Optomechanical Dicke phase transition with 20 tweezed atoms

We report on the realization of a mesoscopic, optomechanical Dicke phase transition using a 1D tweezer array in an optical cavity. We use the tweezers to place up to 20 atoms precisely on the nodes of the cavity field at a spacing of 5.5 wavelengths and expose them to a transverse pump field. Above a critical pump strength, the atoms self-organize onto the cavity antinodes with the same phase, leading to a bifurcation with Z₂ symmetry. Using heterodyne detection, we observe the Dicke bifurcation in the cavity field phase as a function of atom number, pump power, and detunings of the atoms and pump to the cavity. We measure the mechanical susceptibility of this system by introducing a bias in the tweezer positions and show that the susceptibility diverges when the system is self-organized. Lastly, we examine the switching dynamics of the cavity field phase due to atom thermal motion.