© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.de The ability to dynamically control the response of a photonic device to electromagnetic radiation is a very powerful concept that has long been a goal of scientists and engineers. Of par- ticular interest is altering the surface reflection coefficient—as is done in the pixel of a spatial light modulator (SLM)—without any moving parts or gratings, opening up a new paradigm for sensing. Metamaterials are engineered electromagnetic mate- rials which enable the precise control over light and are opti- mistic candidates to achieve this degree of freedom. [1–5] Here we report a metamaterial absorber (MMA) 8 × 8 THz SLM in which the reflection and absorption in each pixel is dynami- cally controlled by all-electronic means. Four central frequen- cies in THz were chosen for the MMA-SLM and were arranged in a square array pattern to form a multi-color superpixel. By altering the carrier density between the frequency selective patterned surface and a ground plane, an average modulation index of 62% was achieved across the pixel array with switching speeds of up to 12 MHz. Our demonstration of a multicolor THz MMA-SLM can be leveraged for greater control over light, leading to more compact, efficient and versatile photonic components. The capability to control light dynamically, spatially, and/ or spectrally has shown numerous potential advantages; specifically with regard to spectroscopic and imaging appli- cations. [6–8] Commercially available digital micro-mirror (DMD) [9] and liquid-crystal (LC) [10] devices have shown great promise from the near-infrared to the ultraviolet, but do not operate at longer wavelengths. These devices, however, suffer other significant limitations. DMDs are mechanical and can only operate up to a few kilohertz. Most LCs per- form optimally near 1 kHz and are thus also relatively slow. [6,11] Obtaining spectral information with these devices can be challenging requiring complex, high cost equipment and instrumentation with significantly limited acquisition speeds. [7] If SLMs, however, had a built-in ability to discrimi- nate between different spectral bands at the physical layer, this would yield significant improvements while enabling a host of applications. Metamaterials can be designed to yield highly efficient absorption by careful tailoring of their electric and magnetic response functions. Metamaterial absorbers have sparked sig- nificant interest since their first demonstration [12,13] and hold great potential for use in applications ranging from thermal emitters [14] and energy harvesting, [15] to sensors and narrow band absorbers. [16] Switching between near perfect absorp- tion and mirror-like reflection is possible by functionalizing metamaterial absorbers where both the effective permittivity and permeability can be controlled within the same unit cell. Integrating tunable absorbers as a reflection-based SLM would offer the intensity contrast and low insertion losses found within commerical SLMs, that are currently unavailable at THz frequencies [17–19] due to fundamental limitations of a metas- urface to generate a sufficient magnet response to match the impedance of free space. [20] The electromagnetic properties of metamaterials can be dynamically tuned through a variety of different mechanisms, [21] and made to be both frequency and spatially selective. [22] When implemented as room-temperature THz modulators, [23,24] metamaterial based devices have demon- strated significant advantages in modulation depth, switching speed and spectral sensitivity over alternative architectures such as quantum well structures, which require cryogenic tem- peratures, [25] and LCs which possess relatively slow switching speeds. [26] They are thus attractive candidates to bridge the tech- nological limitations of current state-of-the-art SLMs, especially at THz frequencies where components necessary to efficiently manipulate THz radiation remain in need of substantial devel- opment in order to provide realistic solutions. [27] Here we demonstrate a doped semiconducting metamaterial SLM with multi-color super-pixels composed of arrays of elec- tronically controlled THz metamaterial absorbers. In Figure 1 a we show a photograph of an 8 × 8 pixel implementation of the MMA-SLM. The pixels are tiled in a square array and we modu- late each at a unique frequency in the THz range. The overall SLM system architecture is shown schematically in Figure 1b and consists of metamaterial absorber pixels flip chip bonded to a Silicon chip carrier with routing to bond pads which are wire-bonded to a leadless chip carrier (LCC). Individual pixels are biased through a commercial 64 channel output controller and each THz color pixel consists of approximately 400 indi- vidual metamaterial absorber unit cells. Our metamaterial absorber consists of two metallic layers with a dielectric spacer lying in-between. The top metal layer is patterned in order to respond resonantly to the electric component of an incident electromagnetic wave. A bottom ground plane layer is spaced relatively close to the top layer, thus allowing the external magnetic field to couple, as shown in Figure 1b. Altering the geometry of the metallic pattern and dielectric thickness allows tuning of the impedance and loss, DOI: 10.1002/adom.201300265 Four-Color Metamaterial Absorber THz Spatial Light Modulator David Shrekenhamer, John Montoya, Sanjay Krishna, and Willie J. Padilla* Dr. D. Shrekenhamer, Prof. W. J. Padilla Department of Physics Boston College, 140 Commonwealth Avenue Chestnut Hill, Massachusetts, 02467, USA E-mail: willie.padilla@bc.edu J. Montoya, Prof. S. Krishna Department of Electrical and Computer Engineering Center for High Technology Materials University of New Mexico Albuquerque, New Mexico, 87106, USA Adv. Optical Mater. 2013, DOI: 10.1002/adom.201300265