Coherent Raman Scattering Microscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 120

signal in CARS microscopy. These include epi detection [37, 60], counter-
propagating CARS [61], polarization-sensitive detection [63], pulse-sequenced
detection [64], and the implementation of coherent control techniques [44].
Common to all these techniques is that the suppression of nonresonant back-
ground is accompanied by a significant reduction of the absolute strength of
the Raman-resonant CARS signal resulting in a modest detection sensitivity
for very weak resonances and/or low sample concentrations.

Optical Heterodyne-Detected CARS

Background-free CARS microscopy without any loss of resonant signal strength
can be achieved through phase-controlled OHD schemes. In OHD-CARS, the
measured signal is the interference of a weak resonance CARS field with an
intense reference field at the anti-Stokes frequency. The latter acts as a so-
called local oscillator (LO) field. As Oudar et al. [68] have pointed out, OHD-
CARS allows the recovery of both the real and imaginary parts of the Raman-
resonant susceptibility. Furthermore, it allows a significant enhancement of
the CARS detection sensitivity through amplification of the Raman-resonant
signal.

Weak Raman-resonant CARS, i.e., when χ (3)r << χ (3)nr , can be amplified
by interference with the intense nonresonant CARS that acts as an intrinsic
local oscillator field created within the same sample. For weak resonances, the
second term in (6.10) becomes negligible and the nonresonant background
and the heterodyne mixing term then dominate the total CARS intensity.
As a result, the amplified CARS signal is proportional to the real part of
the resonant susceptibility and falls off linearly with the number density of
Raman scatterers. Such an intrinsic OHD-CARS effect has been exploited
in CARS microscopy for high-sensitivity imaging [69] and microspectroscopy
[70], where shot-noise limited detection sensitivity has been achieved.
In methods based on nonlinear interferometry, originally proposed by
Chang et al. [71], the CARS field generated by the sample is heterodyne
mixed with a well-controlled and stable local oscillator field that is created
in an external nonresonant CARS bulk medium [66] or that is provided by
an additional laser reference beam at the anti-Stokes frequency from a cas-
cading phase preserving chain [67]. Because both the CARS field from the
sample and the LO field are phase and frequency locked, nonresonant back-
ground suppression is achieved by the addition of the sample CARS field with
a reference field of opposite phase [72, 73]. The potential of CARS interferom-
etry for nonresonant background suppression in CARS microscopy has been
demonstrated using detection in the time domain [65, 74] as well as in the
frequency domain [75, 67].
Recently, Silberberg and coworkers have introduced a different approach
for active phase control in CARS [44-47]. By tailoring the spectral phase of a
single ultrashort laser pulse, phase-sensitive detection of the resonant signal
has been demonstrated where the strong nonresonant CARS background of