  Radiation brightening was recently observed in a multi-fluorophore-conjugated
brome mosaic virus (BMV) particle, at room temperature under pulsed excitation.
Based on its nonlinear dependence on the number of fluorophores, the origins of
the phenomenon were attributed to a collective relaxation. However, the
mechanism remains unknown. We present ultrafast transient absorption and
fluorescence spectroscopic studies which shed new light on the collective
nature of the relaxation dynamics in such radiation-brightened,
multi-fluorophore particles. Our findings indicate that the emission dynamics
is consistent with a superradiance mechanism. The ratio between the rates of
competing radiative and non-radiative relaxation pathways depends on the number
of fluorophores per virus. We also discuss the evidence of coherent
oscillations in the transient absorption trace from multi-fluorophore
conjugated which last for $\sim100$s of picoseconds, at room temperature. The
findings suggest that small icosahedral virus shells provide a unique
biological scaffold for developing non-classical, deep subwavelength light
sources, and may open new realms for the development of photonic probes for
medical imaging applications.
