As I understand, the plasma frequency of a plasmonic material can be considered the bulk plasmon frequency. This is because longitudinal oscillations corresponding to bulk plasmons are only existent when the dielectric constant is equal to zero. Another important aspect of bulk plasmons I have learned is that they cannot be excited by light. If these things are true, how can it be possible that metals exhibit an epsilon-near-zero condition at it's plasma frequency? Are bulk plasmons being excited causing this condition or is the plasma frequency not exclusively defined as the bulk plasmon frequency?