**1. Introduction**

When the light is not strong, the optical response of a material usually scales linearly with the amplitude of optical electric field. However, at high optical powers, the optical properties of material will be changed more rapidly and are no longer linearly related to the intensity of the incident light. As result, nonlinear optical effects will occur. Nonlinear optics is the study of how intense light interacts with matter. The goal of nonlinear optics is mainly the investigation of the new phenomena and effects in the interaction process of strong laser and materials, including a deep understanding of the causes and the process regularity and their possible applications in the development of disciplines. Nonlinear optics has a great value and far-reaching scientific significance. In the past two decades, people have made significant progress in nonlinear optical materials. Many optical materials with fast nonlinear response

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and large nonlinear properties have been used in various photonic and optoelectronic applications such as optical communication, optical information processing, optical data storage, pulsed laser deposition, and optical limiters.

Among various nonlinear effects, nonlinear absorption and nonlinear refraction attract more attention. Ultrafast nonlinear absorption properties are of importance since the nonlinearities considerably change the propagation of intense light through the medium, which can induce novel applications in optoelectronics, optical switchers, and limiters, as well as in optical computing, optical memories, and nonlinear spectroscopy. In fact, there is also much intrinsic interest in nonlinear refractive phenomena, particularly self-focusing and self-defocusing.

The search for new materials is one of the defining characteristics of modern science and technology. Of course, in nonlinear optics field, it is the case. Seeking and investigating new nonlinear optical materials with large nonlinear optical properties and fast nonlinear response is still one of the important works concerning nonlinear optics studies. Generally, the nonlinear optical materials should exhibit high transmission at normal light, so as not to degrade normal vision while exhibiting low or high transmission at intense light to serve as optical limiting materials for protecting human eyes and sensors or as saturable absorber for mode locking. In addition, nonlinear optical materials must exhibit fast response over a broad wavelength range and a high damage threshold.

S is a small aperture used for the measurement of nonlinear refraction. D1 and D2 are photodetectors. During the test, the object to be tested moves along the Z axis, and the relationship

ture Z-scan, which can provide the information about the nonlinear absorption. When there

When studying the materials' nonlinear absorption such as saturable absorption (SA) and reverse saturable absorption (RAS), we need to use open aperture Z-scan technique. Normalized open aperture Z-scan data is insensitive to beam distortion and is only a function

> 1 + (*I*/*I s*

intensity. It is assumed that two-photon absorption (TPA) does not take place simultaneously

Here, *z* corresponds to the sample thickness. As shown in **Figure 2**, saturation intensity *I*

be obtained by fitting the experimental curve according to Eqs. (1) and (2). When materials show only RSA or TPA, according to open aperture Z-scan theory, the normalized transmis-

<sup>3</sup>⁄<sup>2</sup> <sup>≈</sup> <sup>1</sup> <sup>−</sup> *<sup>β</sup> <sup>I</sup>*

2 √ \_\_ 2(1 + *z* <sup>2</sup> /*z*<sup>0</sup> 2

<sup>0</sup> *<sup>L</sup>* \_\_\_\_\_\_\_\_\_ *eff*

is the linear absorption coefficient, *I* is the excitation intensity, and *I*

which can provide the information about the nonlinear refraction of materials.

In the case of SA, the nonlinear absorption coefficient may be written as:

with SA. The transmitted intensity is obtained from the equation:

*m*−0 <sup>∞</sup> [−*q*<sup>0</sup> (*z*)] *m* \_\_\_\_\_\_\_ (*m* + 1)

, Z-scan measurement is called open aper-

Nonlinear Optical Response of Noble Metal Nanoparticles

http://dx.doi.org/10.5772/intechopen.80841

45

) (1)

*dz* <sup>=</sup> <sup>−</sup>*<sup>I</sup>* (2)

*S*

) (3)

is the saturation

*S* can

, Z-scan measurement is called closed aperture Z-scan,

between the light intensity and Z value is recorded.

When there is no aperture before photodetector D1

*<sup>α</sup>*(*I*) <sup>=</sup> *<sup>α</sup>* \_\_\_\_\_\_ <sup>0</sup>

\_\_\_ *dI*

sion can be expressed as [12]:

*T*(*z*) = ∑

is an aperture before photodetector D2

**Figure 1.** Setup of the Z-scan technique.

**2.1. Open aperture Z-scan technique**

of nonlinear absorption.

where *α*<sup>0</sup>

The rapid development of nanoscience and nanotechnology has provided a number of new opportunities for nonlinear optics. A growing number of nanomaterials have been shown to possess remarkable nonlinear optical properties; this promotes the design and fabrication of nanoscale optoelectronic and photonic devices. Specially, metal nanoparticles (MNPs) have attracted considerable attention as potential nonlinear optical materials. Among them, gold and silver nanoparticles (NPs) have been paid more attention because they both exhibit a broad surface plasmon resonance (SPR) absorption band in the visible region of the electromagnetic spectrum [1–10].

In the following sections, the technologies used to measure the amplitude of nonlinear absorption and nonlinear refraction of nonlinear optical materials will be introduced. And some investigations concerning nonlinear optical properties of metal nanoparticles will be discussed.
