  We present a simple method to tune optical micro- and nanocavities with
picometer precision in the resonant wavelength, corresponding to an effective
sub atomic monolayer control of the cavity dimension. This is obtained through
resonant photo-electrochemical etching, with in-situ monitoring of the optical
spectrum. We employ this technique to spectrally align an ensemble of resonant
cavities in a permanent manner, overcoming the dimension variability resulting
from current nanofabrication techniques. In a device containing several
resonators, each is individually addressed and tuned, with no optical quality
factor degradation. The technique is general and opens the way to multiple
applications, such as the straightforward fabrication of networks of identical
coupled resonators, or the tuning of chip-based cavities to external
references.
