Ultrafast nonlinear optical response of photoconductive ZnO films with fluorine nanoparticles

C. Torres-Torres; J. H. Castro-Chacón; L. Castañeda; R. Rangel Rojo; R. Torres-Martínez; L. Tamayo-Rivera; A. V. Khomenko

doi:10.1364/OE.19.016346

1. Introduction

Beside their tiny size, one of the most amazing and shocking features of the nanostructured media seem to be their strong and exceptional optical behavior. The morphology, size or density of the Nanoparticles (NPs) confined in a media can originate the modulation of effects or the control of physical perturbations that can be particularly enhanced or inhibited in comparison with phenomena in bulk materials. These characteristics have attracted considerable attention to these nanocomposites, and it has eventually generated numerous potential applications in several areas like medicine [1], photonics [2] and plasmonics [3]; but the distinctive possibility of building nanostructures with a huge number of ultrafast applications makes them especially attractive for all-optical telecommunication devices [4]. A lot of research developed all over the world seem to point out prospects to obtain a fast and powerful nonlinear response of optical media with the combination of optical properties of NPs; added to the versatility of preparation methods used for their fabrication and design of their nonlinear optical features [5]. Several possibilities suggest that temperature changes can easily induce significant changes on the absorptive and refractive properties of nanostructured materials [6]. Furthermore, the duration of the excitation of the particles can be linked to physical mechanisms that generate different effects [7]. Specific morphologies of nanostructures can be responsible of unexpected resonant mechanisms of nonlinear optical absorption [8]; and moreover, it can be possible that completely opposite optical effects can be exhibited by the same nanostructured material only with the modification of their size and particle density [9,10].

Evidently, all of these circumstances imply that the mechanisms associated to the optical properties of nanostructured samples can be controlled in order to enhance their nonlinear optical response [11]. More to the point, it has been showed that control of the electrical parameters of photoconductive materials can generate extraordinary modifications of the transmittance of optical waves [12,13]; and therefore there must be noticeable consequences in the resulting optical Kerr effect for intense optical beams.

The optical Kerr effect is one of the most attractive vectorial properties that can be helpful in order to develop ultrafast photonic applications with nanostructures. Important optical nonlinearities related with electrically conductive nanocomposites have been reported [14]; and it is worth noting that different studies show that the disappearance of additional absorption processes can be useful for potential nonlinear nanophotonic applications [15]. On the other hand, an absorptive nonlinearity can result in other applications, like those related with electrical conductivity [16]. In addition, it has been proved that temperature changes can produce considerable changes of the optical and electrical characteristics of special ZnO films [17], and some applications in solar cells for similar materials have been reported [18]. A better understanding of the combination between optical and electrical properties of nanostructures seems to be attractive.

In this work the ultrafast nonlinear optical response of a photoconductive ZnO thin film with embedded fluorine nanoparticles is studied using a vectorial self-diffraction technique with picosecond pulses at 532 nm, and the time-resolved optical Kerr gate technique with 830 nm femtosecond pulses. A purely electronic nonlinear refraction was measured for the femtosecond pulses, and a two-photon photoconductive effect was identified for the picosecond case. The contribution of the photoconductivity to the absorptive response was evaluated for optical excitations with 50ps pulse duration, and we were able to control the manifestation of these different effects by reducing to the femtosecond regime the pulse duration of the incident pulses.

2. Theory

According to the previous analysis made at reference [8], we investigate the relative contributions of nonlinear absorption and refraction to the response of a thin media, with thickness D, where two incident, and their self-diffracted, waves interact. In our particular case, we consider that an important contribution from the photoconductivity can be present in the absorptive nonlinearity associated to the sample. The circular components of the incident electric field $\vec{E}$ can be expressed as

\vec{E} = {\vec{E}}_{+} + {\vec{E}}_{-} .

The two induced gratings, $Ψ_{α}^{}$ and $Ψ_{K}^{}$ , from nonlinear absorption and induced birefringence, respectively, can be written in terms of the incident and self-diffracted waves as

Ψ_{α} = \frac{2 π D β}{λ} {| {\vec{E}}_{1} + {\vec{E}}_{2} + {\vec{E}}_{3} + {\vec{E}}_{4} |}^{2},

Ψ_{K \pm}^{} = \frac{4 π^{2} D}{n_{0} λ} [A {| {\vec{E}}_{1 \pm}^{} + {\vec{E}}_{2 \pm}^{} + {\vec{E}}_{3 \pm}^{} + {\vec{E}}_{4 \pm}^{} |}^{2} + (A + B) {| {\vec{E}}_{1 \mp}^{} + {\vec{E}}_{2 \mp}^{} + {\vec{E}}_{3 \mp}^{} + {\vec{E}}_{4 \mp}^{} |}^{2}],

where β is the nonlinear absorption coefficient, λ is the wavelength, and $A = Re [6 χ_{1122}^{(3)}]$ and $B = Re [6 χ_{1221}^{(3)}]$ are the components of the third-order susceptibility tensor, χ ⁽³⁾, for an isotropic material [19].

For a photoconductive semiconductor sample where a two photon interaction can take place, as a first approximation, there is a clear relation between the photoconductivity and the two-photon absorption (TPA) coefficient. If we consider that the quantum efficiency of the photoconductivity is 100%, and the following condition is satisfied:

\frac{α_{0}}{β} < I < \frac{1}{β ​ z},

where $α_{o}$ and β are the linear and nonlinear absorption coefficients, respectively, z is the propagation length in the nonlinear media, and I is the total optical irradiance illuminating the sample. Then the photocurrent J in a semiconductor sample can be estimated as [20]

J = \frac{e S}{2 h ν} β ​ I_{0}^{2} z,

where S represents the illuminated area, h is the Plank constant, ν is the optical frequency and e is the charge of an electron.

For such an instance, we write the amplitude transmittance function as

\hat{T} (x, z) = Ψ_{\pm} (x, z) \exp (- \frac{α (I) z}{2}),

with

Ψ_{\pm} (x, z) = Ψ_{α} + Ψ_{K \pm} .

In our case, $α (I) = α_{o} + β I$ . It is possible to calculate the electric field of the transmitted and self-diffracted waves by means of the Fourier transform of the product between the amplitude transmittance function $\hat{T} (x)$ and the incident field $\vec{E}$ . The electric fields thus calculated are

E_{1 \pm} (z) = [E_{1 \pm}^{0} J_{0} (Ψ_{\pm}^{(1)}) + (i E_{2 \pm}^{0} - i E_{3 \pm}^{0}) J_{1} (Ψ_{\pm}^{(1)}) - E_{4 \pm}^{0} J_{2} (Ψ_{\pm}^{(1)})] \exp (- i Ψ_{\pm}^{(0)} - \frac{α (I) z}{2}),

E_{2 \pm} (z) = [E_{2 \pm}^{0} J_{0} (Ψ_{\pm}^{(1)}) + (i E_{4 \pm}^{0} - i E_{1 \pm}^{0}) J_{1} (Ψ_{\pm}^{(1)}) - E_{3 \pm}^{0} J_{2} (Ψ_{\pm}^{(1)})] \exp (- i Ψ_{\pm}^{(0)} - \frac{α (I) z}{2}),

E_{3 \pm} (z) = [E_{3 \pm}^{0} J_{0} (Ψ_{\pm}^{(1)}) + i E_{1 \pm}^{0} J_{1} (Ψ_{\pm}^{(1)}) - E_{2 \pm}^{0} J_{2} (Ψ_{\pm}^{(1)}) - i E_{4 \pm}^{0} J_{3} (Ψ_{\pm}^{(1)})] \exp (- i Ψ_{\pm}^{(0)} - \frac{α (I) z}{2}),

E_{4 \pm} (z) = [E_{4 \pm}^{0} J_{0} (Ψ_{\pm}^{(1)}) - i E_{2 \pm}^{0} J_{1} (Ψ_{\pm}^{(1)}) - E_{1 \pm}^{0} J_{2} (Ψ_{\pm}^{(1)}) + i E_{3 \pm}^{0} J_{3} (Ψ_{\pm}^{(1)})] \exp (- i Ψ_{\pm}^{(0)} - \frac{α (I) z}{2}),

where E _1±(z) and E _2±(z) are the complex amplitudes of the circular components of the transmitted waves beams; E _3±(z) and E _{4 ±}(z) are the amplitudes of the self-diffracted waves, while $E_{1 \pm}^{0}$ , $E_{2 \pm}^{0}$ , $E_{3 \pm}^{0}$ and $E_{4 \pm}^{0}$ are the amplitudes of the incident and self-diffracted waves at the surface of the sample. J_m(Ψ_± ⁽¹⁾) stands for the Bessel function of order m and

Ψ_{\pm}^{(0)} = \frac{4 π^{2} z}{n_{0} λ} [(A + \frac{n_{0} β}{2 π}) \sum_{j = 1}^{4} {| E_{j}_{\pm} |}^{2} + (A + B + \frac{n_{0} β}{2 π}) \sum_{j = 1}^{4} {| E_{j}_{\mp} |}^{2}],

Ψ_{\pm}^{(1)} = \frac{4 π^{2} z}{n_{0} λ} [(A + \frac{n_{0} β}{2 π}) \sum_{j = 1}^{3} \sum_{k = 2}^{4} E_{j \pm} E_{k \pm}^{*} + (A + B + \frac{n_{0} β}{2 π}) \sum_{j = 1}^{3} \sum_{k = 2}^{4} E_{j \mp} E_{k \mp}^{*}]

are the nonlinear phase changes.

3. Experiment

3.1. Processing route of the samples

In order to prepare the ZnO:F thin films, 88 g of zinc (II) pentanedionate [Zn(C₅H₇O₂)₂⋅H₂O] (from Alfa) are dissolved in 250 ml of de-ionized water and stirred for ca. 20 minutes until a complete dissolution was obtained. Subsequently, 100 ml of acetic acid [CH₃CO₂H] (from Baker) and methanol [CH₃OH] (from Baker) were added until a water: acetic acid: methanol 25:10:65 proportion was reached. The mixture was stirred for ca. 20 min. The final molarity of zinc pentanedionate in the starting solution was 0.4 M. The addition of an excess of acetic acid stabilizes the pH value of the solution around 4, avoiding then an early precipitation of the starting solution when fluorine is added. Hydrofluoric acid [HF (aq.)] (48%, from Merck) was used as a doping agent and was diluted in de-ionized water at 10% in volume, then added to the starting solution at a fixed [F]/[Zn] atomic ratio of 25 at. %, that is an optimum value for the deposition of conductive ZnO:F. The acidity of the solution was ca. pH = 4.5 soon after the preparation was accomplished. The final solution was maintained at room temperature in a dark/clean environment for 17 days, before the deposition process. A slight increase in the pH (~pH = 5) of the aged solution was observed when the deposition occurred. The films were deposited on soda-lime glass substrates, which were previously ultrasonically cleaned with trichloroethylene, acetone and methyl alcohol, and dried under a nitrogen flow. The substrates were then placed on a fused tin bath, whose temperature is measured just below the substrate using a thin chromel-alumel thermocouple contained in a stainless steel metal jacket. The substrate temperature was varied from 400 to 525°C, in steps of 25°C. In order to deposit the thin film layers, an optimum solution flow rate of 12 ml/min was selected. Nitrogen was used as carrier gas and the corresponding gas flow was set to 10 ml/min. The deposition time was fixed for every deposition temperature, in such a way that the same film thickness was obtained in all experiments. The selected film thickness was in the order of 500-550 nm. The thickness and the roughness of the films were measured using a profilometer (Dektak IIA, resolution: 0.5 nm) located next to the deposition area. The surface morphology of the films was determined by scanning electron microscopy (SEM) using a field emission FEI-XL30 operating at voltages lower than 3 keV. The samples were not coated with any metallic material; instead the films were imaged using low acceleration voltages. In Fig. 1 is illustrated the experimental setup required for the preparation of the thin films.

Fig. 1 Schematic diagram of the experimental setup for the processing route of the samples.

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3.2. Picosecond Nonlinear Optical Response

A vectorial self-diffraction experiment [21] was performed in order to identify the sign and the physical mechanism responsible for the nonlinear response of the sample. Figure 2 shows the scheme of our experimental set up. We used a λ/2 phase retarder to rotate the polarization of one of the beams. Two polarization analyzers, in front of the photodetectors PD1-4, allow the measurement of the orthogonal polarization components of the diffracted beams associated with the sample. Self-diffracted and transmitted optical signals were measured at 532nm with the second harmonic of a Nd:YAG laser system with 50 ps pulse duration. The pulse energy at the output of the laser system was 0.7 mJ with linear polarization. A beam splitter, BM, allows the illumination of the nonlinear media by two beams with an irradiance ratio 1:1. L represents the focusing system lenses, M1-3 are mirrors and the radius of the beam waist at the focus in the sample was measured to be 0.4 mm.

Fig. 2 Setup for the picosecond self-diffraction experiment.

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3.3. Femtosecond Nonlinear Optical Response

A standard configuration for the time resolved Kerr gate technique (OKG) [22] allows us to measure the optical Kerr effect of the samples. We used a Ti:sapphire laser with λ = 830 nm, 80 fs pulses, 3 nJ maximum pulse energy and a repetition rate of 94 MHz. Figure 3 shows the experimental setup for our Kerr gate experiments, where BS is a beam splitter, M1-6 are mirrors. A half wave plate, λ/2, with a polarizer, P, are used for controlling the plane of polarization of the probe beam. L represents the focusing system. Pump and probe beams, with an irradiance relation of 15:1 and their linear polarizations making a 45° angle, are focused on the sample with a spot size of 80 μm. A polarization analyzer with its transmission axis crossed respect to the initial polarization of the probe beam, is placed before the photodetector PD1. The probe beam energy is captured using a lock-in amplifier. By delaying the probe beam with respect to the pump beam, we can observe a change in the transmittance of the system and measure the decay of the induced birefringence in the sample.

Fig. 3 Setup for the femtosecond Optical Kerr Gate experiment.

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3.4. Photoconductivity measurements

Only for the picosecond pulses we were able to stimulate the photoconduction on the sample. Using a digital Fluke multimeter we measured separately the modification of the value of the electrical conductivity exhibited by the sample during the propagation of 532 nm light in two different regimes. For the first a continuous wave laser with 100mW power was used; on the other case, the Nd:YAG laser described in the last section was employed. The incident polarization of the beam was chosen to coincide with the path in measurement. The metallic electrodes used for these experiments were in direct contact with the sample; they were located in the neighbor of the diameter of the incident beam. Before measuring photoconductivity of the samples, we observed a slow dynamic phenomena associated with free carriers when propagating the picosecond optical beam. Our experimental data were acquired until the current in the sample was at equilibrium and its resistance did not change with time.

4. Results

Figure 4 shows the linear absorption spectrum obtained for the thin film sample. One can clearly observe an absorbing edge towards the UV that starts at 370 nm, associated with the substrate absorption. The thin film is then transparent above about 400 nm.

Fig. 4 Linear absorption spectra.

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Figure 5 shows a representative Scanning Electronic Microscopy (SEM) image performed in the resulting sample of ZnO:F. The image shows evidence of the nanostructured morphology of the ZnO thin solid film.

Fig. 5 Typical SEM micrograph for ZnO:F thin film.

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Experimental results for the self-diffraction experiments are shown in Fig. 6 . The self-diffraction efficiency η, represents the ratio between the self-diffracted and transmitted irradiances; φ represents the angle between polarization planes of the incident beams. We consider the Fresnel losses for the beams in each layer and we obtain the nonlinear optical coefficients for the samples from a fit to the data using Eqs. (8)-(11). The ratio between the transmitted and self-diffracted beams allows us to calculate the absorptive and refractive contributions to the nonlinearity. In Fig. 6 the marks represent the experimental data, and the continuous line represents the fit to the data.

Fig. 6 Self-diffraction efficiency exhibited by the samples.

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In order to achieve high peak irradiance values while minimizing the thermal load to the sample, we used fs pulses at 830 nm to modulate the phase of an optical signal. In addition we determine the electronic nonlinearity in this regime, without the concurrent effect of a thermal process. Figure 7 shows the data obtained from the transmittance Kerr gate experiments as a function of probe delay.

Fig. 7 Kerr transmittance versus probe delay in the femtosecond gate experiment

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The results show a nonlinear response that raises and decays within the duration of the pulse, indicating and ultrafast response time. The results for the reference material carbone disulfide, CS₂, are also shown in Fig. 7. The CS₂ is a well-known nonlinear material with third-order nonlinear susceptibility $| χ_{1111}^{(3)} | = 1.9 \times 10^{- 12}$ (esu) [19]. Because the Kerr gate signal arises from |χ⁽³⁾|, in order to resolve a possible contribution to the measured Kerr signal from nonlinear absorption, we conducted standard pump-probe experiments for quantifying the femtosecond nonlinear absorption on the sample. From the results, we did not observe TPA even with the highest energies (3nJ) available from our laser system. Table 1 summarizes the resulting parameters, which have an error bar of approximately ±10%

Table 1. - Optical nonlinearities exhibited by the samples.

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For the experiment associated to the photoconduction, the maximum measured modification on the conductivity of the sample was about 4.7 mho/cm ± 5%. This result was performed with the 55% of transmission of a single picosecond beam with 1.8 mJ of pulse energy absorbed in the sample by a focused beam with 0.4 mm diameter. The photocurrent associated with this value was also calculated using the β coefficient reported in Table 1 and Eq. (5); we estimate that this parameter is about 2 × 10⁸ A/m². This result is in agreement with the photocurrent that can also be estimated with the average of the electric field associated with the incident beams in the self-diffraction experiment, which gives 3.9 × 10⁷ A/m² as a result. A stable measurement of 0.195 mho/cm ±5% for the electrical conductivity of the sample was obtained during the propagation of the 100 mW CW laser at a 532 nm wavelength when the illuminated area was 0.33 cm². This last result is equal with the conductivity of the sample in darkness. We did not detect any important optical absorption for the fs pulses up to a 3nJ pulse energy, and as it was expected, we could not measure any induced photoconductivity for this regime.

5. Discussion

From Fig. 6 it can be clearly observed the null self-diffractive response for orthogonal polarizations of the beams. We assume that this situation ought to be related with the fading of the absorptive grating associated with the intensity fringes in the interference region of the beams. We alternatively confirmed this assertion by the evaluation of a null transmittance signal for the time-resolved optical Kerr gate performed with the ps pulses. When an intense picosecond beam propagates through the sample, a decrease in conductivity of ZnO can be present by a reduction of both the density of free electrons and of the effective conducting thickness of the thin film [23–25]. This mechanism can be very different for certain nanostructured materials in comparison to bulk semiconductors. For nanostructured materials belonging a large surface to volume ratio, such as our ZnO:F films, a strong photoconductivity effect could be expected. However a slow photoconductive dynamic could be still exhibited [26]. It has been reported that after the maximum of the observed charge transfer-process in ZnO compounds, most of the photocurrent can decay within some microseconds [27]. But surprisingly, for ZnO nanowires, the mechanism of photoconductivity can appear in less than 1 ns [28]. Besides, it has been demonstrated that diffusion of electrons can make experimentally feasible to observe modification in the Kerr nonlinearities for some optical media [29]. Far from the band edge in the bandgap of the sample, as it is our case, we assume that the change in the refractive index related with the TPA seems to be much greater than the nonlinear change in refractive index given by the optical Kerr effect. Therefore, the influence of the nonlinear optical absorptive properties eventually eliminates the possibility of generating an induced birefringence. By contrast, Fig. 7 clearly shows that the response time for the nanocomposite was quasi-instantaneous, and because the carrier lifetime is also much shorter than the pulse duration, thus the optical Kerr effect can be observed avoiding the participation of an absorptive nonlinearity in the third order response. Another argument that makes us believe that the photoconductivity in this sample results from a TPA interaction, is that the absence of an absorptive contribution to the nonlinearity with the fs pulses, results in an absence of an induced photocurrent in this nonresonant femtosecond regime. The lack of a dependence of the absorption on the incident intensity of the femtosecond optical signal was corroborated by a pump-probe method, so we can claim that a pure electronic response was exhibited by the sample during the fs excitations. Measurements with a multiwave mixing technique at 830nm wavelength with the 80fs pulses were performed; however the self-diffraction originated was not strong enough to determine the nonlinear optical parameters. As a comparative result we measured the nonlinear optical response of a pure ZnO film and we could not detect any important nonlinear third order signal; nevertheless the photoconductive properties are approximately the same for the ZnO sample with or without the fluorine NPs. So, the strong modification on the nonlinear response of the pure ZnO film was associated to the inclusion of the fluorine NPs, while the photoconductive properties were linked to the ZnO matrix. Since photoconductivity and nonlinear absorption can be avoided by the 830 nm ultra-short pulses, an enhancement in the ultrafast refractive optical nonlinearity was achieved with the femtosecond pulses. We believe that the important photoconductive response induced by the ps pulses and the significant optical Kerr effect at the fs regime, can generate potential applications for development of all-optical switches with photoconductive functions, transparent electrodes for solar cells or photoelectrodes producing an electrical current through an external circuit in combination with a TPA generated by ultrafast optical pulses.

6. Conclusion

A clear modification in the nonlinear optical properties of a ZnO thin film was obtained by the inclusion of fluorine NPs prepared by a chemical spray deposition technique. The photoconductive and nonlinear optical properties of the resulting sample were measured, and completely different behaviors were exhibited by changing the pulse duration, and wavelength of the excitation. For picosecond pulses, with 532 nm and 50 ps pulse duration, a multiwave mixing technique allows us to identify a dominant nonlinear absorption effect that can be accompanied by photoconduction; meanwhile the manifestation of a refractive nonlinearity in this regime was not detected. This nonlinear absorptive effect was associated with a photocurrent generated by the optical irradiation. With a 830 nm femtosecond excitation, an important Kerr signal originated by an induced birefringence into the sample was observed. We estimate that these significant results related with the inhibition and enhancement of different effects on the sample can bring remarkable changes in the optical properties of the propagation of a beam, enabling transient electrical and optical signal modulation.

Acknowledgments

We kindly acknowledge the financial support from IPN through grant SIP20110803; from COFAA-IPN, from UNAM, from CICESE, from ICyT-DF through grant PIUTE10-129; from CONACyT through grant 82708, from the Instituto de Física-BUAP, and from the SEP México through grant PROMEP/103.5/09/4194.

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	Nonlinear optical response at λ = 532 nm, t = 50 ps			Nonlinear optical response at λ = 830 nm, t = 80fs
	β [m/W]	n₂ [m²/W]	$\| χ^{(3)} \|$ [esu]	β [m/W]	n₂ [m²/W]	$\| χ^{(3)} \|$ [esu]
ZnO:F	−6 × 10⁻⁹	-	3.7 × 10⁻¹⁰	-	4.47 × 10⁻¹⁵	6.7 × 10⁻⁹

Ultrafast nonlinear optical response of photoconductive ZnO films with fluorine nanoparticles

Abstract

1. Introduction

2. Theory

3. Experiment

3.1. Processing route of the samples

3.2. Picosecond Nonlinear Optical Response

3.3. Femtosecond Nonlinear Optical Response

3.4. Photoconductivity measurements

4. Results

5. Discussion

6. Conclusion

Acknowledgments

References and links

Cited By

Figures (7)

Tables (1)

Equations (13)

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