**2. Theories of CARS process**

170 Photonic Crystals – Innovative Systems, Lasers and Waveguides

methods with molecular specificity, high spatial and temporal resolution, but without any

Now some imaging techniques without any extrinsic labels have been developed, such as the harmonic generation microscopy (second harmonic generation, SHG [9-13] and third harmonic generation, THG [14, 15]), infrared microscopy [16], Raman microscopy [17-19], Stimulated Raman Scattering (SRS) microscopy [20] and coherent anti-Stokes Raman scattering (CARS) microscopy [21, 22]. The specificity and contrast of SHG and THG imaging arise from the unique non-centrosymmetric structure of specific biological tissues, such as the collagen fibrils, microtubules and myosin, but the structural information can not be obtained at molecular level. Because of longer wavelength involved and vibrational absorption of the common solvent, the spatial resolution of infrared imaging technique is too low. With the Raman scattering spectrum, we can distinguish different molecules [17-19]. However, higher average power laser is necessary due to small Raman scattering crosssection and autofluorescence background that limit its applicability in life science. Both SRS and CARS are essentially Raman scattering, the molecules of a sample can be distinguished by their active vibrational modes of specific molecular bonds, which provide the necessary imaging contrasts and specificities [20-26]. As a coherent resonance-enhance process, the CARS signal is not only many orders higher than spontaneous Raman scattering signal but also well oriented in emission direction, which significantly reduce the integration time. The wavelength of CARS signals is blue-shift relative the excitation, the interference of strong one-photon fluorescence background can be avoided. Furthermore, as a nonlinear optical process, the CARS signals are generated in a small focal volume under the tight-focusing condition. Thus, the CARS microscopy can provide high temporal and spatial resolution, and strong 3D sectioning imaging ability. The information of molecular distributions of a

biological sample can be obtained without any disturbance on sample.

and had a great prospect in biology, medicine and life science [22, 33].

In 1965, CARS was first reported by P. D. Maker and R. W. Terhune [27]. They found that a very strong signal could be obtained when two coherent light beams at frequencies ω1 and ω2 were used to drive an active vibrational Raman mode at frequency ΩR=ω1-ω2. It was named as the CARS process, whose signals are 105 stronger than spontaneous Raman scattering process [28]. As a tool for spectral analysis, the CARS spectroscopy was extensively studied and used in physics and chemistry [29-32]. The first CARS microscopy with noncollinear beams geometry was demonstrated by M. D. Duncan et al in 1982 [21]. In 1999, A. Zumbusch and his colleagues revived the CARS microscopy by using an objective with high numerical aperture, collinear beams geometry and detection of CARS signals in forward direction that boosted wide research in the improvements of CARS microscopy [22]. The distinguished research works of Prof. S. Xie and his colleagues proved that the CARS microscopy was a very effective noninvasive optical microscopic approach with high spatial resolution and imaging speeds matching those of multi-photon fluorescence microscopy

In a traditional CARS microscopy, the contrast and specificity are based on single or few chemical bonds of molecule due to the limitation of laser line-width. It is not adequate to distinguish various biological molecules with just single chemical bond. In order to effectively distinguish different molecules, a method for simultaneously obtaining the complete molecular vibrational spectra is required. For this purpose, many methods for extending the simultaneously detectable spectral range of CARS spectroscopy and

extrinsic labels.

As a coherent Raman scattering process, CARS is a typical three-order nonlinear optical process. In order to well understand the mechanism of the CARS process, the brief classical and quantum mechanical discussions of Raman scattering are necessary. Based on the theoretical analysis of Raman scattering, a theoretical description of the CARS process and the conditions for the generation of CARS signals will be outlined. The general classical description will give an intuitive picture. And the quantum mechanical description will enable the quantitative analysis of the CARS process. Here, we just carry out the qualitatively theoretical analysis, and the descriptions of physical mechanism will be highlighted. The emphases are the main characteristics of a CARS microscopy, such as the conditions for generation of CARS signals, and generation and suppression of NRB noise.
