Dynamic Manipulation of Infrared Radiation with MEMS Metamaterials

The advent of metamaterials has ushered in a new era of designer electromagnetic materials and has realized novel responses ranging from negative refractive index [ 1–4 ] and superlensing [ 7 ] to perfect absorption [ 8,9 ] and cloaking. [ 5,6 ] Metamaterials are fashioned from ‘artifi cial atoms’, which are engineered to yield a specifi c response to the electric and magnetic components of light, the properties of which are preserved in a macroscopic medium fabricated from their individual units. Electromagnetic properties achieved by metamaterials derive from the geometry of their unit cells, as opposed to the bandstructure of their makeup. Metamaterials are thus a bottom-up design paradigm for the construction of advanced materials and hold great potential for applications spanning the electromagnetic spectrum. Although metamaterial have realized novel electromagnetic properties, some which have been diffi cult to achieve with natural materials, the ability to dynamically control these responses in real-time would offer signifi cant advantages enabling metamaterials to transition into state-of-the-art devices. Indeed in the past several years, tunable metamaterials have become an important development permitting real time tuning of various electromagnetic responses. Although much work has been carried out in the THz regime, [ 11–17 ] and lower frequencies, [ 18–20 ] there has been limited success in the infrared range. [ 21 ] Photoexcitation or electrical depletion of extrinsic carriers in semiconductor substrates has been proven to be an effective way for achieving tunability at THz but is extremely diffi cult to achieve at shorter infrared wavelengths, which requires much higher doping densities thus leading to high currents and fi elds which tend to severely limit device lifetimes. Temperature controlled tunable metamaterial responses have been demonstrated in many frequency regimes; [ 15,16,21,22 ] however the long response time prevents their practical use. Ferroelectric materials have been used to experimentally show tuning only at microwave frequencies, [ 18 ] while liquid crystals [ 23,24 ] may be implemented across many frequency bands. Although metamaterials have demonstrated tuning by modifi cation of substrate properties a viable alternative is to change the distance between the metamaterial elements, or the substrate, thereby altering the optimized resonant response and/ or the local dielectric environment, thus providing tunability. Thus by fabricating metamaterials as microelectromechanical systems (MEMS) we may achieve mechanically actuated tuning. [ 25–27 ] In this communication we experimentally realize an electrically tunable MEMS metamaterial that effectively manipulates radiation in the mid-infrared wavelength range. The metamaterial consists of an array of suspended metaldielectric elements above a metal ground plane on a carrier substrate. A voltage applied between the metallic metamaterial array and the bottom ground plane layer permits adjustment of the distance between them thus greatly altering the electromagnetic properties. The device functions in refl ection mode and experimentally demonstrates infrared refl ectivity with a modulation index of 56% at a wavelength of 6.2 μ m. Our device is compatible with MEMS commercial foundries and can be incorporated with existing devices to achieve high speed infrared light modulation. To date there have been several demonstrations of MEMS techniques utilized to achieve tunable response at THz and lower frequencies [ 28–31 ] and near infrared wavelengths [ 32 ] as well as for constructing diffractive gratings. [ 33 ] In our study, the metamaterial unit cell (shown in Figure 1 ) consists of four layers above a carrier substrate. The top most metal layer is a Babinet metamaterial and is designed to yield an electrically resonant response. [ 34,35 ] The Babinet metamaterial, and its underlying dielectric layer, are suspended on a supporting structure and are spaced above a ground plane, below which lies the carrier substrate. The ground plane is thicker than the penetration depth of light and thus our device is opaque to infrared radiation. Figure 1 A,B shows the two states of the device which we term the “snap-down” and “snap-back” confi gurations. Our MEMS metamaterial has been designed to achieve a refl ectivity minimum (highly absorbing confi guration) in the snap-down state, i.e. under an applied bias, whereas in the snap-back state (no bias) the metallic-dielectric layer is suspended above the ground plane (with an air gap in-between) and a high refl ectivity (low absorption) is intended. The mechanism of tunability, shown in Figure 1 C,D, is through electrostatic force provided by application of a voltage bias between the top and bottom metallic layers. Details regarding the fabricational process are given in the supplementary materials. In Figure 1 E,F we show SEM images of the fabricated sample which consists of the metamaterial array and eight cantilever arms (two arms per side lying around the perimeter) which provide a spring like restoring force thus enabling the device to be adjustable. As bias is applied to the top suspended metamaterial layer the electrostatic force brings it closer to the ground plane. Notably, when the electrostatic force reaches a certain critical value, the suspended metallic-dielectric layer suddenly comes into contact with the underlying ground plane layer: a phenomenon known as snap-down. [ 27 ] After the applied voltage is removed, the cantilever arms restore the array to its initial suspended state. DOI: 10.1002/adom.201300163 Dynamic Manipulation of Infrared Radiation with MEMS Metamaterials