Biophotonics_Concepts_to_Applications

9.9 Stimulated Raman Scattering Spectroscopy

Stimulated Raman scattering(SRS) spectroscopy offers an alternative analytical
tool to CARS [ 50 – 52 ]. In SRS spectroscopy, two laser beams are used to coherently
excite the sample. One laser provides the pump beam with frequencyωpand the
other laser provides the Stokes beam with frequencyωs. When thebeat frequency
Δω=ωp−ωs(also called theRaman shift) is equal to a particular molecular
vibration of the sample, SRS signals are produced as a result of the nonlinear
interaction between the photons and the molecules. These SRS signals include both
stimulated Raman loss(SRL) andstimulated Raman gain(SRG). Advantages of
SRS that enable straightforward chemical analyses are that it does not have a
resonant background signal, it yields an identical spectrum as Raman scattering,
and it is linearly proportional to the concentration of the analyte. SRS has a major
advantage in imaging speed over Raman scattering and gives an improved image
contrast and spectralfidelity compared to CARS. However, a challenge to its use is
the need for an expensive and sensitive ultrafast dual-wavelength tunable laser
source.

9.10 Photon Correlation Spectroscopy

Photon correlation spectroscopy(PCS) is based on dynamic light scattering
techniques for probing time variations of the density or concentrationfluctuations
of small particles in a highly diluted suspendingfluid or polymers in a solution [ 53 –
55 ]. Dynamic light scattering measures the Brownian motion of the suspended
particles and relates this motion to their sizes. For a liquid of a given viscosity,
larger particles will have a slower Brownian motion than smaller ones. The sizes for
the particles being analyzed range from 1 nm to 10μm. The particle size is cal-
culated from the translational diffusion coefficient D by using the Stokes-Einstein
equation

dH¼

kBT

3 pgD

ð 9 : 10 Þ

Here T is the temperature (K),ηis the viscosity of the suspending solution, and kB
is Boltzmann’s constant. The parameter dHis called thehydrodynamic diameter,
which is the diameter of a sphere that has the same translational diffusion speed as
the particle being analyzed.
A basic setup for PCS is illustrated in Fig.9.20a. First a light pulse from a laser
is injected into a solution and as it hits the molecules in the liquid sample the light
travels in all directions due to Rayleigh scattering. The scattered light is collected
by a photodetector, such as a photomultiplier tube, and then is directed to a signal
analysis unit. The pulsing process is repeated at short time intervals. The scattering

282 9 Spectroscopic Methodologies