Fig. 5: Ultrastrong coupling in cavities fully filled with hBN.
From: Microcavity phonon polaritons from the weak to the ultrastrong phonon–photon coupling regime
a Reflectivity spectra of a cavity fully filled with hBN (illustrated by inset) as a function of cavity thickness \({L}_{{{{{{\rm{cav}}}}}}}\). The dashed curves show the cavities’ eigenmode frequencies \(\,{\omega }_{+}^{(j)}\) and \({\omega }_{-}^{(j)}\) as a function of \({L}_{{{{{{\rm{cav}}}}}}}\). Red and yellow symbols show the spectral position of the experimental reflectivity dips extracted from panel (c). The reddish areas in a and b mark the Reststrahlen band of the in-plane phonon of hBN. b Reflectivity spectra of a bare cavity (illustrated by inset) as a function of cavity thickness \({L}_{{{{{{\rm{cav}}}}}}}\). The dashed curves show the cavities’ eigenmode frequencies \(\,{\omega }_{{{{{{\rm{cav}}}}}}}^{(j)}\) as a function of \({L}_{{{{{{\rm{cav}}}}}}}\). c Experimental reflectivity spectra of cavities fully filled with hBN (vertically offseted). Cavity thicknesses (from bottom to top) are: 1,080 nm, 1,500 nm, 1,665 nm, 1,900 nm, 2,100 nm, 2,500 nm.
