474
OPTICS LETTERS / Vol. 39, No. 3 / February 1, 2014
Enhanced optical limiting in polystyrene–ZnO nanotop composite films P. C. Haripadmam,1 Honey John,2 Reji Philip,3 and Pramod Gopinath1,* 1
2
Department of Physics, Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, India Department of Chemistry, Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, India 3
Light and Matter Physics Group, Raman Research Institute, Sadashivanagar, Bangalore 560 080, India *Corresponding author: [email protected] Received September 19, 2013; revised December 4, 2013; accepted December 16, 2013; posted December 18, 2013 (Doc. ID 197982); published January 20, 2014
In this work we investigate the optical limiting property of polystyrene–zinc-oxide (ZnO) nanotop composite films, using an open aperture Z-scan technique. The nanocomposites are prepared for different loading concentrations of ZnO and are fabricated using spin and dip coating techniques. On exposing the films to a pulsed nanosecond laser at 532 nm, the nonlinear absorption (NLA) coefficient is found to be greater for spin-coated films compared to dipcoated films. The measured NLA coefficient is found to be enhanced with an increase in loading concentration of ZnO in the monomer for both spin- and dip-coated films. © 2014 Optical Society of America OCIS codes: (190.4180) Multiphoton processes; (190.4360) Nonlinear optics, devices; (160.4236) Nanomaterials; (160.4330) Nonlinear optical materials; (160.5470) Polymers; (310.1515) Protective coatings. http://dx.doi.org/10.1364/OL.39.000474
Optical limiters have been utilized in a variety of circumstances where a decreasing transmission with increasing excitation is desirable. This is the main reason that optical limiting (OL) materials are being investigated with great enthusiasm. The OL properties of colloids of semiconductors, metals, and carbon nanotubes [1–3] were reported initially and, later, interest shifted to polymer nanocomposite films [4–6] for device fabrication. These optical limiter films are suitable for various laser pulse shaping applications, passive mode locking, pulse smoothening, etc. One of the most important applications of optical limiters, however, is eye and sensor protection in optical systems, such as direct viewing devices (telescopes, gun sights, etc.), focal plane arrays, and night vision systems [7]. Zinc-oxide (ZnO) is a II–VI wide bandgap semiconductor with a direct bandgap of 3.3 eV [8], and it is widely studied for nonlinear optical applications in the form of colloids and films [9,10]. The commonly used techniques for obtaining polycrystalline films of ZnO are pulsed laser deposition and RF magnetron sputtering [11]. Very few reports have appeared on the effect of fabrication techniques on the nonlinear optical properties of ZnO [12,13]. The requirement of a nanocomposite film for device fabrication prompted us to incorporate ZnO in a polystyrene matrix, prepare films using both spin- and dip-coating techniques, and to study their OL properties. ZnO nanoparticles having an average crystallite size of 23 nm are synthesized from zinc acetate dihydrate (Merck) with polyvinyl pyrrolidone (PVP) as a capping agent. The obtained nanoparticles exhibit the structure of a top, as shown in Fig. 1, and are named ZnO nanotops. Detailed synthesis and property evaluation are reported elsewhere [14]. Composites of the ZnO nanotops with polystyrene are prepared by an in situ bulk polymerization technique using benzoyl peroxide as the initiator. The loading concentration of ZnO nanotops in the styrene monomer is varied in steps from 0.1% to 2% by weight. Uniform films are fabricated on a glass substrate using a spin-coating technique with a spin speed of 0146-9592/14/030474-04$15.00/0
1000 rpm and having the number of coatings as three. Keeping the number of coats as three, composite films are also fabricated by a dip-coating technique, with dipping speed of 5000 μm∕s and retrieval speed of 500 μm∕s. The room temperature absorption spectra (Cary 100 Bio UV–Visible spectrophotometer) of the polymer nanocomposite films fabricated for a loading concentration 0.5% of ZnO, using spin-coating and dip-coating techniques, is shown in Fig. 2. The spectra show a slight shift in the absorption edge (368 nm) to the longer wavelength region compared to that of the ZnO colloid (360 nm) [14]. It is clear from the spectra that spin-coated films are better ultraviolet (UV) light absorbers than dip-coated films. The room temperature photoluminescence (PL) spectra (Fluorolog) of ZnO nanotops dispersed in water for an excitation wavelength of 325 nm is shown in Fig. 3(a), whereas Fig. 3(b) represents the PL spectra of the composite films fabricated by both techniques for a ZnO loading concentration of 0.5 wt. %. The spectra show two emission peaks, one in the UV region and the other in
Fig. 1. Transmission electron micrograph of an individual ZnO nanotop. © 2014 Optical Society of America
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS
Fig. 2. UV–Visible absorption spectra of spin- [curve (a)] and dip-coated [curve (b)] films for 0.5% loading of ZnO nanotops.
the visible region. The peak in the UV region corresponds to the typical exciton emission or near-band-edge emission. This emission is attributed to photogenerated electron recombination with holes in the valence band or in traps near the valence band. The second peak is observed as visible emission. This emission is also called deep-level emission and it is related to oxygen vacancies, but the corresponding mechanism is controversial and not yet clear [15]. For the films, there is a shift of about 10 nm in the visible emission peak with respect to the colloid, whereas a very slight shift is observed for the UV emission. Since visible emission is weak, there is a chance of reduction in defect states of the nanoparticles. The defect states in ZnO nanoparticles can be modified by (i) the polymer matrix acting as a surface passivator to fill the defects in the nanoparticles [15] or (ii) the fabrication technique modifying the defect states on the surface, thereby quenching the visible emission. Nonlinear absorption (NLA) and nonlinear refraction are optical nonlinearities with several potential applications. The third-order nonlinear susceptibility is a complex quantity with NLA contributing to the imaginary part and nonlinear refraction to the real part, given by the following expressions: 3 χ 3 χ 3 R iχ I ;
(1)
n20 ε0 c2 β ω
(2)
2 χ 3 R 2n0 ε0 cγ;
(3)
χ 3 I
where χ 3 is the third-order nonlinear susceptibility, χ 3 R is its real part, and χ 3 I is the imaginary part. β represents the third-order NLA coefficient and γ is the nonlinear refraction coefficient [16]. NLA can happen due to either saturable or reverse saturable absorption. Saturable absorbers show an enhanced transmittance with increase in incident intensity of light on the material. Reverse saturable absorbers exhibit a decrease in transmittance with increase in incident intensity, because the excited-state absorption cross section is larger than the ground-state absorption cross section for these materials and they exhibit OL properties.
475
Fig. 3. PL spectra of (a) ZnO nanoparticles dispersed in water and (b) polystyrene–ZnO nanotop composite films fabricated with the spin coating [curve (1)] and dip-coating [curve (2)] techniques, for ZnO concentration of 0.5%. The excitation wavelength is 325 nm.
In order to study the OL behavior of polystyrene–ZnO nanotop composite films, we performed open aperture Z-scan [17] measurements using a Q-switched Nd:YAG laser (7 ns, 532 nm, single shot), as shown in Fig. 4. The beam waist ω0 is calculated to be 26 μm and the Rayleigh length z0 is evaluated by using the equation [13] z0 πω20 ∕λ, which is found to be 3.9 mm, greater than the thickness of the sample, which is an essential requirement for the validity of Z-scan experiments. The data obtained from the open aperture experiment was analyzed using the procedure described by Bahae et al. [17]. The numerical plot for a third-order NLA process is found to fit well with the experimental data, according to the equation
1 Tz; S 1 1∕2 π q0 z; 0
Z
× exp−τ2 dτ;
∞
−∞
ln1 q0 z; 0 (4)
where q0 z; 0 βI 0 Leff , with β referring to the NLA coefficient and I 0 to the incident irradiance at the focus (z 0). The effective sample thickness, Leff , is given by
Fig. 4. Normalized Z-scan transmittance of spin-coated and dip-coated films for maximum loading concentration (2 wt. %) of ZnO in monomer, for an input fluence of 150 μJ∕cm2 .
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OPTICS LETTERS / Vol. 39, No. 3 / February 1, 2014
Fig. 5. Variation of with loading concentration of ZnO in the monomer for spin-coated [curve (a)] and dip-coated films [curve (b)].
Leff
1 − exp−αL ; α
(5)
where L is the film thickness and α is the linear absorption coefficient [16]. The effective third-order NLA coefficient calculated is 1.8 × 10−8 m∕W for the spin-coated film and 0.46 × 10−8 m∕W for the dip-coated film, for the maximum loading concentration of ZnO in the monomer (2 wt. %). The saturation intensity for the spin-coated film is 4.9 × 1012 W∕m2 with a linear transmittance of 59% and that of the dip-coated film is 7.9 × 1012 W∕m2 with a linear transmittance of 73%. This shows that the films behave as better OLs compared to the colloid reported earlier [14]. The NLA coefficient evaluated for various films plotted as a function of concentration for spin- and dip-coated films is shown in Fig. 5. The increase in the ZnO concentration in the monomer increased the NLA coefficient of the films. The enhanced value of the NLA coefficient of the spin-coated films compared to the dip-coated films effectively shows that the fabrication technique significantly affects the nature of the film formation, which in turn affects the nonlinearity. To find the reason for the difference in limiting properties of the films with respect to fabrication technique, a better understanding of the basic mechanisms of the spin- and dip-coating techniques is required. In the spin-coating process, a solution is first deposited on the substrate, and the substrate is then accelerated rapidly to the desired rotation rate. Liquid flows radially, owing to the action of centrifugal force, and the excess is ejected off the edge of the substrate. The film continues to thin slowly until disjoining pressure effects cause the film to reach an equilibrium thickness or until it turns solid-like due to a dramatic rise in viscosity through solvent evaporation. The final thinning of the film is usually due to solvent evaporation if volatile solvents are used [18,19]. In our case the polymer matrix does not get evaporated; instead, it gets solidified due to the rise in viscosity caused by the completion of polymerization of styrene to polystyrene and thereby forms a welldispersed polymer nanocomposite film. On the other hand, in dip coating, the substrate is normally withdrawn vertically from the liquid bath at a particular speed. The moving substrate entrains the liquid in a fluid mechanical
boundary layer that splits into two, above the liquid bath surface, the outer layer returning to the bath [20,21]. In the case of composite films, dip coating does not help to complete the polymerization and, hence, there is no increase in viscosity during the dipping process, leading to excess draining of the composite solution back into the liquid bath. Thus, for the same number of coating layers, a greater amount of composite may get coated on the substrate during spin coating compared to dip coating. To confirm this fact, we conducted thermogravimetric analysis of the composite films. Equal weights of the spin- and dip-coated films were heated from 30°C to 800°C. When the temperature reached 800°C, the polymer matrix was charred off and the residue was ZnO. The residue of the dip-coated film (2.25%) was found to be lower than that of spin-coated film (8.34%), confirming that the number of ZnO particles available for laser exposure will be smaller for dip-coated film, resulting in a lower value of NLA coefficient, compared to spincoated film. This may be due to the fact that the heavy ZnO nanoparticles are being moved toward the bottom of the film and may drop out along with the outer layer of the composite solution due to gravity, whereas gravity has little role in spin-coated films. When the Z-scan of pure polystyrene film was measured, no trace of nonlinearity was observed. Therefore, it is confirmed that the exhibited absorptive nonlinearity arises from only the incorporated ZnO nanotops. To conclude, polystyrene–ZnO nanotop composite films have been fabricated using spin- and dip-coating techniques. The spin-coated films were found to be better OLs compared to the dip-coated films. Results also indicate that, by selecting the proper fabrication technique, efficient OL films can be obtained. The authors thank SAIF, IIT Madras, for HRSEM measurements. They also thank Prof. V. P. Mahadevan Pillai, Department of Optoelectronics, Kerala University, for providing facilities for photoluminescence studies. References 1. N. Venkatram, D. N. Rao, and M. A. Akundi, Opt. Express 13, 867 (2005). 2. R. F. Souza, M. A. Alencar, E. C. da Silva, M. R. Meneghetti, and J. M. Hickmann, Appl. Phys. Lett. 92, 201902 (2008). 3. J. Wang, Y. Chen, and W. J. Blau, J. Mater. Chem. 19, 7425 (2009). 4. R. Sreeja, J. John, P. M. Aneesh, and M. K. Jayaraj, Opt. Commun. 283, 2908 (2010). 5. H. Shen, B. L. Cheng, G. W. Lu, D. Y. Guan, Z. H. Chen, and G. Z. Yang, J. Phys. D 39, 233 (2006). 6. X. Sun, R. Q. Yu, G. Q. Xu, T. S. A. Hor, and W. Jia, Appl. Phys. Lett. 73, 25 (1998). 7. L. W. Tutt and T. F. Boggess, Prog. Quantum Electron. 17, 299 (1993). 8. V. Srikant and D. R. Clarke, J. Appl. Phys. 83, 5447 (1998). 9. M. K. Kavitha, H. John, and P. Gopinath, Mater. Res. Bull. 49, 132 (2014). 10. A. Thankappan, S. Divya, S. Thomas, and V. P. N. Nampoori, Opt. Laser Technol. 52, 37 (2013). 11. A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72, 126501 (2009).
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS 12. L. Irimpan, A. Deepthy, B. Krishnan, L. M. Kukreja, V. P. N. Nampoori, and P. Radhakrishnan, Opt. Commun. 281, 2938 (2008). 13. T. Ning, P. Gao, W. Wang, H. Lu, W. Fu, Y. Zhou, D. Zhang, X. Bai, E. Wang, and G. Yang, Opt. Mater. 31, 931 (2009). 14. P. C. Haripadmam, M. K. Kavitha, H. John, B. Krishnan, and P. Gopinath, Appl. Phys. Lett. 101, 071103 (2012). 15. H. Xiong, J. Mater. Chem. 20, 4251 (2010). 16. R. L. Sutherland, Handbook of Nonlinear Optics (Marcel Dekker, 2003).
477
17. M. S. Bahae, A. A. Said, and E. W. van Stryland, IEEE J. Quantum Electron. 26, 4 (1990). 18. D. B. Hall, P. Underhill, and J. M. Torkelson, Polym. Eng. Sci. 38, 2039 (1998). 19. N. Sahu, B. Parija, and S. Panigrahi, Indian J. Phys. 83, 493 (2009). 20. N. V. Kaneva and C. D. Dushkin, Bulg. Chem. Commun. 43, 259 (2011). 21. C. J. Brinker, G. C. Frye, A. J. Hurd, and C. S. Ashley, Thin Solid Films 201, 97 (1991).
OPTICS LETTERS / Vol. 39, No. 3 / February 1, 2014
Enhanced optical limiting in polystyrene–ZnO nanotop composite films P. C. Haripadmam,1 Honey John,2 Reji Philip,3 and Pramod Gopinath1,* 1
2
Department of Physics, Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, India Department of Chemistry, Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, India 3
Light and Matter Physics Group, Raman Research Institute, Sadashivanagar, Bangalore 560 080, India *Corresponding author: [email protected] Received September 19, 2013; revised December 4, 2013; accepted December 16, 2013; posted December 18, 2013 (Doc. ID 197982); published January 20, 2014
In this work we investigate the optical limiting property of polystyrene–zinc-oxide (ZnO) nanotop composite films, using an open aperture Z-scan technique. The nanocomposites are prepared for different loading concentrations of ZnO and are fabricated using spin and dip coating techniques. On exposing the films to a pulsed nanosecond laser at 532 nm, the nonlinear absorption (NLA) coefficient is found to be greater for spin-coated films compared to dipcoated films. The measured NLA coefficient is found to be enhanced with an increase in loading concentration of ZnO in the monomer for both spin- and dip-coated films. © 2014 Optical Society of America OCIS codes: (190.4180) Multiphoton processes; (190.4360) Nonlinear optics, devices; (160.4236) Nanomaterials; (160.4330) Nonlinear optical materials; (160.5470) Polymers; (310.1515) Protective coatings. http://dx.doi.org/10.1364/OL.39.000474
Optical limiters have been utilized in a variety of circumstances where a decreasing transmission with increasing excitation is desirable. This is the main reason that optical limiting (OL) materials are being investigated with great enthusiasm. The OL properties of colloids of semiconductors, metals, and carbon nanotubes [1–3] were reported initially and, later, interest shifted to polymer nanocomposite films [4–6] for device fabrication. These optical limiter films are suitable for various laser pulse shaping applications, passive mode locking, pulse smoothening, etc. One of the most important applications of optical limiters, however, is eye and sensor protection in optical systems, such as direct viewing devices (telescopes, gun sights, etc.), focal plane arrays, and night vision systems [7]. Zinc-oxide (ZnO) is a II–VI wide bandgap semiconductor with a direct bandgap of 3.3 eV [8], and it is widely studied for nonlinear optical applications in the form of colloids and films [9,10]. The commonly used techniques for obtaining polycrystalline films of ZnO are pulsed laser deposition and RF magnetron sputtering [11]. Very few reports have appeared on the effect of fabrication techniques on the nonlinear optical properties of ZnO [12,13]. The requirement of a nanocomposite film for device fabrication prompted us to incorporate ZnO in a polystyrene matrix, prepare films using both spin- and dip-coating techniques, and to study their OL properties. ZnO nanoparticles having an average crystallite size of 23 nm are synthesized from zinc acetate dihydrate (Merck) with polyvinyl pyrrolidone (PVP) as a capping agent. The obtained nanoparticles exhibit the structure of a top, as shown in Fig. 1, and are named ZnO nanotops. Detailed synthesis and property evaluation are reported elsewhere [14]. Composites of the ZnO nanotops with polystyrene are prepared by an in situ bulk polymerization technique using benzoyl peroxide as the initiator. The loading concentration of ZnO nanotops in the styrene monomer is varied in steps from 0.1% to 2% by weight. Uniform films are fabricated on a glass substrate using a spin-coating technique with a spin speed of 0146-9592/14/030474-04$15.00/0
1000 rpm and having the number of coatings as three. Keeping the number of coats as three, composite films are also fabricated by a dip-coating technique, with dipping speed of 5000 μm∕s and retrieval speed of 500 μm∕s. The room temperature absorption spectra (Cary 100 Bio UV–Visible spectrophotometer) of the polymer nanocomposite films fabricated for a loading concentration 0.5% of ZnO, using spin-coating and dip-coating techniques, is shown in Fig. 2. The spectra show a slight shift in the absorption edge (368 nm) to the longer wavelength region compared to that of the ZnO colloid (360 nm) [14]. It is clear from the spectra that spin-coated films are better ultraviolet (UV) light absorbers than dip-coated films. The room temperature photoluminescence (PL) spectra (Fluorolog) of ZnO nanotops dispersed in water for an excitation wavelength of 325 nm is shown in Fig. 3(a), whereas Fig. 3(b) represents the PL spectra of the composite films fabricated by both techniques for a ZnO loading concentration of 0.5 wt. %. The spectra show two emission peaks, one in the UV region and the other in
Fig. 1. Transmission electron micrograph of an individual ZnO nanotop. © 2014 Optical Society of America
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS
Fig. 2. UV–Visible absorption spectra of spin- [curve (a)] and dip-coated [curve (b)] films for 0.5% loading of ZnO nanotops.
the visible region. The peak in the UV region corresponds to the typical exciton emission or near-band-edge emission. This emission is attributed to photogenerated electron recombination with holes in the valence band or in traps near the valence band. The second peak is observed as visible emission. This emission is also called deep-level emission and it is related to oxygen vacancies, but the corresponding mechanism is controversial and not yet clear [15]. For the films, there is a shift of about 10 nm in the visible emission peak with respect to the colloid, whereas a very slight shift is observed for the UV emission. Since visible emission is weak, there is a chance of reduction in defect states of the nanoparticles. The defect states in ZnO nanoparticles can be modified by (i) the polymer matrix acting as a surface passivator to fill the defects in the nanoparticles [15] or (ii) the fabrication technique modifying the defect states on the surface, thereby quenching the visible emission. Nonlinear absorption (NLA) and nonlinear refraction are optical nonlinearities with several potential applications. The third-order nonlinear susceptibility is a complex quantity with NLA contributing to the imaginary part and nonlinear refraction to the real part, given by the following expressions: 3 χ 3 χ 3 R iχ I ;
(1)
n20 ε0 c2 β ω
(2)
2 χ 3 R 2n0 ε0 cγ;
(3)
χ 3 I
where χ 3 is the third-order nonlinear susceptibility, χ 3 R is its real part, and χ 3 I is the imaginary part. β represents the third-order NLA coefficient and γ is the nonlinear refraction coefficient [16]. NLA can happen due to either saturable or reverse saturable absorption. Saturable absorbers show an enhanced transmittance with increase in incident intensity of light on the material. Reverse saturable absorbers exhibit a decrease in transmittance with increase in incident intensity, because the excited-state absorption cross section is larger than the ground-state absorption cross section for these materials and they exhibit OL properties.
475
Fig. 3. PL spectra of (a) ZnO nanoparticles dispersed in water and (b) polystyrene–ZnO nanotop composite films fabricated with the spin coating [curve (1)] and dip-coating [curve (2)] techniques, for ZnO concentration of 0.5%. The excitation wavelength is 325 nm.
In order to study the OL behavior of polystyrene–ZnO nanotop composite films, we performed open aperture Z-scan [17] measurements using a Q-switched Nd:YAG laser (7 ns, 532 nm, single shot), as shown in Fig. 4. The beam waist ω0 is calculated to be 26 μm and the Rayleigh length z0 is evaluated by using the equation [13] z0 πω20 ∕λ, which is found to be 3.9 mm, greater than the thickness of the sample, which is an essential requirement for the validity of Z-scan experiments. The data obtained from the open aperture experiment was analyzed using the procedure described by Bahae et al. [17]. The numerical plot for a third-order NLA process is found to fit well with the experimental data, according to the equation
1 Tz; S 1 1∕2 π q0 z; 0
Z
× exp−τ2 dτ;
∞
−∞
ln1 q0 z; 0 (4)
where q0 z; 0 βI 0 Leff , with β referring to the NLA coefficient and I 0 to the incident irradiance at the focus (z 0). The effective sample thickness, Leff , is given by
Fig. 4. Normalized Z-scan transmittance of spin-coated and dip-coated films for maximum loading concentration (2 wt. %) of ZnO in monomer, for an input fluence of 150 μJ∕cm2 .
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OPTICS LETTERS / Vol. 39, No. 3 / February 1, 2014
Fig. 5. Variation of with loading concentration of ZnO in the monomer for spin-coated [curve (a)] and dip-coated films [curve (b)].
Leff
1 − exp−αL ; α
(5)
where L is the film thickness and α is the linear absorption coefficient [16]. The effective third-order NLA coefficient calculated is 1.8 × 10−8 m∕W for the spin-coated film and 0.46 × 10−8 m∕W for the dip-coated film, for the maximum loading concentration of ZnO in the monomer (2 wt. %). The saturation intensity for the spin-coated film is 4.9 × 1012 W∕m2 with a linear transmittance of 59% and that of the dip-coated film is 7.9 × 1012 W∕m2 with a linear transmittance of 73%. This shows that the films behave as better OLs compared to the colloid reported earlier [14]. The NLA coefficient evaluated for various films plotted as a function of concentration for spin- and dip-coated films is shown in Fig. 5. The increase in the ZnO concentration in the monomer increased the NLA coefficient of the films. The enhanced value of the NLA coefficient of the spin-coated films compared to the dip-coated films effectively shows that the fabrication technique significantly affects the nature of the film formation, which in turn affects the nonlinearity. To find the reason for the difference in limiting properties of the films with respect to fabrication technique, a better understanding of the basic mechanisms of the spin- and dip-coating techniques is required. In the spin-coating process, a solution is first deposited on the substrate, and the substrate is then accelerated rapidly to the desired rotation rate. Liquid flows radially, owing to the action of centrifugal force, and the excess is ejected off the edge of the substrate. The film continues to thin slowly until disjoining pressure effects cause the film to reach an equilibrium thickness or until it turns solid-like due to a dramatic rise in viscosity through solvent evaporation. The final thinning of the film is usually due to solvent evaporation if volatile solvents are used [18,19]. In our case the polymer matrix does not get evaporated; instead, it gets solidified due to the rise in viscosity caused by the completion of polymerization of styrene to polystyrene and thereby forms a welldispersed polymer nanocomposite film. On the other hand, in dip coating, the substrate is normally withdrawn vertically from the liquid bath at a particular speed. The moving substrate entrains the liquid in a fluid mechanical
boundary layer that splits into two, above the liquid bath surface, the outer layer returning to the bath [20,21]. In the case of composite films, dip coating does not help to complete the polymerization and, hence, there is no increase in viscosity during the dipping process, leading to excess draining of the composite solution back into the liquid bath. Thus, for the same number of coating layers, a greater amount of composite may get coated on the substrate during spin coating compared to dip coating. To confirm this fact, we conducted thermogravimetric analysis of the composite films. Equal weights of the spin- and dip-coated films were heated from 30°C to 800°C. When the temperature reached 800°C, the polymer matrix was charred off and the residue was ZnO. The residue of the dip-coated film (2.25%) was found to be lower than that of spin-coated film (8.34%), confirming that the number of ZnO particles available for laser exposure will be smaller for dip-coated film, resulting in a lower value of NLA coefficient, compared to spincoated film. This may be due to the fact that the heavy ZnO nanoparticles are being moved toward the bottom of the film and may drop out along with the outer layer of the composite solution due to gravity, whereas gravity has little role in spin-coated films. When the Z-scan of pure polystyrene film was measured, no trace of nonlinearity was observed. Therefore, it is confirmed that the exhibited absorptive nonlinearity arises from only the incorporated ZnO nanotops. To conclude, polystyrene–ZnO nanotop composite films have been fabricated using spin- and dip-coating techniques. The spin-coated films were found to be better OLs compared to the dip-coated films. Results also indicate that, by selecting the proper fabrication technique, efficient OL films can be obtained. The authors thank SAIF, IIT Madras, for HRSEM measurements. They also thank Prof. V. P. Mahadevan Pillai, Department of Optoelectronics, Kerala University, for providing facilities for photoluminescence studies. References 1. N. Venkatram, D. N. Rao, and M. A. Akundi, Opt. Express 13, 867 (2005). 2. R. F. Souza, M. A. Alencar, E. C. da Silva, M. R. Meneghetti, and J. M. Hickmann, Appl. Phys. Lett. 92, 201902 (2008). 3. J. Wang, Y. Chen, and W. J. Blau, J. Mater. Chem. 19, 7425 (2009). 4. R. Sreeja, J. John, P. M. Aneesh, and M. K. Jayaraj, Opt. Commun. 283, 2908 (2010). 5. H. Shen, B. L. Cheng, G. W. Lu, D. Y. Guan, Z. H. Chen, and G. Z. Yang, J. Phys. D 39, 233 (2006). 6. X. Sun, R. Q. Yu, G. Q. Xu, T. S. A. Hor, and W. Jia, Appl. Phys. Lett. 73, 25 (1998). 7. L. W. Tutt and T. F. Boggess, Prog. Quantum Electron. 17, 299 (1993). 8. V. Srikant and D. R. Clarke, J. Appl. Phys. 83, 5447 (1998). 9. M. K. Kavitha, H. John, and P. Gopinath, Mater. Res. Bull. 49, 132 (2014). 10. A. Thankappan, S. Divya, S. Thomas, and V. P. N. Nampoori, Opt. Laser Technol. 52, 37 (2013). 11. A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72, 126501 (2009).
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS 12. L. Irimpan, A. Deepthy, B. Krishnan, L. M. Kukreja, V. P. N. Nampoori, and P. Radhakrishnan, Opt. Commun. 281, 2938 (2008). 13. T. Ning, P. Gao, W. Wang, H. Lu, W. Fu, Y. Zhou, D. Zhang, X. Bai, E. Wang, and G. Yang, Opt. Mater. 31, 931 (2009). 14. P. C. Haripadmam, M. K. Kavitha, H. John, B. Krishnan, and P. Gopinath, Appl. Phys. Lett. 101, 071103 (2012). 15. H. Xiong, J. Mater. Chem. 20, 4251 (2010). 16. R. L. Sutherland, Handbook of Nonlinear Optics (Marcel Dekker, 2003).
477
17. M. S. Bahae, A. A. Said, and E. W. van Stryland, IEEE J. Quantum Electron. 26, 4 (1990). 18. D. B. Hall, P. Underhill, and J. M. Torkelson, Polym. Eng. Sci. 38, 2039 (1998). 19. N. Sahu, B. Parija, and S. Panigrahi, Indian J. Phys. 83, 493 (2009). 20. N. V. Kaneva and C. D. Dushkin, Bulg. Chem. Commun. 43, 259 (2011). 21. C. J. Brinker, G. C. Frye, A. J. Hurd, and C. S. Ashley, Thin Solid Films 201, 97 (1991).
