Revisiting strong coupling between a single molecule and surface plasmons

Strong coupling between a single molecule and surface plasmons is reexamined with a microscopic classical formulation in local and nonlocal responses of metal. In the case of local response, we show that strong single molecule-plasmon coupling can occur in the UV range for a silver particle with a molecule-metal separation of 1 nm or smaller, where the real part of the dielectric function of silver approaches -1. With the nonlocal response consideration, strong coupling happens at shorter molecule-metal distances. The result shows that Rabi splitting can occur even without a resonator.