Strong optical nonreciprocity in a photonic crystal composed of spinning cylinders

Abstract

Moving media break time-reversal symmetry and exhibit intriguing optical nonreciprocity. This nonreciprocity is usually weak due to the much lower moving speed of media relative to the speed of light. We demonstrate that strong optical nonreciprocity can emerge in a two-dimensional photonic crystal composed of spinning dielectric cylinders. The photonic crystal supports two types of chiral modes at the Brillouin zone center: hybridized multipole modes and symmetry-protected bound states in the continuum (BICs), both of which carry intrinsic spin angular momentum. For finite wavevectors near the zone center, the BICs transform into quasi-bound states in the continuum (QBICs). Under oblique incidence of circularly polarized plane waves, the photonic crystal exhibits nonreciprocal transmission and absorption that are significantly enhanced at the frequencies of these hybridized multipole modes and QBICs. Furthermore, the high quality factors of the QBICs enable sharp transitions in nonreciprocity. Our work uncovers strong chiral light-matter interactions in periodic moving structures, with potential applications in nonreciprocal light manipulation. The mechanism may also be generalized to other classical wave systems, such as phononic crystals.

Figures (4)

• Figure 1: (a) Schematic of the photonic crystal composed of spinning dielectric cylinders arranged in a square lattice with period D = 600 nm. The cylinder in each unit cell has the radius $a$ = 200 nm and the height $l$ = 390 nm, and its spinning angular velocity is $\mathbf{\Omega}=\varOmega \hat{z}$. (b) Electric field norm and spin density of the eigenmodes at $\mathrm{\Gamma}$ point of the 6th, 7th and 8th bands (counted from bottom to top) in (c). (c) Band structure of the photonic crystal with rotation speed $\Lambda$=0. The color of the bands denotes the quality factor of the eigenmodes. The bands are labeled by their dominant multipole components. (d) Band structure of the photonic crystal with rotation speed $\Lambda$=0.01.
• Figure 2: (a) Schematic of the photonic crystal under the forward incidence of a circularly polarized plane wave. (b) Transmission spectra of the photonic crystal under forward ($T_{\mathrm{f}}$) and backward ($T_{\mathrm{b}}$) incidences. We set the normalized spinning speed $\Lambda=0.01$. The black solid line denotes the result of the stationary photonic crystal. (c) Transmission contrast $T_{\mathrm{f}}-T_{\mathrm{b}}$ as a function of the normalized spinning speed $\Lambda$.
• Figure 3: (a) Schematic of the photonic crystal under the oblique incidence of a circularly polarized plane wave with incident angle $\theta$. (b) Transmission spectra of the photonic crystal under forward ($T_{\mathrm{f}}$) and backward ($T_{\mathrm{b}}$) incidences. We set the normalized spinning speed $\Lambda$=0.01. The black solid line denotes the result of the stationary photonic crystal. (c) Transmission contrast $T_{\mathrm{f}}-T_{\mathrm{b}}$ for different incident angles $\theta=5^\circ,10^\circ, 15^\circ$.
• Figure 4: (a) Schematics for demonstrating CD and nonreciprocity in the photonic crystal composed of spinning cylinders. Systems B1 and B2 can be transformed into each other through a mirror operation with respect to xoz-plane and a $180^\circ$ rotation with respect to y-axis. (b) Absorption spectra of the photonic crystal ($\Lambda=0.01$) under the incidence of circularly polarized lights. The black line denotes the absorption dissymmetry factor $g$.