Fluorescence enabled phonon counting in an erbium-doped piezo-optomechanical microcavity.

Converting phonons to photons with optomechanical interaction provides a pathway to realize single phonon counting, which is instrumental in the quantum applications of mechanical systems such as entanglement generation, thermometry, and study of macroscopic quantum phenomenon. In this process, the key requirement is high-extinction, narrow-bandwidth, and stable filtering of the parametric optical pump. Here, we propose to lift this necessity by counting fluorescence emission from a rare earth embedded optomechanical cavity. By doing so, we show that an equivalent filtering effect can be achieved due to spectral hole burning and cavity Purcell effect. To demonstrate this, we designed, fabricated, and characterized an integrated piezo-optomechanical Fabry-Perot cavity on the erbium-doped thin-film lithium niobate platform. By collecting fluorescence from the optomechanical sideband, we show that 93 dB suppression of the pump can be achieved with 10 dB loss of signal, resulting in an increase of 83 dB in sideband-pump ratio. Our results facilitate a route to realize filterless single phonon counting and also create new opportunities to study the interaction between solid state emitters and mechanical systems.