where ϵ is the permittivity inside the cavity, and V is the effective mode volume. Alternatively, the electric field strength ε = ω c A 0 can be used as a measure of coupling strength, which is common in experiment. In state-of-the-art cavity designs, such as those from gold or silver NPoM cavities, the local electric field can vary from 1 to 10 V/nm, which is well within the cavity parameters used in the present work (to be discussed in more detail later).