EurekAlert  September 23, 2020 Ultrastrong coupling is a distinct regime of electromagnetic interaction that enables a rich variety of intriguing physical phenomena. An international team of researchers (Sweden, Russia, Poland) used a plasmonic nanorod array positioned at the antinode of a resonant optical Fabry-Pérot microcavity to reach the ultrastrong coupling (USC) regime at ambient conditions and without the use of magnetic fields. From optical measurements they extracted the value of the interaction strength over the transition energy as high as g/ω ~ 0.55, deep in the USC regime, while the nanorod array occupies only ∼4% of the cavity volume. They indirectly observed […]