Maximum Refractive Index Research Articles

A Surface Plasmon Resonance Bio-Sensor Based on Dual Core D-Shaped Photonic Crystal Fibre Embedded With Silver Nanowires for Multisensing

In this paper, for sensing and monitoring the biochemical analyte dissolved in liquid, antigen-antibody interaction or protein-DNA/RNA binding process, we design a surface plasmon resonance refractive index based biosensor by using a dual core D-shaped six-fold photonic crystal fibre which is embedded with silver nanowires for multi-detection. We numerically analyze both the dispersion relations and the loss spectra for various analytes by finite element method. This optical fibre bio-sensor monitors the changes of the refractive index for different analytes by measuring the spectral shifts of the fibre loss peaks at their resonance wavelengths. With the wavelength interrogation method, we find that the proposed biosensor with two sensing channels exhibits a maximum refractive index sensitivity of 3400 nm/RIU and a resolution of $2.94\times 10^{-5}$ RIU for a large sensing range from 1.35 to 1.50, which covers most known analytes of proteins, viruses or DNA/RNA. By utilizing 200 nm silver nanowires in the sensing channels, the sensitivity can be enhanced up to 4000 nm/RIU. Due to its special two-channel design for multi-sensing, it is possible to distinguish/study the binding possibility/capability of unknown analyte with two different target proteins simultaneously. Further, by introducing another critical channel, the confinement loss for either channel I or channel II can be greatly enhanced for high accurate result and more reliable sensing. Moreover, we numerically prove that the diameter of nano silver wires has great influences on the sensing peaks and sensitivity of the proposed biosensor.

Spatial tailoring of the refractive index in infrared glass-ceramic films enabled by direct laser writing

The development of infrared gradient refractive index (GRIN) components relies on the ability to modify the refractive index and dispersion properties of suitable host materials with a high spatial selectivity and a sufficient magnitude of change. We present a novel multi-step approach to induce local refractive index changes in chalcogenide optical materials. Films with thicknesses between 1 and 40 µm fabricated from multi-component GeSe2-As2Se3-PbSe (GAP–Se) glass-ceramic materials were irradiated with continuous-wave and nanosecond-pulsed laser light, and post-processed with heat-treatments. A maximum local refractive index change of Δn = 0.088 across a broad spectral range in the infrared was realized. Spatial control of the refractive index variation was achieved through thermally-induced crystallization of a Pb-rich crystal phase. The magnitude of the index change scaled with the laser power and the exposure dose while the material maintained the required optical quality. The material performance validated in this study for thick films (20–40 µm films) reconfirms our ability to extend results from thin GAP-Se films towards novel optical designs.

Combining whispering gallery mode optofluidic microbubble resonator sensor with GR-5 DNAzyme for ultra-sensitive lead ion detection

Lead ions are deleterious pollutants that often reach drinking water, and can cause significant harm to humans (particularly children). An ultra-sensitive lead ion detection method using a whispering gallery mode (WGM) optofluidic microbubble resonator and the classic GR-5 DNAzyme is proposed in this paper. With the auxiliary piranha and Ploy-l-lysine solution, GR-5 DNAzyme was successfully modified on the inner wall of a microbubble. The mode field distribution of the microbubble was analysed, and the optofluidic sensor with thin wall exhibited a maximum bulk refractive index sensitivity of 265.2 nm/RIU. Lead ions at concentrations ranging from 0.1 pM to 100 pM were tested using the proposed WGM optofluidic sensor. The noise was decreased to 2.43 fM using the self-referenced differential method. Thus, a limit of approximately 15 fM was obtained for the detection of lead ions using the WGM optofluidic biosensor. Eight competing metal ions were also used to evaluate the selectivity of the proposed sensor, with results indicating that it has high selectivity for lead ions. Finally, the sensor performance is verified using real samples.

Tunable Dual-Band Ultrasensitive Stereo Metamaterial Terahertz Sensor

To overcome the effect of analyte thickness and limitation of single wavelength sensing, we propose an ultra-sensitive stereo metamaterial biosensor with double resonance frequencies based on coupling electric resonance and magnetic resonance. The electromagnetic analysis demonstrates that the double resonance coupling, induced by the stereo double-layer structure, can significantly improve sensitive biosensing at the terahertz frequency. The results show that by changing the size of the structure, each resonance frequency can be independently tuned in the range of 0.5–1.8 THz and the maximum refractive index sensitivity is 930.4 GHz/RIU. These results have significant implications for the detection of samples with different frequency points. The proposed ultra-sensitive stereo metamaterial structure has excellent potential for application in biomolecule detection and differentiation.

A high-performance refractive index sensor based on a triangular-lattice photonic crystal with defect and surface modes

The refractive index sensing capabilities of a 2D triangular-lattice photonic crystal (PhC) with defective and surface modes are studied. These are composed of silicon columns arranged in a liquid environment. Through changing the rods’ shape on the PhC’s surface from a circle to an ellipse and optimizing the sizes of two adjacent ellipses, the surface localized mode can be generated. Combining the central defective mode with surface modes, an ultra-small three-working-wavelength PhC refractive index sensor is proposed. The shift of the resonant wavelengths influenced by the slight alteration of the protein’s thickness attached by the special rods is studied, and it is found that a minimum alteration of the protein’s thickness of about 4 nm can be distinguished, and a maximum refractive index sensitivity of 0.516 is obtained. Three kinds of analytes can be detected in real-time and at the same time, which are bounded around the input/output surfaces and the central rods through biological specific binding.

Anisotropic nonlinear refractive index measurement of a photorefractive crystal via spatial self-phase modulation.

We show that the refractive index modification photoinduced in a biased nonlinear photorefractive crystal can be accurately measured and controlled by means of a background incoherent illumination and an external electric field. The proposed easy-to-implement method is based on the far-field measurement of the diffraction patterns of a laser beam propagating through a self-defocusing medium undergoing spatial self-phase modulation. For various experimental conditions, both saturation intensity and maximum refractive index modification have been measured. We also clearly evidence and characterise the anisotropic nonlinear response of the crystal in the stationary regime.

Femtosecond laser direct writing of diffraction grating and its refractive index change in chalcogenide As2Se3 film.

Chalcogenide glasses possess novel optical property in infrared range, which make them ideal candidates for photonic devices by laser direct writing. Precisely control of the refractive index manipulation in the material is the key to achieve high quality optical devices. In this work, diffraction gratings in chalcogenide As2Se3 thin films were fabricated with femtosecond laser direct writing and its refractive index change was carefully studied. Grating diffraction efficiency was measured from visible to near-infrared light by using multiple single-wavelength lasers and a supercontinuum laser. Clear diffraction patterns and high diffraction efficiency indicated the good optical quality of the prepared gratings. Results show that the grating with period of 5 µm inscribed under pulse energy of 30 nJ demonstrated a 1st-order diffraction efficiency of 30% at 808 nm testing wavelength, due to the changes of refractive index and absorption. The relationship of the change of refractive index and absorption coefficient of film under different laser irradiation intensity is carefully studied. The maximum refractive index change was estimated to be 0.087 at 808 nm.

Simultaneous measurement of temperature and refractive index with F–P microcavity sensor based on graded-index few mode fiber

In this paper, a Fabry–Perot (F–P) microcavity sensor based on graded-index few mode fiber is proposed and fabricated. One end of the graded-index few mode fiber is etched by hydrofluoric(HF) acid to form an air microcavity. The end with air microcavity is spliced with single-mode fiber, and another end is cut flat to make sensing head. Due to the generation of reflection surface, the F–P microcavity structure is formed between the surfaces of microcavity and the fiber tip. Simulation and experimental results show that the reflection spectrum will shift regularly with the change of external temperature, and the extinction ratio of that will also change regularly with the change of external solution concentration. The temperature and refractive index can be measured by detecting the wavelength shift and extinction ratio change in spectrum. When the temperature changes between 30∘C∼90∘C, the temperature sensitivity can be up to 10.52pm/∘C. When the refractive index changes between 1.3331∼1.3568, the maximum refractive index sensitivity is -16.03dB/RIU. This fiber F–P microcavity sensor has different responses to the change of temperature and liquid refractive index, it can be used to measure both parameters simultaneously.

Generation and manipulation of the Fano resonance with high refractive index sensitivity based on the square split ring dimer

The Optical properties of the Square Split Ring dimer (SSRD) nanostructure is studied by the finite element method (FEM). Fano resonance can be excited in the visible and near-infrared bands by changing geometrical parameters of the SSRD or the arrangement of two square split rings (SSR). The production mechanism of Fano resonance is illustrated by using the plasmon hybridization theory (PHT). At the meantime, a pair of reverse current circulation is formed in the SSRD nanostructures and the maximum magnetic field enhancement reaches 45.69. In addition, calculation results show that the maximum refractive index sensitivities and the figure of merit (FOM) of SSRD systems reached 2043 nm/RIU and 57.9, respectively. Based on these features of the SSRD system, it may have potential application in sensor devices and surface-enhanced Raman scattering (SERS).

Enlarge-Tapered, Micro-Air Channeled Cavity for Refractive Index Sensing in SMF

We report a compact, highly sensitive all SMF refractive index (RI) sensor based on a waist enlarged tapered (WET) fiber probe with a micro-air cavity (MAC) located at the tip. The fabrication of WET probe has been accomplished using a commercial splicer machine and the MAC has been realized using ultrashort pulsed laser based micromachining. The WET fiber section facilitates excitation of cladding modes while the MAC in conjunction with the curved tip, provides a convenient platform for achieving core mode as well as cladding mode interference within the probe. The probe, with an optimized WET fiber section, exhibited maximum refractive index (RI) sensitivity of 123.7 nm/RIU with a negligible temperature sensitivity of 15.34 pm/°C.

Saturation and Refractive Index Geometry Effects on Localization of a Spatial Soliton in a Waveguide with Parabolic Rectangular Index Profile

In this paper, we have investigated the effects of well numbers, maximum refractive index variation Δn0, saturable nonlinearity β and soliton amplitude N on behavior of a spatial soliton propagating through a waveguide having a parabolic rectangular index profile. The aim of this work is to find the optimal values for these parameters in order to achieve the best soliton behavior. We found that the best system is one with N = 3, β = 0.4 and well number = 20. Knowing these optimal values helps experimenters to select the optimal system. In addition, to study the behavior of the propagating soliton, localization of the soliton was investigated under variations of a variety of the above-mentioned quantities. Our results show that localization of a soliton for the propagation distance z 10, the effective width (Weff) of the soliton shows an oscillatory behavior. The amplitude of Weff oscillations with increasing well number decreases, and the soliton becomes more localized. In contrast, with increase of β, Weff oscillations increase.

Double Fano resonances and electromagnetically induced transparency-like optical response achieved by breaking the symmetry of two double-split rings

In recent years, various nanostructures related to Fano resonance have been proposed by many researchers due to their outstanding optical properties. A structure consisting of two double-split rings (TD-SR) is researched by using finite element method in this paper. When the incident light is normal to the structure surface, a magnetic Fano resonance can be generated by the interference of bright electric mode and dark magnetic mode. In addition, when the symmetry of the structure is destroyed along x-axis or y-axis, double Fano resonances will be generated. At the same time, the maximum refractive index sensitivity (S) and the maximum figure of merit at resonance mode can reach 1200 nm/RIU and 30.61, respectively. More interestingly, the electromagnetically induced transparency (EIT)-like behavior can be induced when the cavity of inner split ring is biased and the corresponding electric and magnetic field enhancements can reach 229 and 52, respectively. The optical properties of our nanostructure make it has important application value in the fields of multi-wavelength sensor, ultra-sensitive biosensor, surface enhanced spectroscopy and slow light.

Three‐dimensional densification measurement of Vickers‐indented glass using digital holographic tomography

AbstractWe have used digital holographic tomography (DHT) to study the refractive index (density) changes occurring under Vickers indentations in silica, soda‐lime, and non‐alkaline aluminoborosilicate glasses. The measurements confirm that the maximum refractive index (density) is not constant but increases with load up to 300 gf. At higher loads (500 gf and 1 kgf), a subsurface (median) crack in soda‐lime glass is observed with no apparent surface trace. The appearance of this crack reduces the maximum refractive index (density) observed. In addition, the vertical cross section of the 3D refractive index (density) map has been successfully obtained using a square fiber sample and lateral observation. These results clearly demonstrate a promising potential of DHT to evaluate the shape and the density distribution of the structurally modified zone in a Vickers‐indented glass.

High-sensitivity quasi-periodic photonic crystal biosensor based on multiple defective modes.

The sensitivities of the octagonal quasi-periodic photonic crystal (QPC) defective modes are theoretically studied. The octagonal QPC biosensors are composed of silicon columns arranged in a liquid background. By designing a defect structure, a variety of localized modes with different spatial symmetries and field profiles are obtained, and a maximum refractive index sensitivity 800nm/RIU is achieved around 1500nm transmission peak when the central rod's size equals 100nm, and the corresponding detection limit reaches 0.00042. The liquid can flow freely among the rods through the entire structure, so it is convenient to monitor the concentration of protein in the liquid environment dynamically. The influence of the protein's thickness to the shift of the resonant wavelength is also studied, where a minimum protein's thickness of less than 10nm can be detected by optimizing the central column's size to be 400nm, and the spatial field profiles of different resonant modes are analyzed to explain the corresponding sensitivities.

In-plane electrical bias tunable optical properties of 1T-TaS2 [Invited

Electrically tunable optical properties have been demonstrated in many solid-state materials such as semiconductors, transparent conductive oxides and graphene. However, their tunability is limited in the visible range due to the requirement of extremely large charge build-up or high capacitive fields. Here, we propose strongly correlated materials for circumventing such limitations. 1T-TaS2, a strongly correlated material exhibiting charge density order at room temperature, allows tuning of its optical properties with an in-plane electrical bias. The electrical bias causes the charge density waves to slide and thereby alter their coherence and condensation. As a result, the optical conductivity or dielectric function of this layered material changes with an in-plane bias. Here, we report measured anisotropic dielectric functions of mechanically exfoliated thin films of 1T-TaS2 and their electrical tunability. We observe a maximum refractive index change on the order of 0.1 in the visible range with DC and AC in-plane biases.

Refractive index sensor based on a solid-core photonic crystal fiber interferometer

Photonic crystal fiber interferometers are widely used for sensing applications. In this work, solid core-Photonic crystal fiber based on Mach-Zehnder modal interferometer for sensing refractive index was presented. The general structure of sensor applied by splicing short lengths of PCF in both sides with conventional single mode fiber (SMF-28). To apply modal interferometer theory; collapsing technique based on fusion splicing used to excite higher order modes (LP01 and LP11). Laser diode (1550 nm) has been used as a pump light source. Where a high sensitive optical spectrum analyzer (OSA) was used to monitor and record the transmitted. The experimental work shows that the interference spectrum of Photonic crystal fiber interferometer exhibits good sensitivity to refractive index variations. The response of the PCFI is observed for a range of refractive index values from (1.33 to 1.38), the position of the interference peaks is found to be shifted to longer wavelength with refractive index increasing. A different length of PCFs (2, 3, 4) cm were used, and the maximum refractive index sensitivity of (7.5 pm / RIU) is achieved with a PCF length of 4 cm. This refractive index sensor has distinguished properties as that it small size, high sensitivity, fast response time, design flexibility, and immunity to electromagnetic interference.

Strong Fano Resonance Excited in an Array of Nanoparticle-in-Ring Nanostructures for Dual Plasmonic Sensor Applications

A large-area (over 1 cm2) nanoparticle-in-ring array (NPIRA) with strong Fano resonance is fabricated by a low-cost shadow sphere lithography (SSL) method. Controlled narrow nanogaps of 8–20 nm are formed between the nanoparticles (NPs) and rings. Quadrupole modes are excited in the nanogaps and interact with the dipole mode of the NPs and ring, resulting in two Fano resonances. The two Fano resonances are in the visible and near-infrared region, respectively, and show a maximum refractive index sensitivity of 425 nm/RIU and a figure of merit of 4.7–7.1 RIU–1. In addition, the NPIRAs show enhanced Raman intensity due to the strong electric fields confined in the nanogaps. The structure has great potential for the dual plasmonic sensor applications, i.e., surface-enhanced Raman scattering and refractive index sensing, and provides a robust sensing performance for each.

Optical properties of As2S3 layers deposited from solutions obtained by chemical reaction

This paper deals with three approaches for the preparation of arsenic sulfide layers from solutions. First approach employs arsenic sulfide solutions obtained by dissolving powder of arsenic sulfide glass in n-propylamine. Second approach, a novel one, employs amorphous arsenic sulfide precipitated by chemical reaction of arsenic trichloride and ammonium sulfide. The precipitate was dissolved in n-propylamine. Third original approach relays on the same chemical reaction carried out in a mixture of n-propylamine and water that prevents the precipitation. By using all the approaches input arsenic sulfide solutions with a concentration of 0.33 mol/l were fabricated and applied onto glass slides by dip-coating method with withdrawing velocities of 50, 100, 200, and 250 mm/min. Applied layers dried in vacuum at 60 °C for 1 h and thermally treated at 180 °C for 30 s were characterized by optical and atomic force microscopy as well as by transmission spectroscopy in a wavelength range of 300–2500 nm. Refractive indices, thicknesses and band gaps were estimated from measured spectra. A maximum refractive index of about 2.15 at 600 nm and thicknesses up to 220 nm were determined on layers fabricated from input solutions obtained by dissolving of arsenic sulfide glass. Arsenic sulfide layers prepared on the basis of the arsenic sulfide precipitation exhibit refractive indices up around 1.90 and thicknesses up to 410 nm. Photonic band gap values on a level of 2.2 eV have been determined on these layers. On the other hands, composite layers prepared by reaction of arsenic trichloride and ammonium sulfide in the solution of n-propylamine and water exhibited low transparencies, refractive indices around 1.7 and thicknesses of about 2 µm.

Third order nonlinear optical properties of monoclinic and orthorhombic CuNb2O6 under CW laser illumination

Third-order nonlinearity of monoclinic and orthorhombic phase of CuNb2O6 is studied in low power regime of 532 nm by Z-scan technique. In open aperture, monoclinic CuNb2O6 possess stronger saturable absorption due to the distorted structure arising from unequal arrangement of copper atoms. Closed aperture exposes the self-defocusing nature and orthorhombic phase exhibits stronger nonlinear refraction. Variation in third-order NLO coefficient due to change in phase and morphology of CuNb2O6 confirms the influence of sintering temperature and time. Microrod monoclinic CuNb2O6 possess maximum nonlinear absorption coefficient (6.47 × 10−3 cm/W) and pure orthorhombic CuNb2O6 attains maximum nonlinear refractive index (5.13 × 10−8 cm2/W) along with higher third-order nonlinear optical susceptibility (1.91 × 10−6 esu). Higher thermal nonlinearity resulted in stronger optical limiting action with pure orthorhombic CuNb2O6 (5.16 mW, 1.26 mW) compared to other phases. For understanding the material suitability for laser safety device, nonlinearity and limiting action of CuNb2O6 thin films were demonstrated. Earlier formation of monoclinic phase with morphology change (700 °C) and transition of monoclinic rich mixed orthorhombic phase to pure orthorhombic phase (900 °C) with increase in sintering time (3–12 h) was confirmed by XRD, FTIR and SEM analysis. Thus orthorhombic CuNb2O6 material with stronger self-defocusing and optical limiting action makes it more preference than monoclinic CuNb2O6 for third-order NLO applications.

Optical study on single-layer photoluminescent graphene oxide nanosheets through a simple and green hydrothermal method

Single-layer and colloidal graphene oxide (GO) nanosheets were produced by a facile and low-cost hydrothermal route using only water as the solvent. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) images indicated that the obtained GO nanosheets are single-layer with an average diameter of ∼1 nm. The GO nanostructures display relatively good quantum yield (about 3% at the excitation wavelength of 360 nm) and nearly excitation-independent PL spectrum. Four types of oxygen-containing functional groups (COOH, CO, COC, and CO) that affect the fluorescence of GO nanosheets were confirmed by the FTIR spectroscopy. Considering the material response to the electromagnetic wave in the infrared region, we calculated optical constants of the obtained GO nanosheets using Kramers-Kronig relations. The constants include the refractive index, n(ω), and the extinction coefficient, k(ω), which are the real and imaginary parts of the complex function of the refractive index. The most important results are that the CO vibrational energy is maximum and occurs at higher frequencies, and the maximum refractive index is related to the CO bond.