Ultrastrong Coupling Enhancement with Squeezed Mode Volume in Terahertz Nanoslots

Abstract

Metallic nanogaps have emerged as a versatile platformfor realizingultrastrong coupling in terahertz frequencies. Increasing the couplingstrength generally involved reducing the gap width to minimize themode volume, which presents challenges in fabrication and efficientmaterial coupling. Here, we propose employing terahertz nanoslots,which can efficiently squeeze the mode volume in an extra dimensionalongside the gap width. Our experiments using 500 nm wide nanoslotsintegrated with an organic-inorganic hybrid perovskite demonstrateultrastrong phonon-photon coupling with a record-high Rabisplitting of 48% of the original resonance (& omega; = 0.48 & omega;(0)), despite having a gap width 5 times larger than previouslyreported structures with & omega; = 0.45 & omega;(0). Mechanismsunderlying this effective light--matter coupling are investigatedwith simulations using coupled mode theory. Moreover, bulk polaritonanalyses reveal that our results account for 68% of the theoreticalmaximum Rabi splitting, with the potential to reach 82% through furtheroptimization of the nanoslots.