Abstract
Planar metasurface based quarter-wave plates offer various advantages over conventional waveplates in terms of compactness, flexibility and simple fabrication; however they offer very narrow bandwidth of operation. Here, we demonstrate a planar terahertz (THz) metasurface capable of linear to circular polarization conversion and vice versa in a wide frequency range. The proposed metasurface is based on horizontally connected split ring resonators and is realized on an ultrathin (0.05λ) zeonor substrate. The fabricated quarter waveplate realizes linear to circular polarization conversion in two broad frequency bands comprising 0.64–0.82 THz and 0.96–1.3 THz with an insertion loss ranging from −3.9 to −10 dB. By virtue of ultrathin sub wavelength thickness, the proposed waveplate design is well suited for application in near field THz optical systems. Additionally, the proposed metasurface design offers novel transmission phase characteristics that present further opportunities to realize dynamic polarization control of incident waves.
Highlights
The metasurface waveplate is fabricated on the 23-μm-thick zeonor film
The presented metasurface waveplate was characterized by measuring the complex transmission coefficients along x and y-axis individually
The photoconductive antenna (PCA) based emitter emit THz pulse mainly polarized along y-direction
Summary
The proposed metasurface is based on modified electric split ring resonators (SRR) having additional split gaps at the sides of the slit ring[19]. For incident fields polarized along y-axis, the split gaps at the centre and sides of the SRR act as capacitors connected in series with the inductance formed by the SRR split ring. For the x-polarized field, the SRR split gap electrodes act as cut-wire arrays connected in parallel with the wire grating formed by SRR split rings. The cut wire arrays normally work as a series LC circuit as a result of large parallel inductance resulting from the wire grating formed by the split rings, the unit cell acts as a parallel LC network, exhibiting a resonant bandpass transmission response[23,24]. The metasurface dimensions are optimized for a resonance frequency of 0.89 THz. For x-polarized fields, the transmission resonance frequency depends on the width “w1” of centre split gap electrodes that act as cut-wires. 0.96 THz, the phase difference again maintains a nearly constant value between −97° to −90°
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