**3.2. Nonlinear optical studies of Ag nanoparticles**

**Figure 10.** Optical limiting capability changing with particle size.

52 Laser Technology and its Applications

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*. The absorption region of gold nanoparticles with different sizes is shown in **Figure 11**. 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 15- and 25-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 energy will first transfer to the core (black part in **Figure 11**) of gold nanoparticles. The transferring energy from gold nanoparticle to solvent decreases, which will obstruct the surrounding solvent to form

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*. For gold nanoparticles, it is 25 nm. The investigation may be helpful for the synthesis of metal nanoparticles for optical limiting. Undoubtedly,

bubbles. The obstruction makes the optical limiting weaker in larger particle.

more studies are required to find out the exact reasons for this behavior.

**Figure 11.** Absorption regions in gold nanoparticles with different sizes.

For Au nanoparticles, there is significant overlap between the interband absorption and the plasmon resonance absorption, which decreases substantially the efficiency of plasmon excitation. In the case of Ag, however, the interband transition absorption at about 320 nm is far from its SPR wavelength of about 400 nm. 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, 22–31].

For example, Gurudas et al. studied the picoseconds optical nonlinearity in silver nanodots prepared by pulsed laser deposition at 532 nm [22]. 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. At 532 nm, the SA and RSA are found to be dependent on sample properties. 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. Zheng et al. have investigated the shape-dependent NLO behaviors of nanostructured Ag nanoplates, nanowires, and nanoparticles suspensions (as shown in **Figure 12**) and their silica gel glass composites at both 532 and 1064 nm [23, 24] by using Z-scan.

NLO abilities of the nanostructured suspensions are found to be shape-dependent at 532 and 1064 nm. **Figure 13** shows the OA Z-scan experiment results of the nanostructured Ag in aqueous suspensions. All the samples exhibit typical RSA, but the deepness of the valleys differs from one to another, indicating different RSA abilities. As shown in **Figure 14**, they also investigated the NLO properties of the Ag/silica gel glass nanocomposites. 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

**Figure 12.** SEM images of the investigated Ag (a) nanoplates, (b) nanowires, and (c) nanoparticles.

gel glass composite and Ag nanowires (NWs) suspension at both 532 and 1064 nm in Ref. [24]. In the Ag NWs suspension, only RSA is found, while in the Ag NWs/silica gel glass composite, a switch from SA to RSA is observed. 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. Unnikrishnan et al. studied nonlinear optical absorption in Ag nanosol at selected wavelengths of 456 nm (inside the SPR band), 477 nm (on the edge of SPR), and 532 nm (outside the SPR) using open aperture Z-scan technique [25]. They all found a flip over from SA to RSA behavior at higher input excitation. Similar switching behaviors have also been previously

All the above studies were conducted at 532 nm far from the SPR wavelength of 400 nm. While at resonant wavelength, Ganeev and Ryasnyansky have investigated the nonlinear optical absorption of Ag nanoparticles [28]. They found that Ag nanoparticles exhibit either SA for 1.2-ps pulsed laser or RSA for 8-ns pulsed laser. In fact, the nonlinear optical properties

In 2012, we studied the nonlinear absorption of Ag nanoparticles using open-aperture Z-scan method with femtosecond laser pulses at 400 nm [29]. As shown in **Figures 15** and **16**, when laser intensities are relatively weaker, Ag nanoparticles show SA, but when laser intensities are strong, a switch from SA to RSA occurs. 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. The peak of the SPR of the Ag nanoparticles is at about 400 nm near the laser excitation. 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,

**Figure 15.** Open-aperture Z-scan curves at relatively low energies (46, 73, 92 nJ) using 1- and 10-Hz pulsed laser.

by Anija [26] and for Ag nanoparticles in PMMA by

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observed for Ag nanoparticles in ZrO2

Deng [27] under nanosecond laser pulse at 532 nm.

of materials depend strongly on wavelength and pulse width.

**Figure 13.** OA Z-scan curves of the investigated nanostructured Ag suspensions at 532 (a) and 1064 nm (b).

two symmetrical humps flanking the valley near the focus, where hump indicates SA and valley RSA. When the input energy increases gradually from 0.6 to 1.5 mJ, the NLA signals switch from SA to RSA. Besides, they have studied the NLO properties of Ag nanowire/silica

**Figure 14.** OA Z-scan curves of the investigated Ag/silica gel glass nanocomposites at 532 (a) and 1064 nm (b).

gel glass composite and Ag nanowires (NWs) suspension at both 532 and 1064 nm in Ref. [24]. In the Ag NWs suspension, only RSA is found, while in the Ag NWs/silica gel glass composite, a switch from SA to RSA is observed. 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.

Unnikrishnan et al. studied nonlinear optical absorption in Ag nanosol at selected wavelengths of 456 nm (inside the SPR band), 477 nm (on the edge of SPR), and 532 nm (outside the SPR) using open aperture Z-scan technique [25]. They all found a flip over from SA to RSA behavior at higher input excitation. Similar switching behaviors have also been previously observed for Ag nanoparticles in ZrO2 by Anija [26] and for Ag nanoparticles in PMMA by Deng [27] under nanosecond laser pulse at 532 nm.

All the above studies were conducted at 532 nm far from the SPR wavelength of 400 nm. While at resonant wavelength, Ganeev and Ryasnyansky have investigated the nonlinear optical absorption of Ag nanoparticles [28]. They found that Ag nanoparticles exhibit either SA for 1.2-ps pulsed laser or RSA for 8-ns pulsed laser. In fact, the nonlinear optical properties of materials depend strongly on wavelength and pulse width.

In 2012, we studied the nonlinear absorption of Ag nanoparticles using open-aperture Z-scan method with femtosecond laser pulses at 400 nm [29]. As shown in **Figures 15** and **16**, when laser intensities are relatively weaker, Ag nanoparticles show SA, but when laser intensities are strong, a switch from SA to RSA occurs. 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. The peak of the SPR of the Ag nanoparticles is at about 400 nm near the laser excitation. 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,

**Figure 13.** OA Z-scan curves of the investigated nanostructured Ag suspensions at 532 (a) and 1064 nm (b).

54 Laser Technology and its Applications

two symmetrical humps flanking the valley near the focus, where hump indicates SA and valley RSA. When the input energy increases gradually from 0.6 to 1.5 mJ, the NLA signals switch from SA to RSA. Besides, they have studied the NLO properties of Ag nanowire/silica

**Figure 14.** OA Z-scan curves of the investigated Ag/silica gel glass nanocomposites at 532 (a) and 1064 nm (b).

**Figure 15.** Open-aperture Z-scan curves at relatively low energies (46, 73, 92 nJ) using 1- and 10-Hz pulsed laser.

**Figure 16.** Open-aperture Z-scan curves obtained at energy of 162 nJ using (a) 1-, (b) 10-, (c) 20-, and (d) 40-Hz pulsed laser.

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.

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. (5). Solid lines in **Figures 17** and **18** are the theoretical fitting. It can be found that the theoretical fit is in good agreement with the experimental results, indicating the model used is reasonable. For Pt nanoparticles, the SPR wavelength is at about 215 nm far from 532 nm, which implies that the ground plasma bleaching cannot occur in Pt nanoparticles. 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 fact, the proposal has been proved by Qu and Ganeev et al.

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**Figure 17.** The normalized open-aperture Z-scan curve at lower fluences.

**Figure 18.** The normalized open-aperture Z-scan curve at higher fluences.

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

**4. Conclusions**

#### **3.3. Nonlinear optical studies of Pd and Pt**

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 [46–49], RSA [50] and SA [13, 51–53]. Correspondingly, Pt nanoparticles can be used in optical limiting [50, 53, 54] and mode locking [52, 53].

Even though platinum's SPR at 215 nm [55] lies away from excitation wavelength of 532 nm, as shown in **Figure 17**, we still observed SA at lower fluences, which is usually linked to SPR in gold nanoparticles [13]. Furthermore, as shown in **Figure 18**, we found the changeover from SA to RSA at higher input bump intensities.

**Figure 17.** The normalized open-aperture Z-scan curve at lower fluences.

**Figure 18.** The normalized open-aperture Z-scan curve at higher fluences.

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. (5). Solid lines in **Figures 17** and **18** are the theoretical fitting. It can be found that the theoretical fit is in good agreement with the experimental results, indicating the model used is reasonable. For Pt nanoparticles, the SPR wavelength is at about 215 nm far from 532 nm, which implies that the ground plasma bleaching cannot occur in Pt nanoparticles. 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 fact, the proposal has been proved by Qu and Ganeev et al.

### **4. Conclusions**

**Figure 16.** Open-aperture Z-scan curves obtained at energy of 162 nJ using (a) 1-, (b) 10-, (c) 20-, and (d) 40-Hz pulsed

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

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 [46–49], RSA [50] and SA [13, 51–53]. Correspondingly, Pt

Even though platinum's SPR at 215 nm [55] lies away from excitation wavelength of 532 nm, as shown in **Figure 17**, we still observed SA at lower fluences, which is usually linked to SPR in gold nanoparticles [13]. Furthermore, as shown in **Figure 18**, we found the changeover

nanoparticles can be used in optical limiting [50, 53, 54] and mode locking [52, 53].

laser.

asymmetrical curves.

56 Laser Technology and its Applications

**3.3. Nonlinear optical studies of Pd and Pt**

from SA to RSA at higher input bump intensities.

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.

[8] Francois L, Mostafavi M, Belloni J, Delouis J-F, Delaire J, Feneyrou P. Optical limitation induced by gold clusters. 1. Size effect. The Journal of Physical Chemistry. B. 2000;**104**:

Nonlinear Optical Response of Noble Metal Nanoparticles

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[9] Francois L, Mostafavi M, Belloni J, Delaire JA. Optical limitation induced by gold clusters. 2. Mechanism and efficiency. Physical Chemistry Chemical Physics. 2001;**3**:4965-4971 [10] Gao Y, Song Y, Li Y, Wang Y, Liu H, Zhu D. Large optical limiting of the [60]fullerenesubstituted tripyridine palladium nanoparticles. Applied Physics B. 2003;**76**:761-763 [11] Sheik-Bahae M, Said AA, Van Stryland EW. High-sensitivity, single-beam n2 measure-

[12] Sheik-Bahae M, Said AA, Wei TH, Hagan DJ, Van Stryland EW. Sensitive measurement of optical nonlinearities using a single bean. IEEE Journal of Quantum Electronics. 1990;

[13] Gao Y, Zhang X, Li Y, Liu H, Wang Y, Chang Q, et al. Saturable absorption and reverse saturable absorption in platinum nanoparticles. Optics Communications. 2005;**251**(4-6):

[14] Hodak JH, Henglein A, Hartland GV. Tuning the spectral and temporal response in PtAu core–shell nanoparticles. Journal of Chemical Physics. 2001;**114**(6):2760-2765 [15] Zhang Y-x, Wang Y-h. Nonlinear optical properties of metal nanoparticles: A review.

[16] Elim HI, Yang J, Lee JY, Mi J, Ji W. Observation of saturable and reverse-saturable absorption at longitudinal surface plasmon resonance in gold nanorods. Applied Physics Letters.

[17] Karthikeyan B, Anija M, Philip R. In situ synthesis and nonlinear optical properties of Au: Ag nanocomposite polymer films. Applied Physics Letters. 2006;**88**(5):053104-053107

[18] Wu DJ, Liu XJ, Liu LL, Qian WP. Third-order nonlinear optical properties of gold nano shells in aqueous solution. Applied Physics A: Materials Science & Processing. 2008;**92**(2):

[19] Polavarapu L, Xu QH. A single-step synthesis of gold nanochains using an amino acid as a capping agent and characterization of their optical properties. Nanotechnology. 2008;

[20] Seo JT, Yang QG, Kim WJ, Heo J, Ma SM, Austin J, et al. Optical nonlinearities of Au nano

[21] Lee YH, Yan YL, Polavarapu L, Xu QH. Nonlinear optical switching behavior of Au nanocubes and nano-octahedra investigated by femtosecond Z-scan measurements. Applied

[22] Gurudas U, Brooks E, Bubb DM, Heiroth S, Lippert T, Wokaun A. Saturable and reverse saturable absorption in silver nanodots at 532 nm using picosecond laser pulses. Journal

particles and Au/Ag coreshells. Optics Letters. 2009;**34**(3):307-309

6133-6137

**26**(4):760-769

429-433

279-282

ments. Optics Letters. 1989;**14**(17):955-957

RSC Advances. 2017;**7**:45129-45144

2006;**88**(8):083107-083110

**19**(7):075601-075607

Physics Letters. 2009;**95**(2):023105-023105

of Applied Physics. 2008;**104**(7):073107-073125
