Abstract-Terahertz Waveguides: Broadband Single‐Mode Hybrid Photonic Crystal Waveguides for Terahertz Integration on a Chip

Haisu Li, Mei Xian Low, Rajour Tanyi Ako, Madhu Bhaskaran, Sharath Sriram, Withawat Withayachumnankul, Boris T. Kuhlmey, Shaghik Atakaramians,

https://onlinelibrary.wiley.com/doi/abs/10.1002/admt.202000117

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.

Abstract-Terahertz sensing of 7 nm dielectric film with bound states in the continuum metasurfaces

Applied Physics Letters

Yogesh Kumar Srivastava,  Rajour Tanyi Ako,  Manoj Gupta, Madhu Bhaskaran, Sharath Sriram,  Ranjan Singh,

Nanometer-scale thin film sensing with quasi-BIC Fano resonance. (a) Optical image of the fabricated TASR metamaterial. The inset depicts the geometrical parameters of the unit cell, gap g = 3 μm, width w = 6 μm, length l = 60 μm, periodicity p = 75 μm, and asymmetry d = 10 μm. (b) Image of the fabricated metamaterial showing robustness and flexibility. (c) The Q factors of quasi-BICs of an ideal (dots, PEC) and a realistic (star, metallic) metamaterial array with varying asymmetry d. Inset: simulated transmission spectra of the metallic TASR metamaterial with an asymmetry of d = 10 μm. (d) and (e) Change in the simulated transmission amplitude (ΔT) and phase (Δϕ, degree) on coating Ge of thicknesses ranging from 7 to 20 nm on the TASR metamaterial.

https://aip.scitation.org/doi/abs/10.1063/1.5110383

The fingerprint spectral response of several materials with terahertz electromagnetic radiation indicates that terahertz technology is an effective tool for sensing applications. However, sensing few nanometer thin-films of dielectrics with much longer terahertz waves (1 THz = 0.3 mm) is challenging. Here, we demonstrate a quasibound state in the continuum (BIC) resonance for sensing of a nanometer scale thin analyte deposited on a flexible metasurface. The large sensitivity originates from the strong local field confinement of the quasi-BIC Fano resonance state and extremely low absorption loss of a low-index cyclic olefin copolymer substrate. A minimum thickness of 7 nm thin-film of germanium is sensed on the metasurface, which corresponds to a deep subwavelength scale of λ/43 000, where λ is the resonance wavelength. The low-loss, flexible, and large mechanical strength of the quasi-BIC microstructured metamaterial sensor could be an ideal platform for developing ultrasensitive wearable terahertz sensors.

The authors acknowledge valuable and timely assistance from Zhang Qiannan in performing the thickness measurements of the analyte layer using Atomic Force Microscopy. Y.K.S., M.G., and R.S. acknowledge the research funding support from the Ministry of Education AcRF Tier 1 Grant No. RG191/17 and Tier 2 Grant No. MOE2017-T2-1-110. S.S. and R.S. acknowledge support from an RMIT Foundation Research Exchange Fellowship. This work was performed in part at the Micro Nano Research Facility at RMIT University in the Victorian Node of the Australian National Fabrication Facility (ANFF).

Abstract-Dielectrics for Terahertz Metasurfaces: Material Selection and Fabrication Techniques

Rajour Tanyi Ako, Aditi Upadhyay, Withawat Withayachumnankul,   Madhu Bhaskaran, Sharath Sriram,

Correct choice of dielectric materials can overcome low efficiency and low operational bandwidth in terahertz metasurface devices. A guide to their selection based on properties and fabrication compatibility is presented. State‐of‐the‐art examples of dielectrics demonstrated as spacers and substrates and as resonant structures are covered, highlighting that selection and handling of dielectric materials can determine the performance of terahertz devices.

https://onlinelibrary.wiley.com/doi/10.1002/adom.201900750

Manipulation of terahertz radiation opens new opportunities that underpin application areas in communication, security, material sensing, and characterization. Metasurfaces employed for terahertz manipulation of phase, amplitude, or polarization of terahertz waves have limitations in radiation efficiency which is attributed to losses in the materials constituting the devices. Metallic resonators‐based terahertz devices suffer from high ohmic losses, while dielectric substrates and spacers with high relative permittivity and loss tangent also reduce bandwidth and efficiency. To overcome these issues, a proper choice of low loss and low relative permittivity dielectric layers and substrates can improve field confinement and reduce dissipation. Alternatively, replacing metallic resonators with a moderate relative permittivity dielectric material that supports cavity mode resonances also reduces dissipation due to the absence of conduction current. Herein, an overview of dielectric materials employed as spacers and dielectric resonators is provided, and the fabrication methods employed to realize these devices at the terahertz frequency range are also presented. Material selection guidelines, material‐specific and application‐specific fabrication quality metrics are outlined, and new techniques are proposed.

Abstract-Terahertz multiple modes defined by fractal symmetry in complementary meta-atoms

Zhidong Gu, Zhenyu Zhao, Hui Zhao, Wei Peng, Jianbing Zhang, Hongwei Zhao, Rajour Tanyi Ako, and Sharath Sriram

 Schematic representation of CSRR design. (a) Fractal meta-atoms of CSRR under different symmetric conditions: O-gap, U-gap, and C-gap, respectively, and fractal levels. (b) Pattern direction of fractal meta-atom, of which the z direction is the <100>-crystallographic orientation of SI-GaAs. P: lattice period, g: gap-size, r1: outer-radius, r2: inner-radius. (c) The top-view optical image of meta-atom. (d) Diagram of terahertz transmission spectroscopy

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-10-4138

Low quality (Q) factors of the intrinsic inductive–capacitive (LC) mode as well as the parasitic dipole oscillation mode restrict high-resolution sensing using split-ring resonators (SRR). Although the trapped Fano-mode of the high-Q factor is found in asymmetric SRR, the conventional design limits the scaling down of resonators. As such, excitation and manipulation of multiple trapped modes of SRR is significant for driving innovative designs of terahertz metamaterials and metasurfaces. In this work, we present a novel approach to manipulating multiple terahertz modes by increasing the fractal levels as well as the geometric symmetry of complementary SRR. It is found that the multiple trapped modes become achievable only in the case that the gap of adjacent fractal SRR opposes each other. By increasing the fractal level, the intrinsic resonance modes change slightly, and more trapped modes appear in between the frequency range of the two major intrinsic modes. The map of surface currents and magnetic field distribution reveal that intrinsic LC resonance in the first or second level SRR dominates the intrinsic modes. By contrast, the trapped mode arises from the hybridization of high-order localized dipole oscillation as well as the multiple localized LC resonances. These findings create new design opportunities for scalable metasurfaces across the terahertz spectrum and beyond, with ability to create high-resolution sensors.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Demonstration of group delay above 40 ps at terahertz plasmon-induced transparency windows

Zhenyu Zhao, Hui Zhao, Rajour Tanyi Ako, Jianbing Zhang, Hongwei Zhao, and Sharath Sriram

. (a) Schematic diagram of a unit cell of proposed binary MM, in which p = 420 µm, r = 90 µm, w = 4 µm, h = 60 µm, g = 15 µm, respectively. l is the length of the metallic arc. (b) Microscopic images (VHX-500, Keyence Inc.) of unit cell of as-fabricated binary MM. (c) A schematic diagram showing terahertz transmission and group velocity delay through a periodic array of binary MM, as measured using a THz-TDS. KTHz refers to the wave vector of incident THz pulse. EX and HY refer to the electrical and magnetic component of the incident electromagnetic waves, respectively. XYZ are the coordinate axis in free space.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-19-26459

Herein, we demonstrate one of the highest terahertz group delay of 42.4 ps achieved experimentally at 0.23 THz, on a flexible planar metamaterial. The unit cell of metasurface is made up of a textured closed cavity and another experimentally concentric metallic arc. By tuning the central angle of the metallic arc, its intrinsic dipolar mode is in destructive interference with the spoof localized surface plasmon (SLSP) on textured closed cavity, which results in a plasmon-induced transparency phenomenon. The measured transmittances of as-fabricated samples using terahertz-time domain spectroscopy validate numerical results using extended coupled Lorentz oscillator model. It is found that the coupling coefficient and damping ratio of SLSP relies on the radius of the ring structure of textured closed cavity. As a consequence, the slow light maximum values become manoeuverable in strength at certain frequencies of induced transparency windows. To the best of our knowledge, our experimental result is currently the highest value demonstrated so far within metasurface at terahertz band.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement