Nanotechnologies / Materials
Sensing

Development of dynamic metasurfaces for highly-efficientterahertz chiral sensing
Department of Systems Innovation, Graduate School of Engineering Science
Associate Professor Yosuke Nakata
https://researchmap.jp/yosuke_nakata

Nanotechnologies / Materials

Abstract
The terahertz frequency range has attracted much interest fromthe perspective of sensing technology because there are giantand bio-molecule vibrations in this frequency range. Measuring
the response for the left- and right-circular polarizations is needed
to detect the chirality of some molecules whose mirrored structure
differs from the original structure. Therefore, a technique to switch
the rotational direction of circular polarization is strongly demanded. In this study, we realize a device that can efficiently convert
an incident linear polarization into a circular polarization and dynamically reverse its rotation direction. The device realizes desired
functionality by deforming a metallic pattern (Fig. 1). The pattern
deformation is achieved by introducing vanadium dioxide (Fig.
2a), of which resistivity changes with temperature. In the end, wedemonstrated its highly-efficient operation of terahertz circular-polarization switching.

▍ Significance of the research and Future perspective
Because the developed device converts the incident wave
based on reflection, it achieves highly-efficient conversion, which
has been impossible for conventional transmissive devices in principle. As shown in Fig. 2b, a power conversion efficiency of over
80% was successfully achieved when vanadium dioxide is in bothinsulator (OFF) and metallic (ON) states, exceeding the theoreticallimit for single-layer transmissive devices. This device is applicableto highly-efficient terahertz chiral sensing and also communications
utilizing polarization degrees of freedom.

▍ Background & Results
In the terahertz frequency range, demands for high-sensitivity
sensing and high-speed communication require the developmentof dynamic devices. However, due to various limitations, conventional dynamic devices such as semiconductor and liquid-crystal
devices cannot be directly used in the terahertz region. In addition,terahertz waves have much longer wavelengths compared to light,
so the devices tend to become thick. To solve these problems,
artificially-structured surfaces called metasurfaces are attractingattention. Metasurfaces often leverage metallic structures, which
produce strong responses despite their subwavelength thickness.
Additionally, metasurfaces can incorporate dynamic materials thatcan change their response depending on external stimuli. In thisstudy, we realized a dynamic metasurface with vanadium dioxide
for highly-efficient switching of terahertz circular polarization.
Figure 1 shows the schematic of the operation principle of thedevice. In the left of Fig. 1, metallic crosses with different lengthsare arranged on the top surface of a dielectric substrate with a metallic bottom ground. The anisotropy of the cross induces the conversion from an incident linearly polarized wave into a reflected circularly-polarized one. When an external stimulus is applied to thestructure, the metal pattern is deformed to be shorted. This deformation reverses the direction of rotation of the reflected circularpolarization. The pattern deformation is implemented by introducing vanadium dioxide between the metal crosses (Fig. 2a). Whenthe temperature is increased, vanadium dioxide changes frominsulator (OFF) to metallic (ON) states, resulting in metallic patterndeformation. Figure 2b represents the power conversion efficiency
from linear polarization into circular polarizations for the OFF andON states of the device. The efficiency is beyond the principal limitof single-layer transmissive metasurfaces. In other words, we caninvert the rotational direction of terahertz circular polarization while
maintaining high efficiency.

Figure 1: Operation principle. A linearly-polarized terahertz wave enters the metasurface. The rotation direction of the reflected terahertz circular polarization isswitched by deforming the metallic pattern.

Figure 2: Physical device and its characterization: (a) Photomicrograph of themetasurface. (b) Power conversion efficiency from linear polarization to circularpolarization. The efficiency sign (±) represents the difference between right- andleft-circular polarizations. The shaded region is beyond the theoretical limit for
single-layer transmissive devices.

P a t e n t
Tr e a t i s e

Kobachi, Mitsuki; Miyamaru, Fumiaki; Nakanishi, Toshihiro et al. Dynamic quarter-wave metasurface for efficient helicity inversion of polarization beyond
the single-layer conversion limit. Advanced Optical Materials. 2022, 10(2), p. 2101615, doi: 10.1002/adom.202101615

URL
K e y w o r d terahertz technique, metamaterials, metasurfaces, vanadium dioxides