  We investigated the possibility of using charged luminescent nanoparticles as
nanoprobes for studying the evolution scenarios of surface and internal
structure of slowly evaporating free (light-absorbing) microdroplets of
suspension. Three concentrations (1, 10 and 50 mg/ml) of luminescent
nanoparticles were used. Single microdroplets were kept in a linear
electrodynamic quadrupole trap and the luminescence was excited with a CW IR
laser with an irradiance of ~50 W/mm2. Since the microdroplet acted as an
optical spherical resonance cavity, the interaction of nanoparticles with light
both reflected and modified the internal light field mode structure. Depending
on the nanoparticle concentration used, it led, among others, to a very
significant increase in modulation depth and narrowing of spherical cavity
resonance maxima (morphology dependent resonances - MDRs) observed both in
luminescence and scattering, the abrupt changes in the ratio between the
luminescence and the scattering and the bi-stability in luminescence signal.
The observed phenomena could be attributed to the interaction of optical MDRs
with nanoparticle lattice shells forming and changing their structure at the
microdroplet surface. In this way, the formation and collapse of such lattices
could be detected.
