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real part of the complex dielectric constant, Re ε m < 0. High Q-factors, i.e. small
damping requires small imaginary parts of ε m . Gold, silver and also copper fulfil
these requirements in the visible and near infrared range. Silver and gold are the
most common materials in plasmon supported optics and spectroscopy. There is a
strong interest in extending the working range of plasmonics by looking for new
materials. For an overview of plasmonic materials see [56].
Properties of surface plasmons and related local optical fields in the vicinity of
plasmonic structures are not only determined by their material properties, but also
by the morphology of the structures. For example, so-called hot spots which provide
extremely strong SERS enhancement always exist for aggregates formed by silver
or gold nanoparticles but not for isolated particles [57, 58]. For illustration, Fig. 2.4
compares SERS experiments performed on isolated gold nanospheres and on small
aggregates formed by these spheres.
In agreement with theory, enhancement factors for isolated gold spheres have
been inferred to be 10 3 − 10 4 by comparing surface enhanced Raman signals with
non-surface enhanced standards. These SERS enhancement factors are too small
to measurably populate the first excited vibrational state and the anti-Stokes spectrum appears at the expected relatively low signal level (Fig. 2.4b). In particular, the
high frequency modes are not seen on the anti-Stokes side due to their low thermal population. This situation changes when the particles form aggregates. Now a
strong anti-Stokes signals appears, in particular also for higher frequency Raman
modes. This is an indication of a very strong Raman process that populates the first
excited vibrational state. The difference in SERS enhancement levels for isolated
Fig. 2.4 Stokes and anti-Stokes SERS spectra of crystal violet attached to isolated and aggregated
gold nanospheres. Strong anti-Stokes signals for aggregated spheres indicate population of first
excited vibrational states beyond Boltzmann distribution. This population pumping due to a very
strong SERS Stokes process indicates a dramatic increase in local field enhancement for nanoparticle
aggregates versus isolated particles (Reprinted with permission from [58])
K. Kneipp et al.
real part of the complex dielectric constant, Re ε m < 0. High Q-factors, i.e. small
damping requires small imaginary parts of ε m . Gold, silver and also copper fulfil
these requirements in the visible and near infrared range. Silver and gold are the
most common materials in plasmon supported optics and spectroscopy. There is a
strong interest in extending the working range of plasmonics by looking for new
materials. For an overview of plasmonic materials see [56].
Properties of surface plasmons and related local optical fields in the vicinity of
plasmonic structures are not only determined by their material properties, but also
by the morphology of the structures. For example, so-called hot spots which provide
extremely strong SERS enhancement always exist for aggregates formed by silver
or gold nanoparticles but not for isolated particles [57, 58]. For illustration, Fig. 2.4
compares SERS experiments performed on isolated gold nanospheres and on small
aggregates formed by these spheres.
In agreement with theory, enhancement factors for isolated gold spheres have
been inferred to be 10 3 − 10 4 by comparing surface enhanced Raman signals with
non-surface enhanced standards. These SERS enhancement factors are too small
to measurably populate the first excited vibrational state and the anti-Stokes spectrum appears at the expected relatively low signal level (Fig. 2.4b). In particular, the
high frequency modes are not seen on the anti-Stokes side due to their low thermal population. This situation changes when the particles form aggregates. Now a
strong anti-Stokes signals appears, in particular also for higher frequency Raman
modes. This is an indication of a very strong Raman process that populates the first
excited vibrational state. The difference in SERS enhancement levels for isolated
Fig. 2.4 Stokes and anti-Stokes SERS spectra of crystal violet attached to isolated and aggregated
gold nanospheres. Strong anti-Stokes signals for aggregated spheres indicate population of first
excited vibrational states beyond Boltzmann distribution. This population pumping due to a very
strong SERS Stokes process indicates a dramatic increase in local field enhancement for nanoparticle
aggregates versus isolated particles (Reprinted with permission from [58])