  Gold nanoparticles emit broad-band upconverted luminescence upon irradiation
with pulsed infrared laser radiation. Although the phenomenon is widely
observed, considerable disagreement still exists concerning the underlying
physics - most notably over the applicability of concepts such as multiphoton
absorption, inelastic scattering, and interband and intraband electronic
transitions. Here, we study single particles and small clusters of particles by
employing a spectrally resolved power-law analysis of the irradiation-dependent
emission as a sensitive probe of these physical models. Two regimes of emission
are identified: at low irradiance levels of kW/cm2, the emission follows a
well-defined integer-exponent power law suggestive of a multiphoton process.
However, at higher irradiance levels of several kW/cm2, the nonlinearity
exponent itself depends on the photon energy detected, a tell-tale signature of
a radiating heated electron gas. We show that in this regime, the experiments
are incompatible with both interband transitions and inelastic light scattering
as the cause the luminescence, while they are compatible with the notion of
luminescence linked to intraband transitions.
