Fano resonant metasurfaces at terahertz and infrared regime

Abstract

Metamaterials has transformed our perception of artificial materials being simply just man-made materials constructed from natural materials. In the context of metamaterials, "artificial" materials can also be referred to as materials that have gone beyond human's imagination and are not existent in nature. They are subwavelength in scale and manifest extraordinary material properties. In this regard, metamaterials can be engineered to manifest Fano resonance through symmetry breaking of the metamaterial system. It is a scattering phenomenon with an asymmetric lineshape and possesses exceptional characteristics such as high-quality (Q) and strong electromagnetic field confinement. Thus, it has great potential for applications in sensing, dynamic switching and enhanced luminescence. For practical applications of Fano resonance across the electromagnetic spectrum, we investigated the decay trend of high-Q Fano resonance and established the universal behaviour of Fano resonance from terahertz (THz) to near-infrared (NIR) regime. This suggests that the scattering phenomenon of Fano resonance is prominent irrespective of the dimensions of the metasurfaces and hence, appropriate for scalability of devices. In addition, to improve the performance of the device in terms of Q-factor, we also demonstrated and showed that by inverting the unit cell configuration of complementary asymmetric dipole bars, we are able to achieve an enhancement of the Q-factor in the NIR frequency. Multipole analysis reveals that linewidth narrowing of Fano resonance is due to the contributions from the magnetic quadrupole, but for non-inverted unit cell configuration, the toroidal dipole acts to broaden the linewidth. As proof of concept of the capability of Fano resonance, we demonstrated an ultrafast all-optical switching of Fano resonance using THz asymmetric split ring resonators (ASRR). The incorporation of germanium, or semiconductor as an optically active medium serves as an avenue to achieve switchable and dynamic tuning of Fano resonance. When free carriers are photoexcited above the conduction band, germanium transits from a semi-metallic to metallic state and shunts the capacitive split gap of THz ASRR. The transmission of Fano resonance is modulated and recovers within 17 ps, hence achieving the effect of photoswitching. The photoswitching of Fano resonance in THz band is beneficial in telecommunications when a signal transmission has to be routed around a fault in the line or employed in logic gates operation. Photoswitch can also be utilized in electric field sensing or security purpose where a modulation of transmission signal can trigger the alarm. The multipurpose of Fano resonance is associated to its non-radiative nature, high-Q and strong confinement of electric field. Therefore, it is with great ambitious that we hope our findings will promote further research in Fano resonant metasurface, so as to fully exploit the potential of Fano resonance. We envision that Fano resonance could be the ideal solution of modern technologies.