# All-Optical Tuning of Fano Resonance for Quasi-BIC and Terahertz Sensing Applications
## 1 Introduction  
Terahertz (THz) sensing technology has the advantages of low photon energy, strong penetrability, wide frequency band, and strong anti-interference ability, and has shown great application potential in non-destructive testing [1], label-free genetic diagnostics [2], and biomedical imaging [3]. However, the sensitivity of THz sensing is not high at present, due to the lack of high-power radiation sources and the weak intermolecular or intramolecular vibration of analytes in THz band, which greatly limits its development in the sensing field. Resonant metasurfaces structures can effectively enhance the local optical field and promote the interaction between light and matter at specific frequencies, thus becoming an effective way to improve sensitivity [4]. This requires the introduction of asymmetry in the geometric cell tailoring of the subwavelength structural platform so as to form Fano resonance, in which an asymmetric linear shape is generated by the interference coupling between a continuous state mode with a wide resonance bandwidth and a discrete state mode with a narrow resonance bandwidth [5; 6; 7]. Recently, a variety of asymmetric structures have been derived, including ring/disk cavities, holes, dolmen structures, and compound sub-structures [8; 9; 10]. However, these require high processing accuracy, which needs to reach the order of 100 nanometers. In recent years, the concept of BIC originated from the concept of quantum mechanics and was gradually introduced into metasurface structures. The ideal BIC structure theoretically does not produce radiation leakage, has an infinite Q factor, and extremely narrow resonance [11; 12], so it has great potential in sensing, lasing, filters, and nonlinear optics [13; 14]. However, for sensing, the ideal BIC is usually hidden in the normally propagating electromagnetic spectrum and cannot be identified [15]. Therefore, in practical applications, it is necessary to perturb the symmetry-protected BIC state to convert it into a quasi-BIC mode in order to construct a radiation channel with the outside. Among them, the quasi-BIC structure constructed by toroidal dipole (TD) not only has a high Q factor but also can strengthen the interaction between light and matter through the near-field enhancement effect, thus providing a unique advantage for sensing [5; 16; 17]. Quasi-BICs have been realized in symmetric dielectric dimer/trimeric nanobar structures [18]; asymmetric dipole rods [5], pillars [19], rings [16]; and other metasurface structures. In addition to introducing symmetry breaking in the structure, the method of inducing the quasi-BIC state can also change the absorption of the material or adjust the incident angle of the light source [16; 19]. The above-mentioned tuning methods are all called parametric tuning BIC. In general, the closer the system is to the BIC state, the greater the influence of asymmetry on it. The micron-scale deviation will even cause the Q factor to decrease by an order of magnitude. Once the structure is determined, it can only support a single BIC state. This is undoubtedly a great challenge to the manufacturing process. In addition, the surface roughness, limited area, and material properties of the sample can lead to unpredictable radiative and non-radiative losses during design, hence the device needs to be dynamically tuned. To overcome this issue, methods based on electrical or optical dynamic BIC modulate of metasurfaces have been gradually developed in recent years. [PERSON] et al. introduced structured graphene into the metal metasurfaces to form graphene-metal metasurfaces, tuned the Fermi energy of graphene to change the free electron concentration to adjust it from semiconductor-like to metal-like, and finally realized free switching between BIC and quasi-BIC of resonance mode [20]. [PERSON] et al.