Metasurfaces, two-dimensional equivalents of metamaterials, are engineered surfaces consisting of deep subwavelength features that have full control of the electromagnetic waves. Metasurfaces are not only being applied to the current devices throughout the electromagnetic spectrum from microwave to optics but also inspiring many new thrilling applications such as programmable on-demand optics and photonics in future. In order to overcome the limits imposed by passive metasurfaces, extensive researches have been put on utilizing different materials and mechanisms to design active metasurfaces. In this paper, we review the recent progress in tunable and reconfigurable metasurfaces and metadevices through the different active materials deployed together with the different control mechanisms including electrical, thermal, optical, mechanical, and magnetic, and provide the perspective for their future development for applications.

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Tunable and reconfigurable metasurfaces and metadevices

We present the design, fabrication, and realization of an electrically tunable metamaterial operating in the mid-infrared spectral range. Our devices combine intersubband transitions in semiconductor quantum-wells with planar metamaterials and operate in the strong light-matter coupling regime. The resonance frequency of the intersubband transition can be controlled by an external bias relative to the fixed metamaterial resonance. This allows us to switch dynamically from an uncoupled to a strongly coupled system and thereby to shift the eigenfrequency of the upper polariton branch by 2.5 THz (corresponding to 8% of the center frequency or one full linewidth) with a bias of 5 V.

Tunable metamaterials based on voltage controlled strong coupling

Design, fabrication and characterization of miniature RF MEMS switched capacitor based phase shifter

The design of a metamaterial-based tunable band elimination filter is described on silicon coplanar wave- guide (CPW) using complementary split ring resonators (CSRRs) and radio frequency microelectromechanical systems (MEMS) varactors to ensure compatibility with integrated circuit fabrication technology. CSRRs are patterned on both the active signal line and ground on both sides. A cantilever bridge is suspended on the signal line to realize variable capacitance. The rejection band of the filter is tuned by moving the MEMS bridge up or down using electrostatic actuation. A stopband around the center frequency of 35.32 GHz with a rejection ratio of $\sim 20.19$ dB is obtained when the bridge is pulled down. Another stopband around the center frequency 38.80 GHz is observed with a rejection ratio of $\sim 18.29$ dB when the bridge is allowed to remain in its up state without electrostatic actuation. Simulation for RF behavior is performed using ANSOFT High Frequency Structural Simulator v13. Numerical extraction of the filter parameters has been done using Agilent ADS (2009) and COVENTORWARE (2010) for mechanical and electromechanical characterization. This is the first reported tunable CPW filter having CSRR on the signal line and ground plane on silicon substrate using MEMS technology. [2014-0365]

Ka-Band Tunable Filter Using Metamaterials and RF MEMS Varactors

A band-stop filter using a metamaterial structure (complementary U-shaped split resonator; CUSR) on a silicon substrate with a 13% tuning range is presented for Ka band applications. The metamaterial structure is used as a frequency-selective geometry on a coplanar waveguide (CPW) and tunability is achieved with the help of MEMS bridges. The rejection in the stop band is around 25 dB for the entire tuning range. A low insertion loss of 0.5 dB is obtained in the pass band. A simple electrical model of the proposed device and the design guidelines are presented. The filter is realized by a novel fabrication methodology involving the micromachining of two bonded silicon wafers and initial fabricated results are reported.

http://iopscience.iop.org/article/10.1088/0960-1317/22/4/045004/pdf

A tunable band-stop filter using a metamaterial structure and MEMS bridges on a silicon substrate

Distributed MEMS transmission line (DMTL) phase shifter on low resistivity silicon for Ku band is presented. The loss relating to the conductivity of silicon and carriers at the silicon-oxide interface is solved by completely removing the silicon as well as the oxide from underneath the CPW t-line. The only silicon left is to provide anchorage to movable sections of the CPW. This also results in a two movable plate system which simultaneously improves the actuation voltage and the switching speed. A phase shifter based on the two movable plate system is designed and a phase shift of 180° is obtained at Ku band with an insertion loss of 1.16dB for low resistivity silicon (10–20 Qcm). A fabrication technique is proposed and initial fabrication results of the design are presented.

RF MEMS DMTL phase shifter on low-resistivity silicon for Ku band with reduced substrate loss

Wide band MEMS resonators banking on the principle of amplitude dependent stiffness nonlinearity as reported in few recent publications can stretch the frequency response. A recent report by our group on four internal proof mass based membrane resonator has been observed to vibrate below 350 Hz with more than three closely placed peaks within 2kHz in the frequency response without introducing non-linearity. However, improvements in the design of such wide band MEMS resonators operating in the linear regime are required, so that the lower frequency range can be reduced further. In this paper we have optimized the lateral dimensions and displacements of the internal proof-masses by introducing a figure of merit to estimate the performance of these resonators. The figure of merit incorporates three parameters viz. power output per unit area, lower cut off frequency and no peaks in the operating frequency range within 2 KHz (most of the practical vibration sources lie within it). It has been observed that the resonator with four internal proof-masses each of dimensions 3×2mm2placed 0.5mm apart from each other shows highest figure of merit.

Samir K. Lahiri

Papers

A tunable band-stop filter using a metamaterial structure and MEMS bridges on a silicon substrate

RF MEMS DMTL phase shifter on low-resistivity silicon for Ku band with reduced substrate loss

Design optimization of a wide band MEMS resonator for efficient energy harvesting