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Journal of Nanoscience and Nanotechnology Vol. 6, 1990–1994, 2006

Nonlinear Optical Absorption and Switching Properties of Gold Nanoparticle Doped SiO2–TiO2 Sol–Gel Films N. Venkatram1 , R. Sai Santosh Kumar1 , D. Narayana Rao1
∗ , S. K. Medda2 , Sucheta De2 , and Goutam De2 RESEARCH ARTICLE

1

2

School of Physics, University of Hyderabad, Hyderabad 500046, India Sol-Gel Division, Central Glass and Ceramic Research Institute, Jadavpur, Kolkata 700032, India

The nonlinear optical absorption and switching properties of sol–gel derived of Au nanoparticle Delivered by Ingenta to:plasmon absorption positions are different Au-surface doped SiO2 –TiO2 sol–gel films having Tohoku University reported in this paper. The Au nanoparticles are embedded in SiO2 and SiO2 –TiO2 mixed glassy IP : 130.34.69.82 film matrices with different refractive index values. To study the nonlinear absorption properties, Wed,are 05used. Jul 2006 04:34:36 lasers with three different wavelengths The optical switching behavior is studied by using the pump-probe technique with 532 nm as the excitation wavelength. Ground state conduction band, surface plasmon band, and the free carrier band are taken as three level model to explain theoretically the obtained RSA and SA behaviors.

Keywords: Saturable Absorption, Reverse Saturable Absorption, Optical Switching, Surface Plasmon Resonance.

1. INTRODUCTION Metal nanoparticles exhibiting wide variety of nonlinear optical (NLO) properties have attracted attention as potential nonlinear optical devices because of their high polarizability and ultrafast nonlinear response.1 The most conspicuous manifestation of confinement in the optical properties of metal nanoparticles is the appearance of morphological resonance, the surface plasmon resonance (SPR) that strongly enhances their linear and nonlinear responses around specific wavelengths.2 The impact of dielectric confinement on the nonlinear optical response of metal nanoparticle has been extensively studied using different techniques.3–5 Silver,6
7 copper,8 and alloy nanoclusters9 in semicontinuous thin films, in colloids, and in different glass matrices are extensively studied for the NLO and switching applications.10–12 Different metal alloy nanoclusters9
14 are also studied in this aspect. Among the recent challenges are the preparation and characterization of the well-defined nanoscale particles for nonlinear optical evaluations. Different techniques have been developed for promoting the formation of small metal clusters in various matrices. The sol–gel technique of preparing different metal particles in a desired glass matrix is one of them.15 Different metal particles, organic nanocrystals, fullerenes, ∗

Author to whom correspondence should be addressed.

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J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 7

and nanoparticles doped in sol–gel glasses have been investigated16–18 for the NLO properties. A change of refractive index of the matrix in which the Au nanoparticles are doped would shift the Au-surface plasmon absorption positions in the visible wavelength regions in accordance to the Mie theory.19 It would therefore be useful to study the NLO properties originated from the Au-nanoparticles embedded in dielectric matrices having different refractive index values. In this paper we report the nonlinear optical absorption, and switching behaviors of Au nanoparticles doped sol gel films having different Au-surface plasmon absorption band positions in the visible wavelength region. Our study points out the important role that refractive indices of the matrix around the gold nanoparticles plays in tuning the surface plasmon absorption band and in turn inducing good photo switching of the film.

2. EXPERIMENTAL DETAILS Three sets of Au nanoparticles doped in SiO2 –TiO2 matrix with the composition of SiO2 to TiO2 as 1:0; 1:1; 1:2.33, labeled as Au1, Au2, and Au3, respectively, are deposited on ordinary soda-lime glass substrates. These films were prepared following the hybrid sol–gel approach. The detailed synthesis and properties of the films are reported elsewhere.20 Briefly, the Au-doped coatings were deposited 1533-4880/2006/6/1990/005

doi:10.1166/jnn.2006.319

Venkatram et al.

Nonlinear Optical Absorption and Switching Properties of Gold Nanoparticle

J. Nanosci. Nanotechnol. 6, 1990–1994, 2006

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on 1 mm thick, clean soda-lime–silica glass substrates. Prior to the coating deposition, the glass substrates were cleaned with a warm neutral detergent solution, rinsed with distilled water and isopropanol and finally, boiled in isopropanol for 5 min. The coatings were prepared using the dipping technique (Dip-master 200, Chemat Corporation) with withdrawal velocities in the range 1–3 in min−1 . Prepared coatings were first dried at 60  C for 60 min, then at 90  C for a further 60 min. The dried films were then heated (heating rate 75  C h−1 ) to 500  C with a soaking time of 30 min. Undoped coatings in the range 30–50 nm Fig. 1. TEM image of film Au1 (on the left) along with EDS (on the thick were also prepared on silicon wafers for the mearight). surement of matrix refractive index values. The thickness of the heat-treated Au-doped coating was measured by a technique.21 Three different excitation wavelengths are Surfcorder SE-2300 profilometer (Kosaka Laboratory Ltd., employed in the recording and are derived from 6 ns; Japan). The UV-visible spectra of the coatings deposited 10 Hz Nd:YAG (Spectra-Physics, INDI 40) laser: on soda-lime–silica glass substrates were obtained using a (1) second harmonic at 532 nm, Cary 50 scan spectrophotometer. The molar ratios of SiO2 (2) 532 nm pumped rhodamine B dye laser out put at and TiO2 in the final heat-treated films are controlled in 600 nm, to: Delivered by Ingenta order to obtain different refractive indices for the matrix. (3) Antistokes Raman shifted line at 435 nm from H2 gas Tohoku University The molar ratio of the Au metal to the equivalent oxide filled Raman shifter pumped by 532 nm. IP : 130.34.69.82 (SiO2 + TiO2 ) was constant in all the films, being 3% Au– Wed, 05 Jul 2006 In a04:34:36 typical Z-scan set up, a laser beam with a transverse 97% oxide (SiO2 + TiO2 ). All coatings are visually clear gaussian profile is focused onto the sample using a lens. and transparent and showed uniform coloration due to the The sample is then moved along the propagation direction surface plasmon absorption band arising from the embedof the focused beam. At the focal point, the sample experided Au-nanoparticles. The nominal compositions of the ences maximum pump intensity, which gradually decreases Au-doped coatings and their thickness values, colors, Auin either direction from the focus. An f/24 configuration is SPR positions and matrix refractive index values are preused for the studies. The thickness of the sample is chosen sented in Table I. The refractive index (reported at 550 nm) in such a way that it is smaller than the Rayleigh range of the coating matrices were obtained from the correspondof the focused beam, which is nearly 3 mm. Apertures ing undoped films. XRD and TEM studies confirmed the are introduced in the path for beam shaping and calibrated presence of Au nanoparticles of average particle diameter neutral density filters are used to vary the laser intensity. of about 20 nm inside the glassy coating matrices. TEM The data is recorded by scanning the cell across the focus studies of these films with various SiO2 :TiO2 composiand the transmitted beam was focused onto a photodiode tions are reported in Ref. [20]. One representative TEM (FND-100) with a collecting lens. Signal averaging is done image of Au1 along with energy dispersive spectroscope by a boxcar averager (model SR250), the output of which (EDS) image is shown in Figure 1. In the EDS image of is given to a PC with an ADC card. The sample is transAu1, peaks of Si, O, Cu, and Au can be seen. Si, O, and lated using a PC controlled stepper motor and the data is Au are due to “Au in pure SiO2 ” film and Cu is from collected at steps of 0.375 mm. TEM copper grid. The TEM images are taken on a JEOL 2010 microscope. The red-shifting of Au surface plasmon absorption band position from 542 nm (Au1) to 588 nm 3. RESULTS AND DISCUSSION (Au3) is due to the increase of matrix refractive index valUV-Vis spectra as recorded using Cary UV-Vis (model 50 ues from 1.411 (SiO2 :TiO2 = 1:0) to 1.916 (SiO2 :TiO2 = scan) spectrometer in the wavelength range of 370 nm to 1:2.33). 750 nm is shown in the Figure 2. Clearly, the surface plasThe nonlinear absorption properties of the Au nano mon resonance band (SPR) of the samples varies from films are studied using the standard open aperture Z-scan 542 nm to 588 nm depending on the sample. The absorpTable I. Physical properties and absorption maxima of the films tion maxima of each sample correspond to that of the surstudied. face plasmon band of the Au nanoparticles embedded in Matrix compFilm Absorption Refractive glassy SiO2 –TiO2 hybrid films. Sample osition (molar) thickness maxima Color of index at Open aperture Z-scan curves are recorded at three diflabel SiO2 :TiO2 (nm) (nm) the film 550 nm ferent wavelengths: 600 nm, 532 nm, and 435 nm. These Au1 1:0 550 ± 10 542 Pink 1.411 three wavelengths are chosen in such a way that 600 nm Au2 1:1 350 ± 5 572 Violet 1.713 falls at the lower edge of the SPR absorption peak, while Au3 1:2.33 225 ± 5 588 Blue 1.916 532 nm falls on the higher energy side and 435 nm falls

Nonlinear Optical Absorption and Switching Properties of Gold Nanoparticle

Venkatram et al. High lying Continuum of states

0.7 0.6

Absorbance

Free-carrier absorption

Au3

0.5

Au2 Au1 0.3

1st excited-state Conduction band

0.2

Plasmon absorption

0.1

τsp=1.2 ps Ground-state Conduction band

0.0 400

500

600

700

800

900

Wavelength (nm)

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τfc=6 ns TPA

0.4

Fig. 2. UV-Vis absorption spectra of Au nanoparticle doped SiO2 –TiO2 sol–gel films.

Fig. 3. Energy level diagram explaining the processes leading to the observed nonlinear processes in the gold nanoparticles.

And the intensity transmitted through the sample is given by

well above the SPR band. Excitation at 600Delivered nm shows by Ingenta to: Tohoku dI saturable absorption (SA) in all the samples at all inten- University 2 : 130.34.69.82dz = − sp N1 I − fc N2 I − I sities with the sample getting damaged beyond aIP fluence Wed, 05 nm Jul 2006 04:34:36
2 
−t 2 
−2r 2  of ∼4.4 J/cm2 . On the other hand, excitation at 435 0 exp exp I = I00 with low intensities shows reverse saturable absorption

2 z p2

2 z (RSA) behavior and with increasing intensity (fluence >1.1 J/cm2 ), the samples start showing SA in RSA. This and behavior could be due to the following reasons. At lower 
2 1/2 intensities main contribution could be coming from the z  20  z0 =

z = 0 1 + absorption form the ground state and from the surface plasz0  mon state to the free carrier states showing RSA behavior. At higher intensities one could expect two-photon absorpwhere sp is the surface plasmon absorption cross-section, tion (TPA) that leads to direct pumping of the Au atoms to fc is the free carrier absorption cross-section. N0 , N1 , free carrier band. Since the TPA involves the absorption of N2 are populations of ground state conduction band, first two photons simultaneously the absorption is proportional excited state conduction band, and high lying continuum to I 2 , where I is intensity of the laser pulse. We therefore states respectively, sp is the lifetime of the surface plasexpect the TPA line shape to be much narrower than the mon band and is taken as 1.2 ps,22 fc is the lifetime of the excited state absorption (ESA) process. The saturation of free carrier absorption band and is taken as 6 ns,23 z0 is the free carrier band, i.e., the presence of SA in RSA at the Rayleigh range, 0 is the beam waist at focus, I is higher intensities indicates long life times22 fc = 6 ns) intensity as a function of r
t, and z
I00 is peak intensity for these states. at the focus of the gaussian beam, p is the input pulse Excitation at 532 nm resulted in the observation of RSA width used and  is the two photon cross-section. The behavior at low intensities and appearance of saturation differential equations are solved numerically using Rungein the RSA curve above pump fluencies of 0.337 J/cm2 . Kutta fourth order method. The differential equations are A simple three level model as shown in Figure 3 is used first decoupled and then integrated over time, length, and to explain the processes observed and theoretically fit the along the radial direction. Assuming the input beam to be a obtained data. This model uses the three level rate equagaussian, the limits of integration for r
t, and z are varied tions taking into account the surface plasmon absorption, from 0 to , − to , and 0 to L (length of the sample) free carrier absorption, and two-photon absorption. respectively. Typical number of slices used for r
t, and z are 60, 30, and 5 respectively. The theoretical fits of the sp IN0 dN0 I 2 N obtained data resulted in the determination of the values =− − + 1 dt


2
sp of sp , fc , . The values corresponding to the three films under study are summarized in the Table II. sp IN0 fc IN1 N1 N2 dN1 + = − − The open-aperture Z-scan curves of the samples as dt





sp fc obtained when excited with 532 nm are shown in the IN I 2 N dN2 Figure 4. Figure 4(a) shows the behavior of Au1 sample = fc 1 + − 2 dt


2
fc when excited with 532 nm. This film shows SA behavior 1992

J. Nanosci. Nanotechnol. 6, 1990–1994, 2006

Venkatram et al.

Nonlinear Optical Absorption and Switching Properties of Gold Nanoparticle

Table II. Two photon cross-section (), surface plasmon absorption cross section ( sp ), free carrier absorption cross-section ( fc ) of the gold nanoparticles excited with 532 nm nanosecond pulses. Sample

sp cm2

fc cm2

90 × 10−10 35 × 10−7 17 × 10−7

13 × 10−17 70 × 10−17 43 × 10−17

10 × 10−10 09 × 10−14 07 × 10−14

A L3

PD

PUMP M3 F

L2

BS PROBE

M2

532 nm

Au1 Au2 Au3

 cm/W

S

L1

M1

sp

2.5 2.0

(a)

I00 = 0.06 GWcm–2 β = 9 × 10

–10

cmW

–1

1.5 1.0 0.5

2.0

– 0.4

0.0

0.4

0.8

(b)

I00 = 0.18 GWcm–2 β = 3.5 × 10

–7

cmW

–1

1.5 1.0 0.5 – 0.8 2.5 2.0

– 0.4

I00 = 0.18 GWcm

0.0

0.4

0.8

500

(c)

–2

β = 1.7 × 10–7 cmW–1

1.5 1.0 0.5 – 0.8

– 0.4

0.0

0.4

0.8

Z (cm) Fig. 4. Open aperture Z-Scan curves of (a) Au1, (b) Au2, and (c) Au3 showing SA behavior of Au1 film and SA in RSA of Au2 and Au3 films, solid line is the theoretical fit.

J. Nanosci. Nanotechnol. 6, 1990–1994, 2006

probe intensity (arb. units)

Normalized transmittance

– 0.8 2.5

RSA behavior at low intensities. Due to strong TPA coefficients (), and due to saturation of free carrier band due to longer lifetimes of the high lying free carrier absorption (FCA) at high intensities result as SA in RSA. These films are found to be having high damage threshold withstanding up to 1.1 J/cm2 for 532 nm excitation. Since these materials show very good RSA at lower intensities and SA at higher intensities with 532 nm, they are studied for switching behavior. The set-up to check the behavior of optical switching is as shown in the Figure 5. Both the pump and probe have approximately the same pathlength. Probe intensity have been kept constant and the pump beam intensity is varied through neutral density filters. Spot size of the probe beam was kept smaller than the pump. Plots for the transmitted intensity as a function of the pump intensity are shown in Figure 6, which show low

280

Au2

Au3

240

400

200

300

160

200

120 100

80

0

40 10–3

10–2

10–1

100

10–3

10–2

10–1

100

pump fluence (J / cm2) Fig. 6. Switching behavior of Au sol–gel films at 532 nm excitation wavelength.

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at all intensities (fluence >0.337 J cm−2 ). Since the excitation wavelength 532 nm falls within the Au1 surface plasmon resonance (SPR) band (with maximum at 542 nm), Fig. 5. Schematic of optical switching of Au sol–gel films with 532 we observe SA behavior at all intensities as a result of nm wavelength using Nd-YAG Laser. M1, M2, M3—mirror, BS—beam localization of the excitation energy. splitter, L1, L2, L3—lens, S—sample, PD—photo diode, A—aperture, The SA in RSA behavior can be observed clearly from F—neutral density filter. the plots (Fig. 4(b) and Fig. 4(c)) at higher intensities in the case of samples Au2 and Au3. This could be because the higher excited sates leads to SA behavior. The solid of the excitation wavelength falling on the higher energy curves in the figure are the theoretical fits as obtained by side of the surface plasmon peak leads to energy diffuemploying to:the three-level model while the small circles sion to lower energy side. This diffusion to theDelivered lower band by Ingenta are the experimentally obtained data. Au2 and Au3 show Tohoku and subsequent excitation to higher energy free carrier University RSA at low intensities and SA in RSA at higher intensi: 130.34.69.82 bands due to the ns pumping source leads to RSAIPbehavties. As seen in Table II, higher values of surface plasmon 05 Jul 04:34:36 ior. When the intensity increases further, theWed, saturation of 2006 absorption cross-section ( ) for Au2 and Au3, leads to

Nonlinear Optical Absorption and Switching Properties of Gold Nanoparticle

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transmittance at low pump intensities and high transmittance at high pump intensities. The recordings are taken several times at the same spot to show the repeatability of the data and that the switching is not due to the any damage in the film. These results are consistent with the Z-scan data for Au2 and Au3 as these films show RSA at low intensities and SA behavior at higher intensities. As Au1 showed SA behavior at low and high intensities, we did not observe any switching behavior in Au1. Further, a delay between the pump and probe by more than 6 ns leads to a constant probe output indicating the fast response time of these films, limited by the pulse duration of the laser pulses used in the present experiment, and also confirms the free carrier lifetime assumed while the theoretical fits shown in Figure 4 were developed.

Venkatram et al.

References and Notes

1. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin (1995). 2. P. N. Prasad, Nanophotonics, John Wiley, New Jersey, USA (2004). 3. H. S. Nalwa and S. Miyata (eds.), Nonlinear Optics of Organic Molecules and Polymers, CRC Press, Boca Raton, Florida (1997). 4. D. Ricard, in Nonlinear Optical Materials: Principles and Applications, edited by V. Degiorgio and C. Flytzains, IOS Press, Amsterdam (1995), p. 289. 5. C. Voisin, N. Del Fatti, D. Christofilos, and F. Valle, J. Phys. Chem. B 105, 2264 (2001). 6. Q. F. Zhang, W. M. Liu, Z. Q. Xue, J. L. Wu, S. F. Wang, D. L. Wang, and Q. H. Gong, Appl. Phys. Lett. 82, 958 (2003). 7. A. M. Whelan, M. E. Brennan, W. J. Blau, and J. M. Kellya, J. Nanosci. Nanotechnol. 4, 66 (2004). 8. J. Olivares, J. R. Isidro, R. del Coso, R. de Nalda, J. Solis, C. N. Afonso, A. L. Stepanov, D. Hole, P. D. Townsend, and A. Naudon, J. Appl. Phys. 90, 1064 (2001). 9. R. Philip, G. Ravindra Kumar, N. Sandhyarani, and T. Pradeep, Phys. 4. CONCLUSIONS Rev. B 62, 13160 (2000). 10. D. Ricard, Ph. Roussignol, and C. Flytzanis, Opt. Lett. 10, 511 Nonlinear optical absorption of the Au nanoparticle doped by Ingenta (1985).to: Delivered 11. L. Yang, K. Becker, F. M. Smith, R. H. Magruder, III, R. F. different University SiO2 –TiO2 in the sol–gel films was studied at Tohoku Haglund, Jr., L. Yang, R. Dorsinville, R. R. Alfano, and R. A. Zuhr, pump wavelengths that probes around the surface plasmon IP : 130.34.69.82 J. Opt. Soc. Am. B 11, 457 (1994). absorption of Au. Depending on the pump wavelength Wed, 05 and Jul 2006 12. 04:34:36 T. Tokizaki, A. Nakamura, S. Kaneko, K. Uchida, S. Omi, H. Tanji, intensities used, the films exhibit either SA or RSA behavand Y. Asahara, Appl. Phys. Lett. 65, 941 (1994). ior. Change of behavior from RSA to SA nature in two of 13. F. Lin, J. Zhao, T. Luo, M. Jiang, Z. Wu, Y. Xie, Q. Qian, and these films leads to optical switching. Time delay experiH. Zeng, J. Appl. Phys. 74, 2140 (1993). 14. Y. Hamanaka, A. Nakamura, S. Omi, N. Del Fatti, F. Vallee, and ment showed that the response of this switching behavior C. Flytzanis, Appl. Phys. Lett. 75, 1712 (1999). is in nanoseconds. Thus, through this study we could con15. G. De, J. Sol–Gel Sci. Technol. 11, 289 (1998). clude that the refractive indices of the matrix around the 16. M. R. V. Sahyun, S. E. Hill, N. Serpone, R. Danesh, and D. K. gold nanoparticles play an important role in tuning the surSharma, J. Appl. Phys. 79, 8030 (1996). face plasmon absorption band and in turn inducing good 17. K. Uchida, S. Kaneko, S. Omi, C. Hata, H. Tanji, Y. Asahara, A. J. Ikushima, T. Tokizaki, and A. Nakamura, J. Opt. Soc. Am. B 11, photo switching. 1236 (1994). 18. M. Falconieri, G. Salvetti, E. Cattaruzza, F. Gonella, G. Mattei, Acknowledgments: Department of Science and TechP. Mazzoldi, M. Piovesan, G. Battaglin, and R. Polloni, Appl. Phys. nology (DST), Govt. of India and Council of Scientific and Lett. 73, 288 (1998). Industrial Research (CSIR, India) are thankfully acknowl19. G. Mie, Ann. Phys. 25, 377 (1908). 20. S. K. Medda, S. De, and G. De, J. Mater. Chem. 15, 3278 (2005). edged for supporting the projects under NSTI (project # 21. M. Sheik-Bahae, A. A. Said, T. Wei, D. J. Hagan, and E. W. Van SR/S5/NM-13/2002) and CTSM (project # CMM0022) Stryland, IEEE J. Quantum Electron. 26, 760 (1990). programs respectively. The authors thank Dr. H. S. Maiti, 22. P. Hannaford, P. L. Larkins, and R. M. Lowe, J. Phys B: At. Mol. Director, CGCRI for his kind permission to publish this Phys. 14, 2321 (1981). work. R. S. S. Kumar and S. De acknowledge financial 23. Mark J. Feldstein, Chritine D. Keating, Yish-Hann Liau, Michael J. Natan, and Nobert F. Scherer, J. Am. Chem. Soc. 119, 6638 (1997). support from the CSIR, India.

Received: 23 December 2005. Accepted: 2 March 2006.

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J. Nanosci. Nanotechnol. 6, 1990–1994, 2006