Nonlinear Optical Responses II

Abstract

Some conventional nonlinear optical responses were summarized in Chap. 6. In the present chapter, we will discuss the recent development of nonlinear optical responses. First, phase-matched harmonic conversion of infrared and visible light into extreme ultraviolet (XUV) or soft X-rays is introduced. Here ultrashort laser pulses shown in Chap. 5 can generate even shorter bursts of coherent XUV or soft X-rays. We are, however, not allowed to describe these phenomena by the perturbational methods used in Chap. 6. These higher order harmonic generations will be discussed in Sect. 7.1. Second, these XUV or X-rays in the frequency region are possibly converted into a train of attosecond pulses or a few bursts of attosecond pulses. These will be discussed in Sect. 7.2. Atoms are used as a material system for high-harmonic generation (HHG) of XUV and soft X-rays and for generation of their attosecond laser pulses. Although the efficiency of HHG is rather small, i.e., of the order of 10^-6, the high-order stimulated Raman scattering (HSRS) is much stronger by several orders of magnitude than that of HHG. Usually the rotational and vibrational modes of molecules are used as a material system. Third, the second nonperturvative phenomena of nonlinear optics come from resonant optical pumping of a specified Raman-active phonon mode of molecular rotation and vibration by two incident beams of femtosecond laser pulses. This will be discussed in Sect. 7.3. Fourth, it is possible, in the excitation of crystals, to combine HHG and HSRS. This combined effect can be observed by resonant excitation of the Raman-active mode of lattice vibration in crystals by two intense near-IR femtosecond laser pulses generated from an optical parametric system prepared by Ti:sapphire lasers. Here the frequency difference of the signal and idler beams is chosen nearly equal to the phonon mode. The efficiency of the HSRS is further enlarged by this method. This will be discussed in Sect. 7.4. It is again seen that these nonlinear optical responses cannot be described by the perturbational treatment discussed in Chap. 6.