Séminaire Charles Vernier

Probing the physical properties of matter with light :
how confinement at the nanoscale affects the elastic and optical features of materials

Materials made of noble metals at the nanoscale are famous for their localized surface plasmon (LSPR) that give colloidal solutions of Au or Ag nanoparticles (NPs) bright, tunable colors.

Lesser known are the standing acoustic waves that NPs can sustain in the GHz range. Still, several confined acoustic phonons modes are Raman active and can be detected by using a Brillouin spectrometer [1], needed to access very low frequency range (typically <50GHz). For Au, Ag or Au@Ag NPs, the confined acoustic modes show a clear dependance between the phonon frequency and the size of the NPs (the bigger the NPs, the lower the frequencies of the phonons) and a milder dependance towards the shape of the NPs [2]. Poly- and monocristalline NPs show sizeable differences in their Raman spectra [3], as can be seen on the left side of the figure below where the main mode of polycrystalline, pentatwinned gold nanorods (AuNRs PT) is split into two modes for single crystalline gold nanorods (AuNRs SC). These findings make low frequency Raman spectroscopy a promising tool for structural characterization of NPs.

Another notable feature of noble metals NPs is the ability to confine light in a sub-wavelength space. This can be done by fine tuning of LSPR bands, which vary strongly depending on the shape and size of the NPs. It is also possible to increase light-matter coupling by building nanocavities that confine light in few nm cavities [4], typically by using NPs oligomers or NP on a mirror configuration, as shown on the right side of the figure. Such cavities yield sharp, tunable resonances, whose asymmetry can be controlled. How? More on that at my seminar, on Friday, December 4th.

(1) Duval, E. Far-Infrared and Raman Vibrational Transitions of a Solid Sphere: Selection Rules. Phys. Rev. B
1992, 46 (9), 5795–5797.
(2) Vernier, C.; Saviot, L.; Fan, Y.; Courty, A.; Portalès, H. Sensitivity of Localized Surface Plasmon Resonance
and Acoustic Vibrations to Edge Rounding in Silver Nanocubes. ACS Nano 2023.
(3) Portales, H.; Goubet, N.; Saviot, L.; Adichtchev, S.; Murray, D. B.; Mermet, A.; Duval, E.; Pileni, M.-P.
Probing Atomic Ordering and Multiple Twinning in Metal Nanocrystals through Their Vibrations.
Proceedings of the National Academy of Sciences 2008, 105 (39), 14784–14789.
(4) Baumberg, J. J.; Aizpurua, J.; Mikkelsen, M. H.; Smith, D. R. Extreme Nanophotonics from Ultrathin Metallic
Gaps. Nat. Mater. 2019, 18 (7), 668–678.