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We put forward a dual-mode photonic crystal nanobeam cavity for simultaneous sensing of the refractive index (RI) and temperature (T) designed with the assistance of artificial neural networks (ANNs). We choose the structure of quadratically tapered elliptical holes with a slot to improve the sensitivities of the two modes. To reduce the time consumption of the design, the ANNs are trained to predict the band structure and to inverse design the geometric structure. For the forward prediction and the inverse design neural networks, low mean square errors of 5.1 × 1 0 − 4 and 1.4 × 1 0 − 2 are achieved, respectively. Through a specific design of band properties by the well-trained neural networks, a dual-mode nanobeam sensor with high quality factors of 9.34 × 1 0 4 and 1.55 × 1 0 5 and a small footprint of 23.8 × 0.7 µ m 2 are designed. The RI and T sensitivities of the air mode are 405 nm/RIU and 40 pm/K, respectively, whereas those of the dielectric mode are 531 nm/RIU and 27 pm/K, respectively. The present work shows significance in further research on the design and applications for dual-mode cavities.

A fiber surface plasmon resonance (SPR) sensor is widely used in high-sensitivity refractive index measurement, but there is less research on curvature measurement. In this paper, a single-mode fiber curvature sensor based on SPR is designed and fabricated. By employing bending, the transmitted light in the fiber core leaks into the cladding. A 50 nm gold film is coated outside the cladding, and the evanescent field of the cladding after bending contacts the gold film to cause SPR. When the curvature changes, the coupled cladding mode and intensity are different; that is, the SPR incident angle and evanescent field intensity are different, so as to realize the dual parameters of SPR resonance wavelength and depth of the resonance valley changing with curvature. By experiments, the influence of different cutoff wavelengths of single-mode fiber on the performance of the sensor is studied. The testing results indicate that with the decrease in cutoff wavelength of the single-mode fiber, the valley depth sensitivity of the sensor increases, and the half height width (FWHM) decreases. When the cutoff wavelength of the single-mode fiber is 630 nm, the valley depth sensitivity of the sensor is 0.0088 a . u / m − 1 , the wavelength sensitivity is 0.26 n m / m − 1 , and the average FWHM is only 21 nm. The proposed single-mode fiber curvature sensor based on SPR has a narrow FWHM and an opening threshold. It can also realize no opening threshold by introducing a coreless fiber, which provides a new solution, to the best of our knowledge, for the diversified detection of fiber SPR sensors.

Salinity is an important environmental factor regulating the aquatic system structure of lakes and other water bodies. Changes in salinity, which can be caused by human activities, can adversely impact the life of water organisms. The refractive index, which can be directly related to water salinity, also controls the polarimetric properties of light reflected from the water surface. In this study, polarimetric measurements of smooth water surfaces with different salinity content were performed at different viewing zenith angles in the wavelength range of 450–1000 nm in the specular reflection directions. The results show that the light reflected from the water surface (defined as reflectance factor) in one measurement direction can be replaced by the reflectance factor derived from polarimetric measurements, and if the polarizer absorptance is considered, the average relative difference is less than 3%. The degree of linear polarization (DOLP) was used to retrieve the refractive indices of water with different salinities based on the Fresnel reflection coefficient. The inverted refractive indices not only have high accuracy (uncertainty from 0.9% to 1.8%) but also have a very strong relationship with the water salinity content. Our study shows the possibility of estimating the variation in water salinity using multi-angular polarimetric measurements.

This theoretical investigation proposes the preliminary stage detection of cancer cells using a prism-based surface plasmon resonance biosensor. The surface plasmon resonance (SPR) biosensor is label-free and rapid; it supports real-time investigations, including greater sensitivity, and it has an enormous possibility for detecting different forms of cancer. The refractive index alteration of cancer cells is 1.360-1.401. The performance of the SPR sensor has been analyzed. The different types of cancer cells for skin, cervical, blood, adrenal, and breast (type I and II) have been analyzed, and sensitivities of 292, 384, 229, 242, 254, and 261 (degree/RIU) have been achieved, respectively. A high figure of merit of 115.2

The fiber surface plasmon resonance (SPR) sensor is widely used in high-sensitivity refractive index detection, and there are few reports on SPR sensors used for strain measurement. In this paper, we propose and demonstrate an S-type fiber strain sensor based on SPR. The simulation and testing results indicate that the smaller the vertical axis offset of S-type fiber is, the larger the incidence angle of SPR is, and the closer the working band of SPR is to the shorter wavelength direction. By electrofusion, we fabricated an S-type structure on the single-mode fiber, and by the S-type structure, the high-order cladding mode was excited. A 50 nm gold film was coated on the surface of the fiber cladding behind the S-type structure. The evanescent field of the cladding mode contacted the gold film to produce SPR, and the strain can change the vertical axis offset of the S-type fiber and further change the incidence angle of SPR; hence an S-type fiber strain sensor based on SPR was realized. When the refractive index of the ambient medium is 1.345 RIU, the vertical axis offset and length of the S-shaped structure are 87 and 501 µm, respectively, the resonance wavelength of the fiber SPR strain sensor changes from 648.06 to 631.77 nm with a strain detection range of 0 − 1200 µ ε , and its sensitivity is − 14.38 p m / µ ε . The proposed sensor provides a new solution for the strain measurement of the fiber SPR sensor, which is expected to be used in the fields of engineering, health monitoring, and early warning.

We propose and demonstrate a highly sensitive refractive index (RI) and temperature sensor based on an asymmetric fiber coupler (AFC). The AFC was fabricated by weak fusion of a pre-stretched single-mode fiber and a few-mode fiber. An ultra-sensitivity RI can be achieved near the dispersion turning point (DTP). The proposed RI sensor achieves a high RI sensitivity of − 10 , 662.4 n m / R I U within the RI range of 1.31–1.35. By packaging the AFC into polydimethylsiloxane (PDMS), the temperature sensitivity reaches 11.44 nm/°C. The proposed AFC with high RI and temperature sensitivity can be potentially used in the field of chemical monitoring, biochemical detection, and clinical diagnosis.

A metal–insulator–metal (MIM) waveguide system consisting of a MIM waveguide, a ring cavity, and a semi-ring cavity is proposed. Using the finite element method, the transmission characteristics of the MIM waveguide system are discussed under the different geometry parameters. By detecting the resonance wavelength and varying the refractive index, the sensing performance of the MIM waveguide system is analyzed. The proposed structure can be used as a refractive index sensor with the maximum sensitivity of 2412 nm/RIU. Due to isolating the ring cavity and semi-ring cavity, the independent tuning of double resonances can be realized by changing the refractive index of the insulator in the ring cavity or the semi-ring cavity. Benefiting from two independent refractive index sensing modes, the structure with two isolated resonators can realize the simultaneous measurement of glucose solution concentration and blood plasma concentration. The sensitivity of glucose solution sensing in the ring cavity is 0.13133 nm/(g/L). Meanwhile, the blood plasma concentration detection in the semi-ring cavity is realized with the sensitivity of 0.358 nm/(g/L). The system with two isolated cavities has the potential to be used as an efficient nano sensor, which can achieve simultaneous measurement of two parameters.

The soluble solids content (SSC) is an important factor in the internal quality detection of apples. It is essential to have reliable and high-speed measurement of the SSC. However, almost all traditional equipment is inconvenient and expensive. We designed a handheld nondestructive SSC detector based on near-infrared (NIR) spectroscopy, which is composed of a portable NIR spectrometer, cloud server, smartphone app, and prediction model of SSC. We preprocessed the spectrum with multiplicative scatter correction (MSC), standard normal variable transformation (SNV), and Savitzky–Golay (S–G) smoothing algorithms. Besides, the linear weight reduction of the particle swarm optimization algorithm is carried out, and we establish the model of an extreme learning machine optimized with the improved particle swarm optimization (IPSO-ELM) algorithm. The R 2 , root mean square error of prediction (RMSEP), and residual prediction deviation (RPD) of the model are 0.993, 0.0155, and 11.6, respectively, which are better than the traditional model obviously. In addition, the number of wavelengths reduced from 228 to 70 as the model is simplified with the uninformative variable elimination (UVE) method. The time of training is reduced by 29.30% compared with the original spectrum. It can be verified that the IPSO-ELM model has good prediction performance, and the NIR diffuse reflectance spectroscopy is a reliable nondestructive measurement of SSC in apples.

This work demonstrates an interferometric technique to measure the refractive index (RI) of microliter volume of biosample solutions with the help of an optical fiber negative axicon and Bessel beam with the micrometer-size central spot. The RI measuring device consists of a broadband superluminescent diode (SLD) source along with an optical circulator and spectrometer. The axicon optical fiber probe packaged with a glass slide is connected to a broadband SLD source through an optical circulator, and a drop of microliter bioliquid sample is placed on the packaged probe’s head. The reflected light from the glass–liquid interface couples back to the axicon probe and interferes with the reference beam generated at the air–glass interface of the axicon. The interference spectrum is further analyzed by applying the fast Fourier transform to calculate power from the respective interface and solve the Fresnel equation for the RI measurement. The RI of small sample volume ∼ 2 µ l of glucose solutions and bovine serum albumin protein solutions are reported as an application of the proposed method. This sensing platform shows promising applications in biomedicine for monitoring small volume various biosample concentrations. We also demonstrate RI measurement of flowing liquid sample using the developed setup in case the design can be considered for microfluidic channel research.

We propose an all-dielectric asymmetric ring-cylindrical metasurface. Based on the analysis of transmission characteristics and the calculation of electromagnetic field distribution of the metasurface with this element structure, it is found that the high Q resonance of the ultra-narrowband can be realized when the symmetry of the ring-cylindrical structure is broken. Meanwhile, it is found that the degree of asymmetry of the ring, the refractive index of the material, the radius of the ring, and the substrate have great influence on the Q value and resonant frequency of the metasurface. Our proposed metasurface structure is applied to the detection of biological molecules based on the change in refractive index of biomolecular solutions. The designed metasurface with high sensitivity to detect biomolecules with different refractive indices, the Q value can reach 365.03, and the sensitivity is increased by 90.36 GHz/RIU compared to that without substrate, while the figure of merit value is as high as 100.56, providing label-free detection of biomolecules.

We present a compact 3D diffractive microscope that can be inserted directly in a cell incubator for long-term observation of developing organisms. Our setup is particularly simple and robust, since it does not include any moving parts and is compatible with commercial cell culture containers. It has been designed to image large specimens ( > 100 × 100 × 100 µ m 3 ) with subcellular resolution. The sample’s optical properties [refractive index (RI) and absorption] are reconstructed in 3D from intensity-only images recorded with different illumination angles produced by an LED array. The reconstruction is performed using the beam propagation method embedded inside a deep-learning network where the layers encode the optical properties of the object. This deep neural network is trained for a given multiangle intensity acquisition. After training, the weights of the neural network deliver the 3D distribution of the optical properties of the sample. The effect of spherical aberrations due to the sample holder/air interfaces are taken into account in the forward model. Using this approach, we performed time-lapse 3D imaging of preimplantation mouse embryos over six days. Images of embryos from a single cell (low-scattering regime) to the blastocyst stage (highly scattering regime) were successfully reconstructed. Due to its subcellular resolution, our system can provide quantitative information on the embryos’ development and viability. Hence, this technology opens what we believe to be novel opportunities for 3D label-free live-cell imaging of whole embryos or organoids over long observation times.

The optical properties of biochemical compositions of microalgae are vital for the improvement of biosensor design, photobioreactor design, biofuel, and biophotonics techniques. A combination method using both the double optical pathlength transmission method (DOPTM) and the ellipsometry method (EM) is called DOPTM-EM, and it is used to acquire the optical constants of protein, lipid, and carbohydrate of Haematococcus pluvialis, Nannochloropsis sp., and Spirulina in both a solid state and a solution state within the visible and near-infrared spectral range. For different types of microalgae, the refractive indices of protein and carbohydrate in the solid state are similar to each other, but show an observed difference from lipid in the solid state. The refractive indices of protein and carbohydrate in the solution state presents a visible distinction in the researched spectral range. The absorption indices of protein, lipid, and carbohydrate in the solid state for these three types of microalgae are close to each other in the spectral range of 300–500 nm. However, an observed difference is shown in the spectral range of 500–1700 nm. For ease of application, the refractive index of biochemical composition of microalgae was fitted based on the Sellmeier equation. We believe this work can provide a reference to obtain the optical properties of biomaterial with high accuracy.

Based on double U-groove photonic crystal fiber (PCF), a surface plasmon resonance sensor with dual parametric detection of temperature and refractive index is proposed. The birefringence of PCF is increased by using germanium ions doped in the core and introducing U-shaped notches on both sides of the D-shaped fiber. The polished surface of the PCF is coated with gold film and PDMS as a temperature sensing channel, and the U-shaped groove is coated with gold film as a refractive index sensing channel. Through the design of the sensor, it is finally possible to achieve independent measurement of the two parameters. The sensor has a maximum wavelength sensitivity of 4715 nm/RIU in the analyte refractive index range of 1.32–1.4, and maximum wavelength sensitivity of 18 nm/°C in the ambient temperature range of − 30 ∘ C − 50 ∘ C . The proposed sensor has broad application prospects in scenarios such as blood analysis, DNA hybridization analysis, and microenvironmental cell interactions.

The study focuses on a methodology providing noninvasive monitoring and evaluation of the antitumor effect of traditional Chinese medicine, cantharides complex (canth), on 4T1 breast tumor cells. Digital holographic tomography (DHT) and developed data post-processing algorithms were used for quantitative estimation of changes in optical and morphological parameters of cells. We calculated and compared data on the refractive index, thickness, and projected area of 4T1 breast tumor cells in control untreated specimens and those treated with doxorubicin hydrochloride (DOX), canth, and their combinations. Post-treatment changes in cellular morphology recorded by DHT demonstrated that the two drugs led to noticeably different morphological changes in cells that can be presumably associated with different pathways of their death, apoptosis, or necrosis. The effect of combined treatment with these two drugs strongly depended on their relative concentrations and could lead to changes characteristic either for DOX or for canth; however, being more profound than those obtained when using each drug solely. The results obtained by DHT are in a good correspondence with commonly used cell viability analysis and immunofluorescent analysis of changes in cellular cytoskeleton.

Digital holographic microscopy (DHM) is a technique that has high potential for analyzing biological samples and has been successfully applied to the study of cells and cell lines providing information about important parameters such as refractive index, morphology, and dry mass, among others; it has also found applicability to study the effects of therapeutic treatments. Finding the size and shape of cells is important since they tend to change in the presence of some pathologies. In this research work, we obtain the morphology thickness and refractive index of the A375 melanoma cell line through a slight tilting of the cell in a DHM setup. Further, the development of a novel mathematical expression based on this tilt and in the optical phase difference is presented. We show images of melanoma cells with the refractive index information included, and their morphology thickness as rendered from the holographic phase maps recorded with DHM.

A biosensor platform based on the plasmonic resonance of graphene in the terahertz (THz) range (0.1 to 10 THz) is designed and investigated. The initial design is to create a nanofluidic channel as a sensing layer in the substrate of a biosensor grounded by metal. The sensor consists of a rectangular graphene patch over the substrate, which can be fed by either an external near-field source or an antenna. The presence of molecules in the nanosensing layer causes small changes in the channel’s properties, detectable through the scattering parameters of the designed biosensor. Since biomolecules are poorly absorbed in the initial biosensor, it can be grounded by a graphene sheet that is the same size as the graphene sheet over the substrate, which results in a performance improvement of the biosensor. It is shown that, by increasing the number of graphene sheets between the ground and the patch, high absorption occurs that enhances the sensitivity of the initial surface plasmon resonance THz biosensor. With varying refractive index of the sensing layer (Ns) in the range of 1.3–1.6, the proposed biosensors are investigated and compared with the initial biosensor. It is shown that by applying a graphene sheet between the two graphene sheets in the substrate, a maximum sensitivity of 8470 nm/RIU is achieved, which is a significant improvement, and also a sensitivity improvement of 4130 nm/RIU is achieved at N s = 1.3 . In the final section, it is shown that changing the substrate material from silicon (Si) to silica ( S i O 2 ) brings a significant sensitivity enhancement in the proposed biosensor with three graphene sheets, which accounts for 15410 nm/RIU in the best scenario.

Optical coherence tomography (OCT) can be a valuable imaging tool for in vivo and label-free digital plant phenotyping. However, for imaging leaves, air-filled cavities limit the penetration depth and reduce the image quality. Moreover, up to now quantification of leaf morphology with OCT has been done in one-dimensional or two-dimensional images only, and has often been limited to relative measurements. In this paper, we demonstrate a significant increase in OCT imaging depth and image quality by infiltrating the leaf air spaces with water. In the obtained high-quality OCT images the top and bottom surface of the leaf are digitally segmented. Moreover, high-quality en face images of the leaf are obtained from numerically flattened leaves. Segmentation in three-dimensional OCT images is used to quantify the spatially resolved leaf thickness. Based on a segmented leaf image, the refractive index of an infiltrated leaf is measured to be , deviating only 1.2% from that of pure water. Using the refractive index and a correction for refraction effects at the air-leaf interface, we quantitatively mapped the leaf thickness. The results show that OCT is an efficient and promising technique for quantitative phenotyping on leaf and tissue level.

In this paper, we present theoretical foundations of first-order design of an imaging refractometer. This refractometer may be used to measure the refractive index of liquids over a wide range. Refractive index reconstruction techniques are presented, and analytical reconstruction expressions are derived. The validity of the derived formulas is tested and is in good agreement with Gaussian reduction results. Design examples are provided and discussed. Limitations of the proposed measuring techniques are discussed, and theoretical models as well as numerical examples for the accuracy are presented.

Measurements of the index of refraction of a sample of high-quality, single-crystal germanium using the minimum deviation refractometry method are presented for temperatures near 22°C and for wavelengths in the range 2 to 14 µm. The standard uncertainty for the measurements ranges from 1.5 × 10 − 5 to 4.2 × 10 − 5 , generally increasing with wavelength. A Sellmeier formula fitting the data for this range is provided. Details of the custom system and procedures are presented, along with a detailed analysis of the uncertainty. These results are compared with previous measurements.

A base plasmonic metal-insulator-metal (MIM) waveguide structure consisting of a baffle waveguide and an obround-shaped resonator is designed to produce Fano resonance. The simulation results exhibit that double Fano resonances can be achieved. Based on this structure, an inner obround-shaped resonator is spliced to the former obround-shaped resonator through a slot resonator to form the expanded structure. Then quadruple Fano resonances are produced by the interference between the broadband continuous state arising from the baffle waveguide and the narrowband discrete state arising from the interaction among the inner obround-shaped resonator, the outer obround-shaped resonator, and the slot resonator. The Fano resonance and refractive index sensing characteristics are investigated, and the sensitivity and the figure of merit can reach 1636 nm/RIU and 33562, respectively. Furthermore, the structure filled with blood plasma can be used for detecting plasma concentrations with different refractive indices, and the sensitivity can reach 2.88 n m ⋅ L / g . The proposed structure with the simple baffle waveguide and obround-shaped resonators may have potential applications in biosensing and nanoscale optical sensing.

A silver based surface plasmon resonance (SPR) sensor with dielectric-graphene composite film is presented. The influences of the dielectric layer and graphene on sensitivity and other sensing properties are theoretically calculated and then comprehensively discussed. The refractive index sensitivities for composite silver film based SPR sensors with graphene and dielectric layers could be increased by 29% and 288% more than that of monolayer silver film based SPR sensors, respectively. Further, the sensitivity could be enhanced by 202% when combining the graphene and dielectric layers together. Considering the high adsorptive capacity of graphene for biochemical molecules, the composite silver film with both a dielectric layer and graphene would have great potential application in biochemical sensing fields. Further, bovine serum albumin protein was successfully used to verify the biochemical sensing ability of the proposed SPR sensor. The shift of resonance angle is nearly 3.1 fold that of monolayer silver based SPR sensors.

The refraction of fluorescence from the inside of a sample at the surface results in fluctuations in fluorescence computed tomography (CT). We evaluated the influence of the difference in refractive index (RI) between the sample body and the surroundings on fluorescence CT results. The brightest fluorescent point is away from the correct point on the tomograms owing to the refraction. The speculated position is determined as the exact point if the RI ratio ranges between 0.97 and 1.03 by immersing the body in an RI matching liquid. The results can help in experimental settings of fluorescence CT for acquiring three-dimensional positional information.

Multi-wavelength digital holographic microscopy (MWDHM) provides indirect measurements of the refractive index for non-dispersive samples. Successive-shot MWDHM is not appropriate for dynamic samples and single-shot MWDHM significantly increases the complexity of the optical setup due to the need for multiple lasers or a wavelength tunable source. Here we consider deep learning convolutional neural networks for computational phase synthesis to obtain high-speed simultaneous phase estimates on different wavelengths and thus single-shot estimates of the integral refractive index without increased experimental complexity. This novel, to the best of our knowledge, computational concept is validated using cell phantoms consisting of internal refractive index variations representing cytoplasm and membrane-bound organelles, respectively, and a simulation of a realistic holographic recording process. Specifically, in this work we employed data-driven computational techniques to perform accurate dual-wavelength hologram synthesis (hologram-to-hologram prediction), dual-wavelength phase synthesis (unwrapped phase-to-phase prediction), direct phase-to-index prediction using a single wavelength, hologram-to-phase prediction, and 2D phase unwrapping with sharp discontinuities (wrapped-to-unwrapped phase prediction).

Etched fiber Bragg gratings (EFBGs) have been widely employed for refractive index (RI) measurements that can be used to monitor sugar consumption during the fermentation of alcoholic beverages. EFBGs are obtained by removing the cladding of a fiber Bragg grating, which is traditionally performed by a chemical attack with hydrogen fluoride, an extremely hazardous corrosive substance that causes severe wounds and even death. To overcome such drawbacks, this technical note presents a simple, practical, and low cost method for the diameter reduction of single mode optical fibers by mechanical polishing, employing a small scale computer numerical control device and an ad hoc 3D-printed rod. The sensor probe obtained was tested using sucrose aqueous solutions with RIs between 1.333 and 1.394, measured in an Abbe refractometer. The results show a linear shift of the Bragg wavelength with respect to RI with a correlation of 0.928.

In this paper, we develop a detailed theoretical model for the optical reflectivity of a bio-tissue film confined between two flat interfaces based on the anomalous-diffraction approximation. We consider bio-tissue films consisting of a few layers of spheroidal cells surrounded by extracellular medium. We explore numerically the predictions of our model and compare them with simple effective medium theories, sometimes used as a first attempt to understand the optical properties of biological media. Then, we fit the model to experimental reflectivity-versus-angle-of-incidence curves of confined whole-blood films measured in an internal reflection configuration. Measurements were performed by confining a drop of fresh blood between a prism and a coverslip. Our experimental results show that it is possible to measure the coherent reflectance with small enough error to infer microstructural parameters with a good precision. The errors in measuring the coherent reflectance depend on the reflectivity magnitude. For instance, for a reflectivity of about 0.3 the error is below 2%, and the refractive indices of cells and surrounding medium can be obtained with a precision better than 1%. These results also indicate that the present model can readily be used to figure out the physical changes experienced at the microscale in bio-tissue films during a physicochemical process.

"Cancer is a disease that takes place when human cells grow uncontrollably. When detected and cured early, it can be non-life-threatening. It becomes life-threatening in case of late discovery where it affects the ability of an organ to function. In this work, a symmetric slab waveguide sensor is analyzed for the detection of cancer cells. The covering layers are assumed anisotropic lossy dispersive left-handed materials. Different from other sensors in which the analyte is located in the cladding region where the evanescent field exists, the cancerous cell is placed in the guiding film region that supports the oscillating field. Hence, the proposed sensor avoids the acute weakness of conven- tional optical waveguide sensors. Due to the high localization of the electromagnetic wave in the analyte region, the proposed sensor shows unusual sensitivity enhancement. The results revealed that the sensitivities obtained are 110%, 325%, and 450% for the first, second, and third modes, respectively. The enhancement of the sensitivity of the third mode relative to the conventional waveguide sensors is nearly a factor of 18." "Cancer is a disease that takes place when human cells grow uncontrollably. When detected and cured early, it can be non-life-threatening. It becomes life-threatening in case of late discovery where it affects the ability of an organ to function. In this work, a symmetric slab waveguide sensor is analyzed for the detection of cancer cells. The covering layers are assumed anisotropic lossy dispersive left-handed materials. Different from other sensors in which the analyte is located in the cladding region where the evanescent field exists, the cancerous cell is placed in the guiding film region that supports the oscillating field. Hence, the proposed sensor avoids the acute weakness of conven- tional optical waveguide sensors. Due to the high localization of the electromagnetic wave in the analyte region, the proposed sensor shows unusual sensitivity enhancement. The results revealed that the sensitivities obtained are 110%, 325%, and 450% for the first, second, and third modes, respectively. The enhancement of the sensitivity of the third mode relative to the conventional waveguide sensors is nearly a factor of 18."

Refractometry is important for characterizing the optical performance of materials. The refractive index can quickly be assessed using critical angle or thin-film techniques. However, these methods only assess the material surface. Measurement of bulk refractive index is performed by measuring the refracted angle of a transmitted beam but requires precision sample geometry. The method presented here avoids costly sample preparation by measuring the sample geometry and refracted angle simultaneously, using reflections from the front and back surfaces of a wedge of material. The method is demonstrated for polydimethylsiloxane prepared under a range of curing conditions, and no significant dependence was observed. Spectral dependence is characterized, and Sellmeier coefficients are reported.

Corneal cross-linking (CXL) using UVA irradiation with a riboflavin photosensitizer has emerged as a new treatment paradigm for corneal ectatic disorders. The thickness threshold for protection of intraocular structures has often been challenged with ongoing developments, and corneal thinning becomes an important safety concern, especially for patients with thin corneas. In this study with an ex vivo bovine eye model, we monitored corneal thinning and corneal refractive index changes using optical coherence tomography (OCT) integrated with an adaptation of the optical path length method. CXL experiments were performed based on the standard protocol that includes removal of the corneal epithelium to facilitate diffusion of riboflavin into the stroma. The corneal stromal thickness and group refractive index were measured by a 1310 nm Fourier-domain OCT imaging system at three critical points of the procedure: immediately after epithelial removal, after 30 min riboflavin instillation, and after 30 min UVA irradiation with continuing instillation. We found that the refractive index of the bovine cornea changed significantly from epithelial removal to riboflavin instillation and UVA irradiation, increasing from 1.377±0.005 (mean±standard deviation) after de-epithelization to 1.387±0.003 after 30 min instillation and 1.388±0.008 after subsequent irradiation. The corneas also underwent a considerable decrease (10%-20%) in stromal thickness with thinning of 95±29 μm (mean±standard deviation) after riboflavin instillation and a further decrease (∼5%) with thinning of 42±19 μm after UVA irradiation. Our study highlights the importance of corneal thickness monitoring during CXL, especially after riboflavin instillation when the decrease is the largest, to avoid delivering endothelial cytotoxic doses. An increase in refractive index heightens the concern for corneal thinning and the need for careful monitoring as a safety precaution.

A surface plasmon resonance biosensor based on a graphene-decorated grating excitation structure is proposed in this paper. The biosensor consists of a three-layer structure, including a graphene layer, a grating layer, and a high refractive index layer. The material of the grating layer is silica. The graphene is physically deposited on the grating ridges. An incident light with transverse magnetic polarization is used to excite surface plasmons in the mid-infrared spectral region, which is highly localized at both ends of the graphene layer. The property of the sensor is improved by the high refractive index dielectric layer, which enhances the absorption of incident light and increases the depth of the spectra. The finite-difference time-domain method is used to simulate the property of the sensor. The structure of the sensor could be optimized by changing the structural parameters and comparing the simulation results. The effective refractive index (RI) on the surface and the wavelength of the reflective resonance absorption peak will be changed when the surface of graphene adsorbs the surrounding analyte. The results show that the relationship between the analyte RI and the resonance wavelength is linear. The measurement range of analyte RI is 1–1.8, and the sensitivity is 2780 nm/RIU.

The skillful confinement of light brought by the composite waveguide structure has shown great possibilities in the development of photonic devices. It has greatly expanded the application range of an on-chip system in dark-field imaging and confined the laser when containing an active medium. Here we experimentally proved a stable directional emission in an active waveguide composed of metal and photonic crystal, which is almost completely unaffected by the external environment and different from the common local light field that is seriously affected by the structure. When the refractive index of samples on the surface layer changes, it can ensure the constant emission intensity of the internal mode, while still retaining the external environmental sensitivity of the surface mode. It can also be used for imaging and sensing as a functional slide. This research of chip-based directional emission is very promising for various applications including quantitative detection of biological imaging, coupled emission intensity sensing, portable imaging equipment, and tunable micro lasers.

We have developed a cholesteric liquid crystal (CLC) mirror-based innovative model for detection and visualization of images in turbid media. Due to its unique optical-polarization properties, the CLC mirror is suggested as the basic element of the imaging Stokes polarimeter. The particular design of the proposed polarimeter, coupled with its distinguished operational simplicity, reliability, and real-time operational facilities, promises to fabricate a new generation of the imaging Stokes polarimeter, which can find applications in areas such as diagnostics, biology, astronomy, and remote sensing.

Holographic tomography (HT) is an advanced label-free optical microscopic imaging method used for biological studies. HT uses digital holographic microscopy to record the complex amplitudes of a biological sample as digital holograms and then numerically reconstruct the sample’s refractive index (RI) distribution in three dimensions. The RI values are a key parameter for label-free bio-examination, which correlate with metabolic activities and spatiotemporal distribution of biophysical parameters of cells and their internal organelles, tissues, and small-scale biological objects. This article provides insight on this rapidly growing HT field of research and its applications in biology. We present a review summary of the HT principle and highlight recent technical advancement in HT and its applications.

Field spectral sensors provide real-time, reliable, quantitative monitoring of crop growth. Fitting the continuous growth in the entire growing period from the measurements of limited frequency is helpful to the comparative analysis of interannual growth and fertilizer management in the field. To exploit this capacity, our work presents a model that uses the normalized difference red edge (NDRE) index derived from the field spectral sensor for real-time monitoring of the canopy growth of winter wheat in the whole growing period. We developed this model from experiments in three counties in Hebei province, China, where we obtained the near-infrared and red edge reflectance, grain yield, and canopy parameters for eight growth stages and for various nitrogen ( N ) rates. Given the correlation between effective accumulated temperature and crop growth, we used the growing degree-days as an adjustment parameter to develop models for dynamic monitoring of the NDRE of the winter wheat canopy during the entire growing period. The results show that high determination coefficients ( R 2 = 0.89 to 0.96) are obtained from all models based on relative NDRE and effective accumulative temperature (independent of N fertilization rates). The model based on the rational function is the best of all models tested, with the accuracy for normal and high N fertilization rates being slightly greater than that for low N fertilization rates. Therefore, a relative-NDRE model with the accumulative growing degree-days since sowing could allow monitoring canopy NDRE of winter wheat at any time, which could be helpful for overcoming the shortage of incomparable growth derived from the differences of sensing date, sowing date, and fertilizer, etc.

Holographic tomography allows the 3D mapping of the refractive index of biological samples thanks to reconstruction methods based on the knowledge of illumination directions or rotation angles of the imaged sample. Recently, phase contrast tomographic flow cytometry by digital holography has been demonstrated to reconstruct the three-dimensional refractive index distribution of single cells while they are flowing along microfluidic channels. In this system, the illumination direction is fixed while the sample’s rotation is not deterministically known a priori but induced by hydrodynamic forces. We propose here a technique to retrieve the rolling angles, based on a new phase images similarity metric that is capable of identifying a cell’s orientations from its 3D positioning while it is flowing along the microfluidic channel. The method is experimentally tested and also validated through appropriate numerical simulations. We provide demonstration of concept by achieving reconstruction of breast cancer cells tomography.