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Single-Layered Metasurfaces as Spectrally Tunable Terahertz Half- and Quarter-Waveplates Seojoo Lee, Won Tae Kim, Ji-Hun Kang, Bong Joo Kang, Fabian Rotermund, and Q-Han Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21456 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019
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ACS Applied Materials & Interfaces
Single-Layered Metasurfaces as Spectrally Tunable Terahertz Half- and Quarter-Waveplates Seojoo Lee1†‡, Won Tae Kim2†, Ji-Hun Kang3, Bong Joo Kang2, Fabian Rotermund2* and Q-Han Park1* 1Department
2Department
of Physics, Korea University, Seoul 02841, Korea
of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea
3Department
of Physics and Astronomy, Seoul National University, Seoul 08826, Korea * E-mail:
[email protected],
[email protected] † These authors contributed equally to this work
Abstract We propose a single-layered THz metasurface that acts as an efficient THz waveplate, providing phase retardation of up to 180 degrees with a tunable operation frequency. Designed with the tight coupling of elementary resonators, our metasurface provides extraordinarily strong hyperbolicity that is closely associated with the distance between resonators, enabling both significant phase retardation and spectral tunability through mechanical deformation. The proposed concept of THz waveplates based on relatively simple metastructures fabricated on stretchable PDMS is experimentally confirmed using THz spectroscopy. It is believed that the proposed design will pave the way for a diverse range of THz applications. Keywords: metasurfaces, metamaterials, terahertz spectroscopy, phase retardation, waveplate
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A waveplate, also known as a phase retarder, alters the polarization state of incident light and is at the core of a broad range of disciplines by providing various applications including polarization rotators1-5, optical filters6,7, wavefront deflector8, flat lens9,10, holograms11,12, and directional radiator13,14. Waveplates consist of birefringent materials that have different refractive indices for each orthogonal principle axis, which leads to the axis-dependent phase retardation of incident light during its propagation through the material3,15–19. However, for the terahertz (THz) spectral range, the lack of natural materials that exhibit strong birefringence in this region is an obstacle for waveplate fabrication20-24. Metasurfaces that mimic the birefringence of a crystal have been proposed to overcome this, but a challenge facing the production of THz half-waveplates (HWPs) in a single-layered configuration is that the singlelayered metasurfaces that have been proposed so far are not able to provide a retarded phase of 180 degrees. Because of this, single-layered metasurfaces have generally been utilized as THz quarter-waveplates (QWPs), for which a retarded phase of 90 degrees is sufficient25,26. Multilayered configurations have been designed to overcome this limitation, but this approach requires sophisticated fabrication processes and sacrifices structural simplicity, including the ultrathin nature of metasurfaces. As a result, the adoption of metasurfaces as THz waveplates in various applications has been hampered. In this work, we propose a relatively simple single-layered metasurface that works as a spectrally tunable THz HWP and QWP. Our metasurface contains strongly-coupled ring resonators separated by deep-subwavelength gaps, giving the metasurface an extraordinarily high anisotropic effective permittivity that is closely associated with the gap width. This imbues the metasurface with both strong hyperbolicity, which is required for use as a THz HWP as it leads to the significant phase retardation of transmitted light, and with a tunable
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ACS Applied Materials & Interfaces
operating frequency. To experimentally demonstrate the feasibility of our proposed metasurface, we fabricated it on stretchable PDMS film. The efficient performance of our single-layered metasurface as a THz HWP and QWP and its spectral tunability via mechanical deformation were subsequently confirmed using THz time-domain spectroscopy. The main concept behind the use of single-layered metasurface THz HWPs is to take advantage of the large phase retardation that occurs when metasurfaces have tightly-coupled unit elements. Figure 1a illustrates our proposed metasurface and the phase retardation process. As shown in Figure 1b, our metasurface is composed of rectangular ring resonators periodically arranged with gap widths of gx and gy along the x and y axes, respectively. When the resonators are spaced with a deep sub-wavelength gap (gx ≠ gy
