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HL: Fachverband Halbleiterphysik
HL 33: Optical Properties 2
HL 33.6: Invited Talk
Thursday, September 8, 2022, 16:30–17:00, H32
Ultrastrong light-matter coupling in materials — •Niclas S. Mueller1,2, Eduardo B. Barros3, Florian Schulz4, Holger Lange4, and Stephanie Reich1 — 1Department of Physics, Freie Universität Berlin, Berlin, Germany — 2Present address: NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, United Kingdom — 3Department of Physics, Universidade Federal do Ceara, Fortaleza, Ceara, Brazil — 4Department of Physical Chemistry, University of Hamburg, Hamburg, Germany
Driven by the field of cavity quantum electrodynamics there is an ever-growing quest for systems with extreme light-matter coupling. In the regimes of ultra- and deep strong coupling the coupling strength becomes comparable to the bare excitation energy, leading to exotic phenomena like virtual photons in the ground state and the breakdown of the Purcell effect. Here, we discuss how ultrastrong coupling is systematically achieved in materials, without the need for external cavities. We introduce densely packed supercrystals of gold nanoparticles as an artificial material where the coupling strength can be tuned from ultra- to deep strong coupling. Using a unified theory of dipole-active material excitations, we show that light-matter coupling gets maximized in three-dimensional materials, setting an upper limit for the coupling strength in cavities. From a large set of experimental data, we identify phonons in ferroelectrics, excitons in molecular crystals, and plasmons in metallic supercrystals as excitations where light-matter coupling is so strong that it affects the material ground state, eventually leading to phase transitions and changing the mechanical properties.