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A refractive index sensor based on metal-insulator-metal (MIM) waveguides coupled double rectangular cavities is proposed and investigated numerically using the finite element method (FEM). The transmission properties and refractive index sensitivity of various configurations of the sensor are systematically investigated. An asymmetric Fano resonance lineshape is observed in the transmission spectra of the sensor, which is induced by the interference between a broad resonance mode in one rectangular and a narrow one in the other. The effect of various structural parameters on the Fano resonance and the refractive index sensitivity of the system based on Fano resonance is investigated. The proposed plasmonic refractive index sensor shows a maximum sensitivity of 596 nm/RIU.

Metasurfaces suitable for the terahertz gap are alternatives to naturally occurring materials and could accelerate the development of terahertz flat optics integrated with terahertz continuous-wave sources. However, metasurfaces have yet to be commonly adopted in terahertz devices that require a number of specific material properties because of the many choices in meta-atom design. In this paper, we demonstrate that simple dimension control of a single kind of meta-atom enables the design of a wide range of refractive indices from large positive to negative values in the 0.3-THz band. Measurements by terahertz time-domain spectroscopy verify three kinds of metasurfaces with (1) an extremely high refractive index of 12.3 +  j 0.88 and reflectance of 5.1% at 0.31 THz, (2) a zero refractive index of − 0.44 +  j 0.12 and reflectance of 2.6% at 0.34 THz, and (3) a negative refractive index of − 5.4 +  j 0.32 and reflectance of 22.7% at 0.31 THz. The 0.3-THz band is a frequency band for candidates to 6G wireless communications. Our results offer an accessible material platform for terahertz flat optics such as metalenses, antennas, and phase plates. Terahertz flat optics based on our presented metasurfaces would be a welcome contribution to the development of terahertz industries, including 6G wireless communications.

In this paper, a new sensor structure is designed, which consists of a metal–insulator–metal (MIM) waveguide and a circular protrusion and a rectangular triangular cavity (CPRTC). The characterization of nanoscale sensors is considered using an approximate numerical method (finite element method). The simulation results show that the sharp asymmetric resonance generated by the interaction between the discrete narrow-band mode and the continuous wideband mode is called Fano resonance. The performance of the sensor is considerably influenced by CPRTC. The sensor structure has attained a sensitivity of 3060 nm/RIU and a figure of merit (FOM) of 53.68. In addition, the application of this structure to temperature sensors is also investigated; its sensitivity is 1.493 nm/°C. The structure also has potential for other nanosensors.

The design theory for electromagnetic metamaterials with negative refractive indices by using a distributed transmission-line model is introduced to the design of acoustic metamaterials, and a negative refractive index (NRI) acoustic lens is designed theoretically. Adjustments to the negative refractive indices of metamaterials have been carried out by calculations with numerical simulators in conventional design methods. As the results show, many calculations are needed to determine the shape of the unit structures and there are issues in that it is difficult to design those rigorously, meaning that limitations regarding the degree of freedom in the designs are many. On the other hand, the transmission-line model can rigorously design the unit cell structures of both the negative refractive index metamaterials and the background media with the positive refractive indices by calculations with the design formulas and modifying the error from the theory with a small calculation. In this paper, a meander acoustic waveguide unit cell structure is proposed in order to realize a structure with characteristics equivalent to the model, and the waveguide width and length for realizing an NRI acoustic lens are determined from the design formula of the model. The frequency dispersion characteristics of the proposed structure are also computed by eigenvalue analysis and the error in the waveguide length from the theoretical value is modified by a minor adjustment of the waveguide length. In addition, the NRI acoustic lens is constituted by periodically arranging the proposed unit cell structure with the calculated parameters, and the full-wave simulations are carried out to show the validity of the design theory. The results show that the designed lens operates at 2.5 kHz.

The refractive index is an important optical property of materials which can be used to understand the composition of materials. Therefore, refractive index sensing plays a vital role in biological diagnosis and therapy, material analysis, (bio)chemical sensing, and environmental monitoring. Conventional optical refractive index sensors based on optical fibers and ridge waveguides have relatively large sizes of a few millimeters, making them unsuitable for on-chip integration. Photonic crystals (PCs) have been used to significantly improve the compactness of refractive index sensors for on-chip integration. However, PC structures suffer from defect-introduced strong scattering, resulting in low transmittance, particularly at sharp bends. Valley photonic crystals (VPCs) can realize defect-immune unidirectional transmission of topological edge states, effectively reducing the scattering loss and increasing the transmittance. However, optical refractive index sensors based on VPC structures have not been demonstrated. This paper proposes a refractive index sensor based on a VPC Mach–Zehnder interferometer (MZI) structure with a high forward transmittance of 0.91 and a sensitivity of 1534%/RIU at the sensing wavelength of λ = 1533.97 nm within the index range from 1.0 to 2.0, which is higher than most demonstrated optical refractive index sensors in the field. The sensor has an ultracompact footprint of 9.26 μm × 7.99 μm. The design can be fabricated by complementary metal–oxide semiconductor (CMOS) fabrication technologies. Therefore, it will find broad applications in biology, material science, and medical science.

A terahertz photonic crystal fiber with two sensing channels was designed. Graphene coated on the micro-grooves in the cladding was used as plasma material to introduce tunability. The dispersion relation, mode coupling, and sensing characteristics of the fiber were studied using the finite element method. Ultrahigh sensitivity of 2.014 THz/RIU and 0.734 GHz/°C were obtained for analytes with refractive index in the range of 1.33 to 1.4 and environment temperature in the range of 10–60 °C, respectively. Refractive index resolution can reach the order of 10−5. The dual parameter simultaneous detection, dynamic tunable characteristics, and working in the low-frequency range of terahertz enable the designed photonic crystal fiber to have application prospects in the field of biosensing.

The seawater refractive index is an important parameter in marine environments, with its variations depending on the specific environmental conditions. During practical applications, modulation parameters such as the sampling rate, bandwidth, and filters directly affect the signal-to-noise ratio (SNR) and need to be adjusted in real-time according to the characteristics of the target signal. Low-cost software-defined radio (SDR) offers significant advantages in this regard. This paper proposes an optical coherence measurement method for seawater refractive index changes based on orthogonal demodulation using SDR along with simulation calculations, and the results demonstrate that the resolution of the refractive index change rate is 3.165×10−9 RIU/s, corresponding to a refractive index change resolution of 10−10 RIU (frequency range 1 Hz–100 Hz, measurement range 0.1 m). By adopting SDR as the implementation platform for the demodulation algorithm and using a radio-frequency source to simulate interference signals for demodulating the refractive index variation, the results show that the relative error of the SDR demodulation results is below 0.3%. Additionally, this study developed a software-defined optical coherence measurement system for the seawater refractive index and measured the refractive index changes in deionized water during heating. The experimental results showed that the root mean square error (RMSE) of the refractive index changes obtained through SDR demodulation was 5.68×10−6 RIU. This research provides a novel demodulation method for high-precision measurements of seawater refractive index changes under different marine environments.

In hot-forming, integrating in situ quality monitoring is essential for the early detection of thermally induced geometric deviations, especially in the production of hybrid bulk metal parts. Although hybrid components are key to meeting modern technical requirements and saving resources, they exhibit complex shrinkage behavior due to differing thermal expansion coefficients. During forming, these components are exposed to considerable temperature gradients, which result in density fluctuations in the ambient air. These fluctuations create an inhomogeneous refractive index field (IRIF), which significantly affects the accuracy of optical geometry reconstruction systems due to light deflection. This study utilizes existing simulation IRIF data to predict the magnitude and orientation of refractive index fluctuations. A light deflection simulation run on a GPU-accelerated ray tracing framework is used to assess the impact of IRIFs on optical measurements. The results of this simulation are used as a basis for selecting optimized measurement positions, reducing and quantifying uncertainties in surface reconstruction, and, therefore, improving the reliability of quality control in hot-forming applications.

Accurate estimates of aerosol refractive index (RI) are critical for modeling aerosol‐radiation interaction, yet this information is limited for ambient organic aerosols, leading to large uncertainties in estimating aerosol radiative effects. We present a semi‐empirical model that predicts the real RI n of organic aerosol material from its widely measured oxygen‐to‐carbon (O:C) and hydrogen‐to‐carbon (H:C) elemental ratios. The model was based on the theoretical framework of Lorenz‐Lorentz equation and trained with n‐values at 589 nm (n589nm${n}_{589\\mathrm{n}\\mathrm{m}}$ ) of 160 pure compounds. The predictions can be expanded to predict n‐values in a wide spectrum between 300 and 1,200 nm. The model was validated with newly measured and literature datasets of n‐values for laboratory secondary organic aerosol (SOA) materials. Uncertainties of n589nm${n}_{589\\mathrm{n}\\mathrm{m}}$predictions for all SOA samples are within ±$\\pm $ 5%. The model suggests that n589nm${n}_{589\\mathrm{n}\\mathrm{m}}$ ‐values of organic aerosols may vary within a relatively small range for typical O:C and H:C values observed in the atmosphere. Plain Language Summary Atmospheric aerosol particles play an important role in affecting the climate by interacting with radiation and water. However, we have limited knowledge of the optical properties of atmospheric organic aerosols, which make up a large fraction of sub‐micrometer aerosol particle mass. One of the challenges is that the RI, that is, the intrinsic optical constant of organic aerosol (OA) material, is poorly constrained. The lack of knowledge on the RI of organic aerosols can cause large uncertainties in estimating their optical properties and radiative effects on climate. To address this knowledge gap, a semi‐empirical model is developed and validated that predicts the real RI of OA material based on the widely measured bulk chemical composition in laboratory and field studies. The model predictions suggest that the RI of typical ambient organic aerosols may have relatively small changes, which supports a simplified representation of using a constant n‐value for ambient OA in atmospheric models. Potential applications of the developed model also include improving remote sensing and in situ optical sizing of aerosols. Key Points A new model was developed to predict the real refractive index (RI) of organic aerosols using elemental ratios The model accuracy was validated with measurements of various secondary organic aerosols The model predicts small variation in real RI at 589 nm for typical oxygen‐to‐carbon and hydrogen‐to‐carbon values of organic aerosols in the atmosphere

Quantitative Phase Imaging (QPI) provides unique means for the imaging of biological or technical microstructures, merging beneficial features identified with microscopy, interferometry, holography, and numerical computations. This roadmap article reviews several digital holography-based QPI approaches developed by prominent research groups. It also briefly discusses the present and future perspectives of 2D and 3D QPI research based on digital holographic microscopy, holographic tomography, and their applications.