Surrounding Refractive Index Research Articles

Sensing of mercury and silver ions using branched Au nanoparticles prepared by hyperbranched polyethylenimine fabricated and capped AuNPs seeds

Branched AuNPs usually have two or more local surface plasmon resonance (LSPR) absorption bands due to their structural anisotropy, and the LSPR performance is more sensitive to the changes of environmental refractive index than that of spherical AuNPs. The design and preparation of branched AuNPs as colorimetric probes is expected to improve the selectivity and sensitivity of detection of targets. In this paper, branched AuNPs were innovatively synthesized via hyperbranched polyethylenimine (HPEI) fabricated and capped AuNPs as seeds and cetyltrimethylammonium bromide (CTAB) as template agent. The branched AuNPs were characterized by TEM, DLS, zeta potentials and UV–vis spectra. Using the branched AuNPs as a colorimetric probe, the detection system for Hg2+ and Ag+ showed bright color changes from blue to orange and blue to green based on the morphological evolution of branched AuNPs. The branched AuNPs could selectively detect Hg2+ and Ag+ at concentrations as low as 77 and 140 nM, respectively. Moreover, this unusual colorimetric method has been successfully used in real water samples and has great potential as a simple, rapid, sensitive and selective method for the detection of Hg2+ and Ag+.

Optimization of Fiber Bragg Gratings Inscribed in Thin Films Deposited on D-Shaped Optical Fibers.

A fiber Bragg grating patterned on a SnO2 thin film deposited on the flat surface of a D-shaped polished optical fiber is studied in this work. The fabrication parameters of this structure were optimized to achieve a trade-off among reflected power, full width half maximum (FWHM), sensitivity to the surrounding refractive index (SRI), and figure of merit (FOM). In the first place, the influence of the thin film thickness, the cladding thickness between the core and the flat surface of the D-shaped fiber (neck), and the length of the D-shaped zone over the reflected power and the FWHM were assessed. Reflected peak powers in the range from −2 dB to −10 dB can be easily achieved with FWHM below 100 pm. In the second place, the sensitivity to the SRI, the FWHM, and the FOM were analyzed for variations of the SRI in the 1.33–1.4 range, the neck, and the thin-film thickness. The best sensitivities theoretically achieved for this device are next to 40 nm/RIU, while the best FOM has a value of 114 RIU−1.

Whispering gallery mode splitting on filling-based microresonators for refractive index sensing

This paper reports the whispering gallery mode (WGM) splitting behavior as-observed between pairs of size-matched microspheres with tight-binding positions. Dye-doped microspheres were excited by pulsed lasers in order that the WGM laser spectra could be monitored. Via analysis of experimental results, we investigated the effects of spacing between two coupled microspheres on coupling strength, and found that coupling strength was increased with decreasing spacing between the microspheres through mode splitting spectra. We demonstrated the effects of environment refractive index (RI) of coupling spot on mode splitting spectra through dripping liquid on the slit of the bi-spheres, and observed that coupling strength changes with the influence of RI. Furthermore, temperature-dependent experiments showed that temperature change will not disturb the mode splitting behavior, this structure exhibits potential for self-referencing sensing.

A highly sensitive temperature sensor with a PDMS-coated tapered dispersion compensation fiber structure

A highly sensitive optical fiber temperature sensor with a polydimethylsiloxane (PDMS)- coated tapered dispersion compensation fiber (DCF) structure is proposed, the structure consists of single mode fiber (SMF)- tapered dispersion compensation-single mode fiber (STDS). The sensing principle is based on the interference between DCF and SMF due to a large core diameter mismatch. The interference of this composite structure is easily affected by the surrounding refractive index (SRI), which ultimately results in a change in the interference dip of the transmission spectrum. Thermosensitive PDMS is covers the tapered dispersion compensation region, which can increase the sensitivity of the temperature sensor, protect the microstructure, and improve the reliability of the structure in practical application. Experiment temperature results verified that the maximum temperature sensitivity of the sensor was 0.22 nm/°C in a range of 20 °C–100 °C, with good repeatability in this range. Compared with the original fiber sensors and PDMS-coated fibers, the temperature sensitivity increased by 5.2 times and 2.24 times, respectively. This sensor can be realistically applied to biochemical sensing and industrial structural health detection fields due to its compact structure, high sensitivity, and simple processing.

Review of Helical Long-Period Fiber Gratings

In this paper, comprehensive remarks are given that focus on the main fabrications and wide applications of helical long-period fiber gratings (HLPGs). Firstly, the techniques of fabricating HLPGs by CO2 laser, hydrogen–oxygen flame heating, and arc discharge are summarized. Furthermore, the applications of HLPGs are investigated, i.e., orbital angular momentum (OAM) mode converters, all-fiber band-rejection filters, and sensors for measuring physical perturbation of torsion, strain, temperature, curvature, and surrounding refractive index (SRI). Furthermore, several long-period fiber gratings (LPFGs) of near-HLPG structures with periodic refractive index change along the azimuthal direction are introduced. Lastly, the prospects and key challenges for HLPGs are discussed.

Turn Around Point Long Period Fiber Gratings With Coupling to Asymmetric Cladding Modes Fabricated by a Femtosecond Laser and Coated With Titanium Dioxide

A detailed study of turn around point (TAP) long period fiber gratings (LPFGs) with coupling to the asymmetric cladding modes of a standard single-mode fiber (SMF-28e), fabricated by femtosecond (fs) laser direct writing was realized. The entire fabrication process, including the coating with different titanium dioxide (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) film thicknesses of LPFGs and the corrections needed to achieve coated devices operating precisely in the TAP condition with coupling to the asymmetric cladding modes, was addressed. The significant fabrication details are also given, such as inscription periods, shape and localization of the refractive index modifications across the core. The fabrication process described allows the optimization of the LPFGs sensitivity in regards to the surrounding refractive index (SRI). Optimization of the writing parameters to obtain gratings working at the TAP for two different media surrounding the fiber (water and air) was achieved. It was demonstrated that for a grating period of 191.8 μm, the LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,12</sub> mode exhibits a TAP at 1442.7 nm in air, and for a period of 192.5 μm, the same mode exhibits a TAP at 1448.6 nm in water. The LPFGs operating at the TAP in air and water were coated with 10, 20, and 30 nm thin TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film thicknesses and the spectral behavior characterized. The wavelength sensitivity to the surrounding refractive index (SRI) was assessed in the range between 1.3700 to 1.4120, and a maximum sensitivity of ~8051.4 nm/RIU was measured for the 192.5 μm LPFG coated with a 30 nm thick TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film.

Active Manipulation of Fano Resonance at Visible and Near-Infrared Wavelengths in Metal Plasmonic Nanodevices Using Graphene

Active manipulation of Fano resonance at visible and near-IR wavelengths in metal nanodevices is one of the important challenges for applications such as chemical and biological sensing. Here, we theoretically research an active manipulation of Fano resonance at visible and near-IR wavelengths in gold plasmonic nanodevices with graphene. The surface plasmon resonance of the gold plasmonic nanodevice with graphene has three resonance peaks, and this can be explained by the distribution of the electric field in the nanodevice. The Fano resonance wavelength of the gold plasmonic nanodevice with graphene has a significant blue-shift compared with the gold nanodevices without graphene. Moreover, the Fano resonance depends on the length and position of Au nanorods and the environment refractive index. The figure of merit of the gold nanodevice with graphene can be as high as 44.1, which makes the system suitable for high sensitivity applications. Finally, we actively manipulate the absorption spectrum and the reflected light phase through changing the Fermi energy of graphene. These results suggest an original method for the design of an actively manipulated Fano resonance nanodevice.

Gas environment independent temperature sensor via double-metal surface plasmon resonance.

As the sensitivity of the optical temperature sensor increases, one of the most important noise sources may be from the fluctuation of the surrounding gas environment (refractive index change). In this work, we have designed and fabricated an optical temperature sensor with a device size of 15 μm2. The sensor is constructed by a titanium dioxide grating on top of a double-metal surface plasmon resonance (SPR) structure. Our design can provide minimal gas environment dependence without compromising the performance in terms of temperature sensitivity. In addition, the design also facilitates a generous dimensional tolerance in the device fabrication. Based on the design, a proof-of-concept device has been fabricated and characterized. The obtained sensitivity of the fabricated sensor reaches 135 pm/℃. Meanwhile, the measured resonance wavelength shift is ∼0.004 pm in different gases (air, CH4, and CO2). The presented temperature sensor should be convenient and valuable for high-accuracy temperature measurements and integrated opto-electronic sensing chips.

Surrounding refractive index sensing characteristics of the few-mode cladding-reduced long period fiber grating

In order to improve the sensitivity of the few-mode long period fiber grating (FM-LPFG) in the surrounding refractive index (SRI) measurement, this paper proposes a few-mode cladding-reduced long period fiber grating (FM-CRLPFG) sensor which is fabricated by using CO2 laser. When the cladding radius decreases, a three-layer dielectric fiber model is vital to solve the effective refractive indices of the core modes (LP01 and LP11) in few-mode fiber (FMF). Through analyzing the influences of cladding radius on the resonant wavelengths of core modes LP01 and LP11, it is found that the smaller the radius is, the higher the SRI sensitivity is. In the experiment, the cladding radius was etched by HF solution, and in the meantime the transmission spectrum in corrosion process was observed constantly. Furthermore, by comparing the sensing characteristics of the resonant peak before and after etching, it is found that the SRI sensitivity can be improved effectively by reducing the cladding radius. The SRI sensitivity after etching is about − 850.80 nm/RIU, which is 1.64 times higher than that before etching. The research on the SRI sensing characteristics of FM-CRLPFG can provide guidance for designing the high sensitivity sensor.

Near-infrared plasma cavity metasurface with independently tunable double Fano resonances

We report a metasurface device based on nano metal-dielectric-metal plasmon cavity arrays. Investigation of the proposed device indicates the sharp absorption peak of double Fano resonance and multiple periods of time-domain oscillations caused by the three-dimensional double resonance. By adjusting the structure parameters, different absorption patterns can be regulated independently with high degree of freedom. The metasurface exhibits sensitive behavior to the environmental refractive index and possesses the sensitivity of 667 nm RIU−1 as a refractive index sensor in the mid-near infrared band, which can be further considered for glucose or temperature sensing.

Multi-band, tunable, high figure of merit, high sensitivity single-layer patterned graphene—Perfect absorber based on surface plasmon resonance

This paper propose a perfect absorber with multi-band, adjustable, high figure of merit (FOM) and high sensitivity is based on single-layer patterned graphene surface plasmon resonance (SPR). The wave absorber is composed of a patterned graphene structure etched in a circular shape in the middle and a bottom metal film separated by a SiO2 dielectric layer. It has simple structural features and can greatly simplify the manufacturing process. In the mid-infrared band of 3 μm ~ 5 μm, the numerical results of FDTD method show that the absorbers have three perfect absorption peaks, which are λ1 = 3275.31 nm, λ2 = 3706.12 nm and λ3 = 4481.76 nm, respectively. The absorption rates are 99.44%, 98.22% and 99.10%, respectively. The resonant wavelength of the absorber can be tuned by controlling the Fermi energy level and relaxation time of the graphene layer. In addition, the wave absorber is insensitive to polarization and can keep high absorption in a wide range of incident angles from 0° to 50°. At last, we explore the sensitivity and FOM of the absorber by changing the environmental refractive index. The results show that the sensitivity of its three resonance absorption peaks is 666.75 nm/RIU, 760.50 nm/RIU and 907.88 nm/RIU (RIU is the per refractive index unit), and the FOM is 86.82, 53.03 and 56.14, respectively. Therefore, we believe that the absorber can be used in the fields of narrow-band thermal radiation, narrow-band light detection and narrow-band sensor.

Theoretical analysis of dual-mode long period fiber grating sensor based on dual-peak resonance for dual-parameter measurement

A dual-mode long period fiber grating (DM-LPFG) sensor for simultaneous measurement of the surrounding refractive index (SRI) and temperature is proposed and demonstrated theoretically. It is made by inscribing a LPFG based on the dual-peak resonance in a dual-mode fiber. Known from coupled mode theory, the sensing principle of the sensor is originated from the sensibility of the dual peaks to the SRI. There are three peaks corresponding to the dual-peak resonance and single-peak resonance in different core modes in the transmission spectrum, respectively. And the three peaks have a different response to the SRI and temperature. Through the monitoring of the distance change between dual peaks and the wavelength shift of single peak, the dual-wavelength matrix method can be used to simultaneously measure the SRI and temperature. As the results show, the SRI sensitivities corresponding to the distance between the dual peaks and the resonant wavelength of the single peak are 1940.0nm/RIU and 590.0nm/RIU in the SRI range of 1.33 to 1.37, respectively. Besides, the temperature sensitivities corresponding to the distance between the dual peaks and the wavelength of the single peak are -0.42nm/°C and 0.23nm/°C with the temperature changing from 20°C to 120°C, respectively. It has great potential application in chemical and biological sensing fields, owing to its advantages of high sensitivity and multi-parameter measurement.

Theoretical design of a triple-band perfect metamaterial absorber based on graphene with wide-angle insensitivity

This paper proposes a square ring-shaped triple-band perfect absorber based on graphene with silicon as the substrate, gold as the reflector and a thin dielectric layer of silicon dioxide. After simulation by the FDTD method, the numerical results show that the absorption spectrum based on the periodic array structure of graphene exhibits three perfect absorption peaks in the terahertz band, corresponding to 25.53 μm, 36.44 μm and 53.44 μm, respectively, and they all reach 99% absorption rate. By changing the gate voltage, the material parameters of graphene can be easily adjusted, so as to achieve dynamic control of the absorption characteristics of the absorber. Due to the high symmetry of the structure, it has a good angular polarization tolerance in the far infrared band. In addition, the obvious spectral shift caused by the change of surrounding environment refractive index and the values of sensitivity (S) and figure of merit (FOM) are all indicates that our absorber has certain application prospects in the field of sensing. Furthermore, our absorber has shown great application potential in light detection in infrared and terahertz band as a consequence of the strong tunability.

Distributed Bragg reflectors from colloidal trilayer flake solutions

The fabrication of highly reflective inorganic distributed Bragg reflectors (DBRs) from aqueous solutions of colloidal flakes is demonstrated. Our approach involves the deposition of compact or mesoporous TiO2–SiO2–TiO2 trilayers onto a patterned sacrificial layer. A subsequent etch-release of the patterned flakes into water results in a colloidal flake solution. Drops of this flake-containing solution are then deposited onto another substrate, where they self-assemble into stacks upon solvent evaporation. This method gives rise to high quality DBRs in a much quicker fabrication process compared to previously established methods and produces ordered DBRs of high reflectivity. Changing compact for mesoporous assemblies, the produced DBRs can be used for environmental refractive index sensing. The presented approach may open the way for the on-demand integration of stimuli-responsive DBRs into microfluidic and fiber-end sensing applications or allow the integration of highly reflective colloidal flakes with light emission and detection technologies.

Fiber optic Lossy Mode Resonance based sensor for aggressive liquids

A fiber-optic sensor for chemically aggressive environments, such as concentrated solutions of acids and alkalis, has been manufactured. The sensor is based on a single-mode optical fiber with a chemically etched cladding, coated with a thin film of tin dioxide (SnO2) by metalorganic chemical vapor deposition (MOCVD). In the transmission spectra of a SnO2 coated taper lossy mode resonances (LMRs) were observed, the maximum of which shifted to the long-wavelength part of the spectrum, with an increase in the thickness of the deposited film. At a deposition temperature of 345 °C, it was possible to detect the separation of the transverse electric (TE) and transverse magnetic (TM) LMRs for resonances up to the third order. From the change in the position of the 1-st TE LMR, the refractive index and the molar concentration of acids and alkalis in such aggressive liquids as aqueous solutions of HCl, HNO3, H2SO4, NaOH were determined, which, according to our information, is demonstrated for the first time. The high chemical durability of the coating was confirmed by its insusceptibility to aqua regia (AR). The sensitivity of the sensor to changes in the surrounding medium refractive index (SMRI) and the molar concentration of acids was more than 5260 nm/RIU and 32 nm/M, respectively.

Lossy Mode Resonance Sensors Based on Double ITO/TiO2 and Triple TiO2/ITO/TiO2 Thin Film Coated Single Mode Fiber Taper

Based on the previously developed computer program for rigorous calculation of modes in optical fibers containing layers of various materials of arbitrary, including nanometer, thickness, the calculation and optimization of single-mode double adiabatic tapers covered with one, two and three-layer films: ITO, ITO/TiO2, TiO2/ITO/TiO2 of equal thickness is performed, with the aim of their application as the environment refractive index sensors. It is shown that in the range of wavelengths 1.00–1.6 µm, two, and in particular, three-layer coatings not only can increase the sensitivity, but also significantly reduce the width of the lossy mode resonance (LMR), which, in turn, leads to a significant increase in the figure of merit (FOM) of the sensor, defined as the sensitivity/width of the LMR.

The Influence of Dielectric Environment on Spectral Shift of Localized Plasmonic Resonance

In paper the investigation results of dielectric environment influences on spectral shift of localized plasmonic resonance in a system of self-assembled metallic nanoparticles was introduced. It was shown, that the value of spectral shift of localized plasmonic resonance has a linear proportion towards the changes of environment refractive index. For investigated plasmonic system of self-assembled Au nanoparticles the coefficient of proportionality was determined.

Ultra-sensitivity refractive sensor based on graphene material at mid-infrared wavelengths

As a tunable and electrically adjustable 2D material, graphene opens a way for manipulating mid-infrared waves and corresponding devices. A kind of configuration of tunable diffractive grating based on mono- and multi-layer graphene material working at mid-infrared wavelengths is presented in this paper. A potential application of the diffractive grating based on graphene material is as a highly sensitive sensor for surrounding environment refractive index, and the sensitivity up to 7370 nm/RIU can be achieved, which is the highest sensitivity among refractive sensor proposed before to our best knowledge. This research paves the way toward ultra-sensitive sensor devices at mid-infrared wavelengths.

Lossy mode resonance in an etched-out optical fiber taper covered by a thin ITO layer.

The results of the rigorous calculation of mode fields in double adiabatic, single-mode etched-out optical fiber tapers coated with thin indium tin oxide films are discussed in the context of their application as environment refractive index sensors. It is shown that at only two particular thicknesses covering the homogeneous section of the taper ITO film about 100 nm and 177 nm, the lossy mode resonance is observed in the wavelength range of 1.50-1.55 µm. Moreover, the sensitivity of a sensor based on a 177 nm coating is significantly higher, and the resonance width is significantly lower than that of a sensor with a 100 nm coating. Optimal from the viewpoint of the figure of merit, values for the diameter of a homogeneous section of the etched fiber are defined.

The manufacturing process and spectral features of tilted fiber Bragg gratings

The controllable cross-sensitivities of tilted fiber Bragg gratings (TFBGs) are strongly related to the control of the parameters during the manufacturing process. The TFBG spectrum characterization, which can demonstrate cross-sensitivities, is controlled by the process parameters. Cross-sensitivities analyses in biochemical surrounding refractive index (SRI) (cladding mode) and mechanical sensing (Bragg and ghost mode) are evaluated through processing parameters, including the rotating tilted angles of phase-mask, laser exposure dose time, and reducing fiber diameters. The results of this study show the cross-sensing ability in both biochemical and mechanical sensing that can be caused by a controlled tilted angle was 10° and exposure dose of five minutes while obtaining the cladding and Bragg modes of TFBGs with broad spectral coverage.