Strong Optical Chirality Leveraged by Vertically-Aligned 3D Helical Graphene Metasurfaces

Optical chirality in Terahertz (THz) range remains a large blank page due to materials limitations and their plasmon resonant frequency range. Graphene offers a promising avenue to address this challenge because its plasmon resonant frequency resides in THz range and can be well tunable. In this work, we developed a novel 3D graphene metasurface (vertically-aligned helical graphene). A thin layer photoresist, acting as an actuator, together with graphene, are both self-twisted and curved upward to 3D vertically-aligned direction simultaneously using a self-assembly approach. The chiral properties of the 3D graphene helix were carefully evaluated with simulation. In detail, the resonant frequency of the graphene helix in THz range can be well adjustable from 25 THz to less than 1 THz by changing the size. For a same size and handed helix, under the illumination of different polarized light, the chiral response demonstrates differently, as indicated by the circular dichroism (CD) and g-Factor, an absolute value from the different absorption of the light and a normalization value of CD, respectively. Moreover, the angle of the helix compared with the light versus the CD property are also carefully studied, with CD value of 0.003 and 0.07, and g-Factor of 0.12 and 1.4 when the helix lies down and perpendicular to the substrate, respectively. The simulation results verify and argument our hypothesis that extended the coupling distance between the helix and the light results in larger CD property, meaning a higher degree of optical chirality. This newly developed graphene metasurface and evaluated chirality property can be further used as a THz biosensor for medicine and chemical molecular detection.