Transmission-Reflection-Integrated Multifunctional Passive Metasurface for Entire-Space Electromagnetic Wave Manipulation.

In recent years, many intriguing electromagnetic (EM) phenomena have come into being utilizing metasurfaces (MSs). However, most of them operate in either transmission or reflection mode, leaving the other half of the EM space completely unmodulated. Here, a kind of transmission-reflection-integrated multifunctional passive MS is proposed for entire-space electromagnetic wave manipulation, which can transmit the x-polarized EM wave and reflect the y-polarized EM wave from the upper and lower space, respectively. By introducing an H-shaped chiral grating-like micro-structure and open square patches into the unit, the MS acts not only as an efficient converter of linear-to-left-hand circular (LP-to-LHCP), linear-to-orthogonal (LP-to-XP), and linear-to-right-hand circular (LP-to-RHCP) polarization within the frequency bands of 3.05-3.25, 3.45-3.8, and 6.45-6.85 GHz, respectively, under the x-polarized EM wave, but also as an artificial magnetic conductor (AMC) within the frequency band of 12.6-13.5 GHz under the y-polarized EM wave. Additionally, the LP-to-XP polarization conversion ratio (PCR) is up to -0.52 dB at 3.8 GHz. To discuss the multiple functions of the elements to manipulate EM waves, the MS operating in transmission and reflection modes is designed and simulated. Furthermore, the proposed multifunctional passive MS is fabricated and experimentally measured. Both measured and simulated results confirm the prominent properties of the proposed MS, which validates the design's viability. This design offers an efficient way to achieve multifunctional meta-devices, which may have latent applications in modern integrated systems.

Asymmetric Cross Metasurfaces with Multiple Resonances Governed by Bound States in the Continuum.

The bound state in the continuum (BIC) has paved a new way to achieve excellent localization of the resonant mode coexisting with a continuous spectrum in the metasurface. Here, we propose an all-dielectric metasurface consisting of periodic pairs of asymmetric crosses that supports multiple Fano resonances. Due to the sufficient degrees of freedom in the unit cell, we displaced the vertical bars horizontally to introduce in-plane perturbation, doubling the unit cell structure. Dimerization directly resulted in the folding of the Brillouin zone in k space and transformed the BIC modes into quasi-BIC resonances. Then, simultaneous in-plane symmetry breaking was introduced in both the x and y directions to excite two more resonances. The physical mechanisms of these BIC modes were investigated by multipole decomposition of the scattering cross section and electromagnetic near-field analysis, confirming that they are governed by toroidal dipole (TD) modes and magnetic dipole (MD) modes. We also investigated the flexible tunability and evaluated the sensing performance of our proposed metasurface. Our work is promising for different applications requiring stable and tunable resonances, such as optical switching and biomolecule sensing.

Dual Tunable Electromagnetically Induced Transparency Based on a Grating-Assisted Double-Layer Graphene Hybrid Structure at Terahertz Frequencies.

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices.

High-efficiency, four-channel beam splitter based on a fishnet-shaped continuous metasurface.

Beam splitters play important roles in several optical systems. Due to the growing demand for the miniaturization of optical systems, it is necessary to design beam splitters with nanoscale dimensions to miniaturize the essential components for integrated optical circuits. In this work, we propose and numerically demonstrate a broadband, high efficient, and four-channel beam splitter based on a fishnet-shaped metasurface. The proposed structure is constructed of cruciform AlSb nanoantennas on the PDMS substrate. The simple design can split a beam of light into four beams with equal intensity, it achieves a conversion efficiency above 83%, and an anomalous transmission intensity exceeding 0.8 for the wavelength range of 761-835 nm. In this wavelength range, the beam splitting angle changes from 46.45° to 53.68°. Moreover, the four-channel beam splitter is tunable when the metasurface is designed as a discrete structure. At the wavelength of 874 nm, the beam splitting angle can be adjusted from 56.34° to 46.39° as the period increases from 1050 nm to 1207 nm by stretching the substrate. The presented metasurface might enable promising applications in integrated optical devices, owing to its advantages of multi-channel, wide broadband, high efficiency, and large beam split angle.

Ultra-Short Polarization Rotator Based on Flat-Shaped Photonic Crystal Fiber Filled with Liquid Crystal.

In this study we demonstrate a high-performance polarization rotator (PR) based on flat-shaped photonic crystal fiber. The flat surfaces of the fiber are plated on gold films as electrodes, and the core of the structure is filled with liquid crystal. The polarization rotation characteristics of the flat-shaped fiber can be effectively adjusted by applying external voltage. The optical properties are analyzed using the finite element method (FEM). The results show that the magnitude of the modulation voltage is closely related to the thickness of the flat fiber. When the fiber thickness is 20 µm, only 100 V is required to achieve the highest PR performance. In the wavelength of the 1.55 µm band (~200 nm bandwidth), the conversion length of the PR is only 3.99 µm, the conversion efficiency is close to 100%, and the minimum crosstalk value is -26.2 dB. The presented PR, with its excellent performance, might enable promising applications in the communication system and the photonic integrated circuits.

Numerical analysis of an ultra-broadband and highly efficient beam splitter in the visible region.

We report a quasi-continuous beam splitter with highly efficient equal-power beam splitting in a wide spectral range. It consists of rhombic aluminum antimonide nanorods standing on a silica substrate. Firstly, a beam splitter based on discrete structures is designed, and the structures are optimized to obtain the quasi-continuous beam splitter. The beam splitter achieves a splitting efficiency of over 80% within the region of 675-786 nm (bandwidth = 111 nm), where the splitting angle can vary in the range of 97.2°-121.8°. In particular, the splitting efficiency reaches 93.4% when the wavelength is 690 nm. Overall, the proposed beam splitter potentially paves the way for realizing broadband metasurfaces and high-performance quasi-continuous metasurface-based devices.

Ultra-Broadband and Highly Efficient Beam Splitter Based on Quasi-Continuous Metasurface in the Near-Infrared Region.

Beam splitters are vital components in several optical systems. It is highly desirable, and compact beam splitters with ultra-broadband performances, high efficiencies, and large split angles are still being sought. In this paper, we demonstrate and numerically investigate an ultra-broadband and highly efficient optical beam splitter based on a quasi-continuous metasurface. The proposed design is constructed of quasi-continuous triangle-shaped gallium phosphide nanoantennas on a silica substrate. The simple structure can achieve a conversion efficiency and an anomalous transmission intensity above 90% and 0.8 covering the wavelength range of 1537-1826 nm, respectively. The maximum beam split angle in the operating bandwidth reaches 131.84° at the wavelength of 1826 nm. Particularly, the operating bandwidth is still as high as 125 nm with the anomalous transmission intensity above 0.92 and the conversion efficiency exceeding 99%. Moreover, the results show that the performance of the metasurface-based optical beam splitter can be further enhanced by optimizing structural parameters. We also demonstrate the adjustability of the beam splitter by adding refractive index (RI) materials on the surface of the device. The results show that the incident plane wave can be divided into three beams with intensity adjustability. The presented metasurface is very promising in the fields of multiplexers, interferometers, and optical communications, owing to its advantages of ultra-broadband, highly efficient, and large split angle simultaneously.

Design of Multifunctional Tunable Metasurface Assisted by Elastic Substrate.

Metasurfaces with both multifunctionality and tunability hold great application potential in next-generation optical devices. In this paper, we propose a stretchable metasurface composed of arrays of identical dielectric rectangular resonators embedded in the polydimethylsiloxane (PDMS) substrate. It is shown that the metasurface possesses three functions at the operating wavelength of 532 nm. The switching of functions can be implemented by changing the period Px of the metasurface, induced by stretching the PDMS substrate along the x-direction. When the period Px is less than the operating wavelength of 532 nm, the behavior of metasurface can switch between transmissive window and reflective mirror. When the period Px of the metasurface varies from 532 nm to 700 nm, the metasurface act as a dynamic equal-power beam splitter with conversion efficiency higher than 90%, and the corresponding splitting angle can be adjusted from 90° to around 49.5°. Moreover, we achieve the switching of transmissive window/reflective mirror/split-ratio-variable splitter based on the metasurface consisting of arrays of identical L-shaped resonators embedded in the PDMS substrate.

A Multi-Parameter Integrated Sensor Based on Selectively Filled D-Shaped Photonic Crystal Fiber.

We propose and numerically investigate a multi-parameter integrated sensor based on a selectively filled D-shaped photonic crystal fiber (PCF). The simple structure can be used to comprehensively detect refractive index, magnetic field, temperature, and voltage. According to the surface plasmon resonance and directional coupling effect, the PCF is coated with a gold nano-film to detect the refractive index of the external environment. In addition, magnetic fluid (water-based Fe3O4), toluene, and nematic liquid crystal (NLC E7) are selectively filled into different cladding air holes of the D-shaped PCF to realize the different sensing of the magnetic field, temperature, and voltage. The measurement of refractive index, magnetic field, temperature, and voltage are independent of each other, so these four parameters can be measured simultaneously. The sensing characteristics of the proposed structure are investigated systematically by the finite element method. The results show that the sensitivities of refractive index, magnetic field, temperature, and voltage are 4600 nm/RIU, 1.375 nm/Oe, 15.143 nm/°C, and 0.971 nm/V, respectively. The presented design based on materials selectively filled with D-shaped PCF might enable promising application in multi-parameter optical sensing.

Broadband anomalous reflective metasurface for complementary conversion of arbitrary incident polarization angles.

The abrupt phase changes at the interface can modulate the polarization and wavefront of electromagnetic waves, which is the physical mechanism of the plasmonic metasurfaces. Conventional polarization converters are difficult to obtain pure polarized light, and most of the anomalously reflecting metasurfaces are limited by the specific angle of incident polarization. Here, we present a high-efficient polarization-independent metasurface for broadband polarization conversion and anomalous reflection when a plane wave with an arbitrary polarization angle is incident vertically. We vary the dimensions of the polarization conversion unit cells and arrange them periodically to cover the full 2π phase range of cross-polarized light in two orthogonal directions. The simulation results show that the pure anomalous cross-polarization efficiency is over 80% over a wavelength range from 1400nm to 1800nm. In particular, the metasurface can realize the complementary conversion of polarization angle for incident light at any polarization angle, and deflect it to a specific angle. Our design provides strategies for miniaturization and integration of polarization conversion devices and systems.

A nanopillar-modified high-sensitivity asymmetric graphene-GaN photodetector.

Integration of two-dimensional (2D) materials with three-dimensional (3D) semiconductors leads to intriguing optical and electrical properties that surpass those of the original materials. Here, we report the high performance of a GaN nanopillar-modified graphene/GaN/Ti/Au photodetector (PD). After etching on the surface of a GaN film, GaN nanopillars exhibit multiple functions for improving the detection performance of the PD. Under dark conditions, surface etching reduces the contact area of GaN with the graphene electrode, leading to a reduced dark current for the PD. When illuminated with UV light, the nanopillars enable an enhanced and localized electric field inside GaN, resulting in an ∼20% UV light absorption enhancement and a several-fold increased photocurrent. In addition, the nanopillars are intentionally etched beneath the metal Ti/Au electrode to modify the semiconductor-metal junction. Further investigation shows that the modified GaN/Ti/Au contact triggers a prominent rectifying I-V behaviour. Benefiting from the nanopillar modification, the proposed PD shows a record large detectivity of 1.85 × 1017 Jones, a small dark current of 5.2 nA at +3 V bias, and a nearly three order of magnitude rectification ratio enhancement compared with non-nanopillar PDs. This pioneering work provides a novel nanostructure-modifying method for combining 2D materials and 3D semiconductors to improve the performances of electronic and optoelectronic devices.

All-Dielectric Huygens' Metasurface for Wavefront Manipulation in the Visible Region.

All-dielectric Huygens' metasurfaces have been widely used in wavefront manipulation through multipole interactions. Huygens' metasurfaces utilize the superposition between an electric dipole and a magnetic dipole resonance to realize transmission enhancement and an accumulated 2π phase change. Benefiting from this unique property, we design and numerically investigate an all-dielectric Huygens' metasurface exhibiting high-efficiency anomalous refraction. To suppress the substrate effect, the metasurface structure is submerged in a dielectric plate. We strategically placed two elements in four short periods to form a unit cell and adjusted the spacing between the two elements to effectively inhibit the interaction between elements. At the operating wavelength of 692 nm, the obtained anomalous transmission efficiency is over 90.7% with a diffraction angle of 30.84°. The performance of the proposed structure is far superior to most of the existing phase-gradient metasurface structures in the visible region, which paves the way for designing efficient beam deflection devices.

All-Dielectric Phase-Gradient Metasurface Performing High-Efficiency Anomalous Transmission in the Near-Infrared Region.

We propose and numerically demonstrate a phase-gradient metasurface with high anomalous transmission efficiency and a large anomalous refraction angle that consists of discontinuous regular hexagonal nanorods supported by a silica substrate. The metasurface achieves high anomalous transmission efficiency and a full 2[Formula: see text] phase shift for the wavelength range of 1400-1600 nm. At a central wavelength of approximately 1529 nm, the total transmission efficiency reaches 96.5%, and the desired anomalous transmission efficiency reaches 96.2%, with an anomalous refraction angle as large as 30.64. With the adjustment of the period and the number of nanorods per periodic interval, the anomalous transmission efficiency exceeds 69.6% for a large anomalous refraction angle of 68.58. The superior performance of the proposed design may pave the way for its application in optical wavefront control devices.

High-Efficiency, Dual-Band Beam Splitter Based on an All-Dielectric Quasi-Continuous Metasurface.

Metasurface-based beam splitters attracted huge interest for their superior properties compared with conventional ones made of bulk materials. The previously reported designs adopted discrete metasurfaces with the limitation of a discontinuous phase profile. In this paper, we propose a dual-band beam splitter, based on an anisotropic quasi-continuous metasurface, by exploring the optical responses under x-polarized (with an electric field parallel to the direction of the phase gradient) and y-polarized incidences. The adopted metasurface consists of two identical trapezoidal silicon antenna arrays with opposite spatial variations that lead to opposite phase gradients. The operational window of the proposed beam splitter falls in the infrared and visible region, respectively, for x- and y-polarized light, resulting from the different mechanisms. When x-polarized light is incident, the conversion efficiency and total transmission of the beam splitter remains higher than 90% and 0.74 within the wavelength range from 969 nm to 1054 nm, respectively. In this condition, each array can act as a beam splitter of unequal power. For y-polarized incidence, the maximum conversion efficiency and transmission reach approximately 100% and 0.85, while the values remain higher than 90% and 0.65 in the wavelength range from 687 nm to 710 nm, respectively. In this case, each array can be viewed as an effective beam deflector. We anticipate that it can play a key role in future integrated optical devices.

Ultra-broadband large-angle beam splitter based on a homogeneous metasurface at visible wavelengths.

Metasurface-based beam splitters with high efficiency, large split angle, wide bandwidth and easy fabrication are highly desirable and still in pursuit. In this paper, we propose a heuristic scheme for designing an ultra-broadband high-efficiency power beam splitter based on a homogeneous metasurface. The conversion efficiency and total transmission intensity of the power splitter stays higher than 95% and 0.66 within the wavelength region from 604 nm to 738 nm, respectively. Particularly, the conversion efficiency can maintain greater than 99% in 58 nm bandwidth. The angle between two split beams can reach a maximum of 157.82° at the wavelength of 738 nm. In addition to simplified design and easy fabrication, the proposed power beam splitter possesses high robustness as well. We expect that our design can pave a new way for realizing high-performance metasurface-based beam splitters.

Mechanisms of 2π phase control in dielectric metasurface and transmission enhancement effect.

Metasurfaces, two-dimensional structures composed of nanoantennas in an array configuration, can be used to fully control electromagnetic waves, which requires a 2π phase shift. Herein, we apply the silicon metasurface as an example to interpret the mechanisms of full 2π phase coverage. It is found that the mechanism varies from Fabry-Pérot resonance to Mie resonance as the period increases for a metasurface with certain height. Particularly, there is a transition region between these two types of resonance. We present the corresponding periods and wavelength regions of the different mechanisms when considering the phase-gradient metasurface with at most three diffraction orders. Moreover, the transmission enhancement of metasurface is investigated. The transmission efficiency can be effectively improved when the nanoantenna is changed from a uniform structure to a gradient-index one. We expect that the results can simplify the design process and provide a reference for the future design of all-dielectric metasurface with 2π phase control.

All-dielectric colored truncated cone metasurfaces with silicon Mie magnetic resonators.

We propose high-index truncated cone silicon metasurfaces based mainly on magnetic Mie resonances. From numerical simulation, the intensity of the reflection peak reaches almost 90%, and the full width at half-maximum (FWHM) of the reflectance spectrum is 43 nm. Specific colors covering the entire visible spectrum with saturation close to 1 are available by selecting appropriate geometric dimensions and period of the structure. In summary, the structural colors achieved by the proposed metasurfaces are superior to previous research in comprehensive aspects of reflection peak, the FWHM of the reflectance spectrum, and the saturation of the color. Furthermore, the proposed structure works with a low aspect ratio of 0.46, which largely relieves the difficulty of process manufacturing.

Realization of perfect selective absorber based on multipole modes in all-dielectric moth-eye structure.

Perfect absorbers play crucial roles in optical functional devices. Among various types of absorbers, moth-eye structures are known for their excellent absorbing efficiency. In this paper, we apply an electromagnetic multipole expansion method to treat an isolated all-dielectric moth-eye structure as a large particle and calculate various electric and magnetic multipole modes within the moth-eye structure. In periodical array, the multipole modes within each particle interact with each other. These constructive or destructive interactions cause shifts in the multipole resonant peaks. The multipole modes inside the particle array introduce reflecting peaks for loss-less materials. The absorption enhancement inside moth-eye structures can be attributed to the electric field enhancement resulting from these electric and magnetic multipole modes. Based on our theoretical study, we propose a near-ideal selective absorber based on moth-eye array, which achieves near 100% absorption within wavelength range from 400 nm to 1500 nm while achieving near 0% absorption over about 1700 nm.

Efficient Polarization Beam Splitter Based on All-Dielectric Metasurface in Visible Region.

In this paper, we present an all-dielectric gradient metasurface, composed of periodic arrangement of differently sized cross-shaped silicon nanoblocks resting on the fused silica substrate, to realize the function of polarization split in visible region. The cross-shaped silicon block arrays can induce two opposite transmission phase gradients along the x-direction for the linear x-polarization and y-polarization. By properly designing, the metasurface can separate the linearly polarized light into x- and y-polarized ones, which propagate at the same angle along the left and right sides of the normal incidence in the x-z plane. Particularly, when a beam with the polarization angle of 45.0° is incident on the proposed device, the x- and y-polarized transmitted ones possess nearly equal intensity within the wavelength range from 579 to 584 nm. We expect the proposed polarization beam splitter can play an important role for future free-space optical devices.

Highly sensitive surface plasmon resonance biosensor based on a low-index polymer optical fiber.

A highly sensitive refractive index sensor based on surface plasmon resonance in a side-polished low-index polymer optical fiber is proposed for biosensing. Benefitting from the low refractive index of the fiber core, the sensitivity of the device can reach ~44567 nm/RIU theoretically for aqueous solutions, at the expense of a lowered upper detection limit that is down to ~1.340. The sensor is fabricated by coating 55-nm-thick Au-film on the polished surface of a graded-index perfluorinated polymer optical fiber. Results show that the sensor exhibits a sensitivity of ~22779 nm/RIU at 1.335 with a figure of merit of 61.2. When employed for glucose sensing, the sensor presents an averaged sensitivity of 24.50 nm/wt%, or 0.46 nm/mM. This device is expected to have potential applications in cost-effective bio- and chemical-sensing.