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The optical Kerr effect exhibited by a nickel doped zinc oxide thin solid film was explored with femto- and pico-second pulses using the z-scan method. The samples were prepared by the ultrasonic spray pyrolysis technique. Opposite signs for the value of the nonlinear refractive index were observed in the two experiments. Self-defocusing together with a two-photon absorption process was observed with 120 ps pulses at 1064 nm, while a dominantly self-focusing effect accompanied by saturated absorption was found for 80 fs pulses at 825 nm. Regarding the nanostructured morphology of the resulting film, we attribute the difference in the two ultrafast optical responses to the different physical mechanism responsible of energy transfer generated by multiphoton processes under electronic and thermal effects.
© 2013 Optical Society of America
1. Introduction
One of the most important aims that material nanosciences and nonlinear optics share in common, is the feasibility for manipulating gigantic amounts of information as fast as possible [1]. Since the nonlinear optical properties of nanoparticles depend strongly on size, then the performance of potential applications based on these materials is notably dependent on the processing route followed for their preparation [2]. Zinc oxide (ZnO) materials can be considered an interesting topic of research because they present a wide and direct band gap, large exciton binding energy, and the facility to control their nucleation sites to tailor their optical and electronic characteristics [3]. ZnO thin solid films have been extensively considered as appropriate candidates to implement distinct optical functions [4]; besides, the conductivity in ZnO doped media can be also taken into account as an attractive feature for circuit designing with photoconductive advantages [5]. Given the special characteristics of ZnO nanostructures, numerous electronic [6], biomedical [7] and nanophotonic applications [8] based on them, have been proposed. A good example is nickel-doped ZnO, which has demonstrated the possibility of strongly modifying the structure and morphology of ZnO media, with the assistance derived by the coupling of exciton-resonance to improve plasmonic or local field effects [9,10]. Certainly, self-focusing and self-defocusing phenomena have been previously reported for ZnO thin films [11,12]; however, the change seemed to happen only between on- and off-resonance conditions [13,14]. In this work we study the nonlinear optical response of a nickel-doped ZnO thin solid film using femto-, pico- and nano-second experiments. By employing non-resonant infrared irradiation, self-focusing and self-defocusing effects were detected. Furthermore, a saturable absorption contribution was identified under femto-second experiments, while a strong two-photon absorption (TPA) process was observed for pico- and nano-second interactions. We estimate that the differences observed in the sign of the electronic optical Kerr effect exhibited by the sample in the different time regimes, originates from dissimilar multi-photonic non-resonant optical contributions related to the Ni doping in the ZnO nanostructures.
2. Experiment
2.1 Sample preparation
ZnO:Ni thin solid films were prepared by an ultrasonic spray pyrolysis technique as it has been previously described [15]. The deposition started from zinc (II) acetate dihydrate [Zn(C2H3O2)2•2H2O] (from Alfa, 98%), dissolved in a mixture of acetic acid [CH3CO2H] (from Baker, 98%), and methanol [CH3OH] (from Baker, 98%) (12.5:487.5, volume proportion) at 0.1 M. Nickel(II) acetylacetonate [Ni(C5H7O2)2] (from Alfa, 98%) was dissolved in a mix of deionized water and acetic acid (1:1, volume proportion) at a 0.2 M concentration, and was prepared in order to be used as the doping solution. A [Ni]/[Zn] atomic percent ratio of 5.0 at% was used. The soda-lime glass substrates were cleaned prior to deposition. Then the substrates are placed on a fused tin bath, its temperature was measured just below the substrate using a chromel-alumel thermocouple, which is contained in a stainless steel metal jacket. The substrate temperature (Ts) was varied from 400 to 500 °C in 50 °C steps, within an accuracy of ± 1 °C. Pure N2 (from PRAXAIR, 99.997%) was used as the solution carrier and director gas, with flow rates of 3.5 and 0.5 L/min, respectively. The materials were analyzed by a KLA profilmeter (Tencor model P15 with a resolution of 1.0 nm) on a step formed during deposition.
2.2 Nonlinear optical response
Z-scan studies [16] were carried out using two different laser sources in order to resolve the third order optical nonlinearities. In one set up we employed the second harmonic of a Quanta Nd-YAG laser system with 8.7 GW/cm2, 120 ps at a 1064 nm wavelength, and 1 Hz repetition rate; on the other hand we used a Ti:sapphire laser system with 0.87 GW/cm2, 80 fs pulses at a 825 nm wavelength. For the picosecond experiments, single-pulses focused to a beam waist of 22 μm ± 0.5 μm with a maximum pulse energy of 4 μJ were employed. For the femtosecond experiments, 3 nJ pulses with a beam waist of 47 μm ± 0.5 μm, and a 94 MHz repetition rate were employed. The open and closed aperture configurations in z-scan experiments were separately obtained under the same energy and irradiance conditions for each temporal regime. Considering an instantaneous response and decay times relative to the pulse width of the laser, the peak-on-axis refractive index change at the focus, represented by Δno in this experiment, can be expressed as [16]:
here n2 represents the nonlinear refractive index and I represents the peak irradiance of the Gaussian beam at the focus. For the closed-aperture z-scan technique, it has been shown [16] that for a Gaussian beam with waist radius, wo, travelling in the + z direction, the geometry-independent normalized transmittance, T, as a function of the sample position, z, and the on-axis optical phase shift of the beam ΔΦo, can be written as [16]:with , , and λ the laser wavelength, all in free space. In addition, the peak nonlinear phase change ΔΦ0 is:where Leff is the effective length given by , with L the sample length, and αo the linear absorption coefficient.Alternatively, for the open z-scan technique, the transmittance can be approximated as [16],
with β as the nonlinear absorption coefficient.3. Results
The film thickness was measured using a surface profilometer, resulting in a thickness of approximately 500 nm ± 1.5 nm. Figure 1 shows the representative linear optical absorption spectrum obtained for the ZnO:Ni thin film samples. It is possible to observe an absorbing edge towards the UV that starts below 400 nm. The thin film is notably transparent for the visible wavelengths and also for the near infrared spectrum as it is illustrated in the plot.
Figure 2 shows a typical Scanning Electronic Microscopy (SEM) image performed in the ZnO:Ni sample. The image denotes evidence of the nanostructured morphology of the ZnO:Ni thin solid film. The average grain size of the features observed in the ZnO:Ni thin film is between 100 nm and 200 nm.
Regarding the nonlinear optical measurements, Fig. 3 shows the experimental results for the closed aperture z-scan results for the 830 nm femtosecond pulses. Open-aperture z-scan experiments are presented in Fig. 4. A very weak but noticeable saturated absorption effect is clearly seen for this pulse duration and wavelength.
Fig. 3 Femtosecond closed aperture z-scan results at 825 nm. Marks represent experimental data and solid lines are numerical fits to the data using the theory described in the text.
Fig. 4 Femtosecond open aperture z-scan results at 825 nm. Marks represent experimental data and solid lines are numerical fits to the data using the theory described in the text.
Figures 5 and 6 depict the z-scan results for the case of picosecond 1064 nm pulses.
Fig. 5 Picosecond closed aperture z-scan results at 1064 nm. Marks represent experimental data and solid lines are numerical fits to the data using the theory described in the text.
Fig. 6 Picosecond open aperture z-scan results at 1064 nm. Marks represent experimental data and solid lines are numerical fits to the data using the theory described in the text.
The results plotted in Fig. 3 clearly show the signature of a positive nonlinear refractive index n2 for the fs pulses, i.e. a pre-focal minimum followed by a post-focal maximum. In Fig. 5 for the picosecond pulses, a negative n2 results from a pre-focal maximum followed by a post-focal minimum. Figure 6 on the other hand shows the signature of a clear TPA process, an on-focus reduction in transmittance, while Fig. 4 exhibits a weak saturable absorption behavior displaying an increase in transmittance. Figures 3–6 also show the best fits obtained to the experimental data, from which the nonlinear parameters are extracted. Table 1 summarizes the resulting parameters, which have an error bar of approximately ± 0.3%.
Table 1. Optical Kerr effect exhibited by the samples
In order to further investigate the nonlinear response of the material, we employed nanosecond pulses at 1064 nm. The pulses employed, from a Nd-YAG laser (Continuum Model SL II-10), have 1 ns pulse duration, in the single shot regime. In this case, an input-output experiment was performed to detect the presence of any nonlinear absorption processes. Figure 7 shows the result for the transmitted irradiance as a function of input irradiance, where a strong TPA process is clearly seen.
The fit of the data was performed using the expression for the transmitted irradiance I(L) in the presence of nonlinear optical absorption [16]:
where β represents the nonlinear absorption coefficient; Io is the total irradiance of the incident beam and L is the thickness of the nonlinear optical media. Considering that spectroscopic measurements give α0 = 5 × 102 m−1 at 1064 nm, the best fit for the nonlinear absorptive coefficients for the samples are shown in Table 1. The error bar in the fitting of the experimental data for the nanosecond transmittance experiment is around ± 10%.4. Discussion
The results show different signs for the nonlinear refractive index measured at 825 nm with fs, and that measured at 1064 nm with ps pulses. In both cases we are in the off-resonance regime, so similar results could be expected. A residual absorption originated by the tail of the absorption edge, or by the product of some gap states produced by the nanostructured character of the sample could be expected and deriving in a saturable effect. Moreover, for the fs pulses, an accumulated pulse to pulse thermal effect could in principle contribute to the nonlinear optical response. In this case, the observed positive n2 under the presence of a thermal contribution could be an indication of a positive thermo-optic coefficient at 825 nm as it has been previously reported in ZnO thin solid films [17]. A comparative femtosecond z-scan experiment was performed after decreasing the incident irradiance and the results indicated a slight reduction in the nonlinear optical refraction together to an inhibition of the nonlinear optical absorption effect. We consider that a low thermal contribution could be present under MHz rate repetition rates as it has been previously reported [18]. Based on the so-called two-parabolic band and the Kramer-Kroning relation previously described [16], n2 should be negative if the excitation photon energy is higher than 0.7 bandgap energy of the semiconductor; however, there is evidence that the dispersion of the nonlinear refractive index of nanoestructured semiconductor materials is different to that of the bulk material; for instance, it has been previously reported a strong dependence of the nonlinear optical response on the Stark effect exhibited by nanometer-size semiconductor crystallites [19]. It has been reported that doping process can generate a modification in the energy states of a ZnO sample and even of a red shift of the band gap [17]. This situation could be potentially responsible for a weak nonlinear optical absorption that should be developed as a saturable absorption in our case.
For the ps pulses, pulse to pulse effects are avoided by the low repetition rate (1 Hz) employed. Intra-pulse thermal effects can be ruled out since a similar experiment with longer 233 ps pulses yields the same results. It is well known that an enhancement in the optical properties of doped semiconductive materials can be caused by the change in the morphology and structure of the media, but also by the physical mechanisms related to the shift of optical resonances [20]. Quantum confinement phenomena have been also reported as a strong influence in the resulting nonlinear optical properties for resonant irradiation [21]. However, although an ultrafast optical interaction is capable of generating electronic excitations that present well defined optical resonances, excitonic contributions may be also responsible for the resulting optical nonlinearities [22,23]. We can observe from the results shown in Table 1 that the magnitude in the nonlinear refractive coefficient is higher for femto- than for pico-second pulses. Considering the distinct physical mechanism that originates the third order optical nonlinearities, dissimilar non-resonant conditions seem to be present for the correspondent multiphotonic interactions. Nonlinear Kramer-Kroning relations can be used to predict that an enhancement in the nonlinear optical absorption for a specific wavelength usually leads to a decrease in the nonlinear optical refraction associated to any particular material, in agreement with other experiments in nanocomposites [24]. This could in principle explain that the significant TPA found in our pico- and nano-second results, with a consequent decrease of the nonlinear optical refraction at this wavelength. As a comparative result, we studied the nonlinear optical response of a pure ZnO film, without finding any significant nonlinear third order signal under similar experimental conditions reported in these experiments. From this we conclude that the nanostructured morphology resulting in the sample after nickel doping ought to be responsible for the change in the structural and optical properties of the thin films. We believe that the significant nonlinear optical refraction exhibited by the sample has potential for application in the development of all-optical systems with phase modulation functions generated by ultrafast optical pulses.
6. Conclusion
A clear difference in the nonlinear optical properties of a ZnO:Ni thin film was obtained for femto- and pico-second z-scan experiments. We assumed that the change in the measured self-focusing and self-defocusing effects has to do mainly with the resulting structure and morphology associated to the Ni doping in the ZnO nanostructures prepared by the ultrasonic spray pyrolysis technique. In the nanosecond regime a strong two-photon absorption process was observed.
Acknowledgments
We kindly acknowledge the financial support from Instituto Politécnico Nacional, COFAA-IPN, CICESE, and from CONACyT through grant 102937.
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