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

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Light emission from gold nanoparticles under ultrafast near-infrared  excitation: thermal emission, inelastic light scattering or multiphoton  luminescence?