Vol.:(0123456789) 1 3 Applied Physics A (2018) 124:105 https://doi.org/10.1007/s00339-017-1506-0 Amplitude modulation in infrared metamaterial absorbers based on electro‑optically tunable conducting oxides D. C. Zografopoulos 1  · G. Sinatkas 2  · E. Loti 3  · L. A. Shahada 3  · M. A. Swillam 4  · E. E. Kriezis 1  · R. Beccherelli 1 Received: 12 August 2017 / Accepted: 20 December 2017 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract A class of electro-optically tunable metamaterial absorbers is designed and theoretically investigated in the infrared regime towards realizing free-space amplitude modulators. The spacer between a subwavelength metallic stripe grating and a back metal relector is occupied by a bilayer of indium tin oxide (ITO) and hafnium oxide ( HfO 2 ). The application of a bias voltage across the bilayer induces free-carrier accumulation at the HfO 2 /ITO interface that locally modulates the ITO permittivity and drastically modiies the optical response of the absorber owing to the induced epsilon-near-zero (ENZ) efect. The carrier distribution and dynamics are solved via the drift–difusion model, which is coupled with optical wave propagation studies in a common inite-element method platform. Optimized structures are derived that enable the amplitude modulation of the relected wave with moderate insertion losses, theoretically ininite extinction ratio, sub-picosecond switching times and low operating voltages. 1 Introduction Metamaterials are artificial electromagnetic structures, based on subwavelength periodic elements, with unprec- edented electromagnetic properties, which are generally unattainable in natural materials [1, 2]. Among the various metamaterial components thus far demonstrated, “perfect” absorbers, i.e., devices that absorb 100% of the incoming electromagnetic radiation at a resonant wavelength [3], have shown signiicant potential in applications such as solar energy harvesting, thermal emitters in thermophotovoltaic cells [4], local heating, and photo-catalysis [5]. Their operation is based on the excitation of a resonant cavity, typically formed by a back metal relector and an array of metallic subwavelength elements, at the so-called critical-coupling regime, i.e., when the radiative decay rate equals that of losses in the metallic and, potentially, dielec- tric parts of the device [6]. This condition ensures perfect absorption and is fulilled for deeply subwavelength resonant cavities, thus leading to thin-ilm devices in the range of tens of nanometers when operating in the visible or infrared spectrum. Apart from 100% absorption, such metamaterial devices can feature also wide-angle operation, polarization- selective or -independent operation, while their design is relatively straightforward and can be scaled to a vast part of the electromagnetic spectrum, spanning from the visible spectrum to microwaves [7]. The key properties of critical absorbers, i.e., resonant frequency and linewidth, depend on the selection of the dielectric materials that ill the resonant cavity. In some cases, the electromagnetic properties of such materials can be dynamically controlled by an external stimulus, e.g., tem- perature variation, illumination with intense laser spots, or application of an electric signal, thus enabling the design of tunable absorbers exhibiting a much higher level of func- tionality compared to their static counterparts. To this end, This report was made possible by a NPRP award [NPRP 7-456- 1-085] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. * D. C. Zografopoulos dimitrios.zografopoulos@artov.imm.cnr.it 1 Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica e Microsistemi (CNR-IMM), 00133 Rome, Italy 2 Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, 54124 Thessaloníki, Greece 3 Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O. Box 2713, Doha, Qatar 4 Department of Physics, School of Science and Engineering, The American University in Cairo, New Cairo 11835, Egypt