Ultrahigh-Q Resonance in Bound States in the Continuum–Enabled Plasmonic Terahertz Metasurface

The study of optical resonators is of significant importance in terms of their ability to confine light in optical devices. A major drawback of optical resonators is the phenomenon of light emission due to their limited capacity for light confinement. Bound states in the continuum are gaining significant attention in the realization of optical devices due to their unique ability for reducing light scattering via interference mechanisms. This process can potentially suppress scattering, leading to improved optical performance. Using this concept, a metasurface having two elliptical silicon (Si) resonators nonidentically angled to create an outof-plane asymmetry is studied. Various parameters are optimized by employing a genetic algorithm (GA) to subsequently achieve a high-Q factor at terahertz frequencies. Herein, the device is fabricated using a novel method, and a thick high-index resonator is achieved. Terahertz measurements are carried out to validate the results. It is indicated in the experimental results that plasmons appear at the top surface of the metasurface and create strong sharp resonances that are sensitive to the external environment. Owing to strong field confinement ability, and high-Q factor, the metasurface is sensitive to its surrounding environment and can be essentially employed in terahertz sensing applications.Md Saiful Islam, Aditi Upadhyay, Rajour Tanyi Ako, Nicholas P. Lawrence, Jakeya Sultana, Abhishek Ranjan, Brian Wai-Him Ng, Nelson Tansu, Madhu Bhaskaran, Sharath Sriram, and Derek Abbot

Broadband and wide-angle reflective linear polarization converter for terahertz waves

Polarization control of electromagnetic waves has wide applications in the field of communications, imaging, and remote sensing. Recent designs of periodic two-dimensional devices or metasurfaces employed for polarization control are limited in efficiency, bandwidth, and allowable incidence angle. This is attributed to high dissipation in the dielectric material used and to less-optimal device configuration. We propose and experimentally validate a reflective linear polarization converter metasurface with high efficiency, wide bandwidth, and wide acceptance angle in the terahertz regime. Our device is composed of three layers: an array of oriented metallic T-shaped resonators, cyclic olefin copolymer (COC) as a low loss dielectric layer, and a ground plane. For the normal and 45° incidence angles, a fabricated sample shows a bandwidth of 95% and 100%, with the average polarization conversion ratio above 80%, covering a frequency range of 0.38–1.07 and 0.36–1.08 THz, respectively. The wide-angle stability is attributed to a phase difference between a single resonance along the T-shaped resonator and a smooth phase response in the low-loss COC dielectric layer. For broad bandwidth performance, a resonator arm extending to adjacent unit cells introduces the fundamental resonance at a lower frequency, while the packed unit cell size shifts the grating lobe onset to a higher frequency. These design aspects can significantly improve the performance of other metasurfaces operating in any frequency range.Rajour Tanyi Ako, Wendy S.L. Lee, Madhu Bhaskaran, Sharath Sriram, and Withawat Withayachumnanku

Frequency-Selective-Surface-Based Mechanically Reconfigurable Terahertz Bandpass Filter

Frequency-tunable filters are in demand for applications requiring high spectral selectivity. To this end, frequency-selective surfaces (FSSs) have been widely applied for spatial filtering, where varactors well serve the purpose for electronic reconfiguration atmicrowave frequencies. For terahertz applications, however, varactors are unavoidably lossy owing to their relatively small cross section required to yield small values of capacitance. In this work, we propose a terahertz bandpass multilayer FSS with a finesse around 20, with 80% power transmission. Its operation frequency is mechanically tunable in a 40% frequency range by varying an interlayer spacer thickness. The experimentally validated bandwidth is 31% due to the limitation in measurement apparatus, and the power transmission is above 75%. The filter is insensitive to misalignment and can maintain tunability under oblique angles of incidence. This proposed filter outperforms a series of notable reconfigurable filters available in the literature in terms of tunable range and insertion loss. The proposed design is among the first few reconfigurable terahertz filters, and the design methodology can be readily applied to other FSS-based structures.Xiaojing Lv, Member, IEEE, Rajour Tanyi Ako, Member, IEEE, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, Fellow, IEEE, and Withawat Withayachumnankul, Senior Member, IEE

Terahertz transmissive half-wave metasurface with enhanced bandwidth

Abstract not availableXiaolong You, Rajour T. Ako, Wendy S.L. Lee, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, and Withawat Withayachumnanku

Broadband terahertz transmissive quarter-wave metasurface

Polarization conversion devices are key components in spectroscopy and wireless communications systems. Conventional terahertz waveplates made of natural birefringent materials typically suffer from low efficiency, narrow bandwidth, and substantial thickness. To overcome the limitations associated with conventional waveplates, a terahertz quarter-wave metasurface with enhanced efficiency and wide bandwidth is proposed. The transmissive quarter-wave metasurface is rigorously designed based on an extended semi-analytical approach employing network analysis and genetic algorithm. Simulation results suggest that the design can achieve linear-to-circular polarization conversion with a 3-dB axial ratio relative bandwidth of 53.3%, spanning 205 GHz–354 GHz. The measurement results confirm that the proposed design enables a 3-dB axial ratio from 205 GHz to at least 340 GHz with a total efficiency beyond 70.2%, where the upper frequency bound is limited by the available experimental facility. This quarter-wave metasurface can cover an entire terahertz electronics band and can be scaled to cover other nearby bands under the same convention, which are technologically significant for future portable systems.Xiaolong You, Rajour T. Ako, Wendy S. L. Lee, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux and Withawat Withayachumnanku

Terahertz reflectarray with enhanced bandwidth

Reflectarrays offer unique potential for beamforming at terahertz frequencies as they combine the advantages of low‐profile of phased arrays and high‐efficiency of parabolic antennas. However, one challenge associated with reflectarrays is their bandwidth limitation resulting from the nonlinear phase response. To enhance bandwidth, a single‐layer stub‐loaded resonator is proposed for constructing reflectarrays. This resonator design shows a smooth and near‐linear phase response with a complete 360° phase coverage at and around the design frequency. To demonstrate its capability in realizing beamforming, a focusing reflectarray is then constructed using the proposed resonator as a building block. The measured results reveal that the 3 dB relative bandwidths of the reflectarray for the transverse electric (TE)‐ and transverse magnetic (TM)‐polarized excitations are 23.3% and 23.9%, respectively, while retaining an efficiency of 71.9% for the TE polarization and 71.0% for the TM polarization at the center frequency of 1.00 THz. The simulation bandwidth of this proposed focusing reflectarray is over twice that of an existing dielectric resonator reflectarray. The proposed resonator has a potential to enhance the bandwidth of terahertz reflectarrays for various beamforming functions.Xiaolong You, Rajour T. Ako, Wendy S.L. Lee, Mei Xian Low, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, and Withawat Withayachumnanku

Terahertz Reflectarray with Enhanced Bandwidth

Reflectarrays offer unique potential for beamforming at terahertz frequencies as they combine the advantages of low‐profile of phased arrays and high‐efficiency of parabolic antennas. However, one challenge associated with reflectarrays is their bandwidth limitation resulting from the nonlinear phase response. To enhance bandwidth, a single‐layer stub‐loaded resonator is proposed for constructing reflectarrays. This resonator design shows a smooth and near‐linear phase response with a complete 360° phase coverage at and around the design frequency. To demonstrate its capability in realizing beamforming, a focusing reflectarray is then constructed using the proposed resonator as a building block. The measured results reveal that the 3 dB relative bandwidths of the reflectarray for the transverse electric (TE)‐ and transverse magnetic (TM)‐polarized excitations are 23.3% and 23.9%, respectively, while retaining an efficiency of 71.9% for the TE polarization and 71.0% for the TM polarization at the center frequency of 1.00 THz. The simulation bandwidth of this proposed focusing reflectarray is over twice that of an existing dielectric resonator reflectarray. The proposed resonator has a potential to enhance the bandwidth of terahertz reflectarrays for various beamforming functions.Xiaolong You, Rajour T. Ako, Wendy S.L. Lee, Mei Xian Low, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, and Withawat Withayachumnanku

Broadband single-mode hybrid photonic crystal waveguides for terahertz integration on a chip

Broadband, low‐loss, low‐dispersion propagation of terahertz pulses in compact waveguide chips is indispensable for terahertz integration. Conventional 2D photonic crystals (PCs) based terahertz waveguides are either all‐metallic or all‐dielectric, having either high propagation losses due to the Ohmic loss of metal, or a narrow transmission bandwidth restricted by the range of single‐mode operation in a frequency range defined by the PC bandgap, respectively. To address this problem, a hybrid (metal/dielectric) terahertz waveguide chip is developed, where the guided mode is completely confined by parallel gold plates and silicon PCs in vertical and lateral directions, respectively. A unique multiwafer silicon‐based fabrication process, including gold–silicon eutectic bonding, micropatterning, and Bosch silicon etching, is employed to achieve the self‐supporting hybrid structure. Theoretical and experimental investigations demonstrate that the hybrid waveguide supports a single‐mode transmission covering 0.367–0.411 THz (bandwidth of 44 GHz, over twice wider than that of all‐silicon PC waveguides) with low loss (below 0.05 dB mm−1) and low group velocity dispersion (from −8.4 to −0.8 ps THz−1 mm−1). This work enables more compact, wideband terahertz waveguides and auxiliary functional components that are integratable in chips toward ultra‐high‐density integrated terahertz devices in particular in the field of wireless communications.Haisu Li, Mei Xian Low, Rajour Tanyi Ako, Madhu Bhaskaran, Sharath Sriram, Withawat Withayachumnankul ... et al