High-sensitivity refractive index sensor based on strong localized surface plasmon resonance.

This study proposes two types of composite structures based on gold nano circular and nano square rings on a gold thin film for plasmonic refractive index sensing. The finite-difference time-domain method was used for simulation and analysis. The nano square ring composite structure showed superior performance, with five surface plasmon resonance modes, and a peak sensitivity and figure of merit in a liquid environment of 1600 nm/RIU and 86R I U -1, respectively. The sensing performances of localized surface plasmon resonance modes of both structures are superior to those of the propagating surface plasmon resonance modes. The proposed composite structures can provide a reference for refractive index sensing and have broad application prospects in bio-chemistry.

Intensity controlled and multi-multiplexed full-space achromatic metalens and modulated orbital angular momentum.

The Fabry-Perot (F-P) interference model was used to make a 6-layer metasurface with cross-polarization that can be changed by heat. The fundamental concept behind the metasurface is to utilize the selectivity of linearly polarized waves by a metal grating to achieve broadband and efficient polarized conversion (PC). It also uses the thermal conversion properties of vanadium dioxide (VO2) to control the amplitude of terahertz (THz) waves in a dynamic way. While achromatic metalenses have been extensively studied, altering the entire spatial incidence angle remains relatively uncommon. Enter modulated orbital angular momentum (MOAM), a promising approach for applications such as holographic encryption, optical communication, and imaging. However, achieving MOAM multidimensional multiplexing has proven to be a significant challenge. In response to this challenge, we have integrated the transmission phase into the metasurface design. This new idea makes it possible to make a full spatially achromatic metalenses with angular multiplexing and makes MOAM multidimensional multiplexing easier by letting you switch between frequency, angle, and MOAM modes. This pioneering approach unveils new prospects for enhancing the capacity, rate, and quality of information exchange in domains such as optical encryption, optical imaging, optical communication, and other related technological fields.

Switchable bi-functional metasurface for absorption and broadband polarization conversion in terahertz band using vanadium dioxide and photosensitive silicon.

We proposed a bi-functional switchable metasurface based on vanadium dioxide (VO2) and photosensitive silicon. The metasurface functions as a transmissive polarization converter in its insulating state with asymmetric transmission characteristics. It attains a remarkable polarization conversion rate (PCR) surpassing 90% and a notable maximum asymmetric transmission (AT) parameter value of 0.73. This performance is observed within the frequency range from 4.31 to 7.86 THz. Dynamic regulation of PCR and AT can be achieved by adjusting the conductivity of photosensitive silicon. To illustrate the underlying factor behind the broadband polarization conversion, the surface current distribution is analyzed at 5.96 THz and 6.08 THz. On the other hand, when VO2is in the metallic state, the metasurface transforms into a bidirectional absorber with near-perfect absorption in both illumination directions. Under forward incidence of terahertz waves, the absorption rates for the transverse electric and transverse magnetic waves are 99.3% at 3.54 THz and 93% at 3.56 THz, respectively. The physical mechanism of near-perfect absorption is explained using impedance matching theory and the electric field distribution. This research expands the applications of transmissive polarization converters within multifunctional metasurfaces, providing new avenues for their practical implementation.

Dynamically tunable terahertz quadruple-function absorber based on a hybrid configuration of graphene and vanadium dioxide.

A quadruple-function dynamically tunable terahertz absorber that uses a hybrid configuration of graphene and vanadium dioxide is proposed in this paper. The absorber achieves dynamic conversion of four functions in one structure: ultra-broadband, broadband, single-frequency narrowband and dual-frequency narrowband, by utilizing the electrical control properties of graphene and the phase-shifting properties of vanadium dioxide. Furthermore, the paper also reveals the physical mechanism of the proposed absorber through the electric field distribution and impedance matching theory. In addition, the influences of the Fermi energy level of graphene and the electrical conductivity of vanadium dioxide on the absorption spectra are investigated, demonstrating the structure's dynamic tunability. Due to the above features, the designed absorber is expected to have potential applications in terahertz imaging, modulation and filtering.

Dual-function tunable metasurface for polarization-insensitive electromagnetic induction transparency and dual-band absorption.

In this paper, we propose a dual-operating mode metasurface based on graphene and vanadium dioxide (VO2), which can switch operating modes by changing the temperature. At room temperature (25 °C), the metasurface can generates a polarization-insensitive electromagnetically induced transparency (EIT)-like effect that can be modulated by changing the Fermi energy level (EF) of graphene (through adding external voltage). In addition, the theoretical results derived from the two-particle model are in good agreement with the simulation results based on the finite element method. At high temperature (68 °C), the metasurface mode of operation can be changed to a dual-band absorber, providing absorption of 78.6% and 99.9% at 1.13 THz and 2.16 THz, respectively. Both absorption peaks can be dynamically tuned by changing theEFof graphene. The metasurface is also simultaneously polarization insensitive and has a wide incidence angle. The proposed metasurface can be used as a slow light device with a maximum group delay of 0.5 ps at room temperature and as a refractive index sensor with a maximum sensitivity of 0.5 THz/RIU at high temperature. The designed metasurface offers a new way for designing multifunctional terahertz devices, slow light devices, and refractive index sensors.

A Tunable Terahertz Metamaterial Absorber Composed of Hourglass-Shaped Graphene Arrays.

In this paper, we demonstrate a tunable periodic hourglass-shaped graphene arrays absorber in the infrared (IR) and terahertz (THz) frequency bands. The effects of graphene geometric parameters, chemical potentials, periods, and incident angles on the pure absorption characteristics are studied by using the Finite Difference Time Domain (FDTD) method. In addition, this paper also analyzes the pure absorption characteristics of bilayer graphene arrays. The simulation results show that the maximum absorption reaches 38.2% for the monolayer graphene structure. Furthermore, comparing the bilayer graphene structure with the monolayer structure under the same conditions shows that the bilayer structure has a tunable dual-band selective absorption effect and has a higher maximum absorption of 41.7%. Moreover, it was found that there are dual-band tunable absorption peaks at 26 µm and 36.3 µm with the maximum absorption of 41.7% and 11%. The proposed structure is a convenient method which could be used in the design of graphene-based optoelectronic devices, biosensors, and environmental monitors.

Theoretical Investigation of a Highly Sensitive Refractive-Index Sensor Based on TM0 Waveguide Mode Resonance Excited in an Asymmetric Metal-Cladding Dielectric Waveguide Structure.

This study proposes a highly sensitive refractive-index (RI) sensor based on a TM0 waveguide mode resonance excited in an asymmetric metal-cladding dielectric waveguide structure, where the analyte serves as the guiding layer. By scanning the wavelength at fixed angles of incidence, the reflection spectra of the sensor were obtained. The results showed that the resonance wavelength redshifted dramatically with increases in the analyte RI, which indicates that this approach can be used to sense both the resonance wavelength and the analyte RI. Based on this approach, we investigated the sensing properties, including the sensitivity and figure of merit, at fixed incident angles of 60° and 45°, at which the sensitivity of the sensor reached 7724.9 nm/RIU (refractive index units) and 1339 nm/RIU, respectively. Compared with surface plasmon resonance sensors, which are based on a similar structure, the proposed sensor can accept a more flexible range of incident angles and a wider sensing range of analyte RI. This approach thus has tremendous potential for use in numerous sensing domains, such as biochemical and medical analyses.