Ultrastrong Coupling Enhancement with Squeezed Mode Volume in Terahertz Nanoslots

Metallic nanogaps have emerged as a versatile platform for realizing ultrastrong coupling in terahertz frequencies. Increasing the coupling strength generally involved reducing the gap width to minimize the mode volume, which presents challenges in fabrication and efficient material coupling. Here, we propose employing terahertz nanoslots, which can efficiently squeeze the mode volume in an extra dimension alongside the gap width. Our experiments using 500 nm wide nanoslots integrated with an organic-inorganic hybrid perovskite demonstrate ultrastrong phonon-photon coupling with a record-high Rabi splitting of 48% of the original resonance (Ω = 0.48ω₀), despite having a gap width 5 times larger than previously reported structures with Ω = 0.45ω₀. Mechanisms underlying this effective light--matter coupling are investigated with simulations using coupled mode theory. Moreover, bulk polariton analyses reveal that our results account for 68% of the theoretical maximum Rabi splitting, with the potential to reach 82% through further optimization of the nanoslots.