Low Refractive Index Region Research Articles

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 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.

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.

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.

Refractive index sensor based on multimode interference in a twin-hole fiber

We report a simple refractive index (RI) sensor based on a twin-hole fiber (THF). The THF is fused with the single-mode fiber to realize sensing by the multimode interference. The mode transmission characteristics in the THF are numerically investigated. In the sensing experiment, when the surrounding RI changes, the wavelength shift of the transmission spectrum shows approximate linearity in the low RI region and nonlinearity in the high RI region. When a THF with the length of 10 mm is used, the highest sensitivity can be up to 1351 nm / RIU in the RI range from 1.430 to 1.445. Moreover, the effect of the temperature on the transmission spectrum relative to the RI is much smaller when the external temperature changes from 30°C to 80°C.

Double-sided Polished Ultra-stable and Ultra-sensitive Optical Fiber Sensor

A double-sided polished photonic crystal fiber (DP-PCF) for surface plasmon resonance(SPR) sensing is designed and analyzed by using the finite element analysis software COMSOL 5.5. The suggested design shows strong sensitivity stability in the low refractive index (RI) region ( $1.33\sim 1.39$ ) and ultra-high sensitivity in the high RI region ( $1.39\sim 1.43$ ), respectively. The structural parameters of the proposed sensor are studied to analyze the stability and high sensitivity of the fiber sensor. The introduced DP-PCF sensor offers high sensitivity of 32,434 nm/RIU in the RI range of $1.39\sim 1.43$ due to its special double-sided polishing design.

Surface plasmon resonance sensor based on a D-shaped photonic crystal fiber for high and low refractive index detection

A D-shaped photonic crystal fiber sensor based on surface plasmon resonance effect is proposed in this paper. Experimental results show that the D-shaped sensor can perform sensing detection in high or low refractive index regions. In addition, the refractive index of different analytes can be double calibrated by peaks and troughs. For high refractive index region from 1.3621 to 1.4010, the average sensitivity of the peaks and troughs is only 30.0 nm/RIU and 72.7 nm/RIU, respectively. However, in the low refractive index region of 1.3415–1.3468, the average wavelength sensitivity reaches 842.6 nm/RIU. The research in this paper has important reference value and positive scientific significance for exploring novel D-shaped photonic crystal fiber sensors.

Dispersion tailoring of silicon nanowire optical rectangular waveguide (SNORW)

Dispersion analysis on silicon nanowire optical rectangular waveguide (SNORW) has been presented in this paper by guiding light inside low refractive index region. Dispersion engineering is an essential study to utilize any photonic integrated circuit-based waveguide in linear and nonlinear optical devices. This paper exhibits distinctive dispersion characteristics recognized in the wavelengths of S, C and L bands by tailoring physical parameters of SNORW. Modal investigation and numerical analysis have been carried out by finite element method (FEM), which shows that SNORW configuration enables the flat and low negative dispersion behavior. This paper also demonstrates the utilization of cladding materials and structural parameters of SNORW to tune the magnitude and behavior of dispersion for flat dispersion profile.

Polarization-Dependent Tuning Property of Graphene Integrated Tilted Fiber Bragg Grating for Sensitivity Optimization: A Numerical Study

In this paper, the general characteristics of the polarization-dependent tuning property and sensitivity optimization of the graphene integrated tilted fiber Bragg grating (TFBG) are investigated theoretically. The tuning mechanism of the graphene on mode characteristics and transmission spectrum is thoroughly explored. The results reveal that the graphene induces a stronger influence on the s-polarized mode than its p-polarized counterpart, but the TFBGs with or without graphene in all the polarized states have nearly the same wavelength sensitivity ( $\sim \text{540}\; \text{nm/RIU}$ in average). In contrast, the resonance strength is highly dependent on the tunable state of the graphene, especially for the s-polarized case. In consequence, the normalized power response is increased in the low refractive index region. More importantly, the s-polarized leaky mode resonance that is ignored for regular TFBGs is significantly enhanced (by increasing field content in the fiber region and decreasing the confinement loss) whereas the guided counterpart is suppressed greatly by the graphene with lower chemical potential. The sensitivity of up to $ \text{6595.8 dB/RIU}$ (9.17 times higher than that of the regular counterpart) is achieved in response to an extremely small analyte perturbation ( $ \text{10}^{-\text{4}}$ close to 1.333) based on the graphene enhanced leaky mode resonance. This paper provides a novel technique for significantly optimizing the sensing performance of the TFBG, which has great potential in biological and biochemical sensing applications.

Titanium Dioxide Film Coated Excessively Tilted Fiber Grating for Ultra-Sensitive Refractive Index Sensor

In this paper, we report a sensitive refractive index sensor based on the titanium dioxide (TiO $ _2$ ) film coated excessively tilted fiber grating (Ex-TFG) operating around the dispersion tuning point. The effects of the TiO $ _2$ film on the cladding mode field, transmission, and sensing characteristics of the Ex-TFG are outlined. It is found that the TiO $ _2$ film induces a two-step mode transition process for both TE/TM $ _{0,l}$ and HE/EH $ _{v,l}$ ( $ v=1,2,\ldots$ ) cladding modes. During the first transition, the same optimized film thickness can be obtained for TE $ _{0,l}$ , HE $ _{1,l}$ , and EH $ _{2,l-1}$ cladding modes. The results show that the sensitivity can be optimized based on the combined effects of the intermodal separation, initial wavelength separation, and the variation in the phase matching curve (i.e., $ \partial \Lambda / \partial \lambda$ ). The improved sensitivities are up to $ 32261.2\;\text{nm}/\text{RIU}$ and $ 23183.3\;\text{nm}/\text{RIU}$ by tracking dual resonance band corresponding to the s-polarized TE $ _{0,10}$ and p-polarized EH $ _{2,9}$ cladding modes, respectively, in the low refractive index region. For the single resonance corresponding to the s-polarized HE $ _{1,9}$ cladding mode, the sensitivity as high as $ 25860.0\;\text{nm}/\text{RIU}$ is achieved. The sensitivity is significantly enhanced by a factor of $ 13.2 \sim 18.1$ and $ 3.7 \sim 5.2$ times as compared with the uncoated Ex-TFG and classical TiO $ _2$ film coated long period fiber grating with reduced fiber diameter, respectively. The sensitivity can be further improved, which makes the film coated Ex-TFG very attractive for the refractive index sensing.

Sensitivity Enhancement of Excessively Tilted Fiber Grating by Inner Cladding Perturbation

In this paper, we present theoretically a general method to enhance the sensing performance of the excessively tilted fiber grating (Ex-TFG) by inner cladding perturbation (ICP). The Ex-TFG is tuned to operate at the dispersion tuning point of the s-polarized TE0, m and HE2, m cladding modes. It is found that the sensitivity is resonance strength dependent when the sensing performance is evaluated by power measurement. The results show that the resonance strength and hence the sensitivity can be greatly improved by ICP together with tailoring the grating length. By this approach, we demonstrate, to the best of our knowledge, the highest sensitivity (18 742.3 dB/RIU) in the low refractive index region for the fiber grating-based sensors. In addition to this, the full-width at half-maximum and figure of merit show advantages over their counterparts of standard Ex-TFG. It is believed that this approach can be used to optimize the sensitivity when the Ex-TFG operates at other surrounding refractive index that is even than the cladding refractive index. This makes the Ex-TFG a very attractive platform for a wide range of sensing applications.

A Slotted Photonic Crystal Nanobeam Cavity for Simultaneous Attainment of Ultra-High Q-Factor and Sensitivity

An optical refractive index (RI) sensor is proposed based on a slotted photonic crystal nanobeam cavity. Three rows of parabolic-tapered air-holes are used to design the cavity and an embedded narrow slot is used to obtain a highly localized optical field in a low refractive index region in order to produce a strong light matter interaction. The number and the radii of the air holes, the width of the slot, and the number of rows are optimized to address the most challenging issue of RI sensor design, i.e. to obtain a high Q-factor and sensitivity, simultaneously. The plane wave expansion method has been used to facilitate the design procedure and a 3-D finite difference time domain method has been used to obtain the simulation outcome. The outcomes for the optimized cavity show an ultra-high Q-factor $\sim 4.3\times 10^{7}$ along with a sensitivity ~975 nm/RIU at a wavelength 1534 nm, for which the figure-of-merit is calculated as $2.73\times 10^{7}$ . However, the slot-width can be varied to obtain a Q-factor of $8\times 10^{7}$ and sensitivity of 1100 nm/RIU individually. The device also has an ultra-small mode volume of the order of 0.0253 $(\lambda /n)^{3}$ . Hence, all together, the sensor is expected to be a promising candidate for the lab-on-chip sensor application.

Tuning the resonance of polarization-degenerate LP1,l cladding mode in excessively tilted long period fiber grating for highly sensitive refractive index sensing.

This work presents a sensitive refractive index sensor based on the dual resonance of an excessively tilted long period fiber grating (Ex-TLPFG). The Ex-TLPFG is tuned to couple the guided core mode with only the polarization-degenerate cladding mode LP1,l, which consists of TE/TM0,l and HE2,l vector modes. It is found that the p-polarized LP1,lp mode exhibits a higher sensitivity than that of the s-polarized LP1,ls mode. An optimized sensitivity as high as 12182.9 nm/RIU is achieved for the p-polarized LP1,2p mode at the low refractive index region by tuning the initial resonance condition. The sensing performance is also evaluated through the power measurement method for a single resonance band. It is demonstrated that the improved sensitivity in this work for diameter-reduced Ex-TLPFG is much higher than that for the conventional LPFG based devices, which makes this sensing platform very attractive for a variety of index sensing applications.

Compact eccentric long period grating with improved sensitivity in low refractive index region.

We demonstrate a compact eccentric long period grating with enhanced sensitivity in low refractive index region. With a period designed at 15 µm for coupling light to high order cladding modes, the grating is more sensitive to surrounding refractive index in low refractive index region. The intrinsically low coupling coefficients for those high order cladding modes are significantly improved with the eccentric localized inscription induced by the femtosecond laser. The fabricated grating is compact with a length of 4.05 mm, and exhibits an average sensitivity of ~505 nm/RIU in low refractive index region (1.3328-1.3544). The proposed principle can also work in other refractive index region with a proper choice of the resonant cladding modes.

Graphene oxide functionalized long period grating for ultrasensitive label-free immunosensing

We explore graphene oxide (GO) nanosheets functionalized dual-peak long period grating (dLPG) based biosensor for ultrasensitive label-free antibody-antigen immunosensing. The GO linking layer provides a remarkable analytical platform for bioaffinity binding interface due to its favorable combination of exceptionally high surface-to-volume ratio and excellent optical and biochemical properties. A new GO deposition technique based on chemical-bonding in conjunction with physical-adsorption was proposed to offer the advantages of a strong bonding between GO and fiber device surface and a homogeneous GO overlay with desirable stability, repeatability and durability. The surface morphology of GO overlay was characterized by Atomic force microscopy, Scanning electron microscope, and Raman spectroscopy. By depositing the GO with a thickness of 49.2nm, the sensitivity in refractive index (RI) of dLPG was increased to 2538nm/RIU, 200% that of non-coated dLPG, in low RI region (1.333–1.347) where bioassays and biological events were usually carried out. The IgG was covalently immobilized on GO-dLPG via EDC/NHS heterobifunctional cross-linking chemistry leaving the binding sites free for target analyte recognition. The performance of immunosensing was evaluated by monitoring the kinetic bioaffinity binding between IgG and specific anti-IgG in real-time. The GO-dLPG based biosensor demonstrates an ultrahigh sensitivity with limit of detection of 7ng/mL, which is 10-fold better than non-coated dLPG biosensor and 100-fold greater than LPG-based immunosensor. Moreover, the reusability of GO-dLPG biosensor has been facilitated by a simple regeneration procedure based on stripping off bound anti-IgG treatment. The proposed ultrasensitive biosensor can be further adapted as biophotonic platform opening up the potential for food safety, environmental monitoring, clinical diagnostics and medical applications.

Using Transmissive Photonic Band Edge Shift to Detect Explosives: A Study with 2,4,6-Trinitrotoluene (TNT)

Photonic crystals (PhCs) possess outstanding optical properties that can be exploited for chemical sensing. We utilized a three-dimensional close-packed PhC structure made of functionalized silica nanoparticles. They consist of alternating high and low refractive index regions and have optical properties, such as photonic band structures, that are very sensitive to the change of physical structures. This study used 2,4,6-trinitrotoluene (TNT) to illustrate a detection method based on the transmissive photonic band edge shift (TPBES) due to the binding of TNT with amine anchored on particle surfaces to form Meisenheimer (amine-TNT) complexes. PhCs are exceptionally sensitive to a small change in refractive index caused by surface modification. As a result, they are suitable for sensing specific reactions between an amine and TNT. This method achieved a wide detection range of TNT concentrations from 10–12 to 10–4 M. 2,4-Dinitrotoluene (DNT) and toluene were used as a control and blank, respectively. Because of gravitational sedimentation, the TNT-functionalized particles were self-assembled in pure ethanol. They were measured by UV–visible transmission spectroscopy. A three-dimensional model to simulate the detection system was built using the particles’ center-to-center distance (a) and effective dielectric constant (ε) as a function of the TNT concentrations. Two sets of simulations were performed: the first set involved a parametric sweep of the center-to-center distance of TNT-functionalized crystals using ε = 2.015. The second set involved a parametric sweep of the dielectric constant with a = 263.1 nm. These perturbations yield a TPBES response that is in agreement with our experimental results.

The effects of optical and material properties on designing of a photonic crystal mechanical sensor

Photonic crystal nanocavities, which can be used to localize electromagnetic fields in a low refractive index region, hold great promise in fabricating micromechanical sensors for their potential applications in physical, chemical and biological engineering. In this paper, effects of optical and material properties on the performance of three different types of photonic crystal based mechanical sensors, i.e., force, displacement, and strain sensors, are investigated. The microcantilevers of sensors are made in Si and Si/SiO2 and exhibited in air and water atmosphere. As we found, both the resonant wavelength and the resonant wavelength shift for microcantilever mechanical sensors exhibit a linear relationship to the signal detection parameters (i.e., force, displacement, and strain) both in air and water. The sensitivity of force and strain sensors operated in water is higher than that in air. Si/SiO2 bilayer microcantilever sensor shows a higher sensitivity in force detection, whereas the Si cantilever sensor shows a higher sensitivity in strain and vertical displacement detection. The optical, material and mechanical properties of sensor material have great effect on the sensitivity of the new photonic crystal based microcantilever mechanical sensor.

Refractometer probe based on a reflective carbon nanotube-modified microfiber Bragg grating.

A carbon nanotube (CNT)-modified microfiber Bragg grating (MFBG) is proposed to measure the refractive index with a strong enhancement of the sensitivity in the low refractive index region. The introduction of the CNT layer influences the evanescent field of the MFBG and causes modification of the reflection spectrum. With the increase of the surrounding refractive index (SRI), we observe significant attenuation to the peak of the Bragg resonance, while its wavelength remains almost unchanged. Our detailed experimental results disclose that the CNT-MFBG demonstrates strong sensitivity in the low refractive index range of 1.333-1.435, with peak intensity up to -53.4 dBm/refractive index unit, which is 15-folds higher than that of the uncoated MFBG. Therefore, taking advantage of the CNT-induced evanescent field enhancement, the reflective MFBG probe presents strong sensing capability in biochemical fields.

Slab waveguide theory for general multi-slot waveguide

Optical devices based on slot waveguide are of considerable interest in numerous applications due to the distinct feature of strong electric field confinement in a low-refractive index region. A theoretical model based on multi-slab waveguide theory is used to reveal the physical mechanism of the slot waveguide. The calculation results derived from the basic Helmholtz equation for the conventional single-slot waveguide with a ~2% validation of the effective refractive index are compared to the former experiment results by the Cornell University group. Moreover, we extend the theoretical model to a general multi-slot waveguide. Its electric field distribution and key properties such as optical power confinement factor and enhancement factor in slot are deduced theoretically and fully discussed.

Carbon nanotube-deposited tilted fiber Bragg grating for refractive index and temperature sensing

A tilted fiber Bragg grating (TFBG) deposited with carbon nanotube (CNT) is proposed to simultaneously measure the surrounding refractive index (SRI) and temperature. Assisted by the interaction between the CNT deposition layer and the TFBG’s cladding mode, the increase in SRI could significantly attenuate the coupling intensity of cladding modes, while the resonant wavelengths keep almost unchanged. Therefore, the temperature response of the CNT-deposited TFBG could be further obtained by tracking the shifted resonant wavelength. In addition, the CNT deposition greatly enhances the sensing capability in a low refractive index region of $1\sim 1.428$ , presenting great potentials in biochemical sensing.