Abstract-Deeply Subwavelength Metasurface Resonators for Terahertz Wavefront Manipulation

Mingkai Liu,  Quanlong Yang,  Ahmmed A. Rifat,   Vidur Raj,   Andrei Komar,   Jiaguang Han,   Mohsen Rahmani,   Haroldo T. Hattori,   Dragomir Neshev,   David A. Powell,   Ilya V. Shadrivov,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900736?af=R

Metasurfaces offer a highly flexible platform for controlling the propagation and localization of electromagnetic waves. Due to the relatively large size of commonly used resonators, various undesirable effects including spatial dispersion and spurious diffraction occur, thus limiting the metasurface performance. To overcome these problems, one straightforward approach is to utilize deeply subwavelength metaunits. In contrast to conventional approaches that minimize the resonator size by reshaping the metallic patches, the capacitive gaps are reshaped, an approach which is more robust to material loss, minimizing the problem of overdamping. As an example, a novel design based on interdigital capacitors (meander gap) is introduced with extremely subwavelength gaps for use in the terahertz frequency range. The size of the new resonator can be reduced to below λ/30 in a reflective‐type terahertz metasurface, while maintaining the 2 𝜋$\pi$ phase shift required for full wavefront control. Using an advanced electron‐beam lithography technique, a proof‐of‐concept experiment is performed and a 5 mm × 5 mm beam deflector is fabricated, with the capacitive gaps as small as 300 nm (≈λ/1130). The device performance is characterized using angle‐resolved time‐domain spectroscopy. The study provides useful insight for ultracompact metadevices based on deeply subwavelength metaunits working at terahertz frequencies and beyond.

Abstract-Strong broadband terahertz optical activity through control of the Blaschke phase with chiral metasurfaces

Michael A. Cole, Wen-chen Chen, Mingkai Liu, Sergey S. Kruk, Willie J. Padilla, Ilya V. Shadrivov, David A. Powell


https://journals.aps.org/prapplied/accepted/6e07eAb0N761870c20e140b145fd23b6eadac6cd4

We demonstrate terahertz chiral metamaterials that achieve resonant transmission and strong optical activity. This response is realized in a metasurface coupled to its Babinet complement, with additional twist. Uniquely, the optical activity achieved in this type of metamaterial is weakly dispersive around the resonant transmission maxima, but can be highly dispersive around the transmission minima. It has recently been shown that this unique optical activity response is closely related to zeros in the transmission spectra of circular polarizations through the Kramers-Kronig relations and strong resonant features in the optical activity spectrum corresponding to the Blaschke phase terms. Here we demonstrate how modifying the meta-atom geometry greatly affects the location and magnitude of these Blaschke phase terms. We study three different meta-atoms, which are variations on the simple cross structure. Their responses are measured using terahertz time-domain spectroscopy, and analyzed via numerical simulations.

Abstract-Terahertz focusing of multiple wavelengths by graphene metasurfaces

Liming Liu^1,2,a), Yair Zarate¹, Haroldo T. Hattori², Dragomir N. Neshev^1,3, Ilya V. Shadrivov¹ and David A. Powell^1,3
 HIDE AFFILIATIONS
1 Nonlinear Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia
2 School of Engineering and Information Technology, University of New South Wales, Canberra, ACT 2612, Australia
3 Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), Australian National University, Canberra, ACT 2601, Australia
a) Author to whom correspondence should be addressed. Electronic mail:liming.liu@student.adfa.edu.au
Appl. Phys. Lett. 108, 031106 (2016); http://dx.doi.org/10.1063/1.4940231

http://scitation.aip.org/content/aip/journal/apl/108/3/10.1063/1.4940231?TRACK=RSS

Metasurfaces can achieve nearly arbitrary wavefront control based on manipulation of the wavephase profile. We propose a metasurface based on double graphene cut-wire resonatorswhich can cover the complete 2π phase region with high reflection efficiency. This full phase coverage is essential for efficient wavefront manipulation, without reflecting energy into unwanted channels. A mirror capable of focusing multiple wavelengths is demonstrated numerically based on the proposed structure. The mirror can effectively focus terahertz (THz)waves from 1.2 to 1.9 THz to the same focal point by changing the Fermi level of eachgrapheneresonator separately. The presented metasurface could provide a powerful platform for controlling THz waves, including focusing, beam steering, beam shaping, and holograph