Self-Isolated Raman Lasing with a Chiral Dielectric Metasurface

The light sources that power photonic networks are small and scalable, but they also require the incorporation of optical isolators that allow light to pass in one direction only, protecting the light source from damaging backreflections. Unfortunately, the size and complex integration of optical isolators makes small-scale and densely integrated photonic networks infeasible. Here, we overcome this limitation by designing a single device that operates both as a coherent light source and as its own optical isolator. Our design relies on high-quality-factor dielectric metasurfaces that exhibit intrinsic chirality. By carefully manipulating the geometry of the constituent silicon metaatoms, we design three-dimensionally chiral modes that act as optical spin-dependent filters. Using spin-polarized Raman scattering together with our chiral metacavity, we demonstrate Raman lasing in the forward direction, while the lasing action is suppressed by over an order of magnitude for reflected light. Our high-$Q$ chiral metasurface design presents a new approach toward compactly isolating integrated light sources by directly tailoring the emission properties of the light source itself.

Figure 1

(a) Schematic of self-isolated metasurface laser. The Raman pump (purple) results in stimulated emission at the Stokes frequency (yellow). Left: In a traditional laser cavity, the original lasing mode (red) is destroyed when reflected light reenters the cavity (yellow). Right: Because of spin-selective modal restriction of the metasurface cavity, the lasing mode remains undisturbed upon reflection. (b) Energy diagram for Raman decay of a pump photon into a Stokes photon and a phonon, with a reference legend defining forward (backward) transmission (reflection). (c) The chiral metasurface, comprised of cylinders in a dimer unit cell arranged in a square lattice with periodicity $a=1.2\mu \mathrm{m}$ and antisymmetric notches, where $d=600\mathrm{nm}$, $h=600\mathrm{nm}$ and $d^{\prime }=160\mathrm{nm}$.

Figure 2

Transmission (a)–(c) and differential reflected phase to show bianisotropy (a),(b) for the given unit cell geometry and associated point group symmetry (inset), where $d=600\mathrm{nm}$, $h=600\mathrm{nm}$, and $d^{\prime }=160\mathrm{nm}$. (a) Cylinders notched in the center: Transmission shows low-$Q$ response over which electric and magnetic dipolar modes exist, and the differential reflection reveals weak but nonzero bianisotropy. (b) Translating the notches antisymetrically: Transmission reveals two high-$Q$ resonances corresponding to illumination with $+45°$ and $-45°$ polarization, both of which exhibit strong bianisotropy in the phase of reflection. (c) Inverting every other notch: Differential transmission is observed for illumination for left-handed versus right-handed circularly polarized light, indicating chirality.

Figure 3

(a) Transmittance and reflectance of the signal as a function of the pump power with a left-handed circularly polarized pump. Inset: The plot is recreated with a decibel scale. (b) The imaginary component of the complex eigenfrequencies of the chiral metasurface, plotted as a function of the pump power. Red corresponds to the 1865.3 nm eigenmode and yellow corresponds to the 1865.7 nm eigenmode from Fig. 2.