Refractive Index Region Research Articles

Impact of structural parameters on multimode D-shaped optical fiber refractive index sensors for different materials

In this work, a D-shaped optical fiber sensor has been experimentally developed to compare how the refractive index (RI) of a water analyte (used as the cladding layer over a small length of the stripped D-polished optical fiber) changes due to the inclusion of an unknown concentration of two distinct materials, one with a low RI and the other with a high RI. No metal film is deposited in the proposed D- shaped optical fiber sensor and hence we demonstrate the sensor's ability to detect the lowest concentration of the added material into the water analyte without employing the surface plasmon modes. A side-polished multi-mode optical fibre with various sensing lengths and polishing depths is used to construct the D-shaped fibre sensor. Experimental results verify that in a tunable refractive index (RI), the sensitivity increases with the sensing length due to the increase in the interaction area between the surrounding material and the sensing area. The sensitivity also increases with polishing depth in the high-RI range for materials with higher concentrations. The D-shaped fibre sensor's maximum sensitivity of 25.076 a.u./RIU is attained in the range of low refractive index values (1.3329) to (1.3345) RIU. Nonetheless, its maximum sensitivity drops to 0. 317 a.u./RIU at the high refractive index region of 1.348–1.4048 RIU. This sensor is predicted to have potential uses in low-RI materials with very low concentrations. Therefore, the best D-shape fibre sensor was used to detect the lowest concentration of the sodium chloride solution in the low refractive index range, and it was found that the lowest detected concentration was 0.012% with a high sensitivity of 327.32 a.u./RIU.

Engineering Refractive Index Contrast in Thin Film Barium Titanate-on-Insulator.

Barium titanate-on-insulator has demonstrated excellent vertical optical confinement, low loss, and strong electro-optic properties. To fabricate a waveguide-based device, a region of higher refractive index must be created to confine a propagating mode, one way of which is through dry etching to form a ridge. However, despite recent progress achieved in etching barium titanate and similar materials, the sidewall and surface roughness resulting from the physical etching typically used limit the achievable ridge depth. This motivates the exploration of etch-free methods to achieve the required index contrast. Here, we introduce three etch-free methods to create a refractive index contrast in barium titanate-on-insulator, including a metal diffusion method, proton beam irradiation method, and crystallinity control method. Notably, molybdenum-diffused barium titanate leads to a large index change of up to 0.17. The methods provided in this work can be further developed to fabricate various on-chip barium titanate optical waveguide-based devices.

Non-mechanical multidirectional optical beam steering using fringing fields in liquid crystals

Non-mechanical multidirectional and wide-angle beam-steering devices are of high interest in advanced laser scanning applications. We present a non-mechanical multidirectional beam-steering device utilizing fringing fields in the liquid crystals. An electrically tunable gradient refractive index (GRIN) region is created due to the fringing fields. The direction of GRIN and the steered beam is controlled by modulating the applied voltage across the designed four-electrode system. The presented device demonstrates a continuously varying steering angle up to ±3∘ at a low applied voltage of ±10Vpp. Moreover, the device is compact, cost-effective, and easy to fabricate, and delivers beam steering in eight different directions.

A numerical analysis of a highly sensitive hexagonal plasmonic refractive index sensor

The article proposes and quantitatively investigates the performance characteristics of a surface plasmon polariton based refractive index sensor with metal–insulator–metal arrangement, which demonstrates higher sensitivity than many recent studies. The Finite Element Method (FEM) has been used to quantitatively analyze the refractive index sensor’s transmission properties and sensitivity in various configurations. The structure consists of two symmetrical triangular stubs coupled to a ring-type hexagonal resonator with two slits in opposing arms. The highest sensitivity and FOM observed is 4571.7 nm/RIU and 63.3, respectively. It has a sensing resolution of 2.18×10−7 RIU. Mode 1 has a maximum figure of merit and quality factor of 197.6 and 204.15, respectively. The high quality factor of mode 1 indicates that this sensor is suitable for use as a notch filter. The sensor has been evaluated for temperature sensing, and the temperature sensitivity is significantly higher than that of recent studies. Some sensors claim to be highly sensitive, but they often use a high refractive index region to evaluate the sensor, which is misleading because the sensitivity would be higher in the region with a high refractive index. The effect of adding the two slits in opposing arms is that the figure of merit can be increased by increasing the width of the two slits. Refractive index sensors are useful in medical diagnostics and have a wide variety of applications. The combination of very high sensitivity and FOM in a tiny and compact configuration is ideal for on-chip plasmonic nanosensors.

Design and characteristics of ellipse-assisted ring-core fiber for space division multiplexing

• Elliptical-assisted ring-core fiber with central low-refractive-index elliptical region. • 9 effectively separated spatial modes have uniform Δn eff with values greater than 3.14 × 10 −4 at 1550 nm. • The level of polarization separation is lower than 3.33 × 10 −6 . • Good scalability and fabrication error tolerance. • This fiber design could be promising candidates toward high-capacity SDM systems. We propose a novel ellipse-assisted ring-core fiber characterized by an elliptical region of low refractive index in the core. The proposed fiber proves effective in eliminating the spatial degeneracy of the LP mn mode groups and keeping the birefringence properties at a low level. This fiber, with optimized structural parameters, can guide 9 modes over the entire C-band. The simulation results show that the effective refractive index difference (Δn eff ) between adjacent modes at 1530–1565 nm falls within the scope of (3.1 ∼ 6.64) × 10 −4 . At the same time, the polarization separation level of each mode is lower than 3.23 × 10 −6 , which is 2 orders of magnitude lower than the degree of spatial mode degeneracy separation. This fiber design has good application prospects in space division multiplexing systems.

Refractive index profile measurement of planar optical waveguides based on the near-field technique and digital holography

• A new method is proposed to measure the refractive index profile of planar optical waveguides. • A measurement system is established by integrating the near-field setup and the modified Mach-Zehnder interferometer. • The refractive index profile is quantitatively obtained from two phase difference distributions. • The accuracy of the proposed method is up to 1 × 10 −3 . A method based on the near-field technique and digital holography is proposed to measure the refractive index profile of planar optical waveguides. A measurement system is established by integrating the near-field setup and the modified Mach-Zehnder interferometer. The near-field intensity profile of optical waveguides is recorded by CCD to get the primary refractive index profile. The primary phase difference distribution is calculated based on the relationship between the phase and refractive index. The digital hologram is acquired by the modified Mach-Zehnder interferometer to extract the real phase difference distribution using the hybrid reconstruction algorithm. According to the relationship between the primary and real phase difference distributions, the refractive index profile is quantitatively obtained. To evaluate the proposed method, we measured the refractive index profiles of multimode and single-mode fibers and compared with S14 based on refracted near-field method and SHR-1802 based on digital holography. The proposed method has a measurement accuracy of up to 1 × 10 -3 and is suitable for measuring the positive refractive index region of waveguides.

Inverse-Designed Metaphotonics for Hypersensitive Detection.

Controlling the flow of broadband electromagnetic energy at the nanoscale remains a critical challenge in optoelectronics. Surface plasmon polaritons (or plasmons) provide subwavelength localization of light but are affected by significant losses. On the contrary, dielectrics lack a sufficiently robust response in the visible to trap photons similar to metallic structures. Overcoming these limitations appears elusive. Here we demonstrate that addressing this problem is possible if we employ a novel approach based on suitably deformed reflective metaphotonic structures. The complex geometrical shape engineered in these reflectors emulates nondispersive index responses, which can be inverse-designed following arbitrary form factors. We discuss the realization of essential components such as resonators with an ultrahigh refractive index of n = 100 in diverse profiles. These structures support the localization of light in the form of bound states in the continuum (BIC), fully localized in air, in a platform in which all refractive index regions are physically accessible. We discuss our approach to sensing applications, designing a class of sensors where the analyte directly contacts areas of ultrahigh refractive index. Leveraging this feature, we report an optical sensor with sensitivity two times higher than the closest competitor with a similar micrometer footprint. Inversely designed reflective metaphotonics offers a flexible technology for controlling broadband light, supporting optoelectronics' integration with large bandwidths in circuitry with miniaturized footprints.

Surface-Normal Free-Space Beam Projection via Slow-Light Standing-Wave Resonance Photonic Gratings.

On-chip grating couplers directly connect photonic circuits to free-space light. The commonly used photonic gratings have been specialized for small areas, specific intensity profiles, and nonvertical beam projection. This falls short of the precise and flexible wavefront control over large beam areas needed to empower emerging integrated miniaturized optical systems that leverage volumetric light-matter interactions, including trapping, cooling, and interrogation of atoms, bio- and chemi- sensing, and complex free-space interconnect. The large coupler size challenges general inverse design techniques, and solutions obtained by them are often difficult to physically understand and generalize. Here, by posing the problem to a carefully constrained computational inverse-design algorithm capable of large area structures, we discover a qualitatively new class of grating couplers. The numerically found solutions can be understood as coupling an incident photonic slab mode to a spatially extended slow-light (near-zero refractive index) region, backed by a reflector. The structure forms a spectrally broad standing wave resonance at the target wavelength, radiating vertically into free space. A reflectionless adiabatic transition critically couples the incident photonic mode to the resonance, and the numerically optimized lower cladding provides 70% overall theoretical conversion efficiency. We have experimentally validated an efficient surface normal collimated emission of ≈90 μm full width at half-maximum Gaussian at the thermally tunable operating wavelength of ≈780 nm. The variable-mesh-deformation inverse design approach scales to extra large photonic devices, while directly implementing the fabrication constraints. The deliberate choice of smooth parametrization resulted in a novel type of solution, which is both efficient and physically comprehensible.

Refractive Index Sensing Simulations of CsPbBr3 Quantum Dots/Gold Bilayer Coated Triangular-Lattice Photonic Crystal Fibers

With the incorporation of noble metal materials, photonic crystal fibers (PCFs) could be performed as an effective platform for refractive index sensing of the filling analytes. Furthermore, by coating functional dielectric layers upon the metal surfaces, the resonance energy transfer is modulated from the core mode of the PCFs towards the surface plasmon resonance mode of the metals, and the sensing performance could be boosted. Here, considering that the exciton-plasmon coupling is efficient between perovskite quantum dots (QDs) and gold, a kind of CsPbBr3 QDs/Au bilayer coated triangular-lattice PCFs has been simulated numerically as the refractive index sensors. With the optimization of the QDs and gold layer thicknesses, together with the variation of the central hole size of the PCFs, in the refractive index (RI) region of 1.26 to 1.34, a rather narrow full width at half maximum (FWHM) of the loss spectra was achieved as 13.74nm when the central hole size was 1.28 µm and the highest figure of merit was 63.79RIU (the central hole size was 1.53 µm). This work demonstrates that the analyte identification accuracy was enhanced by FWHM narrowing of the loss spectra; in addition, taking the abundance of the material choice of perovskite QDs into consideration, more analytes could be detected effectively. Moreover, by adopting asymmetric structures, the sensitivity of the PCFs based refractive index sensors could be further improved.

Numerical approach to design the graphene-based multilayered surface plasmon resonance biosensor for the rapid detection of the novel coronavirus

In this article, a graphene-based multilayered surface plasmon resonance (SPR) biosensor of (BK7/WS2/Au/BaTiO3/Graphene) is proposed for the rapid detection of the novel coronavirus (COVID-19). The proposed SPR biosensor is designed based on the angular interrogation attenuated total reflection (ATR) method for rapid detection of the COVID-19 virus. The sensor’s surface plasmon polaritons (SPPs) and the sensing region refractive index (RI) are changed, owing to the interaction of various concentrated ligand-analytes. The specific ligand is mechanized with the proposed sensor surface and the target analyte that has flowed onto the sensing surface. The proposed sensor is capable of detecting the COVID-19 virus rapidly in two different ligand-analytes environments, such as: (i) the virus spike receptor-binding domain (RBD) as an analyte and monoclonal antibodies (mAbs) as a probe ligand, and (ii) the monoclonal antibodies (IgG or IgM) as an analyte and the virus spike RBD as a probe ligand. Due to the binding of the target ligand-analytes, the concentration level of the sensing region is incremented. As the increment in the concentration level, the RI of the sensing medium increases, therefore the change in RI causes the shift in the SPR angle resulting in the output reflectance intensity. The performance of the multilayered SPR sensor is analyzed numerically using the finite element method (FEM) method. Numerically, the proposed sensor provides the maximum angular shift sensitivity at 230.77 deg/refractive index unit (RIU), detection accuracy (DA) at 0.161 deg−1, and the figure of merits (FOM) is at 37.22 RIU−1. In addition, with each additional graphene layer number (L), the proposed sensor exhibits the angular shift sensitivity increment (1 + 0.7L) times. The novelty of the proposed multilayer (BK7/WS2/Au/BaTiO3/Graphene) sensor is highly angular sensitivity, and capable of detecting the COVID-19 virus rapidly without a false-positive report.

Quantitative morphology of femtosecond laser-written point-by-point optical fiber Bragg gratings.

We investigate the morphology of femtosecond laser, single pulse-inscribed, point-by-point (PbP) fiber Bragg gratings. Direct measurement of a PbP grating's refractive index profile was carried out with micro-reflectivity analysis. PbP gratings were imaged at sub-micrometer scale with scanning electron microscopy, Raman and photoluminescence studies were performed to probe the structural and electronic changes. Comparison of results from different characterisation techniques suggests that the creation of an increased refractive index region around the micro-void is due to contributions from both densification and the formation of highly polarizable non-bridging oxygen bonds.

Design and Fabrication of 3-D-Printed High-Gain Broadband Fresnel Zone Lens Using Hybrid Groove-Perforation Method for Millimeter-Wave Applications

This letter presents a novel hybrid design approach for the Fresnel zone lenses (FZL). The method combines the two conventional design approaches: grooved and perforated methods. Perforated lenses minimize the shadow blockage that grooved lenses suffer from, and therefore provide improved antenna gain. However, the fabrication of the low refractive index regions in the perforated lenses is challenging due to the limited manufacturing resolution, which means low refractive index dielectrics cannot be realized. The proposed hybrid FZL design utilizes the perforated method for the inner regions and applies the grooved method for the outer regions. This letter describes the equations used to optimize the design and gives the general design guidelines. A hybrid FZL antenna that operates at the center frequency of 33 GHz is presented. Measurement results show that the hybrid FZL improved the realized gain by up to 3.2 dB and the –3 dB gain bandwidth by up to 2 GHz when compared with the equivalent grooved FZL.

Numerical Analyses of Liquid-Core Fiber Optic SPR Sensor with Nano-Porous Sio2 as Inner Coating

In this paper, a novel type of liquid-core fiber optic SPR sensor with a nano-porous SiO2 inner coating is proposed. The nano-porous SiO2 is deposited on the inner wall of the quartz tube. Due to the flexible and adjustable refractive index of the deposited material, the RI detection range of the sensor is expanded. The simulation results show that the proposed sensor owns good responses in both low and high refractive index regions, with the corresponding RI sensitivities of 1043.71 nm/RIU and 1234.86 nm/RIU respectively, while the FWHM is only about 20 nm. There are some potential applications for the proposed sensor in environmental monitoring, biochemical detection, food safety etc.

Design of ring-core few-mode multi-core fiber with low crosstalk and low bending loss

Aiming at solving the problems of coupling between modes in a core and mode coupling between cores in few-mode multi-core fiber, a fiber with seven cores each with step index is proposed, and each core can support five modes. Each core has a central low refractive index region and a high refractive index ring to ensure that there is a large refractive index difference between modes in the core, so as to reduce the problem of mode coupling. The bending loss of central core and outer core, the crosstalk characteristics between modes and the influence of core parameters on crosstalk performance are simulated and analyzed by the finite element method. The simulation results show that at a wavelength of 1.55 μm and a bending radius of 50 mm, the bending loss of the proposed multi-core fiber is much lower than its attenuation loss, and the crosstalk between the adjacent cores of the five core modes are less than –20 dB/100 km. Therefore, this multi-core fiber can realize independent transmission of the core modes with long-distance under small bending radius.

Simultaneous temperature and humidity insensitive race track ring resonator using silicon and titanium dioxide waveguides in cavity

We present here a method to achieve temperature-insensitive micro-ring resonator using Si and TiO2 in the racetrack cavity by finite-difference time domain (FDTD) method. Temperature-induced spectral shift in one waveguide is exactly compensated by the optimal length of the other waveguide due to opposite signs of the thermo-optic coefficient of the two materials. Over the span of 40 °C relative to room temperature, MRR undergoes temperature-insensitive state having spectral shift as small as 0.5 pm/°C as compared to 72 pm/°C when TiO2 is not used in the racetrack cavity. The effect of changing core width on temperature sensitivity is also illustrated. We find that temperature insensitivity can only be achieved within a range of the core width. Our method outperforms the often used metal-based temperature compensators in terms of high integration density, low cost, and power dissipation. We have also shown that output response is completely insensitive to variations of relative humidity (RH) in 20% to 60% levels. Due to power confinement in core region and small variations of refractive index in the ambient region, output response is completely insensitive to any variation in humidity.

Low loss fishnet metamaterial via self-rolled nanotechnology

We propose and demonstrate the fabrication of a curved fishnet metamaterial integrated into a rolled-up tube (RUT) that consists of eight alternating layers of gold and silicon dioxide. We adopt a self-rolled technique for the fabrication of metal/dielectric microtubes with large enough diameters for optical characterization. We experimentally characterize the fabricated fishnet structure, and by means of numerical calculations, we show that the fabricated structure possesses a negative refractive index with a high figure of merit. We demonstrate that the negative refractive index region can be tuned by precisely controlling the dimensions of the holes forming the metamaterial. The results of this study open up the possibility to obtain a simple, fast, and flexible platform for the fabrication of RUT-based metamaterials for bioimaging and sensing applications.

Low-loss silicon nitride strip-slot mode converter based on MMI.

Slot waveguide has attracted a lot of attention due to its ability to confine light in the low refractive index region, while strip waveguide acts as the basic component of guiding light due to its relatively low optical loss. In the multifunctional photonic integrated chips, it is critical to achieve the low loss transition between the strip waveguide and the slot waveguide. In this work, a silicon nitride strip-slot mode converter with high efficiency, large bandwidth, and large fabrication tolerance are proposed and demonstrated through the numerical investigation and experiments. The coupling efficiency of the mode converter is up to - 0.1 dB (97.7%), which enables the extremely low transition loss between the strip waveguide and the slot waveguide. Moreover, the fabrication process of silicon nitride photonic devices with high performance is introduced, which is fully compatible with the CMOS technology. Photonic devices based on silicon nitride with the characteristics of the low optical loss and the temperature insensitivity represent a new paradigm in realizing silicon-based photonic multifunctional chips.

Monitoring of the Critical Meniscus of Very Low Liquid Volumes Using an Optical Fiber Sensor

A novel sensing system based on single mode optical fiber in reflective configuration has been developed to measure the critical meniscus height (CMH) of low volumes of liquids, which is then used to calculate the contact angle. The sensing system has been designed especially for very low volumes of liquids (e.g. bio-liquids) and the work has demonstrated that measurements are possible with a minimum liquid volume of 5 μL . The sensing system is based on monitoring the spectral variation induced by the difference in the refractive index regions surrounding the fiber tip, at the air-liquid or liquid-liquid interfaces. From the experiments performed in water, (by immersing and extracting the fiber sensor in the liquid sample), it can be concluded that the CMH forming on the fiber decreases as the temperature increases. The change of temperature (in this experiment from 22 to 60 °) does not influence the CMH of the sample used in the evaluation (P3 mineral oil), giving an indication of its thermal stability. In addition, a fixed fiber was used to measure the variation in the liquid level when another fiber is immersed in the liquid. The error in the liquid level obtained in the work was small, at 0.34 ± 0.04 %. Such a sensor, allowing accurate measurements with very small quantities is especially useful where liquid sample volumes are limited e.g. biologically sourced liquids or specialized, expensive industrial material in the liquid phase.

Ultrasensitive Label-Free Biosensor Based on the Graphene-Oxide-Coated-U-Bent Long-Period Fiber Grating Inscribed in a Two-Mode Fiber

In this article, we theoretically and experimentally report a newly-designed biosensor, which consists of a particular graphene-oxide (GO)-coated-U-bent long-period fiber grating inscribed in a two-mode fiber (LPFG-TMF). Due to the enhancement of evanescent field, the bent LPFG-TMF is more sensitive to the surrounding refractive index and it allows an extremely high sensitivity. The impact of bent radius on the sensing performance is discussed. The experimental result demonstrates, the sensitivity increases nonlinearly with the surrounding refractive index and the highest sensitivity reaches 6632.32 nm/RIU in the refractive index region from 1.3731 to 1.3830. To optimize the immune-sensing performance, the goat anti-human IgG is immobilized on the sensor via covalent bond through EDC/NHS cross-linker for human IgG recognition. The GO-coated-U-bent LPFG-TMF with bent radius of 15 mm has gained a low limit of detection (23 ng/ml) in the 3-20 μg/ml. In addition, the repeatability of the sensor is also investigated. With its premium ability of easy fabrication, high sensitivity, and compactness, the proposed sensor is adapted for detection of human IgG in clinical diagnostics, pharmaceutical researches, and bioengineering applications.

Electromagnetic absorption of SRR based double-inverse E-Shaped metamaterial for DCS, EESC, 5G, and WiMAX applications

Herein, a novel double-inverse E-structured resonator metamaterial (MTM) surrounded by split-ring resonators (SRRs) is introduced and proposed. The proposed MTM has working frequency ranges of 1.62 – 1.79 GHz, 2.08 – 2.21 GHz, 4.39 – 4.99 GHz, and 5.65 – 6.09 GHz. In addition, the proposed MTM can be applied in digital cellular system uplinks (DCS) (1.736 GHz), earth exploration-satellite communication (EESC) (2.148 GHz), lower band 5G (4.8 GHz), and WiMAX (5.89 GHz) applications. The electromagnetic radiation reduction performance of the proposed MTM is investigated at 900 MHz and 1800 MHz of GSM cellular bands. The Finite Integration Technique (FIT) based on high-frequency electromagnetic software is utilised for the development of design along with a detailed investigation of the MTM properties. The proposed MTM is formed by developing a double-inverse E-structure in the middle surrounded by SRRs. The resonator structure with SRRs is implemented on an epoxy resin FR4 substrate, which has a compact size of 11.11 × 11.11 mm2. The unit cell has a high effective medium ratio of 14.82. Furthermore, the developed MTM shows a negative refractive index region from 3.124 to 6.98 GHz and double-negative region from 3.94 to 4.57 GHz, 5.26 to 5.49 GHz, and 6.29 to 6.88 GHz. In addition, SAR evaluation is also performed to test the competence of utilising the proposed MTM on modern mobile phones. The MTM has an electromagnetic absorption rate of about 44% and 43% at 900 MHz and 1800 MHz, respectively, for 1g whereas about 43% and 44% at 900 MHz and 1800 MHz, respectively, for 10g. Thus, the performance and characteristics of the developed MTM can be utilised for the above-mentioned applications.