Observation of Perfectly-Chiral Exceptional Point via Bound State in the Continuum

Chirality is one of the fundamentals of nature with strong ties to asymmetry. In wave physics, it is conventionally characterized by asymmetric scattering of circularly polarized waves but suffers from two-state polarization. To overcome the limitation, here we demonstrate the concept of extreme chirality regarding orbital angular momentum (OAM) helicity, originating from a chiral quasibound state in the continuum held by a mirror-symmetry-broken metasurface. Empowered by the intrinsic OAM-selective coupling nature of the metasurface, the system arrives at a peculiar state where the left-handed incident vortex is completely absorbed while the right-handed counterpart is totally reflected, namely, a perfectly-chiral exceptional point. The realization of asymmetric OAM modulation creates the possibility to explore chirality with unlimited states. Our work raises a new paradigm for the study of extreme OAM chirality and enriches the physics of chiral wave-matter interaction.

Figure 1

Schematics of extreme OAM chirality at the perfectly-chiral EP. Upper panel: a chiral QBIC held by the metasurface featuring weak yet extremely chiral radiation at the eigenfrequency $\omega ={\omega }_0+j{\gamma }_r$. Lower panel: extremely asymmetric scattering response at the perfectly-chiral EP in the critical coupling regime (${\gamma }_d={\gamma }_r$).

Figure 2

(a) Left: schematic of metasurface with $C_2$ rotational symmetry and ${\sigma }_{1,2}$ mirror symmetry. ${\theta }_0$ is the span angle of the wall. $R_i$ and $R_e$ are the inner and external radius, respectively. Right: cross section of the metasurface. (b) Left: schematic of the chiral metasurface. Right: cross section. $\mathrm{\Delta }\theta$ denotes the perturbation of span angle of groove 2. Note that (a) and (b) depict the air domains bounded by the fanlike grooves. (c) Radiative quality factor of eigenstates held by the metasurface. (d) Proportion of mode $|-⟩$ in the radiation field. The dashed lines indicate achiral QBICs with equal $|+⟩$ and $|-⟩$. The circles with arrows denote the helicity of OAM in radiation field. The star and red dot respectively mark the BIC and the selected chiral QBIC. (e) Real part of sound pressure $p$ and intensity distribution from the radiation field of BIC (upper panel) and the chiral QBIC (lower panel). See detailed geometrical parameters in Supplemental Material, Sec. A [39].

Figure 3

(a) Theoretical average acoustic energy density (${ϵ}_p$) in the metasurface excited by $|-⟩$ (red line) and $|+⟩$ (blue line) vortex. (b) Theoretical (solid lines) and experimental (circles) reflectivity spectra. (c) Riemann surfaces of the complex eigenvalues of the $S$ matrix versus real ($c_r$) and imaginary part ($c_i$) of the complex sound speed. The real and imaginary parts of the eigenvalues are quantified by the vertical axis and the color bar, respectively. (d) Theoretical (line) and experimental (circles) chirality.

Figure 4

(a) Schematic of the experiment platform. The scan microphone connected to a motor moves in a step of 1 mm within the blue region. (b) Simulated incident and reflected distribution of real part of sound pressure $p$. (c) Experimental scanning results against simulated results of incidence and reflected fields at 150 mm above the metasurface [cross section encircled by the dashed line in (b)]. The upper and lower rows respectively present the amplitude and phase distribution.