Observation of Bogoliubov excitations in exciton-polariton condensates

Einstein's 1925 paper predicted the occurrence of Bose-Einstein condensation (BEC) in an ideal gas of non-interacting bosonic particles(1). However, particle-particle interaction and peculiar excitation spectra are keys for understanding BEC and superfluidity physics. A quantum field-theoretical formulation for aweakly interacting Bose condensed systemwas developed by Bogoliubov in 1947, which predicted the phonon-like excitation spectrum(2) in the low-momentum regime. The experimental verification of the Bogoliubov theory on the quantitative level was carried out for atomic BEC3 using the two-photon Bragg scattering technique(4). Exciton-polaritons in a semiconductor microcavity, which are elementary excitations created by strong coupling between quantum-well excitons and microcavity photons, were proposed as a new BEC candidate in solid-state systems(5). Recent experiments with exciton-polaritons have demonstrated several interesting signatures from the viewpoint of polariton condensation, such as quantum degeneracy at non-equilibrium conditions(6-8), the polariton-bunching effect at the condensation threshold(9), long spatial coherence(10-12) and quantum degeneracy at equilibrium conditions(13). The particle-particle interaction and the Bogoliubov excitation spectrum are at the heart of BEC and superfluidity physics, but have only been studied theoretically for exciton-polaritons(14,15). In this letter, we report the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory.