Core Refractive Index Research Articles

Green synthesis of sacrificial UV-sensitive core and biobased shell for obtaining optically hollow nanoparticles

Hollow nanoparticles have been extensively studied in recent years. Obtaining such structures with biobased materials, following greener synthetic routes, is still challenging, especially if accurate particle dimensions are required. This work reports the use of an innovative hybrid silica core (Si@azo) containing UV-sensitive molecule, wrapped in biobased multilayer shell composed of polysaccharides. It is a promising strategy for obtaining optically hollow nanoparticles. Indeed, Si@azo cores have the ability to be partially degraded when irradiated with UV light. Combined with a well-controlled and monodisperse diameter, they provide a good basis for layer-by-layer assembly, leading to a multilayer shell with controlled composition and thickness. Finally, UV irradiation of such a core–shell structure is harmless to the polysaccharide shell, but does impact the hybrid silica core, as revealed by turbidity measurements, among other. Each step, i.e. core synthesis, shell addition, and core–shell irradiation, has been carefully characterized at the macro (Fourier-transform infrared spectroscopy – FTIR, Dynamic Light Scattering – DLS, Zeta-potential measurement, Surface Plasmon Resonance – SPR, turbidity) and microscale (Transmission and Scanning Electron Microscopies). Emphasis is put on how turbidity measurements can be related to the core refractive index (ncore), giving information on the state of core degradation and whether the core–shell particle is optically hollow.

Mie scattering theory applied to light scattering of large nonhomogeneous colloidal spheres.

Colloidal suspensions made of smart core-shell structures are of current interest in many fields. Their properties come from the possibility of varying the core and shell materials for modifying the composite particles' chemical, biological, and optical properties. These particles are formed with a material with a constant refractive index core and a shell with a refractive index decaying until it matches the solvent refractive index. Poly(N-IsoPropyl AcrylaMide) (PNIPAM) is a typical example of materials forming shells. In this report, we present how to apply Mie scattering theory to predict and understand the static light scattering of large nonhomogeneous colloidal particles with spherical symmetry whose size is comparable with or larger than the light wavelength used for developing scattering experiments, where the Rayleigh-Gans-Debye approximation is not valid. Here, the refractive index decay was approximated by a Gaussian RI profile numerically evaluated through a multilayer sphere. We calculated the form factor functions of suspensions of PNIPAM microgels previously reported and core-shell suspensions made of polystyrene/PNIPAM at 20 and 40 °C synthesized by us. In all the cases, our method succeeded in providing the scattering intensity as a function of the angle. The software for using the numerical method is fairly straightforward and is accessible as an open-source code. The results can not only help predict and understand the photonic properties of microgels with large core-shell structures but also for any particle with a refractive index distribution with spherical symmetry, as in the case of microgels with super chaotropic agents, hollow microgels, or microparticles.

Femtosecond laser writing of robust waveguides in optical fibers with enhanced photosensitivity

We report the femtosecond laser writing of meter-long optical waveguides inscribed through the coating of specifically designed optical fibers. In order to improve the material photosensitivity and to ensure non-guiding optical fibers for subsequent laser processing of the waveguiding core, a depressed refractive index core design is implemented by co-doping a large portion of the optical fiber with germanium oxide and fluorine. The enhanced photosensitivity provided by further deuterium loading these fibers allows laser-writing of large refractive index contrast waveguides over wide cross sections. To mitigate the formation of photoinduced color centers causing high propagation losses in the photo-written waveguides, thermal annealing up to 400°C is performed on polyimide-coated laser-written fibers. Although the refractive index contrast decreases, the propagation losses are drastically reduced down to 0.08 dB/cm at 900nm allowing a robust single-mode guiding from visible to near infrared. Our results pave the way towards the development of a new generation of optical fibers and photonic components with arbitrarily complex designs.

Photonic hook propagation from eccentric microcylinder

The Photonic hook (PH) is an intricately curved photonic nanojet (PNJ) or a highly intense electromagnetic beam featuring a subwavelength waist, whose principal hallmark lies in its capacity to bend light at the nanoscale. According to existing literature, the origin of PH can be attributed to symmetry breaking, whereas symmetrical microstructures predominantly contribute to PNJ formation. This study presents the novel revelation of PH emergence from an isolated eccentric core–shell dielectric microcylinder, achieved through the illumination of a paraxial Gaussian beam (PGB). The eccentrically structured core–shell microscale geometry introduces an additional degree of freedom, influencing PH formation and directly shaping its characteristic parameters. Much like PNJ, the propagation of PH depends on different parameters such as core and shell refractive indices of the micro-structures, microstructure geometry, incident light type, and direction of propagation. A fascinating outcome from our numerical simulations is the switchable occurrence of PNJ and PH from an eccentric core–shell microcylinder by a simple adjustment of eccentricity, either parallel or perpendicular to the PGB’s propagation direction. This computational investigation emphasizes the impact of eccentricity and the incident wave’s beam waist, maintaining a consistent refractive index contrast between the core and shell. The outcomes are interpreted in terms of key parameters governing PH generation characteristics, encompassing FWHM, maximum electric field enhancement, and focal plane. Notably, we have observed the coexistence of whispering gallery modes (WGM) and PH within this system and these modes exhibit high sensitivity to the excitation wavelength. The potential applications of PH are believed to be far-reaching, including areas like optical trapping, sensing, and functioning as a versatile focusing element. This study contributes to the fundamental understanding of PH and illuminates its potential as a robust tool across diverse optical applications.

Optical and geometric parameter extraction across 300-mm photonic integrated circuit wafers

The precise quantification of a dielectric waveguide core thickness, core width, core refractive index, and cladding refractive index across a wafer is critical for greater consistency and accuracy in photonic circuit fabrication. However, accurate wafer-scale measurements of these parameters have not yet been demonstrated. We have previously described a method for extracting these four parameters simultaneously from silicon nitride waveguides using unbalanced Mach–Zehnder interferometers on a single die. In this work, we show that this technique can be scaled to characterize these photonic parameters across an entire 300 mm wafer. The refractive indices of the core and cladding materials are found with relative standard deviations of the mean of 0.07% and 0.03%, respectively. The core width offset (bias) and thickness are found with relative standard deviations of 0.3% (2.6 nm) and 0.5% (1.1 nm), respectively. The extracted parameter maps suggest a radial variation of material indices and a planar variation of geometric parameters. We verify the extracted parameters by accurately predicting the performance of an unbalanced Mach–Zehnder interferometer and the degeneracy between different modes in straight waveguides.

Hollow titanium nitride nanoshells for enhanced plasmon-driven hot electron generation and improved photocatalytic and photovoltaic applications.

Nowadays, plasmonic titanium nitride (TiN) is widely employed as a potential alternative to noble metals in semiconductor-metal hybrid nanoparticles (S-M HNPs) for improving the utilization efficiency of solar energy in photocatalytic and photovoltaic systems. In semiconductor-TiN nanosystems, TiN NPs convert solar energy into highly energetic (hot) electrons that can be transmitted to the attached semiconductor for enhanced applications. In this paper, we propose TiN nanoshells with a nonabsorbing dielectric core as an improved energy conversion component in S-M HNPs, compared to homogenous TiN nanospheres, with higher geometrical optimization flexibility, wider absorption range tuneability, and effective hot electron generation and utilization due to the reduced plasmonic-shell size. For understanding the impact of the core material on the functionality of the nanoshells, we assume three core materials with different refractive indices (air, silica (SiO2), and magnesium oxide (MgO)). The exact Mie theory is utilized to calculate the absorption coefficient and the plasmon field of the proposed TiN nanoshells. To quantify the absorbance effectiveness on the solar spectrum, we calculate a relevant figure of merit (FoM) that depends on the spectral features of the absorption coefficient. By optimizing the geometrical parameters of nanoshells, it is found that hollow TiN nanoshells with the lowest core refractive index exhibit the highest FoM of solar energy absorption. Also, the plasmon field intensity of hollow TiN nanoshells is higher and more concentrated in a smaller volume of TiN material in comparison to the field intensity of other nanoshells (SiO2-TiN and MgO-TiN nanoshells) and TiN nanospheres. Factors affecting the utilization of the generated hot electrons, including the radiative damping of plasmons and the spreading of the plasmon field inside the nanoparticles, have been investigated. In view of the temporal dynamics of hot electrons, it is shown that using the hollow TiN nanoshells with thin shells greatly enhances the effectiveness of the generated hot electrons to reach the attached semiconductor. In fact, the reduced plasmonic-shell thickness results in a trade-off between a longer radiative relaxation time and less solar energy absorption with regard to the selected core material.

The influence of refractive index profile on supercontinuum beams self-cleaning in graded index multimode fiber

We propose a theoretical scheme to improve the fundamental mode energy proportion of the supercontinuum (SC) generation in graded-index multimode fiber (GRIN-MMF). With this scheme, we conduct numerical investigations into the influence of various core refractive index profile parameters on beam-self cleaning of fundamental mode, establishing an optimal parameter range (α=2.0∼2.06). Within this range, the energy proportion of the fundamental mode gradually increases from 32% to 65% as the propagation distance. Additionally, our simulation provides the initial demonstration that the supercontinuum (SC) broadening is correlated with the distribution of core refractive indices within the graded-index multimode fiber.

Mathematical modelling of fiber optic cable with an electro-optical cladding by incommensurate fractional-order differential equations

In this study, the mathematical model through incommensurate fractional-order differential equations in Caputo meaning are presented for time-dependent variables given as the numerical aperture, critical angle, and acceptance angle characteristics of a fiber optic cable with electro-optical cladding. The qualitative analysis including the existence and stability of the equilibrium points of the proposed model has been made according to the used parameters, and then, the results obtained from this analysis are supported through numerical simulations by giving the possible values that can be obtained from experimental studies to these parameters in the model. In this way, a stable equilibrium point of the system for the core refractive index, cladding refractive index and electrical voltage is obtained according to the threshold parameter. Thus, the general formulas for the critical angle, acceptance angle and numerical aperture have been obtained when this fixed point is stable.

The Study on the Fundamental of Fiber Bragg Grating (FBG) Sensing Principle

Abstract: A single mode fibre core is laterally exposed to a periodic pattern of powerful UV laser light to create an optical sensor called an FBG. The exposure results in a long-lasting rise in the fiber's core's refractive index (ncore), producing fixed index modulation known as grating (Λ). A specific wavelength of input light, known as the Bragg wavelength or Bragg related to grating period, must be reflected by the grating inside the fibre optic core while transmitting all other wavelengths. FBG operates as intended by using the regular variations in the single mode fibre core's refractive index, a Bragg reflector can be constructed on an optical fibre. A specific wavelength of light will be reflected and all others will be transmitted as it passes through the FBG. A shift in the wavelength of the light reflected when the temperature or strain surrounding the grating changes is seen.

Моделирование влияния деформации изгиба в одномодовом оптическом волокне на показатель преломления сердечник – оболочка

Рассматривается модель деформированного одномодового оптического волокна. Приводится математическая модель деформационных составляющих. Произведена оценка показателей преломления в трех взаимно перпендикулярных направлениях. Найдены случаи, в которых распространение света по изогнутому волокну происходит как по одноосному кристаллу. Определено, что значения показателя преломления изогнутого оптического волокна отличаются от показателя преломления недеформированного волокна. The article addresses the model of a deformed single-mode optical fiber. A mathematical model of deformation fibers is given. Cases have been found where the propagation of light along a bent fiber occurs as though along a uniaxial crystal. A certain value of the refractive index of a bent optical fiber differs from the refractive index of the undeformed fiber.

Analysis of Plasmonic Sensors Performance Realized by Exploiting Different UV-Cured Optical Adhesives Combined with Plastic Optical Fibers.

Polymer-based surface plasmon resonance (SPR) sensors can be used to realize simple, small-size, disposable, and low-cost biosensors for application in several fields, e.g., healthcare. The performance of SPR sensors based on optical waveguides can be changed by tuning several parameters, such as the dimensions and the shape of the waveguides, the refractive index of the core, and the metal nanofilms used to excite the SPR phenomenon. In this work, in order to develop, experimentally test, and compare several polymer-based plasmonic sensors, realized by using waveguides with different core refractive indices, optical adhesives and 3D printed blocks with a trench inside have been used. In particular, the sensors are realized by filling the blocks' trenches (with two plastic optical fibers located at the end of these) with different UV-cured optical adhesives and then covering them with the same bilayer to excite the SPR phenomenon. The developed SPR sensors have been characterized by numerical and experimental results. Finally, in order to propose photonic solutions for healthcare, a comparative analysis has been reported to choose the best sensor configuration useful for developing low-cost biosensors.

Extra-High Pressure in the Core of Silica-Based Optical Fiber Preforms during the Manufacturing Process

The core refractive index n2 of silica-based optical fiber preform heated to 2000 °C was determined experimentally for the first time. The measurements were carried out in the process of preform temperature reduction. It was shown that n2 could increase up to ~1.75 in the visible spectral range at temperatures of ~2000 °C (n2 ≈ 1.46 at room temperature). This fact suggests that pressures close to or exceeding the ultimate strength of silica glass (~20 GPa) occur in the preform core region. The local extra pressure is argued to be a possible cause of the well-known “starburst” phenomenon at the core–cladding interface of preforms with certain core compositions. The observed effect of radial cracks can be interpreted as the result of silica cladding destruction under the action of extra-high pressure in the core.

Properties of off-axis helical long-period fiber gratings

In this paper, a new four-electrode arc discharge device with large constant temperature region is designed, which is used to prepared high-quality off-axis helical long-period fiber grating. The larger constant temperature heating area is more conducive to releasing the stress of optical fiber, so that the prepared device is less off-axis. In order to show that low off-axis is a key parameter of high-quality off-axis helical long-period fiber grating, the effects of single mode fiber on transmission spectrum of off-axis helical long-period fiber grating under different coupling lengths, pitches, core refractive indexes, cladding refractive indexes, core diameters, cladding diameters and off-axis quantity are simulated by using beam propagation method. Since traditional methods are difficult to measure the off-axis helical long-period fiber grating with small off-axis quantity, the off-axis quantity of the prepared device is estimated by using the method of spectral comparison and back-thrust off-axis quantity in this work. The off-axis helical long-period fiber grating is prepared by using the established processing device. The off-axis quantities of the prepared devices are about 0.12, 0.13 and 0.16 µm, respectively, according to the comparison between the simulated transmission spectrum and the actual spectrum. Finally, experiments on the torsional resistance and repeatability of the off-axis helical long-period fiber grating prepared by the device are carried out. The experimental results show that the prepared grating has certain torsional resistance and good spectral repeatability.

Mode Hybridization in Silicon Core-Gold Shell Nanosphere.

A dielectric core-metal shell nanosphere has attracted scientific and technological interests due to the unique optical resonances arising from the hybridization of surface plasmon modes and cavity modes. The previous studies focus on a low-index dielectric core without its own optical resonances. Here, optical resonances of a core-shell nanosphere with a high refractive index (n≈ 4) core with the lowest order Mie resonances in the visible range are investigated theoretically and experimentally. Scattering and absorption spectra of a core-shell nanosphere for different values of the core refractive index are first analyzed, and there is a transition of the hybridization scheme around n≈ 2. Above the value, a characteristic hybridized mode with strong absorption and weak scattering emerges in the near-infrared range. A core-shell nanosphere composed of a silicon core and a gold shell is prepared, and the resonance modes are studied by single particle scattering spectroscopy and electron energy loss spectroscopy (EELS) in a transmission electron microscope. The core-shell nanospheres exhibit the hybridized modes depending on the core diameter. The hybridized mode as well as the higher order one that is not observable in the scattering spectroscopy is observed in the EELS.

An optofluidic Bragg fiber sensor for estimating adulterants in a temperature-dependent molar fraction of hydrated mono-alcohol fuels

Since, chemically complex environments, the aroma has been a difficult task so far. Therefore, in the present communication, an optofluidic Bragg fiber artificial nose for perceiving the temperature-functional molar fraction of an adulterated binary composition of hydrated mono-alcohols is optimized and reported. The task is theoretically predicted over an optofluidic Bragg fiber sensor having geometrical defects by creating an asymmetry in mid of periodic cylindrical Bragg reflectors. In a cylindrical coordinate system, Henkel function (HF) and transfer matrix technique (TMT) are used to simulate a multilayer concentric hollow-core Bragg fiber (HCBF). The variation in refractive index (RI) of the adulterated binary mono-alcohol fuel is connected to the temperature-functional molar concentration, which is again anticipated by making use of several models, including the most suitable Dale-Gladstone, Lorentz-Lorenz, etc. A prominent sensing signal of which has the full width at half maximum (FWHM) equal to 0.1 nm is observed in the examined photonic bandgap (PBG). The signal is responsive to fluctuations in optofluidic core RI in the vicinity of a structural defect layer. The suggested sensor's temperature-dependent maximum sensitivity (due to varied weather circumstances) for ethanol fuel rather than methanol fuel is 1057.32 nm/RIU. Furthermore, the surface plasmon-based static temperature sensor is compared. Due to the smallest FWHM of output signal around 0.1 nm, other sensing performance metrics such as detection accuracy and quality parameters are also enhanced in the proposed sensor device.

Sensing performance of Bragg fibers having a defect layer utilizing confinement loss analysis for brain cancer cells detection

The sensing performance of the Bragg fiber having a defect layer in the cladding region is optimized for brain cancer cell detection using the refractive index of brain lesions fluids. The optimization is achieved by comparing the confinement loss of low contrast and a high contrast defected Bragg fiber with similar standard Bragg fiber. Using the transfer matrix method and matching the fields at various interfaces equation of confinement loss is obtained in presence of a defect layer. It is observed that the sensing performance of low contrast waveguide is better than high contrast waveguide in the absence of the defect layer but the situation become reversed in presence of a defect layer. Wavelength interrogation shows high sensing performance with the variation of defect layer refractive index while intensity interrogation shows high sensing performance with the variation of core refractive index. Here wavelength interrogation of high contrast defected Bragg fiber is demonstrated to detect normal and abnormal brain tissues due to its feasibility. Our analysis shows that the respective maximum sensitivity, detection accuracy, and quality parameters are 12.1519 nm/RIU, 4365.7235, and 41.6161 RIU−1 in the benign stage corresponding to the wall of brain and 11.0233 nm/RIU, 4242.7191 and 38.8281 RIU−1 in the malignant stage corresponding to low-grade glioma.

Liquid core photonic quasi-crystal fiber plasmonic refractive index sensor for wide refractive index detection range

In this paper, a photonic quasi-crystal fiber plasmonic refractive index sensor is numerically analyzed. The fiber core is infiltrated with six hypothetical liquids, which refractive indices of them vary from 1.44 to 1.49. The reason for filling the core with different refractive index of liquids is that changeable refractive index of the core is an option to adjust the sensor performance at different intervals of analyte refractive index. Due to changes in the core refractive index, a large RI detection range from 1.38 to 1.53 is obtained and the sensor exhibits maximum spectral sensitivity of 10,000 nm/RIU. The properties of the sensor are calculated by the finite element method. The geometrical parameters of the sensor such as analyte height and gold thickness are also evaluated. The proposed sensor has a tunable capability which can be suitable for RI detection of biomedical liquid analytes in various ranges of refractive indices.

Studying the effect of polymethyl methacrylate polymer opticals fibers (POFs) on the performance of composite materials based on the polyether ether ketone (PEEK) polymer matrix.

More recently, various techniques have been implemented for the sensors manufacturing purpose, such as fiber Bragg gratings fibers (FBG) that allows variable core refractive index suitable for a large scale of measurements types, fiber optic evanescent waves (FOEW) for water parameters measurement, microstructured and crystal photonic optical fibers, polymers optical fiber (POFs), and so on. In this perspective, the objective of this work is to study the reliability and the origin of the resistance of each fiber-matrix interface of the composite materials PMMA/PEEK, Topas/PEEK, and Topas-Zeonex/PEEK. The genetic simulation is based on the probabilistic approach of Weibull to calculate the damage at the interface by crossing the two damages of the matrix and the fiber respectively. The results show that the PMMA/PEEK composite is the most resistant to the mechanical stresses applied compared to those Topas/PEEK and Topas-Zeonex/PEEK; these results were confirmed by the level of damage to the interface observed for the studied materials. The performed calculations are in good agreement with the analytical results of Cox, where he demonstrated that Young’s modulus of fibers have an important influence on the shear strength of the fiber-matrix interface of composite materials. Based on the obtained results, the present study gives the opportunity for the proposed materials (PMMA/PEEK and Zeonex/PEEK) to be as potential candidates for the smart digital applications and telecoms aims.

Characteristic analysis of core radius, core refractive index, mismatched phase constant and coupling efficiency about 2 × 2 fiber directional coupler based on MATLAB

When two parallel waveguides are close to each other, the power of the two adjacent waveguides will be periodically converted to form a directional coupled waveguide system (). The transverse coupling between two parallel waveguides can be described by mode coupling theory. However, in practical application, due to the limitation of equipment or manual computing capacity, it is impossible to quickly calculate the coupling efficiency of two optical fibers through the current specific formula. Through the form of MATLAB programming, it can determine whether the fiber is a single-mode fiber, solve the characteristic equation to obtain the propagation constant, integrate the coupling coefficients K12 and K21, solve the coupling efficiency in the fiber, and finally output in the form of graphics. The results show that for the directional coupler composed of two fibers, if the core radius and core refractive index change, the mismatching phase constant and coupling efficiency will change correspondingly, and the calculated results are in good agreement with the predicted results. On this background, this paper proposes the form of MATLAB programming to calculate coupling efficiency. The coupling efficiency of fiber directional couplers is analyzed. The theory gives accurate results within the actual parameters.

Interconnection of Few-Mode Fibers and Photonic Integrated Circuits Using Mode-Field Adapters

We propose a detailed method for the interconnection between optical fibers and waveguides of photonic integrated circuits. Appropriate modal transmission is accomplished by matching the mode field diameters from both waveguide structures. Links from one structure to another are created by an interconnecting waveguide, maintaining a fixed coupling efficiency as its size is modified to adjust to the target waveguide core. This tailored transition acts as a mode field adapter, equalizing the transmission among multiple modes and reducing the mode-dependent losses while coupling. We present an algorithm to design the mode field adapter based on matching the effective mode areas using the power overlap integral. A study case considering a polymer photonic integrated device immediately connected to a few-mode fiber is analyzed. Coupling efficiencies over 90% for every transmitted mode are achieved, showing an evident improvement compared to typical approaches only matching core sizes. Detailed comparison of the results for each transmission mode is presented. This same procedure can be used to interconnect optical waveguides with different refractive index profiles and core geometry.