  Surface plasmons in a centrosymmetric metal nanocrystal have been perceived
as achiral, based on the quantum-mechanical theory of molecular optical
activity. The one-electron theory is insufficient to describe optical activity
in crystals, where spatial variation of polarization states along the light
propagation direction is crucial in dimensions exceeding molecules. Such
spatial-dispersion, which can drastically modify the optical near-field in
plasmonic nanocrystals, has yet to be experimentally proved at the nanometer
scale. Here, the experimental observation of natural circular dichroism of
surface plasmons in spherical gold, silver, and copper nanocrystals in solution
is presented. It originates from spatial dispersion (nonlocality) occurred at a
dimension as small as 5 nanometers in the optical region. In particular, the
electromagnetic pressure or longitudinal volume plasmons result in phase-shift
and polarization rotation of the transverse electric field of incident light.
Dipolar surface plasmons exhibit negative ellipticity and corresponding
negative phase-shift of the electric field (i.e., optically left-handed),
whereas quadrupolar surface plasmons have positive signs in both ellipticity
and phase-shift (i.e., optically right-handed). Structural effects such as
nanocrystal assembly, nanocrystal shape, electron spin-polarization, and
plasmon-molecule interaction are accessed. Volume plasmons inside the
nanocrystal also produce a positive rotational strength for the interband
transition. This study implicates optical symmetry-breaking in isotropic
nanocrystals by the axial polarization parallel to the wavevector, which will
impact on conduction electron dynamics, hot-electron generation, and nonlinear
optical response at the surface of plasmonic nanocrystals as well as optical
excitation of bound d-band electrons.
