Dispersion Control of Chiral Exciton-Polariton Transport with Dielectric Metasurfaces

Technical Summary: Dispersion Control of Chiral Exciton-Polariton Transport with Dielectric Metasurfaces

Problem Statement
Exciton-polaritons (EPs) are hybrid light-matter quasiparticles that offer enhanced transport lengths and strong nonlinearities, making them promising for applications ranging from low-threshold lasing to all-optical switching. However, while organic semiconductors support room-temperature EPs, their excitonic component is typically confined to single molecules, limiting propagation to a few nanometers. Furthermore, while spintronics and valleytronics utilize electron spin and valley degrees of freedom, controlling the spin angular momentum (SAM) of polaritons for selective transport remains a challenge. Specifically, achieving spin-selective transport in organic systems without relying on intrinsically chiral molecules is difficult. The authors aim to demonstrate how to induce and control chiral transport in EPs formed by achiral organic molecules by coupling them to extrinsically chiral optical modes.

Methodology
The study employs a dielectric metasurface composed of tilted silicon (Si) nanorod dimers arranged in a square lattice on a SiO₂ substrate.

• Metasurface Design: The unit cell consists of two Si nanorods (H=90 nm, W=50 nm, L=130 nm) tilted by $\theta = 10^\circ$ relative to each other. This geometry breaks inversion symmetry, transforming a symmetry-protected bound state in the continuum (BIC) into a radiative quasi-bound state in the continuum (quasi-BIC) while also supporting surface lattice resonances (SLRs) via coupling with Rayleigh anomalies.
• Material System: A thin film (470 nm) of an achiral perylene dye (N, N'-Bis(2,6-diisopropylphenyl)-1,7- and -1,6-bis (2,6-diisopropylphenoxy)-perylene-3,4:9,10-tetracarboximide) blended at 30 wt% in polymethyl methacrylate (PMMA) is spin-coated onto the metasurface. This concentration ensures strong light-matter coupling without aggregation.
• Characterization Techniques:
  • Fourier Microscopy: Used for angle-resolved extinction and photoluminescence (PL) measurements to map the dispersion relations of the polariton bands.
  • Circular Polarization Analysis: A quarter-wave plate and linear polarizer were used to separate left-handed (LCP) and right-handed (RCP) circularly polarized emission.
  • Real-Space Imaging: A lens was added to the Fourier microscope setup to capture real-space propagation images of the polaritons.
  • Modeling: The system was modeled using coupled harmonic oscillator Hamiltonians to extract coupling strengths and Hopfield coefficients.

Key Results

1. Strong Light-Matter Coupling: The metasurface supports SLRs and quasi-BICs that spectrally overlap with the perylene dye excitons (at ~2.24 eV). The system achieves strong coupling with coupling strengths of $g_{SLR} = 114$ meV and $g_{qBIC} = 97.5$ meV, satisfying the Savona criterion ($2g_s > |\gamma_c - \gamma_m|$). Lower polariton bands (LPBs) are formed with quality factors ($Q \sim 55$) comparable to the bare photonic modes.
2. Induced Chirality: Although the molecules and the unit cell structure are achiral, the tilted array exhibits strong extrinsic chirality at oblique angles. This results in a momentum-dependent circular dichroism in the emission. The dissymmetry factor ($g_{lum}$) reaches a magnitude of 0.93 at specific wave vectors ($k_x \approx \pm 4.5$ rad/$\mu$m), indicating a highly chiral optical response.
3. Spin-Selective Transport: Real-space imaging reveals that chiral EPs propagate directionally along the lattice axes.
   • LCP emission dominates for positive in-plane momentum ($k_x > 0$), propagating in the $+x$ direction.
   • RCP emission dominates for negative momentum ($k_x < 0$), propagating in the $-x$ direction.
   • The dissymmetry factor ($g_{lum}$) remains high (above 0.6) over propagation distances of up to 40–50 $\mu$m.
4. Propagation Lengths: The characteristic propagation lengths ($L_p$) of these chiral EPs are approximately 6–13 $\mu$m, with some distances exceeding 50 $\mu$m without significant degradation of the dissymmetry. This represents an enhancement of three orders of magnitude compared to the propagation length of bare excitons in the same organic material (which are confined to a few tens of nanometers).
5. Dispersion Control: The propagation length is wavelength-dependent, increasing with wavelength due to the increasing photonic character of the EPs. A local maximum in propagation length ($L_p > 10$ $\mu$m) is observed around 600 nm, attributed to the contribution of SLR-derived EPs which possess narrower momentum linewidths compared to quasi-BIC-derived EPs.

Significance and Claims
The paper claims to constitute the first demonstration of enhanced and selective chiral transport of organic exciton-polaritons driven by strong light-matter coupling in achiral metasurfaces. The work establishes that extrinsic chirality, engineered through the geometry of a dielectric metasurface, can be transferred to hybrid light-matter states, enabling the separation of spin states (RCP vs. LCP) based on propagation direction.

The authors posit that this mechanism allows for the control of chiral EP transport without the need for intrinsically chiral molecules. They suggest that these findings pave the way for "spin-selective polaritonic technologies" using simple metasurfaces. The study bridges the gap between fundamental chiral science and practical optoelectronic devices by demonstrating that the spin angular momentum of EPs can be manipulated and transported over macroscopic distances (relative to excitons) using dispersion engineering. The work does not propose specific future device architectures but highlights the potential for integrating spin-selective transport with functional optoelectronic systems.