Refractive Index Of Surrounding Medium Research Articles

Optimization of Sensors Based on Multimode Interference in Single-Mode–Multimode–Single-Mode Structure

By coating a single-mode-multimode-single-mode (SMS) structure with a complex refractive index thin-film the central wavelength of the transmission and attenuation bands of the optical spectrum can be shifted. By adequate selection of multimode segment length and diameter, the thin-film thickness and the thin-film refractive index, the sensitivity of the device to the surrounding medium refractive index, the thickness of the thin-film or the refractive index of the thin-film can be optimized. Experimental results are corroborated numerically with the transfer matrix method.

Scattering Efficiency and LSPR Tunability of Bimetallic Ag, Au, and Cu Nanoparticles

Scattering efficiencies of Ag–Cu, Ag–Au, and Au–Cu alloy nanoparticles are studied based on Mie theory for their possible applications in solar cells. The effect of size (radius), surrounding medium, and alloy composition on the scattering efficiency at the localized surface plasmon resonance (LSPR) wavelengths has been reported. In the alloy nanoparticles of Ag1−x Cu x , Au1−x Cu x and Ag1−x Au x ; the scattering efficiency gets red-shifted with increase in x. Moreover, the scattering efficiency enhancement can be tuned and controlled with both the alloy composition and the surrounding medium refractive index. A linear relationship which is in good agreement to the experimental observations between the scattering efficiency and metal composition in the alloys are found. The effect of nanoparticle size and LSPR wavelength (scattering peak position) on the full width half maxima and scattering efficiency has also been studied. Comparison of Au–Ag, Au–Cu, and Ag–Cu alloy nanoparticles with 50-nm radii shows the optical response of Ag–Cu alloy nanoparticle with wide bandwidth in the visible region of the electromagnetic spectrum making them suitable for plasmonic solar cells. Further, the comparison of Ag–Cu alloy and core@shell nanoparticles of similar size and surrounding medium shows that Cu@Ag nanoparticle exhibits high scattering efficiency with nearly the same bandwidth.

Liquid level sensor based on an excessively tilted fibre grating

We propose and demonstrate an optical liquid level sensor based on the surrounding medium refractive index (SRI) sensing using an excessively tilted fibre grating (ETFG). When the ETFG submerged in water, two sets of cladding modes are coupled, corresponding to air- and water-surrounded grating structures, respectively. The coupling strengths of the two sets of cladding modes evolve with the submerging length of the grating, providing a mechanism to measure the liquid level. Comparing with long-period fibre grating based liquid level sensor, the ETFG sensor has a much higher SRI responsivity for liquids with refractive index around 1.33 and a lower thermal cross sensitivity.

Mode transition in complex refractive index coated single-mode–multimode–single-mode structure

By coating a single-mode-multimode-single-mode (SMS) structure with a high refractive index thin-film it is possible to obtain a transition of modes for specific combinations of thin-film thickness, thin-film refractive index and surrounding medium refractive index, which permits to develop devices with a high sensitivity to specific parameters. In order to gain a better knowledge of the phenomenon the experimental results are corroborated numerically with the Transfer-Matrix-Method. The influence of losses in the thin-film has also been studied. The results obtained prove that a thin-film coated SMS structure is a simple and cost-effective platform for development of sensors and optical filters.

Liquid Refractive Index Sensor Based on a Polarization-Maintaining Fiber Loop Mirror

A liquid refractive index sensor based on a fiber loop mirror (FLM) combined with an etched polarization maintaining fiber (PMF) is proposed. As the etched section of the PMF is immersed in liquid with different refractive indices, the interference spectrum of the FLM presents high sensitivity to the surrounding-medium refractive index. High sensitivity of 25.29 nm/refractive index unit (RIU) and resolution of $7.91\times 10^{-4}$ RIU are achieved for the proposed refractive index sensor. This sensor is also capable of simultaneous measurement of refractive index and temperature.

Considerations for Lossy-Mode Resonance-Based Optical Fiber Sensor

This paper presents the fabrication of optical fiber relative humidity (RH) sensors by means of polymeric thin-films as lossy-mode resonance (LMR) supporting coatings. In particular, LMRs are observed because of the fabrication of polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) polymeric coatings onto cladding-removed multimode optical fibers on applying the layer-by-layer self-assembly method. More than one LMR can be generated as a function of the coating thickness. The generation of multiple LMRs produces notch band filter with a rejection band in the visible region. To compare the parameters of different LMRs, two devices with PAH/PAA coating thicknesses of 140 and 470 nm, respectively, are fabricated. The thickness and the refractive index of the PAH/PAA film can be modified as a function of the surrounding medium refractive index. Thus, surrounding medium relative humidity can be easily determined through the wavelength shift of the resonance (~ 34 nm between 20% and 80% RH).

Theoretical and Experimental Study on Nonintrusive Light Injection Via Cladding in Plastic Optical Fibers

Based on the fiber macrobending and the refractive index matching technologies, a novel scheme of nonintrusive light injection, namely, the light signal is injected into the fiber core from the fiber cladding without any destruction, is proposed in plastic optical fibers (POFs). Using the ray-tracing method, a 3-D theoretical model is established to characterize the performance of the light injection. The influences of the fiber bending radius, the light source placement, and the surrounding medium refractive index on the light injection efficiency are investigated and assessed. Meanwhile, correlative experiments have also been conducted to contrast theoretical simulations. The experiment results fit theoretical ones well. This nonintrusive light injection technology in POFs might have many potential applications such as the optical signal uploading and downloading, the optical coupling, and the local area networks.

Design rules for lossy mode resonance based sensors

Lossy mode resonances can be obtained in the transmission spectrum of cladding removed multimode optical fiber coated with a thin-film. The sensitivity of these devices to changes in the properties of the coating or the surrounding medium can be optimized by means of the adequate parameterization of the coating refractive index, the coating thickness, and the surrounding medium refractive index. Some basic rules of design, which enable the selection of the best parameters for each specific sensing application, are indicated in this work.

Rational Selection of Nanorod Plasmons: Material, Size, and Shape Dependence Mechanism for Optical Sensors

The localized surface plasmon resonance dependence on surrounding medium refractive index of Ag, Al, Au, and Cu nanoparticles is examined by electrodynamic approach. The refractive index sensitivity and sensing figure of merit (FOM) dependence of selected metal nanoparticles with similar geometry shows that although, sensing relevant parameters are shape (i.e., aspect ratio), and material dependent below the width 20 nm, but above this size these parameters are material independent under similar geometrical conditions. We have concluded that at optimum size, however, Al shows much higher refractive index sensitivity (RIS) in comparison to Au, Cu, and Ag, but FOM is higher for Ag in comparison to other metals. The observed sensing behavior is expected due to parameters like surface scattering, dynamic depolarization, radiation damping, and interband transitions, which may influence the nanorod plasmons.

Tunable plasmonic properties of silver nanorods for nanosensing applications

Localized surface plasmon resonance (LSPR) sensitivity to the surrounding medium refractive index has been studied for silver nanorods using Gans theory including the effect of retardation and surface scattering. The simulation results show the refractive index sensitivity (eV/RIU) maxima positions at width of 9, 6, and 4 nm for aspect ratios of 2, 3, and 4, respectively. Based on the sensing figure of merit (FOM), 9 nm is found to be a significant nanorod width, where the FOM dependence on width with respect to aspect ratio inverts. However, the optimal nanorod width for both the FOM and the modified figure of merit (MFOM) is about 6 nm for aspect ratios of 2, 3, and 4. A comparison with gold shows that silver nanorods exhibit relatively higher FOM and MFOM and thus, making them potential candidates for biochemical nanosensing applications.

A novel high-sensitivity pressure sensor: Thinned fiber Bragg grating with pressure-sensing polymer

Based on thinned fiber Bragg grating and polymer material, we propose a novel high-sensitivity pressure sensor and obtained a new contribution to the pressure sensitivity, which is dependent on the derivative of the effective refractive index (RI) of core mode with respect to the surrounding medium RI, as well as on the relationship between the polymer RI and the pressure; moreover, it is inversely proportional to Young’s modulus of the polymer material. For the polymer with Young’s modulus of 1.0 MPa, the total pressure sensitivity 1.54×10−2 MPa−1 can be obtained.

Transition mode long period grating biosensor with functional multilayer coatings

We report our latest research results concerning the development of a platform for label-free biosensing based on overlayered Long Period Gratings (LPGs) working in transition mode. The main novelty of this work lies in a multilayer design that allows to decouple the problem of an efficient surface functionalization from that of the tuning in transition region of the cladding modes. An innovative solvent/nonsolvent strategy for the dip-coating technique was developed in order to deposit on the LPG multiple layers of transparent polymers. In particular, a primary coating of atactic polystyrene was used as high refractive index layer to tune the working point of the device in the so-called transition region. In this way, state-of-the-art-competitive sensitivity to surrounding medium refractive index changes was achieved. An extremely thin secondary functional layer of poly(methyl methacrylate-co-methacrylic acid) was deposited onto the primary coating by means of an original identification of selective solvents. This approach allowed to obtain desired functional groups (carboxyls) on the surface of the device for a stable covalent attachment of bioreceptors and minimal perturbation of the optical design. Standard 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide / N-hydrosuccinimide (EDC / NHS) coupling chemistry was used to link streptavidin on the surface of the coated LPG. Highly sensitive real-time monitoring of multiple affinity assays between streptavidin and biotinylated bovine serum albumin was performed by following the shift of the LPGs attenuation bands.

Nonlinear optical theory and figure of merit of surface plasmon resonance of gold nanorods

A nonlinear theory for the optical properties of gold nanorods at various aspect ratios is used to calculate the refractive index sensitivity of surface plasmon resonance (M) which is found to be 100 to 1000 nm/RIU and proportional to the aspect ratio (R). Based on Gans theory, the calculated figure of merit, defined by the ratio of M and the resonance spectral width, has a range of 1.0 to 10 and has a maximum value at the optimum aspect ratio of 3.5 to 4.5. Numerical results are fit for analytic equations for the peak wavelength and sensitivity showing their nonlinear dependence on the surrounding medium refractive index (n) and R. The calculated optimal condition for the figure of merit provides useful guideline for the design of biosensors.

Optical Fiber Refractometers based on Indium Tin Oxide Coatings with Response in the Visible Spectral Region

This work presents the fabrication of optical fiber refractometers based on indium tin oxide (ITO) coatings with response in the visible spectral region. ITO thin-films have been sputtered by employing a rotating mechanism that enables the fabrication of smooth homogeneous coatings onto the optical fiber core. The ITO coated optical fiber devices present several resonances in the visible and infra-red region. These resonances show high optical power attenuations (more than 10dB) in the visible spectral region, which produces changes in the colour of the output visible light. Therefore, since these resonances shift as a function of the surrounding medium refractive index (SMRI), it is feasible to determine the refractive index of the outer medium in contact with the ITO coating by simply monitoring the chromatic coordinates of the visible output light.

Fiber Laser Incorporating an Intracavity Microchannel for Refractive Index and Temperature Sensing

In this letter, we present a standard linear cavity fiber laser incorporating a microchannel for refractive index (RI) and temperature sensing. The microchannel of ~6-mum width created by femtosecond laser aided chemical etching provides an access to the external liquid; therefore, the laser cavity loss changes with the liquids of different RIs. Thus, at a fixed pump power, the output laser power will vary with the change of RI in the microchannel. The results show that the proposed sensing system has a linear response to both the surrounding medium RI and temperature. The RI sensitivity of the laser system is on the order of 10-5, while the temperature sensitivity is about 0.02degC. Both sensitivities could be further enhanced by employing a more sensitive photodetector and using higher pump power.

Not-lithographic fabrication of micro-structured fiber Bragg gratings evanescent wave sensors

This work is devoted to present and to demonstrate a novel approach for the fabrication of micro-structured fiber Bragg gratings (MSFBGs) to be employed as technological platform for advanced optochemical sensors. Basically, the MSFBG consists in a localized SRI sensitization of the grating by deep cladding stripping. The introduction of a perturbation or defect along the grating leads to the formation of a defect state inside the FBG spectral response that is tunable through the surrounding medium refractive index. While its spectral features for sensing and communication applications have been widely described and commented elsewhere, here a simple fabrication procedure is presented as suitable technological assessment enabling cost effective and simple MSFBG production. It relies on a two steps technique based on arc-discharge procedure as fiber pre-treatment and mask-less wet chemical etching to locally sensitize the FBG to external refractive index. The new, simple and low-cost approach overcomes some technological drawbacks related to previous fabrication techniques adopting patterned masking procedures during the etching process. This work demonstrates the effectiveness of the proposed method reporting a detailed description of single and two defects MSFBG fabrication.

Determination of height distribution on a rough interface by measuring the coherently transmitted or reflected light intensity

In this work it is shown theoretically and examined experimentally that the measurement of coherently transmitted or reflected monochromatic light intensity from a randomly rough interface as a function of incident angle provides the height distribution on the interface. It is also shown that the spectrum of coherently transmitted or reflected light from a rough interface is modified and the modified spectrum yields the height distribution. The experimental results obtained by applying both methods, in transmission and reflection, on rough surfaces prepared by roughening the sheet glasses by powders of different grain sizes are quite consistent. In addition, the effect of the surrounding medium's refractive index on the roughness measurement is studied by immersing the samples into liquids of different refractive indices. Also, the application range and limitations of the introduced methods are discussed.

High birefringence D-type fibre loop mirror used as refractometer

A high birefringent D-type fibre loop mirror used as refractometer is reported. A chemical etching was applied to the D-type fibre and presented high sensitivity for the surrounding medium refractive index. The refractometer was tested with different refractive indices in liquids and characterized in temperature, showing a linear and independent response to both parameters. With these results the refractometer can perform simultaneous measurement of refractive index and temperature.

Spectral behavior of thin film coated cascaded tapered long period gratings in multiple configurations

In this work the spectral response of cascaded tapered long period gratings coated by nano-sized polymeric films has been investigated as function of the surrounding medium refractive index (SRI). The investigation was aimed to identify the best configuration in terms of coated/not coated areas in order to fully benefit of the SRI sensitivity enhancement due to the modal transition mechanism of nano-coated long period gratings while preserving the fringes visibility.

Controlling gold nanoparticle assemblies for efficient surface-enhanced Raman scatteringand localized surface plasmon resonance sensors

Gold nanoparticle assemblies with different particle densities were immobilized uponfunctionalized glass substrates and their morphology and optical properties are analysedwith TEM and UV–vis absorption spectroscopy. The efficiency of self-assembledgold colloidal nanoparticles as SERS-active substrates at 633 nm excitation line isevaluated by employing p-aminothiophenol. The average enhancements in the range of105–107 are consistent with signals being dominated by molecules adsorbed at junctions insidenanoparticle assemblies. We employ an annealing treatment to induce changes in the initialmorphology of gold assemblies and produce sensitive LSPR substrate. The sensitivity of theas-prepared substrate to transduce changes of the surrounding medium refractive index isfurther assessed.