**3. Nonlinear optical properties of metal nanoparticles**

The stability combined with a large third-order susceptibility and ultrafast response means that metal nanoparticles are very promising materials for the creation of photonic devices. SPR is the main characteristics of metal nanoparticles, which strongly depend on the size, shape, and type of metal nanoparticles and the dielectric parameter of surrounding environments [14]. A huge enhancement of the nonlinear optical response in random media with metal nanoparticles is often associated with optical excitation of the SPR. Typical three metals having plasmon band in visible range are Au, Ag, and Cu. In contrast, metals such as Pt, Pd, and Cr show SPR in the wavelengths shorter than 300 nm. Therefore, large transparency and low propagation losses in the visible and infrared range are expected in Pt, Pd, and Cr nanoparticles. In many studies, composite materials with metal nanoparticles were fabricated by various methods and then generally studied using lasers operating at frequencies corresponding to the spectral range of the SPR, as listed in Ref. [15]. The review [15] has summarized the development of nonlinear optical random metal-dielectric composites based on metal nanoparticles synthesized by ion implantation.

Among the nanoparticles of noble metals, Au and Ag nanoparticles have attracted more interest and initiated more theoretical and experimental studies [1, 2, 16–31], as they both show a strong SPR band in the visible region. Moreover, their SPR absorption band can be tuned across the entire visible spectra by changing the size and shape of nanoparticles. The SPR of metal nanoparticles can lead to many interesting optical properties. For example, ultrafast nonlinear optical absorption is important since it can considerably alter the propagation of intense light in the medium, which can make metal nanoparticles be applied in optoelectronics, optical limiters and switchers, as well as in optical memories, optical computing, and nonlinear spectroscopy.
