Plasma-Mediated Nanocavitation and Photothermal Effects in Ultrafast Laser Irradiation of Gold Nanorods in Water

We present a theoretical and experimental study that reveals the physical mechanism underlying the response of an in-resonance gold nanorod (AuNR) in water to a near-infrared ultrafast laser pulse. Results reveal the presence of two different regimes of interaction, depending on the irradiation fluence. For fluences below 3 mJ/cm2, AuNRs are in the so-called absorption regime and are shown to strongly absorb energy, leading to a fast temperature increase revealed by the onset of characteristic mechanical vibration of the structure. In situ measurement demonstrates a permanent deformation of the AuNRs occurring for fluences over 100 μJ/cm2. In the absorption regime, we show the formation of a nanoscale plasma around the structure, dominated by a photothermal emission from the AuNR, and the generation of a pressure wave. However, no cavitation occurs under the deformation threshold fluence (100 μJ/cm2). For fluences over 3 mJ/cm2, in the near-field regime, the energy transfer is dominated by the enhanced near-field around the particle that directly ionizes and heats a nanoplasma in the surrounding water. We theoretically show that bubbles with diameters ≈ 490 nm can be generated in this near-field regime for an incident fluence of 200 mJ/cm2. In situ optical characterization of the produced bubbles supports this result and shows that bubbles with diameters ≈ 200–600 nm can be generated for fluences ranging 100–400 mJ/cm2. Important shielding of the laser–nanostructure interaction by the surrounding plasma is shown to decrease considerably the near-field enhancement, the energy absorption, and the diameter of the generated bubbles and may explain the smaller bubbles generated around in-resonance 10 × 41 nm2 AuNRs when compared to off-resonance 25 × 60 nm2 AuNRs and 100 nm AuNPs.