Single and multiband THz metamaterial polarisers
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Single and multiband THz metamaterial polarisers BAGVANTH REDDY SANGALA1,2,∗ , ARVIND NAGARAJAN1 , PRATHMESH DESHMUKH1 , HARSHAD SURDI1 , GOUTAM RANA3 , VENU GOPAL ACHANTA1 and S S PRABHU1 1 Department
of Condensed Matter Physics, Tata Institute of Fundamental Research, Mumbai 400 005, India of Physics, P.G. Centre at Wanaparthy, Palamuru University, Mahbubnagar 509 001, India 3 Department of Electrical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India ∗ Corresponding author. E-mail: [email protected] 2 Department
MS received 26 April 2019; revised 29 August 2019; accepted 11 September 2019 Abstract. We report single and multiband linear polarisers for terahertz (THz) frequencies using cut-wire metamaterials (MM). The MMs were designed by finite-element method (FEM), fabricated by electron beam lithography, and characterised by THz time-domain spectroscopy. The MM unit cells consist of single or multiple length cut-wire pads of gold on semi-insulating gallium arsenide (GaAs) for single or multiple band polarisers. For example, a MM with a square unit cell of 50 μm size on 1 mm GaAs substrate with a gold cut wire of 65 μm length, 2 μm width, and 150 nm height gives a resonance around 1.05 THz. The dependence of the resonance frequency of the single-band polariser on the length of the cut-wires was explained based on transmission line model. Keywords. Terahertz; metamaterial; polarisers. PACS Nos 42.25.Ja; 78.67.Pt; 07.57.−c
1. Introduction Linear polarisers are used to select light with electric field oscillations confined to a single plane. A combination of two polarisers can attenuate the light intensity in controlled fashion. In the terahertz (THz) part of the electromagnetic spectrum, the commercially available devices for this purpose are wire grid polarisers (WGP) [1]. The WGPs can be free-standing or grown on a substrate [2]. Linear polarisers were also reported using liquid crystals [3], aligned single wall carbon nanotubes [4], Brewster angle silicon wafers [5], aligned nickel nanoparticles [6], etc. With the motivation of making the WGPs robust and inexpensive, some ideas are reported recently [7–9] and even more inexpensive options using gold colour lines printed on a paper [10], silver nanoparticle ink printer on a paper [11], graphite– lead grids on a sheet of paper [12], etc. were reported. All of these devices work for broad spectrum in THz. For a narrow pass-band polariser with better extinction coefficient, antireflection coating on the WGPs seems to be the option [2]. Other than this, there is no direct device that can work as narrow single-band or multiband linear polariser in the entire electromagnetic spectrum.
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A cut-wire metamaterial (MM) is a modification of the line grid polariser with an electrical dipole resonance when the incident THz radiation has polarisation along the wires and flat spectral response for orthogonal polarisation and it can work as a narrow-band polariser [13]. A similar
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