  We present the first experimental study of optical response of collective
plasmon resonances in regular arrays of nanoresonators to local environment.
Recently observed collective plasmon modes arise due to diffractive coupling of
localised plasmons and yield almost an order of magnitude improvement in
resonance quality. We measure the response of these modes to tiny variations of
the refractive index of both gaseous and liquid media. We show that the phase
sensitivity of the collective resonances can be more than two orders of
magnitude better than the best amplitude sensitivity of the same nanodot array
as well as an order of magnitude better than the phase sensitivity in SPR
sensors.
