Abstract-Strong terahertz magneto-optical phenomena based on quasi-bound states in the continuum and Fano resonances

G. Y. Chen, W. X. Zhang, and X. D. Zhang

Fig. 1 (a) Diagram of the photonic crystal-slab structure and coordinate. Magnetic field is along Z-axis,𝐵=7𝑇.$B=7T.$The photonic crystal is arranged in a square lattice with the lattice constant 𝑎=9.5194𝜇𝑚.$a=9.5194\mu m.$The radii of air cylinders in photonic crystal are 𝑟=0.3*𝑎.$r=0.3*a.$The photonic crystal thickness is 𝑑1=0.3*𝑎.$d_1=0.3*a.$The slab is placed next to the photonic crystal and the thickness is 𝑑2=0.3*𝑎.$d_2=0.3*a.$ (b) Cross sections of the reflection coefficient at 0o$0^{\text{o}}$ and 2o$2^{\text{o}}$ incidence angles show the appearance of the collapsed symmetry-protected BICs to sharp Fano resonance. Blue and red arrows point to the BICs and quasi-BICs. All the incident light in the above figure is linearly polarized light, and the polarization direction of the electric field along the X axis. (c) Eigenfrequency analysis with𝜃$\theta$ for BICs and Fano resonances. Blue and red lines overlap at 𝜆=11.25𝜇𝑚$\lambda \text{=}11.25\mu m$. (d) The quality factor changes with 𝜃$\theta$, and it can be found that the Q factor decreases rapidly along the direction of 𝛤$\Gamma$ point. The frequencies used are consistent with those in Fig. 1(c). (e) The electric field intensity at the 𝜆=12.95𝜇𝑚$\lambda \text{=12}\text{.95}\mu m$ corresponds to the electric field distribution under quasi-BICs. (f) The electric field intensity at 𝜆=11.25𝜇𝑚$\lambda \text{=11}\text{.25}\mu m$corresponds to the electric field distribution under Fano resonances.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-12-16449

We propose new ways to produce strong terahertz (THz) magneto-optical phenomena from monolayer graphene based on bound states in the continuum (BICs) and Fano resonances. The BICs and Fano resonances of radiation modes in the monolayer graphene are realized by designing the photonic crystal slab-graphene-slab structure. Based on them, the magnetic circular dichroism near 100% has been achieved. Importantly, such magneto-optical phenomena can be modulated in intensity and frequency using only electrostatic doping at a fixed magnetic field. Comparing two ways to produce magneto-optical phenomena, it is found that the way based on BICs exhibits some advantages such as good electrical tenability due to narrower resonance width, higher conversion efficiency and more stability with the change of incident angle. These phenomena can appear in a broad THz range by designing the nanostructures, which are very beneficial to polarization conversion and optoelectronic devices.

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