In this paper, a switchable multi-functional wideband metasurface device based on vanadium dioxide (VO2) in the terahertz (THz) region is numerically proposed. By controlling the phase of VO2, the functions of the proposed structure can be reversibly switched between a perfect absorber and two different polarization conversions. When VO2 is in the metallic phase, the absorption response can be achieved with an absorption rate above 0.9 in the range of 1.42 - 4.13 THz for wide-angle incidence, contributed by the highly lossy feature of VO2 metallic. Meanwhile, when VO2 acts as an insulator phase, the proposed metasurface exhibits linear-to-linear polarization conversion in the range of 1.27 - 3.53 THz and linear-to-circular polarization conversion in the frequency ranges of 1.08 - 1.24 THz and 3.65 - 5.27 THz simultaneously. This work opens new avenues for realizing multi-functional wideband metasurface devices in the terahertz range.

Reconfigurable metasurfaces (RMSs) that enable the switching function of absorption and polarization conversion have attracted increasing attention. However, the design of RMSs to achieve wideband and high efficiency for both absorption and polarization conversion functions simultaneously remains a great challenge. Here, we propose the design of a RMS structure with a high-efficiency cross-polarization conversion and nearly perfect absorption. The reconfiguration between different functions of polarization conversion and absorption is obtained based on the reversible insulator-to-metal phase transition of Vanadium dioxide (VO $$_{2}$$2 ). When the VO $$_{2}$$2 is in insulator state, the RMS realizes the cross-polarization conversion function in the wideband of 1.04–3.75 THz with a relative bandwidth up to 113 $$\%$$% due to the multi-resonant modes of electric and magnetic resonances. Meanwhile, the nearly-perfect absorption is achieved in the range of 1.36–3.38 THz with the corresponding relative bandwidth up to 85 $$\%$$% for the VO $$_{2}$$2 in metallic state. Specially, the wideband and high-efficiency performance of these functionalities is maintained for a wide angle incidence. The capability of bi-functional switch and integration with polarization conversion and absorption in a single metasurface structure endowed with both wideband and high-efficiency characteristics for a wide incident angle is very promising for emerging RMS devices in the terahertz region.

Surface plasmon resonance (SPR) based sensors are usually designed using the Kretschmann prism coupling configuration in which an input beam couples with a surface plasmon through a thin metal film. This is generally preferred by sensor developers for building planar devices instead of the Otto prism coupling configuration, which, for efficient coupling, requires the metal surface to be maintained at a distance on the order of the wavelength from the input prism surface. In this paper, we report on the microfabrication and characterization of an Otto chip device, which is suitable for applications of the SPR effect in gas sensing and biosensing.

Developing a millimeter-wave (mm-wave) antenna that enables wide bandwidth with its operating band covering the entire global 5G spectrum is highly desirable but very challenging for achieving both compact size and high-performance antenna. Herein, the mm-wave microstrip patch antenna (MPA) and its multiple-input multiple-output (MIMO) configuration based on the metasurfaces for 5G system applications are proposed and investigated by the simulation method. To improve performance and keep the low-profile and low-cost MPA antenna, square ring resonator (SQRR) metasurface and radiating patch are printed on a single dielectric layer. With the presence of the metasurfaces that acting as a secondary radiator, the performance of the designed antenna is significantly improved with a wide operating band in the range of 23.9-30.7 GHz, high peak gain of 9.4 dBic, and radiation efficiency of above 87%. Based on this design, four-port MIMO antenna configuration is performed for evaluating a MIMO system that realizes the advantage features such as compact size, wide bandwidth covering the entire global mm-wave 5G spectrum band of 24.25-29.5 GHz, and excellent diversity performance characterized by good isolation between the adjacent elements and low envelope correlation coefficient. Thus, the MIMO antenna design is a very promising candidate for 5G MIMO mm-wave applications, specifically in cellular systems.

Polarization-control devices have attracted considerable interest, however, most of the polarization converters operating at lower frequencies have a heavy design and narrow bandwidth which limits their practical applications. Here we report a simple design of an ultra-wideband and lightweight polarization converter for applications in the S- and C-bands. The proposed converter is designed based on a metasurface structure with the dielectric layer modified to hollow structure to obtain a lightweight design even working at such low frequency. Theoretical analysis and simulation results indicate that the converter can convert the orthogonal polarization transformation of reflected wave. Furthermore, the measurement results show good agreement with the simulation results. The proposed polarization converter can achieve a polarization conversion ratio above 90% in an ultra-wide frequency range from 2 to 8.45 GHz due to multi-resonance modes. These performances are going beyond state of the art in terms of bandwidth and lightweight design, thus it can be applied in various applications in the operating bands.

We have experimented with using continuous self-assembled monolayer (SAM) coatings on different surfaces, such as gold and silicon nitride, as anti-stiction coatings for radio-frequency microelectromechanical systems (RF MEMS). SAMs are coated on gold and silicon nitride surfaces in order to fabricate stiction-free gold structures after the wet release process. SAM coatings on gold and silicon nitride surfaces change the contact angle of a water droplet from 70.6° to 112.6° and from 36.7° to 115.8°, respectively. From atomic force microscopy, we can see that there is little change in the roughness of the gold and silicon nitride before and after the SAM coating. Using the electroplating technique and the wet release process, we have fabricated 4 μm thick gold cantilevers and bridges on a gold surface with a 6.5 μm gap from the substrate. Cantilevers and bridges are fabricated with different lengths in the range of 100–1000 μm with separations of 100 μm. It is observed that cantilevers and bridges with a length of 1000 μm on a gold substrate are released and standing free. A 1000 μm long gold cantilever has only 16.5 μm tip end deflection after the wet release process and SAM coating, much smaller compared with the 120 μm tip end deflection with the dry release process. Gold cantilevers, 100–500 μm long, on silicon nitride are released free and gold bridges on silicon nitride are released with a length of 1000 μm with continuous SAMs. The SAMs are coated on gold and silicon nitride surface in order. This method enables the gold cantilever and bridge on the gold or silicon nitride surface to be released with low deflection.