High Effective Refractive Index Research Articles

Dual parameter sensor for RI and temperature detection by cascading Ag/WO3 film PCF and Ag/MoS2/PDMS film PCF.

Multi-parameter surface plasmon resonance (SPR) sensors generally have low detection sensitivity due to detection wavelength limitations. We developed a two-parameter SPR sensor for refractive index (RI) and temperature detection by cascading Ag/WO3 film photonic crystal fiber (PCF) and Ag/MoS2 film PCF together. By using WO3 film with a low effective refractive index to prevent oxidation of silver-based PCF and maintain a wider RI sensing channel detection band; at the same time, MoS2 film with a high effective refractive index is used to modulate the detection range of temperature sensing channel. The detection performance and stability of the two-parameter SPR sensor were verified by software simulation and experiments. The data results indicate that the sensor can perform stable and independent tests during RI and temperature detection. The maximum sensitivity of the sensor within the RI detection range of 1.333 to 1.395 is as high as 6443 nm/RIU; the maximum temperature sensitivity within the temperature detection range of 40 °C∼90 °C is 8.72 nm/°C. This sensor can achieve high-sensitivity RI measurement in a high-temperature environment. This will have broad application prospects in the field of biochemistry.

Study on mode properties of GaAs-based hybrid plasmonic terahertz waveguides

Terahertz (THz) fields are increasingly being used to address the critical challenges associated with achieving high data rates and rapid communication. In this study, a hybrid plasmonic THz waveguide is designed and analysed operating in the 2.5–3.5 THz frequency range. The waveguide is constructed using gallium arsenide as the high-refractive-index core, surrounded by aluminium arsenide and silver placed on a high-density polyethylene (HDPE) substrate. Graphene is strategically positioned between the HDPE layers to enhance light confinement. The mode properties of the proposed waveguide are simulated with Comol Multiphysics using the finite-element method and show unique characteristics. Observation of the simulated results at 2.5–3.5 THz reveals a high effective refractive index of 3.79, a maximum effective mode area of 1.88 mm2, a high birefringence of 0.2, a low dispersion of 0.10 ps THz−1 cm−1, a high mode field diameter of 15.8 mm, a high beat length of 123 mm and a low confinement loss of 1.79 × 10−9 mm−1. These features make the proposed waveguide suitable for applications in photonic integrated circuits for THz communications.

High gain and high-efficiency compact resonator antennas based on spoof surface plasmon polaritons

Abstract We proposed a high gain high-efficiency compact metallic resonator antenna operating at even-mode meander line spoof surface plasmon polaritons (MLSSPPs). The high radiation efficiency is caused by the bulk of fields crowded in lossless air near the antenna rather than in lossy dielectric as in conventional dielectric resonator antennas (DRAs). The proposed antenna also exhibits compact size because of its high effective refractive index. A reliable equivalent circuit model is proposed for the design of the resonator antenna with basic mode of half wavelength resonant mode. As an example, a meander-line SSPP antenna is designed, fabricated and measured. Both the simulated and measured results show the advantages of high efficiency and compact volume. In addition, the antenna achieves higher gain and wider relative bandwidth per wavelength cube volume compared with its counterparts. This method provides a good alternative for designing DRAs.

Ultra-compact electro-optic phase modulator based on a lithium niobate topological slow light waveguide.

Electro-optic modulators (EOMs) are essential devices of optical communications and quantum computing systems. In particular, ultra-compact EOMs are necessary for highly integrated photonic chips. Thin film lithium niobate materials are a promising platform for designing highly efficient EOMs. However, EOMs based on conventional waveguide structures are at a millimeter scale and challenging to scale down further, greatly hindering the capability of on-chip integration. Here, we design an EOM based on lithium niobate valley photonic crystal (VPC) structures for the first time. Due to the high effective refractive index introduced by the strong slow light effect, the EOM can achieve an ultra-compact size of 4 μm×14 μm with a half-wave voltage of 1.4 V. The EOM has a high transmittance of 0.87 in the 1068 nm because of the unique spin-valley locking effect in VPC structures. The design is fully compatible with current nanofabrication technology and immune to fabrication defects. Therefore, it opens a new possibility in designing lithium niobate electro-optic modulators and will find broad applications in optical communication and quantum photonic devices.

Experimental study of SPR sensor performance enhancement by metal oxides

With the widespread application of fiber surface plasmon resonance (SPR) sensors in the field of biochemical sensing, especially in the detection of large molecules such as proteins, the sensitivity of traditional fiber SPR sensors has made it difficult to meet the requirements of detection accuracy in practical applications. This article proposes and proves an effective method for selecting the optimal metal oxide material to improve the sensing performance of SPR sensors. Metal oxides (TiO2 and ZnO) can improve the sensitivity by enhancing the evanescent field of fiber SPR phenomenon. Through simulation, it was found that when the metal oxide film of the composite film sensor maintains the same thickness, the metal oxide with a high effective refractive index (RI) has a better improvement effect on the sensitivity of the SPR sensor. Experimental results show that the maximum RI sensitivity of TiO2 composite film sensors and ZnO composite film sensors in the RI range of 1.333–1.420 is 15,200 nm/RIU and 13,600 nm/RIU, respectively. Compared to silver-based sensors, RI sensitivity has increased by 16.92 % and 4.62 %, respectively. To further verify the enhancement effect of different metal oxides on fiber sensors, another layer of PDMS film is deposited on TiO2 and ZnO films. Within the temperature sensing range, the maximum temperature sensitivity of these two composite film temperature sensors is 4.9 nm/°C and 4.3 nm/°C, respectively. Overall, metal oxides with high RI can improve the sensing performance of fiber optic sensors.

Graphene based waveguide fed hybrid plasmonic terahertz patch antenna

Abstract The terahertz (THz) technology has fascinated lot of attention due to its enormous potential for a wide range of applications in the public, private and enterprise domains. This research work demonstrates the design structure and analysis of graphene based waveguide fed hybrid plasmonic THz patch antenna (HPTPA) constructed at around 3 THz. Hybrid plasmonic THz waveguide (HPTW) as a feeding line for the proposed THz patch antenna can increases antenna efficiency. The graphene is sandwiched between gallium arsenide (GaAs) and silver (Ag) to confine THz waves efficiently. Using mode analysis in finite element method for the proposed HPTW the propagation length, effective refractive index has been thoroughly examined. Based on the finite element method (FEM) approach, the results of the designed graphene-based waveguide fed HPTPA shows a high effective refractive index of 2.9, large propagation length of 230 µm, gain of 2.29 dBi, bandwidth of 200 GHz and efficiency of 85 % has obtained. The proposed graphene-based waveguide fed HPTPA could be beneficial to enable several photonic integrated circuit applications in next-generation wireless communication.

Analysis of Nanoparticle-Embedded-Resin Printing Conditions for High-Aspect-Ratio Metasurfaces

Metasurfaces, composed of periodic nanostructures, have been attractive because of their extraordinary modulation of light propagation. However, conventional electron-beam lithography to fabricate metasurfaces is time-consuming and costly, which prevents commercialization of functional metasurfaces. We investigate nanoimprint lithography-based technique for single-step fabrication of metasurfaces. A high-refractive-index material is granulated and mixed with nanoimprint resin. This mixture results in higher effective refractive index, and thus can be utilized to directly fabricate metasurfaces. Besides, diverse processing conditions are investigated such as swelling effect for the successful replication of high-aspect-ratio nanostructures. Finally, we verify the optimized nanoparticle-embedded resin printing process through the replication of metasurfaces with various dimensions and an optimal design.

Bioinspired Soft Elastic Metamaterials for Reconstruction of Natural Hearing.

Natural hearing which means hearing naturally like normal people is critical for patients with hearing loss to participate in life. Cochlear implants have enabled numerous severe hearing loss patients to hear voice functionally, while cochlear implant users can hardly distinguish different tones or appreciate music subject to the absence of rate coding and insufficient frequency channels. Here a bioinspired soft elastic metamaterial that reproduces the shape and key functions of the human cochlea is reported. Inspired by human cochlea, the metamaterials are designed to possess graded microstructures with high effective refractive index distributed on a spiral shape to implement position-related frequency demultiplexing, passive sound enhancements of 10 times, and high-speed parallel processing of 168-channel sound/piezoelectric signals. Besides, it is demonstrated that natural hearing artificial cochlea has fine frequency resolution up to 30Hz, a wide audible range from 150-12000Hz, and a considerable output voltage that can activate the auditory pathway in mice. This work blazes a promising trail for reconstruction of natural hearing in patients with severe hearing loss.

Compact Dual-Band High-Efficiency Antennas Based on Spoof Surface Plasmon Polaritons

A compact dual-band high-efficiency antenna based on spoof surface plasmon polaritons (SSPPs) is proposed by well exciting both the TM10 mode and the LC resonant mode. Each mode can be tuned separately without affecting the other mode. The whole SSPP slab is utilized as the radiator of the TM10 mode, while the air ring gap, the shorting pin, and a minor part of SSPP slab work as the radiator of the LC resonant mode. The compact size and the high radiation efficiency of the proposed antenna are due to the high field confinement and high effective refractive index of SSPPs. A reliable circuit model is first proposed for an SSPP antenna and applied to design it for dual-band operation. A prototype was fabricated using PCB technology and measured. The proposed antenna achieves a compact electrical size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18\lambda _{\mathbf {0}} \times 0.18\lambda _{\mathbf {0}} \times 0.07\lambda _{\mathbf {0}}$ </tex-math></inline-formula> , which is significantly smaller than its counterparts ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{\mathbf {0}}$ </tex-math></inline-formula> is the wavelength at lower resonant frequency). Our proposed antenna possesses higher relative bandwidth per wavelength square area in comparison with other dual-band antennas. Both the simulated and measured results verified the high total efficiencies of the proposed dual-band antenna. The proposed antenna will have potential applications in wireless communication systems.

Optimization of the nanostructured weakly coupled few-mode fiber for mode-division-multiplexed systems.

The objective of the study is to optimize the optical fiber structure for mode-division multiplexing systems using nanostructurization. The nanostructuring technique allows to fabricate fibers with arbitrarily designed (free-form) refractive index distribution based on two glasses. Three optimization schemes have been proposed. The nanostructuring method allows for designing fibers with optical properties similar and even better parameters impossible to produce by other methods. In this proposal, we examined four linearly polarized (LP) few-mode fibers. We report a high effective refractive index difference between modes while maintaining other important parameters for the weakly coupled approach.

Semiconductor material photonic crystal fiber for mid-infrared supercontinuum generation

In this paper, Al0.24Ga0.76As based holey optical fiber is designed for coherent supercontinuum generation. It is mainly simulated for optical characteristic of lattice microstructures. For computational studies, the introduced design is composed of five loops of air vents with circular and elliptical structures. Flat and anomalous dispersion in the mid-infrared range has been achieved by obtaining a high nonlinear Kerr coefficient 5402 W−1 km−1 at the pump wavelength of 3.25 µm from the proposed microstructure fiber. Thereby, the expansion of the supercontinuum spectrum from 1.70 µm to 8.5 µm has been obtained. With the help of the high effective refractive index at the pump wavelength and the small effective mode region, proper supercontinuum spectrum (SC) is generated. The range of the spectrum of supercontinuum based on fiber length, pump duration and pump energy is simulated. Applications of this work will be advantageous in bio-medical, optical sensing, and wavelength conversion.

Gridization-Driven Mesoscale Self-Assembly of Conjugated Nanopolymers into Luminescence-Anisotropic Photonic Crystals.

Organic semiconducting emitters integrated with butterfly-mimetic photonic crystals (PhCs) are fascinating for dramatic advantages over light outcoupling efficiency and multifunctional strain sensors, as well as the key step toward electrically pumped lasers. Herein, an unprecedentedly direct mesoscale self-assembly into 1D PhCs is reported through a covalently gridization-driven approach of wide-bandgap conjugated polymers. A simple solvent-casting procedure allows for in situ self-assembly of the state-of-the-art conjugated nanopolymer, poly{[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]grid}-co-{[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]grid} (PODPFG), into well-defined multilayer architectures with an excellent toughness (30-40 J m-3 ). This ordered meso-architecture shows a typical Bragg-Snell diffraction behavior to testify the PhC nature, along with a high effective refractive index (1.80-1.88) and optical transmittance (85-87%). The PhC films also exhibit an angle-dependent blue/green photoluminescence switching, an electroluminescence efficiency enhancement by 150-250%, and an amplified spontaneous emission enhancement with ultralow waveguide loss coefficient (2.60 cm-1 ). Gridization of organic semiconductors offers promising opportunities for cross-scale morphology-directed molecular design in multifunctional organic mechatronics and intelligences.

Spectrum Broadening of Supercontinuum Generation by fill Styrene in core of Photonic Crystal Fibers

In this paper, a new photonic crystal fiber with hollow core filled by styrene (PCFS) has been designed for coherent supercontinuum generation. Its main optical characterisitcs are simulated for different lattice microstructure. The PCFS 2,0.3 with lattice pitch L=2 mm, filling factor d/L=0.3 and styrene core of diameter of 3.34 mm, owning the flat and anomalous dispersion in the near infrared range from 1.33 mm, lower confinement loss, higher effective refractive index at pump wavelength, and smaller effective mode area is chosen to invetigate the supercontinuum spectrum (SC). The spectrum broadening of SC depending on the fiber’s length, pump duration and pump energy is simulated and discussed in comperison to that obtained in the photonic crystal fibers with hollow core filled with toluene (PCFT 2,0.3 ).

Novel computational design of high refractive index nanocomposites and effective refractive index tuning based on nanoparticle morphology effect

This study introduces a method to predict the refractive index (RI) of nanocomposites with the Finite Elements Analysis (FEA) based on the Fabry-Pérot interference. The efficacy was verified by comparing the estimated composites’ RI with the available data in the literature. In the experimental verification, the FEA-based prediction showed closer results with the measurement as compared to the effective medium approximation (EMA) approaches, which are prevalently used to predict the physical properties of nanocomposites. Due to the modeling capability, the FEA-method could investigate the effect of the nanoparticle morphology (particle size, shape, and orientation) and distribution. Large particle size, particle agglomeration in high electric-field amplitude region, and particle elongation along the light oscillating direction are found to be the major factors to enhance the RI of composites. The underlying mechanism of RI changing is attributed to the light scattering by embedded nanoparticles, which provides one potential real-time RI tuning schematic.

Spoof plasmonic Brewster angle transmission for broadband electromagnetic energy squeezing in the microwave regime

We demonstrate through numerical experiments and analytical calculations that extreme subwavelength gaps between two corrugated surfaces support high effective refractive index guided modes. The corrugated gap mode is of low loss because it does not rely on plasmonic currents induced inside a metal. This enables guided modes with a much higher effective refractive index than is possible in natural plasmonic materials. These high-index guided modes are incorporated as periodic slots in an opaque screen, which is then shown to support broadband highly transmitting modes at a certain oblique incidence angle in addition to the usual Fabry–Pérot resonances. This anomalously high transmission is the extension of the plasmonic Brewster angle to arbitrarily low frequency, controlled by the geometry of the corrugated slots. We demonstrate the preservation of the shape of a broadband low-frequency pulse transmitted through the slotted screen, opening up the possibility to use the structure for broadband energy squeezing applications in the GHz to THz regime.

Numerical study of a high negative refractive index based tunable metamaterial structure by graphene split ring resonator for far infrared frequency

We present a numerical study of a high negative effective refractive index based actively tunable squared shaped graphene split ring resonator for far infrared frequency spectrum. Graphene split ring resonator (GSRR) structure is investigated for the different cases of the split rings. Tunability of the structure is controlled by varying the different chemical potential of the graphene sheet, which can be controlled by external potential. Transmission, Reflection and Absorption parameters are investigated to check the resonance response. At each and every point of resonance, highly negative refractive index is been observed. The proposed structure is producing the multiple short as well as large band resonance band over 1 THz to 2.5 THz frequency range for the different chemical potential that varies between 0.1 eV to 0.9 eV. The study results inferred that tunable properties of the GSRR structure can help to design the various photonics device such as sensor, modulator and polarizer, etc.

Light waveguiding in bioinspired peptide nanostructures.

Basic optical properties of bioinspired peptide nanostructures are deeply modified by thermally mediated refolding of peptide secondary structure from α-helical to β-sheet. This conformational transition is followed by the appearance in the β-sheet structures of a wideband optical absorption and fluorescence in the visible region. We demonstrate that a new biophotonic effect of optical waveguiding recently observed in peptide/protein nanoensembles is a structure-sensitive bimodal phenomenon. In the primary α-helical structure input, light propagates via optical transmission window demonstrating conventional passive waveguiding, based on classical optics. In the β-sheet structure, fluorescent (active) light waveguiding is revealed. The latter can be attributed to completely different physical mechanism of exciton-polariton propagation, characterized by high effective refractive index, and can be observed in nanoscale fibers below diffraction limit. It has been shown that peptide material requirements for passive and active waveguiding are dissimilar. Original biocompatibility and biodegradability indicate high potential future applications of these bioinspired waveguiding materials in precise photobiomedicine towards advanced highly selective bioimaging, photon diagnostics, and optogenetics.

Performance analysis of a liquid-filled glass prism-coupled metal-clad planar waveguide sensor

The performance of a metal-clad planar waveguide-based sensor coupled with a liquid-filled prism is studied and is compared with a similar waveguide sensor coupled with a solid prism. The characteristic equations of these waveguides are obtained by matching field components at various boundaries, and the reflectivity of these waveguides is obtained using transfer matrix method. The obtained dispersion characteristic shows that the transverse electric field components are loosely bound in liquid-filled prism-coupled waveguide-based sensor. Unlike to solid prism-coupled waveguide sensor, the performance of liquid-filled prism-coupled waveguide sensor increases with increase of effective refractive index. Hence, the proposed liquid-filled prism-coupled waveguide-based sensor shows better performance at high effective refractive index.

SnO2 Inverse Opal Composite Film with Low-Angle-Dependent Structural Color and Enhanced Mechanical Strength.

Structural colors are attracting considerable attention for their advantages of environmental friendliness and resistance to fading. However, the weak mechanical strength and intrinsic iridescent color restrict their widespread application. This article describes a SnO2 inverse opal composite film with low-angle-dependent structural color and enhanced mechanical strength. In the present study, a direct template method was used to prepare SnO2 inverse opals, which were then embedded in polydimethylsiloxane (PDMS). The structural colors of obtained composite films were low-angle-dependent due to light scattering and high effective refractive index. Meanwhile, because of the good physical strength of PDMS, structures of SnO2 inverse opals were provided with effective protection. No specific wavelength shift occurred during stretching, and exhibited excellent cycling stability. All these advantages indicated potential applications in packing and decorating materials.

Metamaterial Demonstrates Both a High Refractive Index and Extremely Low Reflection in the 0.3-THz Band

Communication and imaging in the terahertz waveband have advanced rapidly in offering industrial applications. However, optical elements such as collimated lenses in the terahertz waveband are bulky compared with the wavelength due to the lack of naturally occurring substances with a high refractive index and low loss. It is essential to miniaturize optical elements in the terahertz waveband for industrial application. Metamaterials consisting of subwavelength structures can arbitrarily control permittivity and permeability and provide a range of refractive indices. Here, we demonstrate a metamaterial with both a high refractive index and extremely low reflection consisting of symmetrically aligned paired cut metal wires with 18,800 units on the front and back surfaces of a dielectric substrate. Measurements by terahertz time-domain spectroscopy (THz-TDS) confirm a high effective refractive index of 6.66 + j0.123, extremely low reflection power of 1.16%, and the unprecedented high figure of merit (FOM = |n real/n imag|) of above 300 in the 0.3-THz band. Components with such specifications would enable miniature, high-performance optical elements in the terahertz waveband such as ultrathin flat antennas with high directivity. Further, the concept of the metamaterial with both a high refractive index and extremely low reflection potentially offers a wide range of attractive applications such as solid immersion lenses and cloaking devices.