2 Plasmonics for Enhanced Vibrational Signatures
105
In addition to incoherent, spontaneous Raman scattering, vibrational modes can
be probed by coherent Raman processes, also called stimulated, Raman scattering.
There, molecular vibrations are coherently driven by two interacting fields at frequency differences that match vibrational transitions [10, 11]. In coherent Raman
probing, vibrational information appears in strong and directed coherent optical signals. This makes coherent Raman scattering attractive for fast collection of vibrational information and for vibrational imaging [12]. Moreover, time resolved coherent
Raman techniques, such as impulsive stimulated Raman spectroscopy or femtosecond stimulated Raman spectroscopy can provide information on the pathways and
dynamics of chemical reactions, as they allow to monitor vibrational signatures of
intermediate structures with ultrafast time resolution [13–16].
In this article, we focus on surface enhanced vibrational spectroscopy exploiting
enhanced local fields of plasmonic structures. In Sect. 2.2, we introduce the incoherent effects of surface enhanced Raman scattering (SERS), surface enhanced infraredabsorption SEIRA, surface enhanced pumped anti- Stokes scattering (SEPARS),
and surface enhanced hyper Raman scattering (SEHRS). There, we also explain
the coherent Raman techniques of surface enhanced coherent anti-Stokes Raman
scattering (SECARS) and surface enhanced stimulated Raman scattering (SESRS),
and summarize some important features of these plasmonic supported optical
effects. In Sect. 2.3, we discuss important properties of plasmonic nanostructures and
conditions suitable for enhancing different vibrational spectra obtained in different
wavelength ranges. In Sect. 2.4, we illustrate the capabilities of plasmon supported
vibrational spectroscopy by selected applications, in particular ultrasensitive SEIRA
of protein monolayers, monitoring of catalytic reactions on composite nanostructures
with SERS, and a new class of nanosensors and labels based on SERS-, SEHRS- or
SECARS signals. These examples illustrate how plasmonics transforms vibrational
spectroscopy from a method for chemical structure analysis to a versatile tool providing information on chemical structures and processes along with ultrasensitive
detection limits and nanoscale confinement of spectroscopic information. Section 2.5
gives a brief summary and outlook to potential future developments in the field.
2.2 “Normal” and “Surface-Enhanced” Vibrational
Spectroscopy
Vibrational modes can be probed based on absorption and inelastic scattering of
photons, as discussed above. Due to the Raman scattering process, vibrational information occurring in the infrared range of the spectrum is transferred to the visible,
NIR, or UV range, respectively, depending on the excitation wavelength applied.
Figure 2.1 illustrates some of the different spectroscopic methods for vibrational
probing in an energy level diagram. Usually, in Raman scattering, excitation and/or
scattered photons are not in resonance with any real molecular electronic transition
(see Fig. 2.1). In contrast, in resonance Raman scattering (RRS), the excitation laser,
105
In addition to incoherent, spontaneous Raman scattering, vibrational modes can
be probed by coherent Raman processes, also called stimulated, Raman scattering.
There, molecular vibrations are coherently driven by two interacting fields at frequency differences that match vibrational transitions [10, 11]. In coherent Raman
probing, vibrational information appears in strong and directed coherent optical signals. This makes coherent Raman scattering attractive for fast collection of vibrational information and for vibrational imaging [12]. Moreover, time resolved coherent
Raman techniques, such as impulsive stimulated Raman spectroscopy or femtosecond stimulated Raman spectroscopy can provide information on the pathways and
dynamics of chemical reactions, as they allow to monitor vibrational signatures of
intermediate structures with ultrafast time resolution [13–16].
In this article, we focus on surface enhanced vibrational spectroscopy exploiting
enhanced local fields of plasmonic structures. In Sect. 2.2, we introduce the incoherent effects of surface enhanced Raman scattering (SERS), surface enhanced infraredabsorption SEIRA, surface enhanced pumped anti- Stokes scattering (SEPARS),
and surface enhanced hyper Raman scattering (SEHRS). There, we also explain
the coherent Raman techniques of surface enhanced coherent anti-Stokes Raman
scattering (SECARS) and surface enhanced stimulated Raman scattering (SESRS),
and summarize some important features of these plasmonic supported optical
effects. In Sect. 2.3, we discuss important properties of plasmonic nanostructures and
conditions suitable for enhancing different vibrational spectra obtained in different
wavelength ranges. In Sect. 2.4, we illustrate the capabilities of plasmon supported
vibrational spectroscopy by selected applications, in particular ultrasensitive SEIRA
of protein monolayers, monitoring of catalytic reactions on composite nanostructures
with SERS, and a new class of nanosensors and labels based on SERS-, SEHRS- or
SECARS signals. These examples illustrate how plasmonics transforms vibrational
spectroscopy from a method for chemical structure analysis to a versatile tool providing information on chemical structures and processes along with ultrasensitive
detection limits and nanoscale confinement of spectroscopic information. Section 2.5
gives a brief summary and outlook to potential future developments in the field.
2.2 “Normal” and “Surface-Enhanced” Vibrational
Spectroscopy
Vibrational modes can be probed based on absorption and inelastic scattering of
photons, as discussed above. Due to the Raman scattering process, vibrational information occurring in the infrared range of the spectrum is transferred to the visible,
NIR, or UV range, respectively, depending on the excitation wavelength applied.
Figure 2.1 illustrates some of the different spectroscopic methods for vibrational
probing in an energy level diagram. Usually, in Raman scattering, excitation and/or
scattered photons are not in resonance with any real molecular electronic transition
(see Fig. 2.1). In contrast, in resonance Raman scattering (RRS), the excitation laser,