Radiation-pressure-induced non-Hermitian skin effect in elastic membranes

Abstract

We show that optical forces perpendicular to the direction of the incident light, generated on structures with asymmetric optical scattering, can manipulate longitudinal elastic waves traveling in that same perpendicular direction. When the radiation pressure acts unidirectionally, reciprocity and hence Newton's Third Law are effectively broken. As a result, the waves grow exponentially with position, an instance of the non-Hermitian skin effect. The effect can be enhanced by orders of magnitude to measurable scales in optically dispersive nanostructured membranes. These findings are particularly relevant in the context of lightsails, spacecraft propelled by radiation pressure from high-power lasers. Our discovery showcases a new interaction between radiation pressure and elastic waves, which taps into the rich field of non-Hermitian physics.

Figures (5)

• Figure 1: Left: radiation reflected specularly from a body drives elastic waves within the medium. Right: Radiation pressure perpendicular to the direction of light incidence couples to longitudinal elastic waves propagating in that direction, leading to non-Hermitian effects.
• Figure 2: (a) Flexible membrane modeled as a series of masses and springs with light incident from below. Brightness of red bars represents the laser power intercepted by the springs. Bottom row: rest configuration (all springs have length $l_0$). Middle row: strained configuration. Top row: diffraction gratings with unit cells strained in the $\hat{\mathbf{x}}$ direction compared to the rest structure shown faintly in the background. (b) Radiation pressure can be generated with components parallel and perpendicular to the direction of light incidence when light is scattered asymmetrically. (c) Periodic-boundary-condition modes in the complex-frequency plane (Eq. \ref{['eq:dispersion_complex']}, in units of $\omega_\text{spr}$) form a loop parametrized by $\beta l_0 \in [-\pi,\pi)$ increasing ($\bar{k}_\text{pr} = 0.1$). The (real) eigenfrequencies for the same parameters but in a finite system ($N=60$) with open boundary conditions (OBC) are displayed with rainbow colors (purple $\beta_\text{re}l_0 =0$ to red $\beta_\text{re}l_0 =\pi$). (d) OBC eigenvector (displacement) magnitudes with the associated colors (except the zero-frequency mode).
• Figure 3: Snapshots in time of displacements across a membrane. Each snapshot is indicated by a different color and normalized by the $t=0$ displacement amplitude. For clarity in all simulation results, center-of-mass motion is subtracted off, we set $Q=0$ and plots are truncated to positive displacement values video (the rationale is discussed in Appendix \ref{['app:symmetric']}). (a) Unidirectional radiation pressure with $\bar{k}_\text{pr} = 8e-3$ for a Gaussian initial displacement. (b) Radiation pressure has opposite sign for $n<N_\text{mid}$ and $n>N_\text{mid}$ due to the sail having reflection symmetry about the $\hat{\mathbf{y}}$ axis (represented by the angled reflectors underneath the springs). The magnitude of nonreciprocity varies with the nonuniform laser beam. (c) Symmetric sail irradiated by a uniform laser beam whose spatial intensity distribution is depicted in the red background of the $t=0$ frame ($\bar{k}_\text{pr} = 4e-3$ with $Q = 0$, $\frac{\partial Q}{\partial s}=5e2$ and $I=5GW\per m$). (d) Same as (c) but with a Gaussian laser beam (same total power, but higher peak intensity such that $\bar{k}_\text{pr} = 7e-3$).
• Figure 4: (a) Centrifugal force from lightsail spinning increases with distance from mass $n=N_\text{mid}$. (b) Spin opens a momentum bandgap (Eq. \ref{['eq:spin_dispersion']} with $\bar{k}_\text{spin} = 0.1$). The solid green line and dashed orange line show the positive and negative frequency branches (in units of $\omega_\text{spr}$).
• Figure 5: Derivatives of $Q$ with respect to elongation and wavelength over the Doppler spectrum. Wavelength shift is relative to the laser wavelength. Left: grating optimized for broadband beam-transport stability from Ref. Lin:2026aa. Right: grating optimized for $Q_\text{lin}$ in this work.