Electrically tunable metasurface perfect absorber for infrared frequencies
- Gwanho Yoon†1,
- Sunae So†1,
- Minkyung Kim1,
- Jungho Mun2,
- Renmin Ma3 and
- Junsuk Rho1, 2Email authorView ORCID ID profile
© The Author(s) 2017
Received: 19 October 2017
Accepted: 4 December 2017
Published: 21 December 2017
We theoretically investigate a metasurface perfect absorber based on indium-tin-oxide as active material. Our design scheme relies on conventional metal–oxide–semiconductor model and the Drude model. Inducing a voltage into the device causes a blue-shift of 50 nm in the reflectance spectrum in the infrared region. The total thickness of the device is only 3.5% of the working wavelength λ = 2.56 μm, and the rate of reflectance change reaches 5.16 at λ = 2.56 μm. Because the material that we use has advantages of easy fabrication and fast response, our design approach can be used for numerous applications on active plasmonic sensors and filters.
Metamaterials composed of subwavelength-scale artificial meta-atoms can have extraordinary optical properties that originate from the geometrical configuration of the meta-atoms, rather than from their chemical composition. Many exotic phenomena such as negative refraction, [1–3] super-resolution imaging, [4–6] perfect absorption  and optical cloaking [8, 9] have been demonstrated by controlling the shape of the meta-atoms.
Metasurface perfect absorbers (MPAs) are metamaterial-based devices that can perfectly absorb electromagnetic waves of certain wavelengths. Compared to conventional absorption devices based on multi-layered thin films, the MPA is very thin, usually less than one wavelength. The absorption spectrum of the MPA can be easily tuned by controlling the design of the meta-atoms, so the MPA has many possible practical applications such as plasmonic sensors and filters.
Tunability is an important goal of metasurface research, especially in MPAs. Research on tunable metasurfaces consider electrical, [10–18] thermal,  optical [20, 21] and mechanical [22–25] tuning methods. The electrical method exploits the effect of carrier concentration on the optical properties of materials; this approach controls the optical functionality of the device by applying a voltage. The thermal method exploits the effect of temperature on the phase of special materials such as VO2 and GeSbTe; the phase change induces change in the optical properties. The optical method changes the optical response of the materials by using photon energy to excite electrons from a valence band into a conduction band; this process is fast, but independent and simultaneous control of each meta-atom is a difficult task. The mechanical methods are based on direct transformation of the physical profile of the meta-atoms; these methods include microelectromechanical systems (MEMS). Changing the shape of the meta-atom can generate a huge variation of resonance characteristics, but its working frequency is relatively low, because MEMS technology works on a scale of micrometers.
Electrically tunable metasurfaces (ETMs) are the most promising for practical application, because they are compatible with complementary metal-oxide semiconductors (CMOSs). ETMs use voltage as a power source, so they can be integrated into conventional semiconductor devices. A typical ETM is based on active materials such as indium-tin-oxide (ITO), GaAs or graphene. Of these materials, ITO is the cheapest; it is also easy to deposit on any kind of substrates, by either conventional electron beam evaporation or sputtering. Furthermore, the charge-carrier concentration ρ of ITO can be controlled from 1019 to 1021 cm−3 by annealing it under controlled conditions.
In this work, we propose an ITO-based electrically-tunable metasurface perfect absorber (ETMPA) that works near wavelength λ = 2.56 μm. Our device has a metal–insulator–metal (MIM) structure with square patch antennas to maximize the absorbance. Complex permittivity of ITO is derived using the Drude model, and the effect of external voltage on carrier concentration according is calculated using the conventional metal–oxide–semiconductor (MOS) model. The maximum absorbance reached 98% near λ = 2.56 μm, and the absorption spectrum blue-shifted by 50 nm as the external voltage was increased from 0 to 9 V. The rate of reflectance change reached 5.16 at λ = 2.56 μm. Our device and design approach have possible applications in plasmonic sensors and in active devices based on ETMPA.
N d is an important factor that determines the maximum N(x) as well as the accumulation thickness. As N d increases, the maximum N(x) increases, but the accumulation thickness decreases which explains why the accumulation effect in metal can be ignored. Therefore, N d of the ITO layer should be optimized to maximize the functionality of devices that use it.
In summary, an ETMPA that works in the IR region is presented. Carrier concentration inside the ITO layer is analyzed based on the conventional MOS model, and the corresponding complex permittivity of ITO is derived using Drude modelling. When bias is applied to the device, the minimum point of the reflectance spectrum blue-shifts by 50 nm, and the rate of reflectance change reaches 5.16 at λ = 2.56 μm. Overall thickness of the device is only λ/28. Our design scheme based on ITO can be used to convert passive metasurface devices to active devices. Compared to other tuning mechanisms, our method has advantages of CMOS compatibility and compact size. We believe that our approach provides an intuition to researchers who try to develop active devices based on metasurfaces and plasmonics, such as absorbers, reflectors, sensors and filters.
JR and RM conceived the concept. GY and SS performed simulation on the device and wrote the manuscript. GY, SS, MK, JM analyzed the data. JR guided the entire work. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The authors have no data to share; all are shown in the submitted manuscript.
This work is financially supported by the National Research Foundation Grants (NRF-2017R1E1A1A03070501, NRF-2015R1A5A1037668, CAMM-2014M3A6B3063708) funded by the Ministry of Science, ICT and Future Planning (MSIP) of the Korean government. SS and MK acknowledge Global Ph.D. Fellowships (NRF-2017H1A2A1043322 and NRF-2017H1A2A1043204) from NRF-MSIP of Korean government, respectively.
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