We develop coherent optics of dipole-active, dispersionless excitons in spherical
semiconductor photonic dots (PDs). In the absence of any incoherent scattering,
both the strong and weak coupling regimes can intrinsically be realized simply
by changing the parameters of the dot and surrounding medium. A criterion,
which attributes the transition between these two regimes to a discrete topological
change of the relevant dispersion curves, is found and approximated by an analytic
expression. The transition depends upon the intrinsic radiative lifetime of the PD
photon eigenstates, i.e. it is determined by the parameters of the structure (the
oscillator strength of the exciton–photon interaction, PD radius and the ratio of the
background dielectric constants inside and outside of the dot). We propose the use of
high-precision modulation spectroscopy in order to visualize the above ‘phase’
transition between a well-developed polariton picture (the strong coupling regime) and
weakly-interacting exciton and PD photon states (the weak coupling regime). It is shown
that the radiative decay of optically dressed PD excitons, coherently distributed
among the relevant PD eigenstates, is non-monotonous against the dot radius
a: a size-dependent increase of the effective oscillator strength at small
a
saturates at , and with a
increasing further towards the optical lifetime of excitons starts to increase proportionally to
a, reflecting the ballistic escape of nearly bulk polaritons from the PD. The numerical
simulations are scaled to dispersionless excitons in PDs fabricated from cyanine dye
J
aggregates.