Ultra-strong light–matter coupling for designer Reststrahlen band

B Askenazi,G Beaudoin,A Vasanelli,L C Andreani,I Sagnes,C Sirtori,A Delteil,Y Todorov

doi:10.1088/1367-2630/16/4/043029

Abstract

The strength of the light–matter interaction depends on the number of dipoles that can couple with the photon trapped in an optical cavity. The coupling strength can thus be maximized by filling the entire cavity volume with an ensemble of interacting dipoles. In this work this is achieved by inserting a highly doped semiconductor layer in a subwavelength plasmonic resonator. In our system the ultra-strong light–matter coupling occurs between a collective electronic excitation and the cavity photon. The measured coupling strength is 73% of the matter excitation energy, the highest ever reported for a light–matter coupled system at room temperature. We experimentally and theoretically demonstrate that such an ultra-strong interaction modifies the optical properties on a very wide spectral range (20–250 meV), and results in the appearance of a photonic gap of 38 meV, independently of the light polarization and angle of incidence. Light–matter ultra-strong coupling can thus be exploited to conceive metasurfaces with an engineered reflectivity band.

Highlights

When a material medium is inserted in an optical resonator, its interaction with light can be strongly enhanced and new quasi-particles can appear, the microcavity polaritons [1]
The strength of the coupling is measured by the vacuum field Rabi splitting, ER, which gives the energy separation between the microcavity polariton states when the cavity mode is resonant with the matter excitation energy, Ematter
When the coupling energy becomes comparable to that of the material excitation, the system enters a new quantum electrodynamics regime, the so-called ultra-strong coupling regime [7,8,9]. It has been first observed by coupling an electronic excitation in a quantum well (intersubband (ISB) excitation) and a microcavity mode [10, 11]

Summary

Introduction

When a material medium is inserted in an optical resonator, its interaction with light can be strongly enhanced and new quasi-particles can appear, the microcavity polaritons [1]. ISB transitions in quantum wells are a very interesting system for the observation of the ultra-strong coupling regime and the achievement of high values of the ratio between the Rabi and the matter excitation energy. The use of collective excitations (indicated as MSP in the figure) has allowed achieving a ratio ER Ematter greater than 0.5 at 4 K [20] This increase of the relative coupling strength is associated with the uniform filling of the cavity mode volume with electrons.

From multisubband plasmons to Berreman modes

Findings

Ultra-strong coupling and artificial Reststrahlen band