Acoustic helical dichroism enhanced by chiral quasi-bound states in the continuum

TL;DR

This work addresses the weak acoustic helical dichroism (HD) by engineering quasi-bound states in the continuum (QBICs) within meta-cavities built from Helmholtz resonators. Using full-wave simulations, the authors show that an achiral meta-cavity supports vortex QBICs with high Q but 2D (planar) chirality, which fail to produce HD, whereas a chiral meta-cavity with twisted resonators supports 3D helical QBICs that strongly enhance HD. The enhancement is evidenced by differential absorption of opposite-handed helices and a measurable dissymmetry factor (g_A d 0.04) at the QBIC frequency, as well as the ability of the chiral cavity alone to induce HD. The study identifies two key requirements for strong acoustic HD via QBICs: high Q-factor and 3D chirality of the state fields, offering a route to robust acoustic chiral sensing and OAM manipulation.

Abstract

Acoustic helical dichroism (HD) arises from the interaction between vortex beams carrying orbital angular momentum (OAM) and chiral media, yet such chiral sound-matter interactions are typically weak. Here, we employ quasi-bound states in the continuum (QBICs) in acoustic meta-cavities composed of coupled Helmholtz resonators to enhance acoustic HD. We design both achiral and chiral meta-cavities that support QBICs in the form of vortex states with high Q-factors. Using full-wave numerical simulations, we show that the QBICs in the achiral meta-cavities cannot enhance acoustic HD due to the absence of a chiral wavefront. In contrast, the chiral meta-cavity exhibits a pronounced HD enhancement through the QBICs with a 3D helical wavefront, which can be excited by incident waves either with or without OAM. Our work identifies two essential requirements for enhancing acoustic HD effect via QBICs: a high Q-factor of the states and 3D chirality of the state fields, which usually compromise each other in conventional acoustic resonators. The findings open new avenues for achieving strong chiral sound-matter interactions, with potential applications in acoustic chiral sensing and acoustic OAM manipulation.

Figures (4)

• Figure 1: (a) Achiral meta-cavity composed of four identical Helmholtz resonators with slit openings. (b) Chiral meta-cavity composed of four identical twisted Helmholtz resonators with slit openings. Each resonator of the achiral and chiral meta-cavities has radius $r=50$ mm, height $h=100$ mm, slit width $b=10$ mm, and radial distance $a$ from the central axis. All resonators have a shell thickness $t=5$ mm. The inset in (b) indicates the twist angle of 180° for each resonator. Eigenfrequencies and Q-factors for the lowest four eigenmodes of (c) the achiral meta-cavity and (d) the chiral meta-cavity with $a=72$ mm. The insets show the eigen pressure field on a cross section of the meta-cavities. The Q-factor of dipole modes as a function of the radial distance $a$ for (e) the achiral meta-cavity and (f) the chiral meta-cavity. Line colors indicate the real part of the eigenfrequency.
• Figure 2: (a) Achiral meta-cavity system under the incidence of a vortex guided wave. The meta-cavity is located inside a cylindrical waveguide of radius $R=300$ mm and height $H=2000$ mm. A helix particle (with $D=40$ mm, $d=16$ mm, and $c=50$ mm) is placed at the center of the meta-cavity, as shown by the zoom-in. (b) Three lowest-order guided modes of the cylindrical waveguide. (c) Waveguide transmission for the achiral meta-cavities with $a = 75$ mm, $90$ mm, and $100$ mm, under the incidence of the linear dipole guided wave $p_y$. The inset shows the excited pressure field inside the meta-cavity. (d) Absorption of the helix particle under the incidence of vortex guided waves with $l = \pm1$, where we set $a = 100$ mm. (e) Waveguide transmission (solid red and dashed blue lines) for the achiral meta-cavity ($a = 100$ mm) without the helix, under the incidence of vortex guided waves with $l = \pm1$. The solid magenta and blue lines denote the normalized Stokes parameters $S_3$ of the local velocity field at the center of the meta-cavity. (f) Pressure amplitude and phase of the excited vortex QBICs inside the meta-cavity. The dashed black circles in the profiles of Abs($p$) and Arg($p$) indicate the location of the wavefront Arg($p$) = 0.
• Figure 3: (a) Chiral meta-cavity system under the incidence of a linear dipole guided wave. The waveguide and helix particle are the same as in Fig. \ref{['fig2']}(a). (b) Waveguide transmission for the chiral meta-cavities with radii $a = 75$ mm, $90$ mm, and $100$ mm, under the incidence of the linear dipole guided wave polarized along $y$-direction. (c) Pressure amplitude and phase of the excited vortex QBIC. The dashed circles marked in the profiles of Abs($p$) and Arg($p$) indicate the location of the wavefront Arg($p$) = 0. (d) Normalized Stokes parameter $S_3$ of the velocity field at the center of the meta-cavities with radii $a = 75$ mm, $90$ mm, and $100$ mm. (e) Absorption of the left-handed and right-handed helices for the systems with and without the chiral meta-cavity. We set $a = 100$ mm for this case. (f) Absorption dissymmetry factor $g_A$ for the systems with and without the chiral meta-cavity, corresponding to the results in (e).
• Figure 4: (a) Absorption of the lossy chiral meta-cavity under the incidence of vortex guided waves with $l=\pm 1$. We set the sound speed $v_0=343(1+0.01i)$ m/s for the regions (blue colored) inside the four Helmholtz resonators. (b) Absorption of a single lossy Helmholtz resonator under the incidence of vortex guided waves with $l=\pm 1$. (c) Absorption dissymmetry factor $g_A$ for the systems in (a) and (b).