Giant Optical Nonlinearity from Self-Hybridized Exciton Polaritons in Perovskite Quasi-BIC Metasurfaces

Exciton-polaritons (EPs) are quasi-light quasi-matter particles formed in the strong light-matter coupling regime. Due to their hybrid nature, EPs are excellent candidates for realizing low-photon-number nonlinear optics and bosonic condensation [1]. Previous EP studies have typically involved coupling excitonic materials to external cavities, which can result in weaker interactions due to limited field overlap with the materials. An alternative method to ensure strong light-matter interactions is to use the excitonic material itself to form the cavity [2].
In this work, we adopt this approach and achieve significantly enhanced optical nonlinearity through self-hybridized EPs in a quasi-bound-state-in-the-continuum (quasi-BIC) metasurface made of FAPbBr₃ perovskite. Anti-crossing features are observed in both room-temperature and 6-Kelvin experiments with energy-momentum spectroscopy, confirming the strong coupling nature. Spectral fittings reveal a coupling strength of up to 78.4 meV. At the lower EP branches, we demonstrate strong optical nonlinear absorption and two-photon-absorption up-conversion emission with an ultra-low incident power density threshold ---- 4 orders of magnitude lower than bare perovskite materials. The nonlinear absorption modulation depth is up to 76% at 560 nm wavelength. This provides a promising path toward low-photon-number optical nonlinearity in free-space planar meta-optical devices.
The advantages of our EP nonlinearity platform are threefold. First, the FAPbBr₃ material has a large exciton binding energy and a high internal quantum yield with near-unity absorption efficiency [3]. Second, the quasi-BIC photonic mode offers strong field confinement, perfectly overlapping with the patterned perovskite materials [4]. Third, the asymmetry-enabled tunable cavity quality factors ensure the design freedom to achieve polaritonic critical coupling condition with efficient photon-trapping capability and a rich population of polaritons [2].

References
[1] H. M. Gibbs, G. Khitrova and S. W. Koch, “Exciton-polariton light-semiconductor coupling effects.” Nat. Photonics 5, 273-273 (2011).
[2] T. Weber, L. Kühner, L. Sortino, A. B. Mhenni, N. P. Wilson, J. Kühne, J. J. Finley, S. A. Maier and A. Tittl, “Intrinsic strong light-matter coupling with self-hybridized bound states in the continuum in van der Waals metasurfaces”, Nat. Mater. 22, 970-976 (2023).
[3] R. Su, A. Fieramosca, Q. Zhang, H. S. Nguyen, E. Deleporte, Z. Chen, D. Sanvitto, T. C. H. Liew and Q. Xiong, “Perovskite semiconductors for room-temperature exciton-polaritonics”, Nat. Mater. 20, 1315-1324 (2021).
[4] S. I. Azzam and A. V. Kildishev, “Photonic Bound States in the Continuum: From Basics to Applications”, Adv. Optical Mater., 9, 2001469 (2021).