Biomedical Engineering Reference
In-Depth Information
[2-4] and fluorescence spectroscopy [5-8]) are described in this chapter and
highlighted with a diverse set of applications.
The foundation for the development of these techniques is built on inves-
tigations into photon migration processes [2, 9]. Subsequent, detailed exami-
nation by Everall et al. [10, 11] demonstrated that the inelastically scattered
(Raman) component decays substantially more slowly than its elastically scat-
tered counterpart (i.e. the laser light) due to the regeneration of the Raman
signal from the laser component. Discrimination between diffusely scattered
photons and the ballistic and snake components is achieved by gating the
detector in the temporal or spatial domain.
3.3 Deep Probing Raman Techniques
In practical terms, diffuse scattering in a conventional backscattering Raman
spectrometer results in the overwhelming dominance of the signal from surface
layers over the considerably weaker subsurface layer signals. This bias is a
consequence of the dilution of deep layer Raman signals due to their large
lateral diffusion upon propagation to and from deep layers of the sample. In
contrast, the surface-generated Raman photons are more tightly confined to
the vicinity of the laser deposition area and hence are much more effectively
collected.
3.3.1 Ultrafast Raman Signal Gating
The interfering surface Raman signal can be effectively suppressed using im-
pulsive excitation and fast (picosecond) temporal gating of the backscattered
Raman signal. The viability of this concept with Raman spectroscopy was
first demonstrated by Wu et al. [9] using photon counting detection which al-
lowed non-invasive imaging of a buried object. The recovery of the full Raman
spectrum of a deeply buried layer in a turbid sample was demonstrated using
a Kerr gating approach by Matousek et al. [12] on a two-layer powder sample.
This work combined pioneering research into Raman photon migration by Ev-
erall et al. [10, 11] and Morris et al. [13] with fluorescence rejection methods
developed by Matousek et al. [14-16]. A more extensive description of these
methods and underlying principles can be found in [17].
3.3.2 SORS
The time-resolved Raman approach proved effective in isolating the Raman
spectra of deep layers; however, the complexity of the instrumentation re-
quired and the high peak intensities associated with short-pulse lasers pre-
cluded its wider use. These problems were addressed by utilising the spatial
Search WWH ::
Custom Search