A terahertz in-line polarization converter based on through-via connected double layer slot structures

Single/Dual/Triple Broadband Metasurface Based Polarisation Converter with High Angular Stability for Terahertz Applications

This paper presents design and characterisation of a new compact metasurface based linear polarisation converter for terahertz applications. The metasurface unit cell with periodicity of 0.292λ0 consists of an asymmetrically oriented planar double semicircular goblet-shaped resonators. It is printed on a polydimethylsiloxane (PDMS) dielectric substrate backed by a gold layer that acts as a ground plane. This metasurface structure exhibits a broadband cross-polarisation conversion in the frequency range of 0.72−0.99 THz with a polarisation conversion ratio (PCR) > 95% and angular stability > 40∘ for both TE and TM modes. However, the PCR for the single band is >99% at resonant frequencies of 0.755 and 0.94 THz, while the optimised design shows 100% PCR over a BW of 95 GHz. Furthermore, slight modification and optimisation of the broadband design results in quad-ring and slotted DSGRs that produce dual and triple broadband polarisation conversion, respectively. The quad-ring DSGR performs polarisation conversion for frequency range of 0.70−1.08 and 1.61−1.76 THz while the slotted DSGR shows the triple broadband cross-conversion for frequency range of 0.67−0.85, 1.04−1.11, and 1.62−1.76 THz with PCR > 95%. This design is simple, easy to modify to implement single and multi broadband polarisation conversion with high PCR at terahertz regime. In addition to that, it is easy to fabricate and integrate with other components like multiple-input multiple-output terahertz antennas for mutual coupling reduction.

Wideband dual-polarized linear-circular and linear-cross angular stable THz reflective polarizer based on a modified square loop FSS

A simple wideband reflective type linear-cross and linear-circular polarization converter for terahertz (THz) applications is proposed in this paper. The top frequency selective surface (FSS) consists of a thin gold coated figure of eight square loop with the connected strip line corners on a thin grounded polyamide dielectric substrate. For the y/x-polarized incidence, the design exhibits linear-circular conversion with an axial ratio (≤3dB) from 0.49-0.50, 0.60-0.83, 1.16-1.62, and 1.81-1.85 THz with 2.02%, 32.17%, 33.09%, and 2.19% fractional bandwidth (FBW), respectively. In addition, it also performs the linear-cross conversion with a minimum 90% polarization conversion ratio (PCR) from 0.53-0.56, 0.92-1.07, and 1.69-1.75 THz having 5.50%, 15.08%, and 3.49% FBW, respectively. Multiple plasmonic resonances are the reason behind different polarization conversions and are confirmed with surface current distribution profiles of the FSS and ground. The metasurface's performance is stable up to 45° for both transverse electric (TE) and transverse magnetic (TM) oblique incidences. The polarizer's unit cell architecture is compact with structural dimensions of 0.121×0.121×0.041λL3, where λ_L is the lowest operating frequency's free-space wavelength. The authors believe this design is compact, with angular stable multi-band multi-conversion ability that will significantly impact the THz applications in real time.

Optically tunable terahertz chiral metasurface based on multi-layered graphene

Active manipulation of the polarization states at terahertz frequencies is crucially helpful for polarization-sensitive spectroscopy, having significant applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. The weakness of polarization manipulation provided by natural materials can be overcomed by chiral metamaterials. Chiral metamaterials have a huge potential to achieve the necessary polarization effects, hence they provide the basis for applications such as ultracompact polarization components. Terahertz chiral metamaterials that allow dynamic polarization modulation of terahertz waves are of great practical interest and still challenging. Here, we show that terahertz metasurface based on the four conjugated “petal” resonators integrated with multi-layered graphene (MLG) can enable dynamically tunable chiroptical response using optical pumping. In particular, a change of ellipticity angle of 20° is observed around 0.76 THz under optical pumping by a 980 nm continuous wave (CW) laser. Furthermore, using temporal coupled-mode theory, our study also reveals that the chiroptical response of the proposed multi-layered graphene-based metasurface is strongly dependent on the influence of optical pumping on the loss parameters of resonance modes, leading to actively controllable polarization states of the transmitted terahertz waves. The present work paves the way for the realization of fundamental terahertz components capable for active polarization manipulation.

Electrodynamic resonance of surface conduction and THz transmission through arrays of rectangular apertures in opaque metallic thin films

A modal method is developed analytically to investigate the THz optical transmission and reflection of a metallic thin film perforated by a 2D array of rectangular apertures. For subwavelength apertures, this optical model is interpreted in terms of passive electrical circuits, with interface admittances accounting for the THz surface conduction properties of the metallic film. The reactive component of the admittance of the evanescent diffraction cloud is shown to exhibit resonant behavior governed by the shape factors of the array. Interaction of such an electrodynamic resonance with the Rayleigh diffraction orders may alter their standard Fano profiles. Experimental evidence of the resonance is obtained owing to lineshape analysis of transmittance measurements in the THz range on metallic thin films deposited on a dielectric substrate, both above and below the first Wood-Rayleigh anomaly.

Dual-band transmission polarization converter based on planar-dipole pair frequency selective surface

A novel linear polarization converter operating in C- and X-bands with high polarization conversion ratio is described and demonstrated based on frequency selective surface. The building element is a planar-dipole pair, which is orthogonally printed on a double-layer substrate and vertically connected by a pair of through-via holes functioning as a quasi-two-wire transmission line coupler. A perforated metal shielding layer is sandwiched between the double-layer structure to only support a transverse electric and magnetic (TEM) mode coupling between the top and bottom dipolar components. The front dipole responds to the incident transverse electric (TE) wave, and sends the induced current into the two-wire transmission line to feed the bottom dipole. The bottom dipole is orthogonal or oriented at an arbitrary angle with respect to the top one, and a resultant outgoing transverse magnetic (TM) wave or arbitrary orientation polarized wave can be achieved. In addition, a bidirectional orthogonal polarization converter is realized by using double orthogonally arranged planar-dipole pairs, which are also printed on the same double-layer substrate.