Abstract
Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. Tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals.
Highlights
Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material
At surfaces or on thin layers of polar crystals, strong phonon–photon coupling can lead to surface phonon polaritons and hyperbolic volume phonon polaritons[26,27] that allow for nanoscale concentration of infrared and terahertz fields, which could lead to novel communication and sensing technologies[28,29], in form of nanoresonators[26,30,31] or by coupling the polaritons with plasmonic antennas and metasurfaces[32,33,34,35,36]
To analyze and discuss the dispersion of the microcavity phonon polaritons, ωðkÞ, we extracted the wavevector k from the reflectivity spectra according to k 1⁄4 jπ ; Lcav ð1Þ
Summary
Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals. A detailed study and control of the coupling strength between photons and phonons in classical Fabry–Pérot microcavities is relatively unexplored terrain This might be related with the difficulty to fabricate high-quality thin crystal layers of arbitrary thickness and place them inside the microcavities. The high crystal quality and adjustable layer thickness, both achieved by simple exfoliation, establish microcavities embedding van der Waals materials as a versatile platform for studying and tuning the coupling strengths between photons and optical phonons
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