Nonlinear optical response of platinum nanostructures and application for water detection in transformer oil

Abstract

We report the fabrication of platinum nanostructures on an ITO substrate (Pt-NSs) and the investigation of their nonlinear optical response (NLO). The platinum structures were synthesized using an electrochemical method. The presence of Pt-NSs was confirmed by energy X-ray diffraction analysis (EDAX) and their optical spectrum (UV-vis) displays a localized surface plasmon band centered at 290 nm. A study of their plasmonic behavior was also carried out on the basis of a simulation of Pt-NSs arrays arranged with a spacing of 30 nm, which mimicked the experimental observations. A simulation has been run using Computer Simulation Technology (CST). The absorptive and refractive contributions to the NLO were studied using the Z-scan technique with pulses of 100 fs using an 800 nm Ti-sapphire femtosecond laser. The experimental values of the NLO that were calculated were an absorption coefficient β of <2.74 cm kW⁻¹ and a refractive index for different laser powers of +1.35 × 10⁻⁸ < n₂ < +4.21 × 10⁻⁸ cm² W⁻¹. Owing to these extrinsic properties possessed by Pt-NSs, we further investigated the employment of Pt-NSs for the detection of water in transformer oil as a promising application in the engineering industry.

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A. Introduction

The nonlinear optical response (NLO) of materials owing to the presence of metallic nanoparticles has been studied for several years and utilized in a wide range of applications. Noble metal nanoparticles such as gold (Au) and silver (Ag) are known to have very interesting optical properties owing to the excitation of the resonant collective oscillations of localized surface plasmon resonance (LSPR). Metal nanoparticles (MNPs), in particular platinum nanoparticles (Pt-NPs), can possess a wide range of properties that make them promising materials for all-optical signal processing devices.^1,2 It has been found that the use of MNPs inside a dielectric matrix has the capability of quantum confinement of an electromagnetic field, where the electrons of the metal oscillate under the influence of an electric field. The integration of quantum confinement together with small particles has provided motivation for the exploitation of SPR in different areas of optics including nonlinear optics,^3–5 metamaterials⁶ and plasmonics.^7,8 Much research has reported the utilization of platinum (Pt) and palladium (Pd), which can be tuned spectrally by means of various shape-controlled syntheses⁹ and exhibit broad localized surface plasmons with higher sensitivity.¹⁰

The chemical and physical properties possessed by platinum (Pt) make it a useful material that can be employed in applications that require high chemical stability, low electrical resistance or a high melting temperature. It also plays a crucial role in catalysis and fuel cell applications owing to its ability to facilitate both oxidation and reduction reactions.¹¹ Pt has been found to be a critical component of fuel cell technology, in which Pt acts as the most effective electrocatalyst for the oxygen reduction reaction (ORR) and the oxidation reactions of fuels (including hydrogen, methanol, ethanol and formic acid).¹²

In this study, we aimed to contribute a better understanding of the nonlinear optical (NLO) response of platinum nanostructures on an indium tin oxide (ITO) substrate and utilized their properties for the detection of the contamination of transformer oil by water. Experimental data were obtained by performing a typical Z-scan technique with measurements using open and closed apertures with laser pulses of 100 fs at a wavelength of 800 nm, and different powers of characterization revealed the values of the nonlinearity parameters. The NLO studies observed that the structures of Pt that related to surface plasmon resonance (SPR) as such provided a contribution to the NLO response. Their nonlinear optical properties originated from the fundamental behavior of light in an optical medium, in which the dielectric polarization P responds fundamentally to the electric field E of light. MNPs usually display different characteristics from those of the corresponding bulk phase because of their finite size or, what is equivalent, the large fraction of atoms located near to or at the particle surface, which can lead to significant changes in structural and chemical properties, which are among the factors that enhance the sensitivity of the sensors. A simulation was carried out using Computer Simulation Technology (CST) with a variation in the size of Pt structures at a spacing of 30 nm for better detection in the UV region. For the detection of water in transformer oil, we present an optical sensor concept that utilizes a sensing layer of platinum as a light-propagating layer¹³ and a method of coupling light into a planar waveguide, which enables a simple manufacturing process.

B. Preparation and characterization of samples

The controllable fabrication of platinum nanostructures was experimentally demonstrated via electrodeposition in a liquid-crystalline phase. An electrodeposition process took place using a buffer solution of Pt(NH₃)₂OH at a pH of 5.8. The electrochemical deposition of Pt-NSs was performed in a one-electrode electrochemical cell by chronoamperometry at −0.6 V in connection with Pt/PtCl reference electrodes. An ITO electrode was used as the working electrode, and a platinum foil with an area of 2 cm² was used as the counter electrode. A solution containing a concentration of 40 mM Pt(NH₃)₂OH was prepared by adding ammonia (1 wt%) to a 50 mM solution of platinum until complete dissolution of the precipitates was achieved. Distilled water in a volume ratio of 1 : 12 (Pt(NH₃)₂OH : water) was subsequently added for the chronoamperometric deposition of Pt-NSs on the ITO electrode for a deposition period of 10 minutes. The final product (Pt-NSs) was washed using distilled water and dried at 50 °C in a conventional oven.

The UV-vis absorption spectrum of a Pt-NSs film is shown in Fig. 1. An absorption peak of Pt-NSs was observed at 291 nm, which is similar to those in several previous reports.^14–16

Fig. 1 UV-vis absorption spectrum of Pt-NSs showing an absorption peak at 291 nm.

FESEM images were recorded to study the morphology of the synthesized Pt-NSs (Fig. 2). A low-magnification image of the electrodeposited Pt-NSs shows the presence of such particles with ‘dome’-like structures with a random distribution at low magnification (Fig. 2(a)). This could be ascribed to a cooperative effect of the liquid-crystalline phase soft template and the self-assembly of Pt. As the deposition process evolved over time, aggregated Pt structures disrupted the liquid-crystalline phase template and formed nanoparticles. One of the Pt-NSs structures shown in Fig. 2 is seen to be a structure of ‘dome’-shaped particles. This structure grew during the deposition time, with a recorded size of approximately 67 nm (Fig. 2(b), insets) per ‘dome’ structure. A detailed view (Fig. 2(b)) reveals that two structures sit closely together, which is a drawback of using chemical methods to control the spacing between structures. Herein, on the basis of these two structures, we assumed that the structures could be as close as 30 nm apart, and this value of the spacing was adopted in simulations to analyze the properties of this structure.

Fig. 2 FESEM images show: (a) the distribution of structures of Pt-NSs on ITO; and (b) ‘dome’-like structures of platinum, with a size of approximately 67 nm per ‘dome’. Two structures are shown to be too close together, with an assumed spacing taken to be 30 nm (insets).

EDX analysis was used to determine the elemental compositions of the final products on the substrate. Platinum, sodium, magnesium, silicon, carbon and oxygen were the elements that could be detected (Fig. 3). Silicon contributed to the spectrum owing to the composition of the substrate and the presence of sodium and magnesium was possibly due to the chemical process. The carbon and oxygen signals were most probably due to capping agents on the surfaces of nanostructures.

Fig. 3 EDX analysis spectrum of composition with a percentage of platinum nanostructures of 34%.

C. Simulation studies using computer simulation technology (CST)

To understand the behaviour of these structures, we employed a simulation using Computer Simulation Technology (CST) based on the finite element method (FEM) to find approximate solutions to boundary value problems for partial differential equations. Herein, Pt-NSs size and arrangement has been set to ensure the accuracy of the simulation measurement. Fig. 4 depicts the dimensions of a single platinum structure with a size of 67 nm and a thickness of 55 nm. Fig. 5 shows the absorption spectra that correspond to different sizes of platinum structures. Herein, the sizes ranged from 25 nm, 50 nm, 67 nm (experimental result), 75 nm, and 100 nm to 500 nm. The absorption spectra show a trend of a red shift to longer wavelengths for larger diameters of the structures owing to the dependence on the aspect ratio of the LSPR.¹⁷ Pt-NSs with a size of 67 nm display two vague peaks below 300 nm, which were recorded at 224 nm and 255 nm. Herein, the peaks at 224 nm and 255 nm conceivably originated from intra-band transitions of conduction band electrons to higher energy states. However, upon an increase in the particle size, an absorbance peak emerged in the visible region up to 500 nm.¹⁸ These two peaks are less prominent in comparison with those in UV-vis absorption spectra predicted for an accurate and standard arrangement of Pt-NSs in a plane. The spacing of the arrangement was set at 30 nm (inset image).

Fig. 4 (a) Dimensions of platinum nanostructures and (b) thickness of a single structure obtained from a CST simulation.

Fig. 5 Absorption spectra of platinum nanostructures simulated by CST comparing the spectra for different sizes ranging from 25 nm to 500 nm with the spectrum for a size of 67 nm obtained experimentally. The inset image shows the arrangement of the particles with the spacing set at 30 nm.

D. Third-order nonlinear optical properties

a Z-Scan experimental setup

In order to measure the NL optical properties of the Pt-NSs samples, the basis of our studies was the Z-scan technique, which studies the refractive and absorptive contributions to nonlinearity, as well as determining their magnitude and sign.¹⁹ The Z-scan is a simple, accurate technique, which is accomplished by shifting the position of a NL sample across the focal point of a focused Gaussian beam, and the far-field transmittance is measured through an aperture (close Z-scan).²⁰ This measurement can be assessed by determining its index of nonlinear refraction (NLR) and two-photon absorption (TPA) coefficient.²¹ These two parameters can be used for characterizing the nonlinear optical properties of various nonlinear optical materials, including the nonlinear refractive index (n₂), as well as the Kerr nonlinearity and nonlinear absorption coefficient (β), via the “closed aperture” (CA) and “open aperture” (OA) methods, respectively. Therefore, no aperture is required (OA) to perform measurements of far-field transmittance.²² As the sample moves along the Z-axis with the Gaussian beam, it experiences modulations of phase and intensity that can be observed via measurements of transmittance relative to the sample position (Z). If all the transmitted light is measured, only TPA will be affected and the Z-scan is characterized as open-aperture. The range of typical values of the normalized transmittance is 0.1 < S < 0.5. TPA and NLR of a nonlinear process are only significant in situations that involve high optical intensities.²³

Herein, the NL parameters were investigated by employing the Z-scan technique using a Ti-sapphire laser (Spectra-Physics Tsunami) at a wavelength of 800 nm, with a regenerative amplifier of 100 fs and a repetition rate of 82 MHz. Experiments were conducted on the detection of nonlinear optical signals from Pt samples using an optimal laser beam profile that consisted of an average power of from 10 to 70 mW, an energy of 0.15 nJ, and a beam waist of 8.8 μm with circular symmetry at low irradiance. The experimental arrangement is shown in Fig. 6. In order to measure the pure nonlinear index of refraction, the closed-aperture (S = 0.35) data were divided by the open-aperture (S = 1) parameter to obtain the Z-scan transmission data. The incident laser power was controlled via a polarizer and a lens, where the NL samples were mounted on a translational stage that was maneuvered along the beam propagation direction (Z-axis). The beam waist at Z = 0 was 8.8 μm. The usual procedure for a Z-scan is to divide CA data by OA data to obtain refractive components. These ‘divided Z-scan’ curves revealed the effect of third-order nonlinear refraction alone, which is expressed by a power series of the nonlinear phase shift at the focus ΔΦ₀(t). The nonlinear refractive index n₂ in the expression was calculated as follows: n(I) = n₀ + n₂I, where n₀ is the linear refractive index and I is the intensity of the incident laser light. The nonlinear absorption coefficient β in the expression α(I) = α₀ + βI is determined by an open-aperture Z-scan, where α₀ is the linear (low-intensity) absorption coefficient and β accounts in a phenomenological way for nonlinear processes such as induced absorption (β > 0) or induced transparency (saturation of linear absorption, β < 0).

Fig. 6 Setup for a simple Z-scan experimental technique in which the transmission through the aperture is measured as a function of the sample position (Z).

In order to obtain useful data using the Z-scan technique, the sample was moved forward or backward along the direction of the femtosecond laser beam through its focal point. Nonlinear optical effects occurred in the regions before and after the focal point where the laser intensity was high. Both open- and closed-aperture Z-scans were conducted to distinguish NLA and NLR effects for each nanoparticle layer. In the closed-aperture Z-scan measurements, a peak followed by a valley was observed in the normalized transmittance results, and this trend indicated a negative nonlinear refractive index (which is also known as self-defocusing). In this case, the linear transmission of the aperture was recorded as 0.35.

b Results and discussion

To determine the nonlinear coefficients, open- and closed-aperture Z-scan measurements of Pt-NPs films with a thickness of 55 nm (a thickness obtained by an electrochemical process) were used in NLO studies. From the percentage of power transmitted through a sample containing particles p and the percentage of power transmitted through the substrate material p₀, the linear absorption α was calculated, as shown in eqn (1):²⁴

In the Z-scan experiments, the sample was maneuvered along the direction of the laser beam on the focus axis Z = 0. By monitoring changes in the transmittance through a small aperture placed at the far-field position (CA), the NL absorption amplitude and phase shift were also able to be determined. The experimental results for Z-scan data with an aperture were divided by values obtained without an aperture in order to obtain the absolute nonlinear refraction. The peak followed by a valley in the normalized transmittance results, which were obtained from the closed-aperture curves, indicated that the sign of the nonlinear refractive index was negative, which is referred to as self-defocusing. The value of the linear transmission of the aperture S was 0.35 herein, and the nonlinear refractive index n₂ (cm² W⁻¹) can be obtained from eqn (2) and (3):

ΔT_(p–v) = 0.406(1 − S)0.27ΔΦ°, for ΔΦ° < π (2)

The nonlinear refractive index n₂ can be calculated from the value of the difference between the normalized peak and valley transmittances ΔT_(p–v). The empirical relationship between the induced phase distortion ΔΦ and ΔT_(p–v) for a third-order nonlinear refractive process in the absence of NLA can also be deduced from the following equation:²⁵

ΔT_(p–v) = −0.406(1 − S)^0.25(ΔΦ°) (3)

where S is the transmittance of the aperture in the absence of the sample and ΔΦ° is the on-axis nonlinear phase shift with the sample at the focus Z = 0, respectively.

The sign of ΔΦ was determined from the positions of the peak and valley relative to the Z direction, where ΔT_(p–v) could be defined as the difference between the normalized peak and valley transmittances T_p − T_v, with ΔΦ° as the on-axis phase shift at the focus.

The linear transmittance of the aperture can also be ascertained from:

where r_a is the radius of the aperture and w_a is the radius of the beam at the aperture.

The sample in the measurements acted as a thin lens with a varying focal length as it moved through the focal plane.²⁶ By moving the sample through the focus and placing an aperture before the detector (a closed-aperture configuration), the value of n₂ was calculated by considering three elements. The first element was I₀, which in this case depended on the beam waist w° (ref. 27) of 8.8 μm. The second element was ΔΦ, which could be calculated from the normalized peak and valley transmittances, as indicated in eqn (1). The third element L_eff refers to the effective thickness, which can be calculated from the linear absorption at low incident power as obtained from eqn (1)–(5). The effective thickness L_eff of the sample was defined as:²⁸

The resulting eqn (7) is shown as follows:

where λ is the wavelength of the incident beam and I₀ is the irradiance of the incident beam with the sample at the focus Z = 0.

The open-aperture transmittance is symmetric in relation to the focal point Z = 0, where the minimum transmittance occurred on the basis of the following equation:

where ΔT(Z) represents the peak value of the open-aperture Z-scan curve. The intensity-dependent absorption was measurable as a change in the transmittance through the sample in a closed-aperture Z-scan, although it could be determined more accurately via an open-aperture Z-scan. The values of β and n₂ depended on the light–matter interaction that occurred when a sufficiently high intensity of light was incident on a sample. Thus, the interaction could change the optical properties of the medium. The present study has provided evidence of the values of β and n₂ of Pt-NSs.

Fig. 7 shows the optical transmittance around the focal point of the scanned samples at different intensities, which indicated nonlinear absorption by Pt-NSs films.²⁹ It displayed an increasing trend that indicated different shifts in the peak and distinct values of ΔT (peak-to-valley distance), which denoted the presence of reverse saturable absorption character in all the films.³⁰ An entire open-aperture graph comprised a peak and valley, which were asymmetrical. To determine the refractive nonlinearity, the focus was on the magnitude of the nonlinear refractive index n₂. For this reason, closed-aperture measurements were performed and the normalized transmittance was recorded, as shown in Fig. 8. Pt-NSs films exhibited a post-focal peak and valley, which directly indicated a negative value of n₂ (negative lens). Z-Scan measurements were also performed at different average powers.

Fig. 7 Data points with fitting lines for open-aperture transmittance of Pt-NSs samples exposed to different laser powers.

Fig. 8 Data points with fitting lines for closed-aperture transmittance of Pt-NSs samples exposed to different laser powers.

Table 1 lists the measurement conditions with the associated values of nonlinear absorption, where n₂ decreases with an increase in laser power from n₂ = 4.21 × 10⁻⁸ cm² W⁻¹ to n₂ = 1.35 × 10⁻⁸ cm² W⁻¹ for an increase in power from 10 mW to 70 mW, respectively. By comparing the measured nonlinear absorption of the Pt-NSs thin films with values of the nonlinear absorption of platinum nanoparticles and platinum ions (Pt-NSs) reported recently,³¹ it can be seen that the values increased significantly even though other factors were negligible.

Table 1 Nonlinear refractive index and nonlinear absorption coefficient of Pt-NPs samples subjected to an incident laser beam

Sample	PI (mW)	(nm)	w° (μm)	I0 (mW cm^−2)	α (cm^−1)	ΔΦ°	n2 (cm^2 W^−1)	β (cm W^−1)
Pt(a)	10	800	15.28	49.87	50 × 10^4	0.30	4.21 × 10^−8	0.00274
Pt(b)	30	800	15.28	149.35	49 × 10^4	0.39	1.77 × 10^−8	0.00105
Pt(c)	40	800	15.28	199.14	45 × 10^4	0.47	1.51 × 10^−8	0.00085
Pt(d)	70	800	15.28	348.50	21 × 10^4	1.20	1.35 × 10^−8	0.00047

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E. Pt-NSs used in detection of water

This section demonstrates the use of hybrid Pt-NSs in a waveguide sensor as we used transformer oil as an analytical medium. A polymer-based waveguide sensor was fabricated on a silicon substrate that was coated with silicon dioxide (SiO₂) as an undercladding layer. The thickness of the SiO₂ layer was 8 μm with a refractive index of 1.440 measured at 1550 nm using a Sairon Technology SPA-4000 prism coupler. The waveguide was then coated with platinum followed by an annealing process at 170 °C for 3 hours. A fiber waveguide butt-coupling technique was used to couple light into the proposed channel, as shown in Fig. 9(a). The channel width was fabricated as 10 μm and the spacing as 2 mm where the light was input, as shown in Fig. 9(b). Throughout the optical measurement process, the polarization of the incident light was set to be TE-polarized using a polarization controller (PC). To detect the sensitivity of the platinum waveguide reacts with amount of water in the transformer oil, an amount of oil of 0.5 ml was dropped onto the sensing region, which consisted of different amounts of water in parts per million (ppm), where the water content in oil ranged from 15 to 21 ppm, as well as immiscible water in oil. Extra care was taken in the handling of the launch fiber to ensure that the linear polarization state of the incident light was not scrambled by minor disturbances on the fiber.

Fig. 9 (a) Experimental setup for water contamination in oil and (b) planar waveguide structure with a channel width of 10 μm and a spacing of 2 mm.

The response time of the sensor chip to different amounts of water was measured using repetitive optical power acquisition with a measurement interval of 10 seconds (Thorlabs PM100 USB). The measurements were repeated a few times to ensure the accuracy of the data. All these sensors can be characterized by two important parameters: their sensitivity and operating (or dynamic) range.

The sensing mechanism basically comprised the propagation of an electromagnetic field inside the waveguide core, which was not completely confined. A fraction of the field, which is known as the evanescent field and was generated by SPR in the metal, escaped the core, penetrated and propagated in the outer layer, which herein was a platinum coating. The guided field (inside the core) and the evanescent field were connected by the condition of continuity of the electromagnetic field at the border between the core and the coating. Thus, the guided field would be influenced by any variation in the evanescent field.³²

Fig. 10 shows the change in power with respect to the water content over the recorded time and indicates that the TE power decreased in proportion to the amount of water in the tested oil. This significantly changed the boundary conditions at the guide–cladding interface, which reduced the confinement of the beam in the core (guide). The trend shows a similar reduction in the power for each water content; for some reason, fewer fluctuations occurred at 21 ppm. To investigate the sensitivity of the developed sensors, we plotted a graph, as shown in Fig. 11, of the changes in power against the water content. A linear regression fit reveals that the relationship between the TE power and the water content in ppm was Y = −0.561x + 4.933 as the TE power decreased for water contents from 15 to 21 ppm. New oil (immiscible) as a reference showed that the initial power was −2.0 dBm. It was observed that a linear trend line can be fitted to the experimental data with a value of the correlation coefficient of r > 0.99. Linear fitting of the experimental data indicated that the power sensitivity of the sensor was −0.57679 dB s⁻¹. The sensitivity of this measurement can be attributed to the fact that the difference between the thermo-optical dependences of the core and claddings changed when the amount of water increased, which caused a linear reduction in power. The experiment was repeated several times to ensure the accuracy of the measurements and reproducibility of the system.

Fig. 10 Change in power with respect to the water content.

Fig. 11 Change in TE power with respect to the water content.

F. Conclusions

In conclusion, we used Pt-NSs to investigate nonlinear optical properties at different laser intensities by the Z-scan method using an 800 nm femtosecond laser. Third-order nonlinear optical susceptibilities were calculated using the measured values of n₂ and β, which indicated a significant increase in the third-order nonlinear response. Pt films that were composed of randomly distributed Pt-NSs induced a broad absorption and predominantly contributed to the high nonlinear absorption. Therefore, this indicated that samples with high-intensity platinum nanoparticles would provide a lower nonlinear response in comparison to samples with low-intensity nanoparticles. As such, high nonlinearity and low switching energies are both more advantageous for sensor applications. The fabricated sensors displayed excellent sensitivity to dissolved water in oil, which was much better than those reported in several previous reports carried out in this area. The increase in transmitted power at a low water content was due to a change in the dielectric properties of the Pt-NSs film, where the conductivity possessed by Pt was reduced in the presence of water molecules. This research has succeeded in providing a simple and fast method for determining the nonlinearity of nanostructures, which has resulted in Pt-NSs films becoming a potential choice as a nonlinear optical material in sensor applications.

Acknowledgements

This research was funded by the following grant awarded by the University of Malaya: RU001-2014.

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