**1. Introduction**

168 Photonic Crystals – Innovative Systems, Lasers and Waveguides

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*Applications*, Springer Series in optical sciences, ISBN-10 3-540-71066-3, Springer

Optics, one of the oldest natural sciences, has been promoting the developments of sciences and technologies, especially the life science. The invention of the optical microscope eventually led to the discovery and study of cells and thus the birth of cellular biology, which now plays a more and more important role in the biology, medicine and life science. The greatest advantage of optical microscopy in the cellular biology is its ability to identify the distribution of different cellular components and further to map the cellular structure and monitor the dynamic process in cells with high specific contrast. In 1960's, with the invention of laser, an ideal coherent light source with high intensity was provided. Since then the combination of optical microscopy with laser has been expanded. Many novel optical microscopic methods and techniques were developed, such as various kinds of fluorescence microscopy. In fluorescence microscopy, the necessary chemical specificity is provided by the labeling samples with extrinsic fluorescent probes [1, 2]. With the developments of ultra-short laser, fluorescent labeling technique and modern microscopic imaging technique, the fluorescence spectroscopy and microscopy with high spatial resolution, sensitivity and chemical specificity has become the key promotion of the life science and unveiled many of the secrets of living cells and biological tissues [3, 4]. In particular, the confocal fluorescent microscope (CFM), with the confocal detection [5] and multi-photon excitation [6, 7], can obtain the 3D sectioning images of cells and tissues with high spatial resolution. Today, fluorescence microscopy has become a powerful research tool in life science and has achieved the great triumph. Nevertheless, the disadvantages of fluorescence microscopy, such as the photo-toxicity and photo-bleaching, can not be ignored [8]. Furthermore, some molecules in cells, such as water molecule and other small biomolecules, can not be labeled until now. Finally, for the biological species that do not fluoresce, the extrinsic fluorescent labels will unavoidablely disturb the original characteristics and functions of biological molecules, which will limit the applicability of fluorescence microscopy. Therefore, it is very necessary to develop some complementary

Ultra-Broadband Time-Resolved Coherent Anti-Stokes Raman Scattering

molecules because of the influence of strong NRB.

review and outlook the feasible ways.

**2. Theories of CARS process** 

**2.1 Raman scattering process** 

feasibilities [40-44].

Spectroscopy and Microscopy with Photonic Crystal Fiber Generated Supercontinuum 171

microscopy are presented [34-38]. With the advent and the progress of supercontinuum (SC) generated by photonic crystal Fibre (PCF) pumping with ultra-short laser pulses [39], the broadband CARS microscopy based on SC has been developed that provided better

As well known, CARS microscopy is not background-free [29]. Strong nonresonant background signals (NRB), from the electronic contributions to the third-order susceptibility of sample and the solvent, always company with the CARS signals in a broad spectral range. It often interferences or overwhelms CARS signals from small targets that limits the spectral accuracy, imaging contrast and systemic sensitivity. In the broadband CARS with a broadband pump source, it is not beneficial to simultaneously distinguish various biological

In this chapter we briefly introduce the theoretical basics of the CARS process and characteristics of CARS spectroscopy and microscopy. The classical and quantum mechanical descriptions of Raman scattering and CARS processes are qualitatively reviewed in order to be helpful for understanding the physical mechanisms of these two light scattering processes. The main characteristics and applications of a CARS spectroscopy and microscopy are specifically emphasized, such as the condition of momentum conservation; the generation and suppression of NRB noise. In order to simultaneously obtain the complete molecular vibrational spectra, SC laser is a proper pump source. We will briefly review the history and characteristics of PCF. A summarization of the theoretical analysis of SC generation with PCF pumping by ultra-short (picosecond or femtosecond) laser pulses will be outlined. The necessities, developments and characteristics of the broadband CARS spectroscopy and microscopy will be briefly described. An ultrabroadband time-resolved CARS technique with SC generated by PCF will be especially emphasized. By this method, the complete molecular vibrational spectra without NRB noise can be obtained. During recent years, the ways to improve spatial resolution of CARS microscopy have become one of attractive questions all over the world. We will briefly

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.

When a beam of light passes through a media, one can observe light scattering phenomenon besides light transmission and absorption. Most of elastically scattered photons (so called

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

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.

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

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 microscopy are presented [34-38]. With the advent and the progress of supercontinuum (SC) generated by photonic crystal Fibre (PCF) pumping with ultra-short laser pulses [39], the broadband CARS microscopy based on SC has been developed that provided better feasibilities [40-44].

As well known, CARS microscopy is not background-free [29]. Strong nonresonant background signals (NRB), from the electronic contributions to the third-order susceptibility of sample and the solvent, always company with the CARS signals in a broad spectral range. It often interferences or overwhelms CARS signals from small targets that limits the spectral accuracy, imaging contrast and systemic sensitivity. In the broadband CARS with a broadband pump source, it is not beneficial to simultaneously distinguish various biological molecules because of the influence of strong NRB.

In this chapter we briefly introduce the theoretical basics of the CARS process and characteristics of CARS spectroscopy and microscopy. The classical and quantum mechanical descriptions of Raman scattering and CARS processes are qualitatively reviewed in order to be helpful for understanding the physical mechanisms of these two light scattering processes. The main characteristics and applications of a CARS spectroscopy and microscopy are specifically emphasized, such as the condition of momentum conservation; the generation and suppression of NRB noise. In order to simultaneously obtain the complete molecular vibrational spectra, SC laser is a proper pump source. We will briefly review the history and characteristics of PCF. A summarization of the theoretical analysis of SC generation with PCF pumping by ultra-short (picosecond or femtosecond) laser pulses will be outlined. The necessities, developments and characteristics of the broadband CARS spectroscopy and microscopy will be briefly described. An ultrabroadband time-resolved CARS technique with SC generated by PCF will be especially emphasized. By this method, the complete molecular vibrational spectra without NRB noise can be obtained. During recent years, the ways to improve spatial resolution of CARS microscopy have become one of attractive questions all over the world. We will briefly review and outlook the feasible ways.
