Absorption of Ultrashort Laser Pulses by Plasmonic Nanoparticles: Not Necessarily What You Might Think

Exciting plasmonic nanostructures by subpicosecond laser pulses can generate many interesting phenomena due to hot electrons, which can be further exploited in photonics or in chemical or biomedical applications. In order to quantitatively analyze and optimize these effects, proper evaluation of the light pulse power absorbed by the nanoparticles is highly required. However, in the literature only stationary properties are considered for that purpose. Here, we show that this may be invalid owing to the optical nonlinearity associated with the photogenerated hot electron distribution. We demonstrate through a simple optical transmission experiment the influence of hot electrons on the absorption cross section of gold nanorods, excited by subpicosecond laser pulses tuned to the longitudinal plasmon resonance spectral domain. The partial melting threshold of the nanorods is reached for a peak intensity of 5 GW cm(-2), corresponding to a volume density of energy of 2.2 aJ nm(-3). Below this threshold, the experimental results are interpreted through a model that accounts for the nonthermal nature of the electron distribution and for the multiphoton excitation. The variation of the effective optical absorption cross section, with laser peak intensity reveals a strong and complex nonlinearity, which in addition depends on laser wavelength and nanoparticle shape, being either larger or smaller than the stationary cross section value. Besides, we show that for a given pulse energy the shorter the pulse duration, the greater this deviation. Finally, we illustrate the consequences of this discrepancy through the evaluation of the nanoparticle temperature reached after photothermal conversion.