They traced the enhancement to the nonlinearities of the electrons in Au particles. Among all composite materials, those made out of gold NPs embedded in a dielectric matrix are more important, because of their strong SPR absorption band in the visible region [ 33 ]. The coexistence of unique linear and nonlinear especially third-order optical properties makes the material be well suited for the potential applications ranging from optical limiter [ 20 , 34 ], quantum information processing [ 35 , 36 ], cancer treatment [ 37 , 38 , 39 ], on to all-optical switching [ 33 , 40 , 41 ].
In this direction, Au NPs embedded in dielectric media have been widely put more attention for their SPR, which depends strongly on the NPs environment and geometry [ 42 ]. Many investigations were performed in Au nanoparticles to study the nonlinear refractive index and nonlinear absorption [ 2 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ].
Moreover, the optical limiting of Au nanoparticles has also been studied widely for protection of human eyes and optical devices from laser damage. The contents studied mainly include the effects of sizes, matrices, and shapes on the nonlinear optical properties in Au nanoparticles [ 43 ]. Under nm, ps pulses, they found that the Au NPs exhibited a negative nonlinear absorption, which increases with size and size-independent positive nonlinear refraction. For larger Au nanoparticles than those above, a systematic study of the size-related nonlinear optical properties of triangular Au particles was performed by S.
With the increasing particle size, the absorption peak shifts to longer wavelength.
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The curve of the nm sample showed only the SA component. With the increase of particle size, the SA becomes dominant.
The increase of refractive index is due to the excited electrons, resulting in the self-focusing. It is obvious that the size of Au nanoparticles influences the nonlinear optical properties of Au nanoparticles. Hence, the optical limiting of Au nanoparticles should be size-dependent. They found that the optical limiting threshold and the amplitude depend strongly on size.
The 2. But we believe that the increasing trend of optical limiting capability with size will terminate somewhere, and there should be an optimal size for optical limiting. But when particle size increases further, optical limiting capability decreases instead.
We analyzed the results in terms of the surface layer of nanoparticles. We assume that only surface layer of the particle can respond to outside light. We think of the thickness of the surface layer to be ds.hosylsyla.tk
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When gold nanoparticle has the radius less than ds , light energy can be absorbed by whole particle. Then, the absorbed energy transfers to surrounding solvent and leads to solvent bubbles. Moreover, the larger the size is, the stronger the nonlinear scattering is, and gold nanoparticles exhibit size-enhanced optical limiting. This is in consistent with the results in and nm gold nanoparticles. But when the radius of gold nanoparticle increases to be larger than ds , only outer layer ds can absorb light energy.
The transferring energy from gold nanoparticle to solvent decreases, which will obstruct the surrounding solvent to form bubbles. The obstruction makes the optical limiting weaker in larger particle. Based on the analysis above, we found that, when the radius of nanoparticle is equal to the surface layer ds , the absorption-induced scattering is the strongest, correspondingly the optical limiting is most effective.
That means, the optical limiting optimal size of metal nanoparticles equals to surface layer thickness ds. The investigation may be helpful for the synthesis of metal nanoparticles for optical limiting. Undoubtedly, more studies are required to find out the exact reasons for this behavior. For Au nanoparticles, there is significant overlap between the interband absorption and the plasmon resonance absorption, which decreases substantially the efficiency of plasmon excitation. This makes the plasmon excitation in Ag nanoparticles more efficient than that in Au nanoparticles.
Moreover, the separation of two kinds of absorption facilitates the separate investigation of the nonlinear optical effects arising from interband transitions and those due to SPR. Hence, many groups conducted research on the nonlinear absorption and optical limiting in silver nanoparticles [ 1 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. The nonlinear refraction and nonlinear absorption of these nanoparticle films were measured by using the Z-scan technique.
The broad SPR absorption indicates that there are different-sized and different-shaped nanoparticles in the samples. So by designing properly nanoparticles with different sizes and shapes, it may be possible to use these materials for various applications such as mode locking and optical limiter to protect sensors or eyes from the damage of high-power laser. All the samples exhibit typical RSA, but the deepness of the valleys differs from one to another, indicating different RSA abilities. In contrast to corresponding suspensions, the composites show more complicated OA Z-scan curves and the curves are insensitive to the nanostructured shapes.
To a different extent, all the traces show two symmetrical humps flanking the valley near the focus, where hump indicates SA and valley RSA. When the input energy increases gradually from 0. The origin of this phenomenon was discussed from the viewpoint of electronic dynamics of Ag NWs in liquid and solid-state matrices. The solid-state environment of the gel glass composite greatly enhances the fluorescence of Ag NWs, retards the electronic relaxation process, and results in surface plasmon bleaching, which causes SA.
While at resonant wavelength, Ganeev and Ryasnyansky have investigated the nonlinear optical absorption of Ag nanoparticles [ 28 ].
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They found that Ag nanoparticles exhibit either SA for 1. In fact, the nonlinear optical properties of materials depend strongly on wavelength and pulse width. Moreover, when the repetition rate of pulse laser is high, open-aperture Z-scan curves become asymmetric. The switch and asymmetry were interpreted in terms of plasmon bleaching, free carrier absorption, and migration of Ag nanoparticles.
So the observed SA is due to the ground-state plasmon bleaching. It is the effect that causes an increase in transmission. When the laser intensity increases further, RSA begins to happen because the excitation can easily cause free carrier responsible for the RSA. This is a typical optical limiting effect, which can be applied to protect eyes and sensors from the damage of intense laser. Besides, asymmetrical open-aperture Z-scan curves were observed by using laser with higher repetition rate, which was thought to be due to the migration of nanoparticle following the impulsive optical excitation.
More work is needed to study how laser causes the migration of nanoparticle and make a theoretical fit of the asymmetrical curves. Open-aperture Z-scan curves at relatively low energies 46, 73, 92 nJ using 1- and Hz pulsed laser. Open-aperture Z-scan curves obtained at energy of nJ using a 1-, b , c , and d Hz pulsed laser. It is well known, because both Au and Ag nanoparticles show a strong surface plasmon resonance SPR absorption band in the visible region, they have attracted more interest and initiated more theoretical and experimental studies concerning nonlinear optical properties.
Though the SPR of transition metal nanoparticles is located in the ultraviolet range, the NLO response cannot be resonantly enhanced, but instead, it can be influenced by other nonlinear processes. Pd and Pt nanoparticles have also been found to exhibit interesting nonlinear optical properties such as two-photon or multiphoton processes [ 46 , 47 , 48 , 49 ], RSA [ 50 ] and SA [ 13 , 51 , 52 , 53 ].
Correspondingly, Pt nanoparticles can be used in optical limiting [ 50 , 53 , 54 ] and mode locking [ 52 , 53 ]. To interpret the flip of SA around the beam waist, we phenomenologically combine a saturable absorption coefficient and the two-photon absorption TPA coefficient, yielding the total absorption coefficient as shown by Eq. It can be found that the theoretical fit is in good agreement with the experimental results, indicating the model used is reasonable.
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So we think that the SA in Pt nanoparticles has different origins from that in gold and silver nanoparticles and cannot be interpreted in terms of SPR. Although we have no ideal about the special phenomena, we think that Pt nanoparticles may be employed in not only optical limiting but also mode locking. In this chapter, nonlinear optical properties of metal nanoparticles were reviewed. Most of these studies were conducted in liquid matrices. However, from the viewpoint of practical applications, it is important and indispensable to homogeneously disperse the nanostructured metals in solid-state matrices to avoid their easy agglomeration and instability in suspensions.
In this case, investigation on NLO behaviors of metal nanoparticles in solid matrices becomes the most significant step toward the development of practical optoelectronic components and devices. Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers.