Fig. 5. Modification of the vacuum state by the ultrastrong coupling.
a The normalized vacuum energy variation in the coupled plasmon–cavity system as a function of the bare cavity energy for normal incidence eigenmodes calculated with the coupling strength obtained from fitting of the Lrod = 300 nm system, as well as the vacuum energy variation calculated directly from the measured polariton energies (error bars show 95% confidence intervals for indirect measurements). b The photonic occupancy of the modified ground state in the coupled system as a function of the bare cavity energy calculated with the coupling strength obtained from fitting of the Lrod = 300 nm system.