Strong Refractive Index Research Articles

Cucurbit[7]urils induced bimetallic nanoparticles network for ultra-sensitive detection of Caspase-3 based on surface plasmon resonance

A novel signal amplification method for surface plasmon resonance (SPR) was developed to detect Caspase-3 based on cucurbit[7]urils (CB[7]) and bimetallic nanonetwork. Three-dimensional (3D) bimetallic nanonetwork was formed through CB[7]-integrated self-assembly of gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) (AuNPs-CB[7]-AgNPs) by one-pot synthesis method. Caspase-3 was utilized as a model protein and peptide EEAAADEVDFKKAAAC was used as the substrate. In the presence of Caspase-3, peptide was hydrolyzed and exposed phenylalanine at the N-terminus, which could bind to CB[7] through host-guest interaction. The multivalent receptors CB[7] on bimetallic nanonetwork provide a simple strategy to increase the binding affinities between 3D bimetallic nanonetwork and peptide. Thus, AuNPs-CB[7]-AgNPs could attach to the gold chip with ease. Remarkably, due to the strong optical properties and high refractive index of 3D bimetallic nanonetwork, it has a higher SPR signal enhancement than that of only AuNPs or AgNPs. Therefore, a highly sensitive Caspase-3 sensing strategy could be achieved with a low detection limit of 5.39 pg mL−1, providing a new and facile SPR signal amplification method for detection of cell apoptosis.

Attenuated Total Reflection at THz Wavelengths: Prospective Use of Total Internal Reflection and Polariscopy

Capabilities of the attenuated total reflection (ATR) at THz wavelengths for increased sub-surface depth characterisation of (bio-)materials are presented. The penetration depth of a THz evanescent wave in biological samples is dependent on the wavelength and temperature and can reach 0.1–0.5 mm depth, due to the strong refractive index change ∼0.4 of the ice-water transition; this is quite significant and important when studying biological samples. Technical challenges are discussed when using ATR for uneven, heterogeneous, high refractive index samples with the possibility of frustrated total internal reflection (a breakdown of the ATR reflection mode into transmission mode). Local field enhancements at the interface are discussed with numerical/analytical examples. Maxwell’s scaling is used to model the behaviour of absorber–scatterer inside the materials at the interface with the ATR prism for realistic complex refractive indices of bio-materials. The modality of ATR with a polarisation analysis is proposed, and its principle is illustrated, opening an invitation for its experimental validation. The sensitivity of the polarised ATR mode to the refractive index between the sample and ATR prism is numerically modelled and experimentally verified for background (air) spectra. The design principles of polarisation active optical elements and spectral filters are outlined. The results and proposed concepts are based on experimental conditions at the THz beamline of the Australian Synchrotron.

A Tunable “Ancient Coin”-Type Perfect Absorber with High Refractive Index Sensitivity and Good Angular Polarization Tolerance

In this paper, we design and present a graphene-based “ancient coin”-type dual-band perfect metamaterial absorber, which is composed of a silver layer, silicon dioxide layer, and a top “ancient coin” graphene layer. The absorption performance of the absorber is affected by the hollowed-out square in the center of the graphene layer and geometric parameters of the remaining nano disk. The optical properties of graphene can be changed by adjusting the voltage, to control the absorption performance of the absorber dynamically. In addition, the centrally symmetric pattern structure greatly eliminates the polarization angle dependence of our proposed absorber, and it exhibits good angular polarization tolerance. Furthermore, the proposed “ancient coin”-type absorber shows great application potential as a sensor or detector in biopharmaceutical, optical imaging, and other fields due to its strong tunability and high refractive index sensitivity.

Metal-Organic Frameworks for Preserving the Functionality of Plasmonic Nanosensors.

Preserving the functionality of nanosensors is critical for their reliable performance under harsh environmental conditions. Biofunctionalized plasmonic nanostructures are an important class of bionanoconjugates for biosensing, bioimaging, and nanotherapeutics. Plasmonic nanostructures and biomolecules exhibit poor thermal stability over time. Here, we report a class of metal-organic framework, zeolitic imidazolate framework-8 (ZIF-8), as a protective coating for preserving plasmonic nanostructures and plasmonic bionanoconjugates at elevated temperature. Gold nanobipyramids (AuNBPs) with sharp tips are attractive plasmonic nanotransducers with high sensitivity but are prone to structural change and loss of sensitivity. This work reports the first observation that ZIF-8 can preserve the structure of AuNBPs and their corresponding strong electromagnetic field enhancement and high refractive index sensitivity. In addition, ZIF-8 coating enables nearly 100% retention of biorecognition capability of antibodies immobilized on the AuNBP surface after exposure to 60 °C for 48 h. The efficacy, versatility, and facile implementation of ZIF-8 coating offer great promise for the preservation of nanosensors.

Electrodeposited Negative Index Metamaterials with Visible and Near Infrared Response

AbstractNegative index metamaterials have revolutionized the field of photonics because of their unconventional electromagnetic properties absent in naturally occurring materials. It remains a challenge however, to achieve strong negative refractive index at optical frequencies over large areas in a device compatible fashion. In this work, a scalable method for the straightforward production of fishnet‐like negative index metamaterials combining soft nanoimprinting and electrodeposition is reported. In four simple steps, 2D arrays of pillars surrounded by metal–insulator–metal stacks made of gold and air gaps are created. The proper design of the geometrical features leads to negative indices from the visible to the near infrared, with values from ‐1.2 at 700 nm to ‐2.8 at 910 nm. The metamaterials are prepared on transparent conductive electrodes hence ready for device implementation. In addition, the nanostructures display high optical quality, which is corroborated with numerical simulations under normal and oblique incidence. As a proof of concept, the accessibility to the porosity of the metamaterial by studying the spectral changes produced when infiltrating different liquids through the metallic layers is demonstrated. The resonant wavelength increasing with the refractive index of the media makes the metamaterials potential platforms for both negative index structures and optical sensors.

Electromagnetic momenta for wave-particle systems in vacuum waveguides

Whenever light is slowed down, for any cause, two different formulas give its momentum. The coexistence of those momenta was the heart of the century-old Abraham-Minkowski dilemma, recently resolved for dielectrics. We demonstrate that this framework extends to momentum exchange in wave-particle interaction; in particular to vacuum waveguides of electron tubes (dispersive metallic slow-wave structures). In waveguides, the dilemma can be easily investigated since energy and force are well established through the use of Maxwell equations in vacuum, and since waveguides can have a strong refractive index. Our theory is assessed with simulations validated against measurements from a traveling-wave tube. In addition, we show that the dilemma resolution is not limited to discriminating between kinematic and canonical momenta but also involves a non-negligible flowing momentum from Maxwell’s electromagnetic stress. The existence of two momenta for diverse systems like materials, plasmas and waveguides, for which light velocity modification has entirely different origin, points to the universality of the Abraham-Minkowski dilemma.

Thermo-optic instabilities in asymmetric dual-core amplifiers

Dynamical thermo-optic instabilities in asymmetric dual-core fiber amplifiers are investigated by numerical simulations. A coupled-mode model capable of describing the asymmetric dual-core structure is derived, and structures with various levels of refractive-index asymmetry from 10 − 6 to 10 − 4 are studied. The most viable route to avoid thermally induced power signal field fluctuations is found to be core decoupling via large separation and strong refractive-index asymmetry. A design criterion for this strategy to be effective is suggested based on the numerical results.

A miniature ultra long period fiber grating for simultaneous measurement of axial strain and temperature

A SMF-MMF-SMF-ultra long period fiber grating (SMS-ULPFG) sensor is proposed for the simultaneous measurement of axial strain and temperature. The all-optical fiber sensor was fabricated by periodically splicing a multimode fiber (MMF) and a single mode fiber (SMF). Due to the strong refractive index (RI) modulation ability of the MMF, the sensor forms two resonant dips with smaller dimensions (the period of 1000 µm and total length of 5 mm). The transmission spectra were obtained by experiments and simulations. The results of the performance tests indicate that the uncertainty values of the SMS-ULPFG structure are 3.26 με and 0.23 °C over the ranges of 0–1000 με and 26–118 °C.

Particle image velocimetry measurements of a thermally convective supercritical fluid

The feasibility of particle image velocimetry (PIV) in a thermally convective supercritical fluid was investigated. Hereto a Rayleigh–Benard convection flow was studied at pressure and temperature above their critical values. The working fluid chosen was trifluoromethane because of its experimentally accessible critical point. The experiments were characterized by strong differences in the fluid density from the bottom to the top of the cell, where the maximum relative density difference was between 17 and 42%. These strong density changes required a careful selection of tracer particles and introduced optical distortions associated with strong refractive index changes. A preliminary background oriented schlieren (BOS) study confirmed that the tracer particles remained visible despite significant local blurring. BOS also allowed estimating the velocity error associated with optical distortions in the PIV measurements. Then, the instantaneous velocity and time-averaged velocity distributions were measured in the mid plane of the cubical cell. Main difficulties were due to blurring and optical distortions in the boundary layer and thermal plumes regions. An a posteriori estimation of the PIV measurement uncertainty was done with the statistical correlation method proposed by Wieneke (Measure Sci Technol 26:074002, 2015). It allowed to conclude that the velocity values were reliably measured in about 75% of the domain.

Fiber-Optic Vector Accelerometer Using Orthogonal Bragg Grating Inscription Over Innermost Cladding of a Multi-Clad Fiber

A fiber-optic vector accelerometer is demonstrated based on output power detection of orthogonal fiber Bragg gratings (FBGs). The compact sensing probe consists of a short section of multi-clad fiber (MCF) containing two concatenated, orthogonally positioned FBGs inscription over the core and innermost cladding. We use femtosecond laser side-illumination to ensure the two grating inscriptions are precisely positioned and strong refractive index modulation. Once the configuration was achieved, two well-defined groups of resonances appear on the reflection spectrum separately, corresponding to two “cladding” FBG reflections and two “core” FBG reflections. The two “cladding” resonances are polarized by the asymmetrical grating distribution over the fiber cross-section, thus perceive the vector fiber bend. The key of this configuration is that the orthogonal “cladding” FBGs are highly sensitive to acceleration together with getting the orientation of unknown source in two-dimensional plane. Moreover, the power fluctuations can be referenced out by measuring power of the core mode resonances that are insensitive to fiber bending.

A miniature SMS-LPG bending sensor with high sensitivity based on multimode fiber embedded-LPG

A miniature bending sensor is designed using multimode fiber (MMF) and single mode fiber (SMF). Two kinds of optical fibers are periodically spliced and arranged. MMF provides a strong refractive index modulation with a short length, and a significant resonance peak forms in only 5 periods. The final length of the sensor is only 3 mm. The evolution process and spectrum characteristics of the SMS-LPG are demonstrated through a simulation and experiment. The bending sensitivity is 31.47 nm/m−1 with a bending range of 0-2.44 m−1. The experiment results reveal that the proposed sensor has an insensitive temperature response (˜0.0106 nm/ºC).

Thickness-dependent microwave dielectric and nonlinear optical properties of Bi0.5Na0.5TiO3 thin films

Abstract Lead-free Bi0.5Na0.5TiO3 (BNT) films were grown on Pt(111)/Ti/SiO2/Si and quartz substrates for various thickness by using pulsed laser deposition technique. The impact of thickness on structural, microstructural, Raman-spectroscopy, microwave dielectric and nonlinear optical properties have been investigated in detail. X-ray diffraction pattern of BNT films deposited at 700 °C showed rhombohedral crystal symmetry with a small secondary phase of Bi4Ti3O12 and also revealed that the lattice parameters, lattice volume, strain and crystallite size were enhanced with film thickness. The film thickness, refractive index and optical bandgap have been determined by Swanepoel's envelope method and Tauc relation and are in the range of 208 nm–1300 nm, 3.79 eV–3.28 eV and 2.41–1.80, respectively. The third order nonlinear optical properties of films were estimated by Z-scan technique and the 208 nm film exhibited strong nonlinear refractive index (n2 = 4.62 × 10−6 cm2/W) and nonlinear absorption coefficient (β = 0.796 cm/W). The enhancement in microwave dielectric properties was observed with film thicknesses which were measured by using SPDR technique. The leakage current density exhibited the space charge limited current conduction behavior. The enhanced microwave dielectric and nonlinear optical properties of BNT films are promising for microwave tunable and nonlinear photonic device applications.

Ultrafast Spectrally Resolved Photoinduced Complex Refractive Index Changes in CsPbBr3 Perovskites

The exceptional optoelectronic properties of metal halide perovskites have been illuminated by extensive spectroscopic studies. Recent measurements suggest strong photoinduced refractive index chan...

Strong reverse saturable absorption and negative nonlinear refractive index in S and N co-doped GQDs at 532 nm CW laser

In this study, a bottom up pyrolysis approach is pursued in the preparation of sulphur: nitrogen co-doped graphene quantum dots (SN-GQDs). For the first time, 5V direct current light emitting diode (LED) is used as an excitation source and the influence of solvent’s nature such as dipole moment and polarity on fluorescence is studied. The investigated third order nonlinear optical behaviour reveals a strong reverse saturable absorption (RSA) and negative nonlinear refractive (NLR) index at 532 nm wavelength of continuous wave (CW) laser. The thermal nonlinearity in closed aperture is addressed employing thermal lens model.

Nonlinear absorption and refraction studies of truncated CuNb3O8 with high-repetition rate femtosecond pulses

Copper niobate with CuNb3O8 composition was prepared via one-step solid state reaction (750 °C, 12 h). Powder X-ray diffraction (XRD) confirmed the formation of parental CuNb2O6 phase with traces of CuNb3O8 on initial sintering (till 9 h), which was then transformed into pure CuNb3O8 on higher sintering (12 h). Field emission scanning electron microscope studies (FESEM) illustrates the formation of truncated cubes due to unification of layered structure upon prolonged heating. Third-order nonlinear optical properties of truncated CuNb3O8 was studied by Z-scan technique using Ti: Sapphire laser (800 nm, 150 fs, 80 MHz). Pure CuNb3O8 possessed strong nonlinear absorption (two-photon absorption coefficient of 5.3 × 10−10 m/W) and nonlinear refractive index n2 (2.1 × 10−16 m2/W) compared to mixed copper niobate (CuNb2O6 - CuNb3O8). The third-order nonlinear optical susceptibility of CuNb3O8 was ∼10−11 esu and was higher than the known nonlinear optical (NLO) systems such as lithium niobate and sodium niobate measured at similar pulse duration. Enhancement in nonlinearity arises due to the influence of morphology and here truncated cubes with the platonic solids offer high polarizability as the charges are concentrated in the edges of the system. The truncated cube CuNb3O8 exhibited strong optical limiting action with a low limiting threshold (34.6 μJ/cm2), thus making it appealing for ultrafast optical limiter devices towards photosensitive component protection from Infrared (IR) laser damage.

Synthesis and investigation of nonlinear optical properties of Para Red: Z-scan technique and quantum mechanical calculations

Abstract In this study, we present a systematic study of linear and nonlinear optical properties of Para Red with the aim of Z-scan technique and quantum mechanical calculations. The Z-scan experiments were performed using a 532 nm Nd: YAG (SHG) CW laser beam. Para Red exhibited a strong nonlinear refractive index, nonlinear absorption coefficient and third-order nonlinear susceptibility 3.487 × 10−6 cm2/W, 2.341 × 10−1cm/W and 2.157 × 10−4 esu, respectively. Also, quantum chemical analysis was used for the calculation of the dipole moment μ, dipole polarizability α, anisotropy of polarizability Δα and molecular hyperpolarizabilities (β,γ). The results revealed that Para Red has large first and second hyperpolarizabilities. However, from the obtained results, it was found that Para Red can be a promising material for applications in the development of non-linear optical materials.

Particle size reduction of thallium indium disulphide nanostructured thin films due to post annealing

Chalcogenide semiconductors are widely used, due to their versatile structural, optical and electrical properties. They are used in image sensors, energy conversion, non-volatile memory, and waveguides applications. Due to these versatilities in the properties, nanostructured thin films of TlInS2 are successfully prepared by thermal evaporation. The average particle size changes due to thermal treatment are examined by X-ray diffraction (XRD), and transmission electron microscope (TEM). A sharp reduction in the average particle size of the pristine nanostructured films is reported upon annealing processes at 423 K and 523 K. Besides, thermal treatment temperatures induce changes in the dispersion parameters viz oscillator energy, dispersion energy, high-frequency dielectric constant and dielectric constant. Of these, only the oscillator energy decreases with the annealing temperatures while all the other parameters increase. Moreover, the non-linear optical calculations have revealed that the nanostructured TlInS2 films exhibit high third-order nonlinear optical susceptibility of order 10−11 esu and strong nonlinear refractive index of order 10−10 esu.

Two-dimensional microbend sensor based on long-period fiber gratings in an isosceles triangle arrangement three-core fiber.

To realize the 2D microbend sensor, we design and fabricate two non-orthogonal long-period fiber gratings (LPFGs) in an isosceles triangle arrangement three-core optical fiber which is made in our lab, making an isosceles triangle arrangement three-core optical fiber. To mark two directions without crosstalk, we write two different periods of LPFG in each of the two external cores and the central core, which induces a strong asymmetric refractive index arrangement in the fiber cross section. Theoretical analysis and experimental results verify that the resonant wavelength originates from the tunneling between the LP01 core mode in the center core and the external core. In the confirmation experiments, the proposed sensor can distinguish multiple bending directions and experiences a maximum sensitivity of 3.234 nm/m-1 with a bending range of 0-0.588 m-1.

Enhancement transmission filter using a two-dimensional subwavelength periodic membrane

Enhancement transmission filter using a two-dimensional (2-D) subwavelength periodic membrane is proposed. It can be found that strong refractive-index modulation of the silicon periodic membrane can support the excitation of multiple guided-mode resonances (GMRs) in a reflection band, and every GMR relates a transmission peak on its edge, therefore the overlapping of the edges of these resonances can be tailored to create enhancement transmission. Thelocation of the transmission peak is shifted linearly with a slop of 1.51 as the period is varied. Enhancement transmission with multiple channels near 1310 nm can also be achieved using the interaction of the nondegenerate GMRs at oblique incidence.

Ultrasensitive and label-free detection of γ-aminobutyric acid using fiber-optic interferometric sensors functionalized with size-selective molecular sieve arrays

An ultrasensitive tapered fiber-optic sensor functionalized with size-selective molecular sieve (Fe3O4@SiO2@meso SiO2 microspheres) arrays for label-free γ-aminobutyric acid (GABA) detection has been proposed and experimentally demonstrated. The surface morphology of the Fe3O4@SiO2@meso SiO2 microspheres and optical properties of tapered fiber-optic interferometer endow the as-prepared sensing probe to transfer slight GABA concentration changes into strong refractive index fluctuations with high selectivity (size dominated). Label-free GABA detection with a linear sensitivity of 0.94nm/logM has been achieved. The proposed fiber-optic sensor provides an improved limit of detection up to 3.51×10−13M together with size-based selectivity, making it an appealing solution for rapid, low consumption and highly sensitive detection of analytes at low concentrations in medicine, chemical and environmental monitoring.