Ultrastrong Light-Matter Coupling—Engineering Electronic Wavefunctions with Single Photons

When the coupling between a confined electromagnetic mode and the electronic degrees of freedom of a solid-state system becomes large enough, the light-matter interaction can modify electronic wavefunctions and the related material properties. The hybrid polaritonic quasiparticle generated when the system absorbs a photon gets then dressed by an excitonic wavefunction which doesn’t correspond to any of the unperturbed electronic states. As a first demonstration of this concept we experimentally measured a change of roughly 30% in either direction for the Bohr radius of Wannier excitons in microcavity embedded GaAs quantum wells [1], an effect originally predicted by Khurgin [2].<br/>Investigating this technique with the aim to modify electronic wavefunctions at the single photon level we discovered the effect becomes much more dramatic in systems with continuum electronic degrees of freedom. In these systems the gapless spectra enhance the electronic compressibility, to the point that novel photon-bound excitons can be created around the photo-excited quantum well [3]. Using doped GaAs quantum wells we were then able to spectroscopically observe the existence of the predicted photon-bound excitons stabilised by the presence of the photonic cavity [4].<br/>More recently related effects were also observed in a 2D electron gas, in which the excitation of high-momenta hybrid modes by nano-features of the resonator can launch propagative plasmons acting as a powerful damping channel [5]. This interaction with the propagative nature of the plasmons, similarly to other nonlocal effects in metals [6] or dielectrics [7], reduces the available field concentration and thus the maximal coupling achievable between light and matter, but it also allows to tune the electromagnetic field at the nanoscale.<br/>Solid-state cavity quantum electrodynamics, after having shown its relevance in optoelectronic and more recently in chemistry, is increasingly becoming a tool for quantum material engineering, allowing us to drastically enrich the catalogue of materials available for scientific and technological applications.<br/>[1] Experimental verification of the very strong coupling regime in a GaAs quantum well microcavity. S. Brodbeck <i>et al.</i>, Physical Review Letters <b>119</b>, 027401 (2017)<br/>[2] Excitonic radius in the cavity polariton in the regime of very strong coupling. J. B. Khurgin, Solid State Commun. <b>117</b> 307 (2001)<br/>[3] Strong coupling of ionising transitions. E. Cortese <i>et al.</i>, Optica <b>6</b>, 354 (2019)<br/>[4] Excitons bound by photon exchange. E. Cortese <i>et al.</i>, Nature Physics <b>17</b>, 31 (2021)<br/>[5] Polaritonic nonlocality in light-matter interaction. S. Rajabali <i>et al.</i>, Nature Photonics <b>15</b>, 690 (2021)<br/>[6] Probing the ultimate limits of plasmonic enhancement. C. Ciraci <i>et al.</i>, Science <b>337</b>, 1072 (2012)<br/>[7] Optical Nonlocality in Polar Dielectrics. C. R. Gubbin and S. De Liberato, Physical Review X <b>10</b>, 021027 (2020)