NONLINEAR OPTICS IN NEW NANOMATERIALS HENDRY IZAAC ELIM NATIONAL UNIVERSITY OF SINGAPORE 2005 NONLINEAR OPTICS IN NEW NANOMATERIALS HENDRY IZAAC ELIM (M.Sc Bandung Institute of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2005 ACKNOWLEDGEMENTS I would not have been able to complete this thesis without the support of numerous individuals and institutions. So great is the number in fact that I fear I may fail to recognize all who have contributed to this effort, but in gratitude I attempt to so here. Professor Ji Wei was my academic advisor and was helpful a number of times throughout my academic program. He has a gift for seeing the potential in a person even before they see it in themselves, as was the case with me. His patience, optimism, excitement for the work, and frequent encouragement inspired me to stay true to my objectives despite various adversities. I am grateful to National University of Singapore (NUS) research scholarship program for its role in helping me complete this dissertation after becoming a post graduate student for three and a half years. The services of Femtosecond Laser Spectroscopy Laboratory were essential to the completion of this work. I found the NUS experience unique not only for its academics and research, but also for its ability to attract the very best students in the world. Femtosecond Laser Spectroscopy Laboratory seems particularly gifted in this regard. I am thankful especially to those students with whom I most closely worked for their friendship and selfless contribution to my work. Dr. Ma Guohong taught me all the fine points of pump-probe technique. He Jun shared many hours and countless ideas with me as we grew in knowledge of the Z-scan and optical limiting technique. Weizhe Chen worked with me to produce optical limiting properties of carbon nanocomposites. A.H. Yuwono was a source of frequent i consultations that contributed to the work of TiO2-PMMA nanohybrid materials. Prof. Ji Wei gave generously of his laser knowledge and time to help me achieve the nonlinear optical performance reported in this thesis. To my parents, relatives, and in-laws with whom my family and I could only connect by telephone for the time I’ve spent here at Singapore, I’ve missed you, and I thank you for the tremendous faith you have placed in me. To my son Deniel, I give my eternal love and gratitude. You grew up for the most part in Singapore and went days at a time waiting for me to come home from the lab. You gave many weekends to me to help me realize my dream. Finally, to my wife Susanviani, I thank you for believing in me from the beginning, for believing in our ability to work together as a couple and as a family with a son Deniel toward this goal, for your seemingly endless patience and understanding, and for loving me no matter what the circumstance is. I will love you until the end of time. ii CONTENTS ACKNOWLEDGEMENT i SUMMARY vii LIST OF FIGURES/TABLES x LIST OF PUBLICATIONS xv 1. INTRODUCTION 1.1. Background 1.2. New nanomaterials 1.2.1. Nanohybrid particles of Poly(methyl methacrylate)-TiO2 1.2.2. Carbon nanotubes film 1.2.3. Mono- and multi-functional fullerene (C60) incorporated with polymers 1.2.4. Carbon nanocomposites and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes 1.3. Objectives and scope 1.4. Layout of the Thesis Reference 2. EXPERIMENTAL TECHNIQUES 14 2.1. Z-scan technique 14 2.2. Optical limiting technique 17 2.3. Pump-probe technique 19 Reference 21 iii 3. NONLINEAR OPTICAL PROPERTIES OF POLY(METHYL METHACRYLATE)-TiO2 NANOCOMPOSITES 22 3.1. Introduction 22 3.2. Sample preparation 22 3.3. Characterization and discussion 24 3.4. Conclusion 32 Reference 33 4. ULTRAFAST ABSORPTIVE AND REFRACTIVE NONLINEARITIES IN MULTIWALLED CARBON NANOTUBE FILM 35 4.1. Introduction 35 4.2. Sample preparation 36 4.3. Characterization and discussion 38 4.4. Conclusion 45 Reference 46 5. OPTICAL LIMITING AND Z-SCAN STUDIES OF MONO- AND MULTI-FUNCTIONAL FULLERENE (C60) INCORPORATED WITH POLYMERS 48 5.1. Introduction 48 5.2. Materials 50 5.3. Results and discussion 54 iv 5.3.1. Ultraviolet and visible (UV-vis) absorption spectra 54 5.3.2. Ground state absorption parameters 54 5.3.3. Photoluminescence performance of FCA, Fullerenol, and MPFcontaining polymers versus C60 57 5.3.4. Optical limiting behaviors of FCA, Fullerenol, and MPFcontaining polymers versus C60 60 5.3.5. Polymer concentration dependence of optical nonlinearities in supramolecular fullerene 72 5.3.6. Z-scan studies of mono- and multi-functional fullerene (C60) incorporated with polymers 5.4. Conclusion Reference 73 83 85 6. OPTICAL LIMITING STUDIES OF NEW CARBON NANOCOMPOSITES AND AMORPHOUS SixNy OR AMORPHOUS SiC COATED MULTIWALLED CARBON NANOTUBES 88 6.1. Introduction 88 6.2. Preparation of samples 89 6.2.1. Carbon nanoparticles produced by laser ablation technique 89 6.2.2. Carbon nanocomposites/nanoballs 95 6.2.3. Amorphous SixNy and amorphous SiC coated multiwalled carbon nanotubes 98 v 6.3. Results and discussion 101 6.4. Conclusion 110 Reference 112 7. CONCLUSION 114 vi Summary ____________________________________________________________________ SUMMARY One of the most attractive properties of nanomaterials is their strong nonlinear optical (NLO) behavior. In this study, we report the NLO properties of new nanomaterials such as nanohybrid particles of poly(methyl methacrylate )-TiO2 and multiwalled carbon nanotubes. Another part of the study will concentrate on the optical limiting (OL) properties of fullerene (C60) incorporated with polymers, carbon nanoballs or nanoparticles, and amorphous SixNy or amorphous SiC coated multiwalled carbon nanotubes. A variety of material characterization techniques, such as TEM, SEM, photoluminescence, and UV-visible absorption spectroscopy, have been employed in our study. The NLO properties have been determined with Z-scan and energydependent transmission measurements at different wavelengths. The origins of the observed NLO properties have been revealed by time-resolved pump-probe experiments. Ultrafast optical nonlinearity carried out with 780-nm, 250-fs laser pulses has been observed in a series of thin films containing poly(methyl methacrylate) (PMMA)-TiO2 nanocomposites, which are synthesized by a simple technique of insitu sol-gel/polymerization. The best figures of merit are found in one of the films prepared with a 60% weight percentage of titanium isopropoxide. Transmission electron microscopy (TEM) shows the presence of the 5-nm-diameter particles in the film. The observed optical nonlinearity has a recovery time of ~1.5 ps. These findings suggest the strong potential of PMMA-TiO2 nanocomposites for all-optical switching. By employing femtosecond laser pulses at a wavelength range from 720 to 780 nm, we have observed absorptive and refractive nonlinearities in a film of multivii Summary ____________________________________________________________________ walled carbon nanotubes grown mainly along the direction perpendicular to the surface of quartz substrate. The Z-scans show that both absorptive and refractive nonlinearities are of negative and cubic nature in the laser irradiance range from a few to 300 GW/cm2. The magnitude of the third-order nonlinear susceptibility, χ(3), is of an order of 10-11 esu. The degenerate pump-probe measurement reveals a relaxation time of ~ ps. With nanosecond laser pulses at 532-nm wavelength, we have studied the OL and NLO properties of some new composites which consist of mono-functional 1,2dihydro-1,2-methanofullerene[60]-61-carboxylic acid (FCA), multifunctional fullerenol, and supramolecular C60(HNC4H8NCH3)9 - containing polymers. The OL performance of FCA/poly(styrene-co-4-vinylpyridine) is found to be better than that of its parent C60, while the multi-functional Fullerenol and supramolecular C60(HNC4H8NCH3)9 incorporated with polymers show poorer OL responses. The possible sources for the improvement in the OL are discussed due to triplet-triplet state absorption. By using fluence-dependent transmission measurement with nanosecond laser pulses, we have investigated the OL properties of a series of new carbon nanomaterials as well as amorphous SixNy or amorphous SiC coated carbon nanotubes suspended in distilled water. The observed nonlinearity at 532 nm contributed to OL performance of the carbon nanoparticles, carbon nanocomposites and carbon nanoballs (CNBs) is suggested from optically induced heating or scattering effects. It is found that when the linear transmittance of the carbon nanomaterials is less or equal to 70%, the intensity-dependent transmission of the carbon nanocomposites and carbon nanoballs is comparable to that of C60. While at the 80% linear transmittance, carbon nanocomposites/nanoparticles possess better OL behavior than that of C60. viii Chapter Nonlinear optical properties of Poly(methyl methacrylate)-TiO2 Nanocomposites ___________________________________________________________________________ 3.4. Conclusion In conclusion, large optical nonlinearities in the PMMA-TiO2 nanocomposites have been observed using 780-nm, 250-fs laser pulses, and the nonlinear response time has been found to be about 1.5 ps. The best figures of merit have been found in the film using 60% of TiiP in preparation. The large optical nonlinearities are believed to be caused by two-photonresonant exciton in the nano-sized particles containing TiO2. Finally it should be pointed out that these films of the nanocomposites are fabricated with a simple sol-gel/polymerization method. 32 Chapter Nonlinear optical properties of Poly(methyl methacrylate)-TiO2 Nanocomposites ___________________________________________________________________________ References [3.1] G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan and L.M. Peng, Appl. Phys. Lett. 79, 3702 (2001). [3.2] N. Suzuki, Y. Tomita and T. Kojima, Appl. Phys. Lett. 81, 4121 (2002). [3.3] B. Wang, G.L. Wilkes, J.C. Hedrick, S.C. Liptak and J.E. McGrath, Macromolecules 24, 3449 (1991). [3.4] J. Zhang, S. Luo and L. Gui, J. Mater. Sci. 32, 1469 (1997). [3.5] W.C. Chen, S.J. Lee, L.H. Lee and J.L. Lin, J. Mater. Chem. 9, 2999 (1999). [3.6] J. Zhang, B. Wang, X. Ju, T. Liu and T. Hu, Polymer 42, 3697 (2001). [3.7] L. Lee and W. C. Chen, Chem. Mater. 13, 1137 (2001). [3.8] Q.F. Zhou, Q.Q. Zhang, J.X. Zhang, L.Y. Zhang, and X. Yao, Mater. Lett. 31, 41 (1997). [3.9] S.X. Wang, L.D. Zhang, H. Su, Z.P. Zhang, G.H. Li, G.W. Meng, J. Zhang, Y.W. Wang, J. C. Fan and T. Gao, Phys. Lett. A 281, 59 (2001). [3.10] M. Burgos and M. Langlet, J. of Sol-Gel. Sci. Tech. 16, 267 (1999). [3.11] S. Monticone, R. Tufeu, A.V. Kanaev, E. Scolan and C. Sanchez, Appl. Surf. Sci. 162/163, 565 (2000). [3.12] L. E. Brus, J. Chem. Phys. 79, 5566 (1983), or J Chem. Phys. 80, 4403 (1984), or J. Chem. Phys. 90, 2555 (1986). [3.13] M. Sheik-Bahae, A.A. Said, T. Wei, D.J. Hagan, and E.W. Van Stryland, IEEE J. Quantum Electron 26, 760 (1990). [3.14] Y. Watanabe, M. Ohnishi and T. Tsuchiya, Appl. Phys. Lett. 66, 3421 (1995). 33 Chapter Nonlinear optical properties of Poly(methyl methacrylate)-TiO2 Nanocomposites ___________________________________________________________________________ [3.15] M.E. Schmidt, S.A. Blanton, M.A. Hines and P. Guyot-Sionnest, Phys. Rev. B 53, 12629 (1996). [3.16] For example, M. Sheik-Bahae, D.J. Hagan and E.W. Van Stryland, Phys. Rev. Lett. 65, 96 (1990). [3.17] A. Miller, K. R. Welford and B. Baino, Nonlinear Optical Materials for Applications in Information Technology, edited by Kluwer Academic Publishers, Netherlands, p.301 (1995). 34 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ Chapter ULTRAFAST ABSORPTIVE AND REFRACTIVE NONLINEARITIES IN MULTIWALLED CARBON NANOTUBE FILM 4.1. Introduction The nonlinear optical (NLO) properties of carbon nanotubes have attracted attention because of their potential applications in a wide range of technological devices [4.1-4.11]. In particular, ultrafast NLO responses of single-wall carbon nanotubes (SWNTs) in suspensions and in films have been investigated intensively in recent years [4.12-4.17]. Transient photobleaching has been observed with femtosecond laser pulses at photon energies of 0.8 ~ 1.1 eV (wavelength λ = 1.1 to 1.5 μm), resonant with the lowest inter-band transitions of semiconducting SWNTs. The imaginary part of the thirdorder nonlinear susceptibility, Imχ(3), has been determined to be as large as 10-7 esu [4.16]. Saturable absorption has also been detected at photon energies of ~ 1.47 eV (λ ~ 843 nm), near the second lowest inter-band transitions of semiconducting electronic structure in SWNTs [4.14]. In addition, transient photoinduced absorption has been reported under off-resonant conditions. The resonant saturable absorption is identified as a band-filling mechanism, while the off-resonant photoinduced absorption is attributed to a global redshift of the π-plasmon resonance [4.14]. Note that all the above-mentioned investigations focus on the imaginary part of χ(3) and its picosecond (or subpicosecond) relaxation. It is known that the real part of the third-order nonlinear susceptibility, Reχ(3), is another key parameter if carbon nanotubes 35 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ are to be used as ultrafast switching materials. It is generally accepted that multiwalled carbon nanotubes (MWNTs) behave like metals. We expect that the π-electrons should play an important role in the electronic structures and optical properties of MWNTs. In SWNTs, the π-plasmon resonance is located at ~5 eV and spans the entire visible spectral region with a tail extending to near-infrared wavelengths [4.14]. The π-plasmon is a collective excitation of the π electrons. In a bulk medium, this resonance is not coupled to light. But, in the case of nanoscale objects, it changes into a surface plasmon. In metallic nanoparticles like silver particles, it gives rise to large values of Imχ(3) and Reχ(3) [4.18]. We believe that similar NLO processes should occur in MWNTs. In this chapter, we report the observation of negative absorptive and refractive nonlinearities in a film of MWNTs under off-resonant conditions. These cubic nonlinearities also exhibit ultrafast relaxation. All these findings strongly suggest the potential of MWNT films for alloptical switching. 4.2. Sample preparation The MWNT film grown on quartz substrate was prepared by a method of plasmaenhanced chemical vapor deposition. The details of the preparation were reported elsewhere [4.19,4.20]. Figure 4.1(a) shows a high magnification scanning electron microscopy (SEM) image on the side view of the film and substrate. It shows that the carbon nanotubes, like bushes, are grown mainly in the direction perpendicular to the surface of the substrate, although some of the tubes are entangled with one another. The size distribution of the as-grown sample is displayed in Fig. 4.1(b), showing an average diameter of ~ 40 nm and an average length (or film thickness) of 1.3 μm. 36 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ (a) Counts (%) 50 (b) 40 MWNTs LogNormal Fit 30 20 10 0 30 60 90 Diameter (nm) Fig. 4.1. (a) High magnification SEM image , and (b) diameter distribution with an average diameter of ~ 40 nm of the MWNT film grown on quartz substrate with an average film thickness of 1.3 μm. 37 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ 4.3. Characterization and Discussion The linear and nonlinear optical properties of the MWNT film were examined as the laser light propagates in the axis perpendicular to the quartz substrate (or normal incidence on the film) at room temperature. Figure 4.2 displays the absorption spectrum of the MWNT film at photon energies between and eV. It clearly demonstrates that the optical property of the MWNT film is dominated by an absorption resonance peaked at 5.4 eV. It is assigned as the π-plasmon resonance, which is about 0.4 eV blue shifted in comparison to that of SWNTs measured by Lauret et al. [4.14]. The blue shift is Absorbance (a.u) anticipated since there are more π-electrons in MWNTs than in SWNTs. π-plasmon resonance at λ = ~229 nm MWNTs Photon energy (eV) Figure 4.2 UV-visible absorption spectrum of the MWNT film grown on quartz substrate with an average film thickness of 1.3 μm. 38 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ To minimize average power and reduce accumulative thermal effects, we employed 100-fs laser pulses at kHz repetition rate. The laser pulses were generated by a mode-locked Ti: Sapphire laser (Quantronix, IMRA), which seeded a Ti: Sapphire regenerative amplifier (Quantronix, Titan). The wavelengths were tunable as the laser pulses passed through an optical parametric amplifier (Quantronix, TOPAS). The laser pulses were focused onto the MWNT film with a minimum beam waist of ~ 42 μm. The incident and transmitted laser powers were monitored as the MWNT film was moved (or Z-scanned) along the propagation direction of the laser pulses. Figure 4.3 displays typical open- and closed-aperture Z-scans, showing negative signs for the absorptive and refractive nonlinearities. We assume that the observed absorptive and refractive nonlinearities can be described by Δα = α2 I and Δn = n2 I , respectively; where α2 is the nonlinear absorption coefficient, n2 is the nonlinear refractive index, and I is the light irradiance. Both the α2 and n2 values are extracted from the best fittings between the data and the Z-scan theory [4.21]. We plot the extracted α2 and n2 values as a function of the maximum irradiance for each Z-scan. As shown in Fig. 4.4, it is evident that the α2 and n2 parameters are independent of the irradiance, indicating a pure third-order nonlinear optical process in the irradiance regime up to 300 GW/cm2. No observation of saturation in α2 and n2 clearly demonstrates that the underlying mechanism in MWNTs is different from resonant saturable absorption in SWNTs, in which the saturation in the inter-band transitions of semiconducting electronic structures plays an important role [4.14,4.17]. Within our experimental errors, Fig. 4.3 shows little change in the magnitudes of the measured α2 and n2, as the photon energies are tuned from 1.59 to 1.72 eV (780 to 720 nm), indicating a nonresonant nature of our observed nonlinearities. This is consistent 39 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ with the absorption spectrum in Fig. 4.2, which shows that the probed wavelengths are in the tail of the π-plasmon resonance. 1.20 (a) (b) 720 nm 1.15 I =144 GW/cm 720 nm I =144 GW/cm2 Normalized Transmittance 1.10 1.05 1.00 0.95 0.90 -6 1.20 -4 -2 (c) -6 -2 (d) 780 nm 1.15 -4 780 nm I =101 GW/cm2 I =101 GW/cm2 1.10 1.05 1.00 0.95 0.90 -6 -4 -2 -6 -4 -2 Z (cm) Fig. 4.3. Z-scans of the MWNT film on quartz measured at 720 and 780 nm by using 100-fs laser pulses focused with a minimum waist of 42 μm. The solid lines are the best-fit curves calculated by using the Z-scan theory reported in Ref. [4.21]. The filled and open circles in (a) and (c) are the Z-scans without aperture, while the filled and open circles in (b) and (d) are the Z-scans with aperture. 40 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ α2 (x 10 cm/GW) (a) -1 λ = 780 nm -2 -2 -3 -3 -4 -4 -5 n2 (x 10-4 cm2/GW) -1 50 100 150 200 250 (b) -1 -5 300 -1 λ = 780 nm -2 -2 -3 -3 -4 -4 -5 50 100 150 200 250 -5 300 Irradiance (GW/cm2) Fig. 4.4 Irradiance independence of (a) α2 and (b) n2 values measured at 780 nm. 41 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ At 780 nm, we determine that α2 = -29 cm/GW (or Imχ (3) = - 1.6 x 10-11 esu); and n2 = -3 x 10-4 cm2/GW (or Re χ (3) = -1.7 x 10-11 esu). Hence |χ (3)| = [(Im χ (3))2 + (Re χ (3))2]1/2 = 2.2 x 10-11 esu for the MWNT film. By comparison, a value of 8.3 x 10-13 esu at 800 nm was observed for |χ (3)| in carbon nanoparticles embedded in sol-gel SiO2 glass [4.22]. For a SWNT solution at a concentration of 0.33 mg/mL, it was reported to be x 10-13 esu at 820 nm [4.12]. Our larger value of |χ (3)| is attributed to the high concentration of MWNTs packed in a 1.3 μm-thick layer. In our previously reported closed-aperture Z-scans on solutions of MWNT/polymer composites [4.11], a positive sign of n2 was detected in association with thermally originated nonlinear scattering for optical limiting. The negative sign of the measured n2 (or self-defocusing) implies that thermal effects are insignificant in the MWNT film. In most of semiconductors, ultrafast off-resonant nonlinearities are dominated by two-photon absorption and bound electronic optical Kerr effect (or self-focusing effect). These nonlinear processes result in positive signs for the α2 and n2 nonlinear parameters; and lead to laser-induced damage at high irradiances. On the contrary, the observed nonlinearities in the MWNT film are negative and, therefore, are desirable in optical applications. To evaluate the relaxation time and to gain an insight of the underlying mechanism for the observed cubic nonlinearities, we conducted a degenerate pump-probe experiment at 780 nm with the use of 250-fs laser pulses from a mode-locked Ti:Sapphire laser (Spectro-Physics, Tsunami). Figure 4.5 illustrates the transient transmission signals (ΔT/T) as a function of the delay time for three pump irradiances. With the film excited by the pump pulses, an optically induced transparency occurs. As the pump irradiance increases, the signal increases. All these findings are consistent with our Z-scan 42 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ measurements [4.23]. The transient signals clearly shows there are two components. By using a two-exponential component model, the best fits (solid lines in Fig. 4.5) produce τ1 = ~ 250 fs and τ2 = ~ ps. We believe that τ1 is the autocorrelation of the laser pulses used. The τ2 component is the recovery time of the excited π-electrons in the MWNT film. Its time scale is comparable to the findings for carbon nanoparticles [4.22] and SWNTs [4.12-4.17]. The nature of this relaxation is unclear at present. The relaxation through charge transfer via coherent tunneling processes to neighboring metallic tubes is possible [4.14] and such a relaxation has been observed in C60 fullerene films [4.24]. Further studies far beyond the scope of this investigation are required to unambiguously identify the nature of the relaxation in the MWNT film. 43 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ 28 2.65 GW/cm2 2.35 GW/cm2 1.75 GW/cm2 24 ΔT/T (a.u) 20 16 12 0 Time Delay (ps) Fig. 4.5 Degenerate 250-fs time-resolved pump-probe measurements of the MWNT film preformed at 780 nm with three different pump irradiances. The top and middle curves are shifted vertically for clear presentation. The solid lines are twoexponential fitting curves with τ1 = 250 fs and τ2 = ~ ps. 44 Chapter Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube film ________________________________________________________________________ 4.4. 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New nanomaterials 1. 2 .1. Nanohybrid particles of Poly(methyl methacrylate)-TiO 2 1. 2.2. Carbon nanotubes film 1. 2.3. Mono- and multi-functional. MWNT-a-SiC180 () solution Fig. 6 .11 . Optical limiting responses of MWNTs (●), MWNT-a-SiC80 (□), MWNT- a-SiC180 (), and MWNT-a-SiN180 (Δ) in a constant ~60% linear transmittance at 10 64 nm.