Transition from strong to ultra-strong coupling regime in mid-infrared metal-dielectric-metal cavities

TL;DR

This study demonstrates the transition from strong to ultra-strong coupling in mid-infrared metal-dielectric-metal microcavities, revealing a photonic gap that depends on the Rabi to intersubband transition energy ratio, at room temperature.

Contribution

It experimentally shows the ultra-strong coupling regime in mid-infrared cavities and links the photonic gap to the coupling strength, advancing understanding of light-matter interactions.

Findings

Ultra-strong coupling achieved at room temperature for 11.7 μm wavelength.

Photonic gap opens in polariton dispersion during the transition.

Gap width scales quadratically with the Rabi to intersubband transition energy ratio.

Abstract

We have investigated the transition from strong to ultra-strong coupling regime between a mid-infrared intersubband excitation and the fundamental mode of a metal-dielectric-metal microcavity. The ultra-strong coupling regime is demonstrated up to room temperature for a wavelength of $11.7 \mu$m by using 260 nm thick cavities, which impose an extreme sub-wavelength confinement. By varying the doping of our structures we show that the experimental signature of the transition to the ultra-strong coupling regime is the opening of a photonic gap in the polariton dispersion. The width of this gap depends quadratically on the ratio between the Rabi and intersubband transition energies.