Coherent Raman Scattering Microscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 119

(3)nr

) 2 +|

(3)r

| 2 +2

(3)nr (3)r

( Re )

2

nr

) 2

(

1

r

| 2

|

0

nr (3)r

2 Re( )

-1

-50 0 50

r -( p - S )/cm -1

Fig. 6.4. Typical CARS spectral profile modeled for an isolated Lorentzian Raman
line assuming (6.5) (Γ r =4 . 6cm − 1 , A r =4cm − 1 ) and parallel-polarized input and
CARS fields. According to (6.10), the total CARS spectrum (solid line) is composed
of a constant nonresonant background χ (3)nr 2 , a resonant contribution χ (3)r

2

,

and a heterodyne mixing term of dispersive character, 2 χ (3)nr Re χ (3)r

6.3.2 Increasing the Detection Sensitivity

In condensed-phase CARS, the effects of the nonresonant susceptibility χ (3)nr
are most profound when a sample with weak Raman modes is embedded in
a nonlinear medium. The nonresonant background of the latter can be easily
comparable to or larger than the resonant contribution from the sample of
interest. This is a situation commonly encountered in biological applications
of CARS microscopy. Depending on the experimental situation, the CARS
detection sensitivity to weak resonances can then be restricted either by the
nonresonant background or by the photon shot-noise [62]. To maximize either
the relative or the absolute CARS intensity, nonresonant background sup-
pression schemes [44, 60, 61, 63, 64] and optical heterodyne detection (OHD)
techniques [65-67] have been developed during recent years.

Suppression of Nonresonant Background

It is important to realize that, unlike in fluorescence detection, CARS detec-
tion is not background free. Nonresonant background signals, which provide
no vibrational contrast, and the solvent water that has strong resonant sig-
nals of broad spectral width often overwhelm the CARS signal from small
objects and limit the sensitivity. During recent years, various methods have
been developed that permit ecient suppression of nonresonant background