All-optical ultrafast spectroscopy of a single nanoparticle-substrate contact

Abstract

5 pagesInternational audienceWe present an all-optical method to investigate the GHz dynamics of the elastic contact between a single metallic nanoparticle and a substrate. A resonant excitation mechanism driven by the 82-MHz Dirac comb of the femtosecond oscillator is associated with femtosecond pump-probe experiments performed in a transient reflectivity configuration. This scheme allows us not only to detect the known breathing mode of the nanoparticle but also to unravel the existence of an axial oscillation of the nanoparticle through an intrinsic common-path interferometer.We measured the eigenfrequency and the lifetime of this vertical motion, which are related to the contact stiffness and hysteresis, and to the acoustic leakage at the nanoparticle-substrate interface. A modeling of the axial oscillation in the framework of classical adhesion theories predicts a simple power law dependence of the axial eigenfrequency with respect to the breathing mode frequency. Measurements performed for single particles with radii ranging from 60 to 700 nm are in strong agreement with this prediction

Dynamics of a monolayer of microspheres on an elastic substrate

“…Contact resonances of isolated nanoparticles have been previously demonstrated for spheres as small as 120 nm in diameter, and their frequencies have been shown to scale in agreement with ad-hesive contact models based on Hertzian mechanics [31]. Conversely, at the macroscale, contact springs would be determined by gravity and, possibly, applied lateral static compression [9], rather than by adhesion forces.…”

Acoustic vibrations of metal nano-objects: Time-domain investigations

“…This has been related to lubrication forces [253], an effect that should stimulate further theoretical work. Additional information on nanoparticle-substrate has also been recently obtained from transient reflectivity measurements, which allow observation of a mode associated to the axial oscillation of the nanoparticle normally to the substrate surface, undetectable in transient transmission experiments as it does not affect particle volume and shape [254]. For gold nanospheres on silica, this mode presented a low (<10) quality factor, possibly related to strong hysteresis at the nanoparticle-substrate adhesive contact [254].…”

“…Additional information on nanoparticle-substrate has also been recently obtained from transient reflectivity measurements, which allow observation of a mode associated to the axial oscillation of the nanoparticle normally to the substrate surface, undetectable in transient transmission experiments as it does not affect particle volume and shape [254]. For gold nanospheres on silica, this mode presented a low (<10) quality factor, possibly related to strong hysteresis at the nanoparticle-substrate adhesive contact [254]. The effect of variable particle-substrate interaction concomitant with deposition of single nanoparticles can be suppressed, and a homogeneous environment recovered, using either an optically trapped particle (gold spheres and rods [234]) or investigating a nano-object in suspension upon a trench (gold and copper wires [67,247]).…”

Vibrational dynamics of a two-dimensional microgranular crystal

“…Such spheroidal mode splitting due to symmetry breaking by the substrate is not unexpected but has not been previously reported. Indeed, in a more typical measurement with the laser spot centered on an individual particle [30][31][32] only the S m=0 L modes can be excited due to symmetry constraints. We expect similar mode splitting to take place for S 3 and S 4 modes; however, the signal from those modes is too weak to detect this phenomenon.…”