Sensitivity of CARS, detection

The detection sensitivity of CARS ranges from 0.1-100 ppm (=10 —10 " relative concentrations) depending on the Raman cross sections. Although other spectroscopic techniques such as laser-induced fluorescence or resonant two-photon ionization (Sect. 1.2) may reach higher sensitivities, there are enough examples where CARS is the best or even the only choice, for instance, when the molecules under investigation are not infrared active or have no electronic transitions within the spectral range of available lasers. 

With a detection sensitivity of 10 to 100 ppm, CARS is not as good as some other techniques in monitoring pollutant gases at low concentrations... 

Detection sensitivity is one of the key issues in CARS microscopy. This is an especially acute problem in applications where chemical selectivity of CARS perfectly suits the tracking of small changes in cells related to specific protein and DNA distributions, external drug delivery/distribution, etc. There is, however, a component in CARS signal that is not associated with a particular vibration resonance and therefore does not carry chemically specific information. Unfortunately, in many cases, it can distort and even overwhelm the resonant signal of interest. In modeled approach, the CARS response originates from the third-order nonlinear susceptibility, which... 

Figure 7.15b). Hence, sensitive heterodyne CARS detection is realized with intrinsic interferometric stability. Dne to the interference with the strong external LO, signal amplification of a factor larger than 5000 was achieved (von Vacano et al. 2006a). 

We first review the essentials of the phase distribution of the electric fields at the focus of a high numerical aperture lens in Section II. After discussing the phase properties of the emitted signal, in Section HI we zoom in on how the information carried by the emitted held can be detected with phase-sensitive detection methods. Interferometric CARS imaging is presented as a useful technique for background suppression and signal enhancement. In Section IV, the principles of spatial interferometry in coherent microscopy are laid out and applications are discussed. The influence of phase distortions in turbid samples on phase-sensitive nonlinear microscopy is considered in Section V. Finally, in Section VI, we conclude this chapter with a brief discussion on the utility of phase-sensitive approaches to coherent microscopy. 

In condensed-phase CARS, the effects of the nonresonant susceptibility x(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 suppression schemes [44, 60, 61, 63, 64] and optical heterodyne detection (OHD) techniques [65-67] have been developed during recent years. 

As these examples illustrate, in vivo CARS microspectroscopy with shot-noise limited detection sensitivity allows the noninvasive quantification of densities, chemical composition, and physical state of molecular species inside biological systems ranging from a single lipid monolayer to a complex living cell. 

For a particular biomedical application of CRS microscopy, the best choice whether to use CARS or SRS detection depends on the optimal balance between the pros and cons of each technique regarding its detection sensitivity, image acquisition time, and interpretability of image contrast and spectrum. In the following, we provide a critical discussion of the advantages and disadvantages of both complementary detection techniques ... 

The detection sensitivity being limited at 100 to 1000 ppm for usual gases in flames using background cancellation, an effort has been undertaken in order to understand resonance enhancement mechanisms and in order to apply resonance CARS to trace species detection. The theory is now well understood (8) and an encouraging experimental verification has been reported with detectivity gains of 100 to 1000 (9). However, numerous experimental problems remain to be solved, among which are saturation and laser stability problems (10). 

In conclusion, CARS is now a proven laboratory technique for chemical analysis and temperature measurements. These achievements have come through a very careful engineering of the laser sources and associated optics. The detection sensitivity has been improved... 

---