magnetron sputter technology. The main influence factors for the preparation of SERS substrate were investigated in detail. The analytical enhancement factor of the 3D SERS substrate can reach to 2.6 × 108. The 3D SERS detection platform showed a wide linear detection range of 10-4∼10 mg L-1 and a low limit of detection of 0.1 µg L-1 for melamine. Moreover, such detection platform showed good stability, high reproducibility and high recovery rates for melamine. The 3D Ag@PPC SERS substrate can be easily prepared and engineered, displaying a great potential application in food safety field.

Tailoring of Circularly Polarized Beams Employing Bound States in the Continuum in a Designed Photonic Crystal Metasurface Nanostructure.

We propose a photonic crystal (PC) nanostructure that combines bound states In the continuum (BIC) with a high-quality factor up to 107 for emitting circularly polarized beams. We break the in-plane inversion symmetry of the unit cell by tilting the triangular hole of the hexagonal lattice, resulting in the conversion of a symmetrically protected BIC to a quasi-BIC. High-quality circularly polarized light is obtained efficiently by adjusting the tilt angles of the hole and the thickness of the PC layer. By changing the hole's geometry in the unit cell, the Q-factor of circularly polarized light is further improved. The quality factor can be adjusted from 6.0 × 103 to 1.7 × 107 by deliberately changing the shape of the holes. Notably, the proposed nanostructure exhibits a large bandgap, which significantly facilitates the generation of stable single-mode resonance. The proposed structure is anticipated to have practical applications in the field of laser technology, particularly in the advancement of low-threshold PC surface emitting lasers (PCSELs).

Ultra-Fast Fabrication of Mechanical-Water-Responsive Color-Changing Photonic Crystals Elastomers and 3D Complex Devices.

The traditional fabrication of opal-structured photonic crystals is constrained by the rate of solvent evaporation, a process that is not only time-consuming but also labor-intensive. This study introduces a paradigm shift by incorporating silica nanoparticles (SiNPs) with high zeta potentials and hydrogen bonding capabilities into an elastomeric matrix, resulting in a novel non-close-packed structure. This innovation circumvents the limitations of conventional methods by enabling the rapid formation of photonic inks (PI) into vibrant and luminous photonic elastomers (PEs) within seconds. These PEs demonstrate remarkable mechanochromic properties, exhibiting dynamic color changes across the visible spectrum in response to tensile and compressive deformations. Furthermore, the presence of hydroxyl groups endows the PEs with superior water-responsiveness, which can be finely tuned through the ink formulation. The elimination of solvent evaporation dependency facilitates the fabrication of macroscopic photonic crystal devices with complex geometries using digital light processing (DLP)-based 3D printing. This approach ensures exceptional optical performance and high customization potential. The resulting PEs hold significant promise for applications in smart wearables, soft robotics, and advanced human-machine interface technologies.

Bioinspired Polydopamine Modification for Interface Compatibility of PDMS-Based Responsive Structurally Colored Textiles.

Textiles that can repeatedly change color in the presence of external stimuli have attracted great interest. Effectively designing to produce such functional textiles is essential, yet there remain challenges like producing stable coloration, rapid response, and reverse color changing. Here, the preparation of a magnetic field response (MFR) textile with a fast magnetic field response, brilliant structural coloration, and mechanical robustness is reported. The MFR textile is knitted by incorporating magnetic particles' ethylene glycol (EG) suspension within polydimethylsiloxane (PDMS)-based fibers. A surface modification strategy is designed to prevent EG from seeping out along the PDMS polymer chains. A PDMS fiber is encapsulated in waterborne polyurethane, and a polydopamine joint layer is used. The MFR textile demonstrates magnetic field-triggered structural colors, and the breaking strength and elongation at break of each composite fiber are improved. In addition, multishaped patterns can be printed on the MFR textile with the help of the photo etching technology, which enhances the applications of the new functional textiles.

Infrared Stealth Coating with Tunable Structural Color Based on ZnO Spheres.

It is important to develop low infrared (IR) emissive coating with tunable structure color to improve the infrared-visible stealth performance of military equipment. In this work, uniform ZnO spheres are used as building units to construct photonic structures with both bright adjustable structure color and low IR emissivity due to the relatively high refractive index and low IR emissivity of ZnO. The vivid tunable structural colors are provided by the photonic bandgap of ZnO photonic crystals (PCs) or the quasi-bandgap of amorphous photonic crystals (APCs), respectively. Both ZnO PCs and APCs exhibited low IR emissivity in 3-5 µm. The IR emissivity of 255 nm ZnO PC is 0.483 and the IR emissivity of 255 nm ZnO APC is 0.492 at 25 °C. With the increase of temperature, the IR emissivity of further decreased to 0.295 and 0.312 at 300 °C. These structures can be applied to a variety of surfaces, and all these structures have good thermal and light stability as well. This work may open a simple and effective way to fabricate materials with good infrared-visible stealth performance, expanding the application of ZnO PCs and APCs coatings in the camouflage area.

Unclonable Encryption-Verification Strategy Based on Bilayer Shape Memory Photonic Crystals.

The ability to reversibly exhibit structural color patterns has positioned photonic crystals (PCs) at the forefront of anti-counterfeiting. However, the security offered by the mere reversible display is susceptible to illicit alteration and disclosure. Herein, inspired by the electronic message captcha, bilayer photonic crystal (BPC) systems with integrated decryption and verification modules, are realized by combining inverse opal (IO) and double inverse opal (DIO) with polyacrylate polymers. When the informationized BPC is immersed in ethanol or water, the DIO layer displayed encrypted information due to the solvent-induced ordered rearrangement of polystyrene (PS) microspheres. The verification step is established based on the different structural colors of the IO layer pattern, which result from the deformation or recovery of the macroporous skeleton induced by solvent evaporation. Moreover, through the evaporation-induced random self-assembly of PS@SiO2 and SiO2 microspheres, unclonable structurally colored identifying codes are created in the IO layer, ensuring the uniqueness upon the verification. The decrypted code in the DIO layer is valid only when the IO layer displays the pattern with the predetermined structural color; otherwise, it is a pseudo-code. This structural color-based "decryption-verification" approach offers innovative anti-counterfeiting applications in nanophotonics.

Design and numerical analysis of high contrast ratio and ultra-compact all-optical 2-bit reversible comparator.

In this paper, a novel interference-based nanostructure was designed and simulated to realize an all-optical 2-bit reversible comparator by employing a novel technique. The plane wave expansion (PWE) method was adopted to analyze the encoder design and frequency modes. Aside from downsizing, the finite-difference time-domain (FDTD) method was utilized for the simulation and numerical analysis of the design proposed herein. An ultra-compact nanostructure with a 129.8 µm2 footprint was utilized for the all-optical 2-bit reversible comparator. One of the noteworthy characteristics of the proposed nanostructure was its excellent contrast ratio (i.e., 13.8 dB) in comparison to other nanostructures. The bitrate and delay time in this nanostructure were 3.33 Tb/s and 300 fs, respectively. Based on the findings of the simulations conducted at a central wavelength of 1.55 µm, it is recommended to employ the nanostructure proposed herein during the third telecom window. A photonic crystal nano-resonator was utilized to design the high-performance all-optical 2-bit reversible comparator, which may also be employed in integrated optical circuits (IOCs).

ß-Hydroxybutyrate dehydrogenase functionalized two-dimensional photonic crystals for quantitative and visual sensing of ketone bodies.

ß-Hydroxybutyrate (BHB) is a substantial physiological ketone body. Its elevated concentration causes ketoacidosis, which is a disorder with a high mortality rate. Therefore, there is an urgent need to develop a simple method for the in-situ monitoring of BHB in urine. In this study, a photonic crystal hydrogel (PCH) sensing material for the detection of urinary ketones was prepared by embedding a two-dimensional polystyrene photonic crystal array (PCA) in a hydrogel functionalized with ß-hydroxybutyrate dehydrogenase (BHBDH). BHBDH catalyzes the interconversion between ß-hydroxybutyrate and acetoacetic acid and relies on the cofactor nicotinamide adenine dinucleotide (NAD+) to participate in the reaction process. The catalytic cycle of converting ß-hydroxybutyrate to acetoacetate generates H+, which reduces the electrostatic repulsion between the carboxyl groups in the hydrogel network, ultimately leading to the shrinkage of the hydrogel volume. The hydrogel volume change was detected by measuring the diameter of the Debye diffraction ring, thus reflecting the concentration of BHB. When the concentration of BHB was increased from 0 to 10 mM, the reflection spectrum of PCH shifted for 117 nm within 60 min, consequently, the structural color of PCH changed from red to green and finally to blue. The material was used for quantitative detection of BHB with a detection limit of 48.94 µM. Then it was used for detection in artificial urine samples. While, this smart and reusable sensing material could provide a more convenient and efficient strategy for the ketone body detection in clinical diagnosis and point-of-care monitoring.

Size-Dependent Magnetic Responsiveness of a Photonic Crystal of Graphene Oxide Nanosheets.

A magnetically responsive photonic crystal of colloidal nanosheets can exhibit a controllable structural color, offering diverse potential applications. In this study, we systematically investigated how the lateral sizes of graphene oxide (GO) nanosheets affect their magnetic responsiveness in a photonic system. Contrary to the prediction that larger lateral sizes of nanosheets would be more responsive to an applied magnetic field based on the magnetic energy of anisotropic materials, we discovered that GO nanosheets with larger lateral sizes in the photonic system scarcely responded to a 12 T magnetic field. The lack of magnetic response may be due to the strongly restricted rotational motion of GO nanosheets by mutual electrostatic forces. In contrast, GO nanosheets with medium lateral sizes readily responded to the 12 T magnetic field, forming a uniaxially oriented structure that resulted in a vivid structural color. However, smaller GO nanosheets displayed a less vivid structural color, possibly because of less structural ordering of GO nanosheets. Finally, we found that the photonic crystal of GO nanosheets with optimized lateral sizes responded effectively to the 12 T magnetic field across various GO concentrations, resulting in a vivid and tunable structural color.

Design and simulation of a compact polarization beam splitter based on dual-core photonic crystal fiber with elliptical gold layer.

For the polarization multiplexing requirements in all-optical networks, this work presents a compact all-fiber polarization beam splitter (PBS) based on dual-core photonic crystal fiber (PCF) and an elliptical gold layer. Numerical analysis using the finite element method (FEM) demonstrates that the mode modulation effect of the central gold layer effectively reduces the dimensions of the proposed PBS. By determining reasonable structural parameters of the proposed PCF, the coupling length ratio (CLR) between X- and Y-polarized super-modes can approach 2, achieving a minimal device length of 0.122 mm. The PBS exhibits a maximum extinction ratio (ER) of - 65 dB at 1.55 µm, with an operating bandwidth spanning 100 nm (1.5-1.6 µm) and a stable insertion loss (IL) of ~ 1.5 dB at 1.55 µm. Furthermore, the manufacture feasibility and performance verification scheme are also investigated. It is widely anticipated that the designed PBS will play a crucial role in the ongoing development process of miniaturization and integration of photonic devices.

High-Sensitivity Refractive Index Sensor with Dual-Channel Based on Surface Plasmon Resonance Photonic Crystal Fiber.

In order to achieve a high-precision synchronous detection of two different refractive index (RI) analytes, a D-type surface plasmon resonance (SPR) photonic crystal fiber (PCF) RI sensor based on two channels is designed in this paper. The sensor uses a D-shaped planar region of the PCF and a large circular air hole below the core as the sensing channels. Surface plasmon resonance is induced by applying a coating of gold film on the surface. The full-vector finite-element method (FEM) is used to optimize the structural parameters of the optical fiber, and the sensing characteristics are studied, including wavelength sensitivity, RI resolution, full width at half maximum (FWHM), figure of merit (FOM), and signal-to-noise ratio (SNR). The results show that the channel 1 (Ch 1) can achieve RI detection of 1.36-1.39 in the wavelength range of 1500-2600 nm, and the channel 2 (Ch 2) can achieve RI detection of 1.46-1.57 in the wavelength range of 2100-3000 nm. The two sensing channels can detect independently or simultaneously measure two analytes with different RIs. The maximum wavelength sensitivity of the sensor can reach 30,000 nm/RIU in Channel 1 and 9900 nm/RIU in Channel 2. The RI resolutions of the two channels are 3.54 × 10-6 RIU and 10.88 × 10-6 RIU, respectively. Therefore, the sensor realizes dual-channel high- and low-RI synchronous detection in the ultra-long wavelength band from near-infrared to mid-infrared and achieves an ultra-wide RI detection range and ultra-high wavelength sensitivity. The sensor has a wide application prospect in the fields of chemical detection, biomedical sensing, and water environment monitoring.

Rapid Construction of PVA@CDs/SiO2 Fluorescent/Structural Color Dual-Mode Anti-Counterfeiting Labels via Spray-Coating Method.

A method of sequential spraying of polyvinyl alcohol with carbon quantum dots (PVA@CDs) aqueous suspension and SiO2 aqueous suspension is proposed to rapidly prepare multicolor dual-mode anti-counterfeiting labels. With the optimization of the concentration (15%) of colloidal microspheres in the SiO2 aqueous suspension as well as the spraying process parameters (spray distance of 10 cm, spray duration of 3 s, and assembly temperature of 20 °C), different-sized SiO2 microspheres (168 nm, 228 nm, and 263 nm) were utilized to rapidly assemble red, green, and blue photonic crystals. Furthermore, the tunable fluorescence emission of carbon quantum dots endows the labels with yellow, green, and blue fluorescence. The constructed dual-mode labeling was used to develop an anti-counterfeiting code with dual-channel information storage capabilities and also to create dual-mode multicolor anti-counterfeiting labels on various packaging substrates. This work provides a novel solution for anti-counterfeiting packaging and information storage.

Excitation/emission-enhanced heterostructure photonic crystal array synergizing with "DD-A" FRET entropy-driven circuit for high-resolution and ultrasensitive analysis of ctDNA.

Circulating tumor DNA (ctDNA) is an emerging biomarker of liquid biopsy for cancer. But it remains a challenge to achieve simple, sensitive and specific detection of ctDNA because of low abundance and single-base mutation. In this work, an excitation/emission-enhanced heterostructure photonic crystal (PC) array synergizing with entropy-driven circuit (EDC) was developed for high-resolution and ultrasensitive analysis of ctDNA. The donor donor-acceptor FÖrster resonance energy transfer ("DD-A" FRET) was integrated in EDC based on the introduction of simple auxiliary strand, which exhibited higher sensitivity than that of traditional EDC. The heterostructure PC array was constructed with the bilayer periodic nanostructures of nanospheres. Because the heterostructure PC has the adjustable dual photonic band gaps (PBGs) by changing nanosphere sizes, and the "DD-A" FRET can offer the excitation and emission peak with enough distance, it helps the successful matches between the dual PBGs of heterostructure PC and the excitation/emission peaks of "DD-A" FRET; thus, the fluorescence from EDC can be enhanced effectively from both of excitation and emission processes on heterostructure PC array. Besides, high-resolution of single-base mutation was obtained through the strict recognition of EDC. Benefiting from the specific spectrum-matched and synergetic amplification of heterostructure PC and EDC with "DD-A" FRET, the proposed array obtained ultrasensitive detection of ctDNA with LOD of 12.9 fM, and achieved the analysis of mutation frequency as low as 0.01%. Therefore, the proposed strategy has the advantages of simple operation, mild conditions (enzyme-free and isothermal), high-sensitivity, high-resolution and high-throughput analysis, showing potential in bioassay and clinical application.

Innovative Refractive Index Sensor Utilizing Terahertz Spectrum for Early Cancer Detection: a Photonic Crystal Fiber Approach.

In this article, we have presented a new cancer sensor with a square core Photonic Crystal Fiber (PCF) to detect the cancerous tissues of the cervix, breast, and skin. This process is thus streamlined and separated by PCF due to its excellent detection characteristics. All required configurations using the finite element method are developed, and various performances of the model are studied using MATLAB. The results depict a mathematical analysis regarding the effectiveness of the sensor within the frequency range of 1.0-2.8 THz. Its relative sensitivity becomes around 99.85% at 2.2 THz with 8.49 × 10-14 dB/m for CL. This PCF has a spot size 3.06 × 10-4 µm that further contributes an effective area of 9.078 × 10-8 m2. Moreover, it has a very small EML of 0.00182 cm-1. This device uses the unique photonic properties of cancer cells to provide quick, reliable, and really very accurate methods for cancer cell identification, such as in breast, cervical, and skin cancers. Due to small size and flexibility, only minimally invasive operations are possible. Real-time monitoring can also be provided, hence improving immediate evaluation and therapy efficacy. This article introduces a novel integration of PCF technology with THz radiation to create a highly sensitive sensor for early cancer detection. By utilizing THz waves' non-invasive and high-resolution properties, this sensor overcomes the sensitivity limitations of traditional methods. It also addresses scattering issues from conventional air hole shapes through optimized geometric configurations, setting a new standard in biomedical sensing and potentially revolutionizing early cancer diagnostics.