  Metallic optical systems can confine light to deep sub-wavelength dimensions,
but verifying the level of confinement at these length scales typically
requires specialized techniques and equipment for probing the near-field of the
structure. We experimentally measured the confinement of a metal-based optical
cavity by using the cavity modes themselves as a sensitive probe of the cavity
characteristics. By perturbing the cavity modes with conformal dielectric
layers of sub-nm thickness using atomic layer deposition, we find the
exponential decay length of the modes to be less than 5% of the free-space
wavelength (\lambda) and the mode volume to be of order \lambda^3/1000. These
results provide experimental confirmation of the deep sub-wavelength
confinement capabilities of metal-based optical cavities.
