W. M. Zhu

Micromachined tunable metamaterials: a review

This paper reviews micromachined tunable metamaterials, whereby the tuning capabilities are based on the mechanical reconfiguration of the lattice and/or the metamaterial element geometry. The primary focus of this review is the feasibility of the realization of micromachined tunable metamaterials via structure reconfiguration and the current state of the art in the fabrication technologies of structurally reconfigurable metamaterial elements. The micromachined reconfigurable microstructures not only offer a new tuning method for metamaterials without being limited by the nonlinearity of constituent materials, but also enable a new paradigm of reconfigurable metamaterial-based devices with mechanical actuations. With recent development in nanomachining technology, it is possible to develop structurally reconfigurable metamaterials with faster tuning speed, higher density of integration and more flexible choice of the working frequencies.

Microelectromechanical Maltese-cross metamaterial with tunable terahertz anisotropy.

Dichroic polarizers and waveplates exploiting anisotropic materials have vast applications in displays and numerous optical components, such as filters, beamsplitters and isolators. Artificial anisotropic media were recently suggested for the realization of negative refraction, cloaking, hyperlenses, and controlling luminescence. However, extending these applications into the terahertz domain is hampered by a lack of natural anisotropic media, while artificial metamaterials offer a strong engineered anisotropic response. Here we demonstrate a terahertz metamaterial with anisotropy tunable from positive to negative values. It is based on the Maltese-cross pattern, where anisotropy is induced by breaking the four-fold symmetry of the cross by displacing one of its beams. The symmetry breaking permits the excitation of a Fano mode active for one of the polarization eigenstates controlled by actuators using microelectromechanical systems. The metamaterial offers new opportunities for the development of terahertz variable waveplates, tunable filters and polarimetry.

Micromachined switchable metamaterial with dual resonance

We experimentally demonstrate a micromachined switchable metamaterial with dual mode resonance which is induced at THz regime under oblique incidence. Here, we explore, both theoretically and experimentally, the dynamic dual mode switching by reshaping metamaterial elements using micromachined actuators. The mode switching allows robust control over the transmission and the reflection of the metamaterial at 0.76 THz and 1.16 THz. Such switchable dual mode metamaterial promises wide applications in optical switches, tunable filters, and THz detectors.

Polarization dependent state to polarization independent state change in THz metamaterials

We experimentally demonstrated a polarization dependent state to polarization independent state change in terahertz (THz) metamaterials. This is accomplished by reconfiguring the lattice structure of metamaterials from 2-fold to 4-fold rotational symmetry by using micromachined actuators. In experiment, it measures resonance frequency shift of 25.8% and 12.1% for TE and TM polarized incidence, respectively. Furthermore, single-band to dual-band switching is also demonstrated. Compared with the previous reported tunable metamaterials, lattice reconfiguration promises not only large tuning range but also changing of polarization dependent states, which can be used in photonic devices such as sensors, optical switches, and filters.

A Flat Lens with Tunable Phase Gradient by Using Random Access Reconfigurable Metamaterial

The first demonstration of an optofluidic metamaterial is reported where resonant properties of every individual metamolecule can be continuously tuned at will using a microfluidic system. This is called a random-access reconfigurable metamaterial, which is used to provide the first demonstration of a tunable flat lens with wavefront-reshaping capabilities.

Design and fabrication of a novel piezoelectric multilayer actuator by thick-film screen printing technology

A novel structured multilayer piezoelectric lead zirconate titanate (PZT) actuator that combined the merits of high displacement/voltage sensitivity, high resonance frequency and low costs has been designed and successfully fabricated using the thick-film screen printing technology. Its structural, dielectric, and piezoelectric properties were also characterized. Experimental results show that the average grain size, dielectric, and piezoelectric properties of the PZT thick-film multilayer actuators are strongly dependent on processing conditions. Under the driving voltage of 10 V, a displacement value of 1.57 μm is obtained, and the corresponding resonance frequency is 56.9 kHz. Only a few layers are required to construct this multimorph actuator, instead of tens or even hundreds of layers that a conventional stacked multilayer actuator requires in order to realize a large displacement, thus greatly reducing manufacturing costs. The multimorph actuator presented in this paper provides a valuable alternative for actuator applications beyond those available with the popular bimorph and longitudinal multilayer actuators, such as for the second-stage track following control in computer hard disk drives.

Micromachined optical well structure for thermo-optic switching

This letter demonstrates the thermo-optic switching function using an adjustable optical well structure, which is constructed by a thin air gap sandwiched between two micromachined hemicylindrical prisms. The device is etched on a silicon-on-insulator wafer within a footprint of 400×400μm2. In experiment, it measures an extinction ratio of 30.2dB and a switching time of 2.2μs. Compared with the other demonstrated switches that have optical barrier structures, this device is unique in the working principle and optical design, and shows various merits such as high extinction ratio, fast speed, low power consumption, and small size.

A micromachined optical double well for thermo-optic switching via resonant tunneling effect

This letter presents the thermo-optic switching characteristics of an optical double-well structure, which has a high-low-high-low-high refractive index construct formed by micromachined silicon prisms and air gaps. Analysis shows such structure features full transmission (i.e., on state) and requires low refractive index change for switching function. The device is fabricated on silicon-on-isolator wafer using deep etching process. In experiment, it measures a switching speed of 1μs and an extinction ratio of 30dB. Compared with the other micromachined switches, this device utilizes different physical principle and processes various merits such as fast switching speed and low power consumption.

A pseudo-planar metasurface for a polarization rotator

New demonstrations of effective interaction between light and artificially electromagnetic interface, or the metasurface, have stimulated intensive research interests on control of light to realize applications in beam steering, optical imaging and light focusing, etc. Here we reported a new type of planar metasurface of which every individual metamolecule is single metallic layer with stereo structure and the metasurface is name as Pseudo-Planar Metasurface (PPM). The metamolecule of the PPM is a chiral structure and therefore derives significant optical activity.

A MEMS tunable metamaterial filter

This paper presents a terahertz tunable metamaterial filter using microelectromechanical systems (MEMS) structures. The metamaterial unit cells are tuned by micromachined comb-drive. The metamaterial slab works as a notch filter of THz region, which can lower the transmission of interest frequency down to −70 dB. In the experiment, it measures the tuning range of filter frequency from 3.32 to 3.80 THz. The tunable metamaterial filter has better tunability compared with traditional active metamaterial because the optical property of metamaterial is more sensitive to the change of the unit cell structure.