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Abstract
Nanostructures with remarkable optical nonlinearity and ultrafast light response have enabled numbers of applications in the fields varying from physics to biochemistry, and even quantum science. Maximally increasing the optical nonlinearity and simultaneously reducing the light response time of nanostructure is regarded as a great challenge for the optics from both fundamental and applied. In this paper, we report unprecedentedly huge third-order nonlinearity and ultrafast property in the nanosystem of Au nanodisks with ∼7 nm thickness. Their thin thickness brings about large electric field enhancements, leading to the third-order nonlinearity susceptibility (χ(3)) reaching the order of 10−18 m2/V2 (∼10−10 esu), which is 1-3 orders of magnitude larger than that of other shapes of Au nanostructures in the same solution environment, such as nanospheres, nanorods and triangle nanoprisms. Furthermore, optical Kerr measurements demonstrate their optical response time is as fast as ∼100 fs. Our findings demonstrate the thin Au nanodisk can be a suitable candidate for ultrafast nonlinear optical devices, such as the all-optical switches and all-optical signal processing devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optics and optical nonlinear technologies have brought about both fundamental and applied advances of great importance in various fields, including physics, [1–6] biochemistry [7–9] and quantum information, [10–13] etc. For example, materials with large third-order nonlinearity have been applied to generate entangled quantum source and construct integrated quantum circuits based on four-wave mixing effect. [10] Meanwhile, based on second-harmonic generation (SHG), second-harmonic imaging microscopy has been a viable microscope imaging technique for realizing the visualization of cell and nanostructured materials that is widely applied in biological and material science. [14–16] In order to further promote the development of nonlinear optics, as well as extend its applications, three directions should be specially addressed. [17] First, to meet the demand of manufacturing highly integrated device and functional optoelectronic chips, fabricating nanoscale nonlinear materials is of notable priority. Second, maximally increase and optimize the optical nonlinearity of materials is also important. Third, since the physical essence of nonlinear optical effect or device is light-matter interaction, reducing the light response time of materials can undoubtedly help to improve device performances. Specifically, since the ultrafast response characteristic is crucial for the application of ultrafast optical signal processing [18,19] and ultrafast all-optical switching, [20,21] the demand for materials with ultrafast optical response is dramatically increasing. Thereby, obtaining a nanostructure with ultrafast optical response together with huge optical nonlinearity can intrinsically facilitate the development of nonlinear optics.
Due to the devotion of tremendous research efforts, it is found that plasmonic nanostructures possess naturally advantages to meet the three requirements mentioned-above. First, numerous plasmonic nanostructures have exhibit their superiority for device miniaturization and integration, so a large variety of plasmonic nanodevices or optoelectronic chips have been demonstrated, including surface-enhanced SHG, [22–24] high-harmonic generation based on plasmonic waveguide, [25,26] and all-optical modulation devices. [27–29] Second, it is proved that the optical response time in plasmonic nanostructures is on the timescale of plasmon decay, which is about several tens of femtoseconds [1,30] and is much faster than most of other nonlinear optical nanomaterials including indium antimonide (InSb), [31] alkaline niobate [32] and borates, [33] etc. Third, the optical nonlinearity favors large local electric field enhancements, which is the instinct advantage of plasmonic nanostructures. Therefore, plasmonic nanostructures with diverse types have been investigated, including nanoparticle, [34,35] nanosphere, [36] nanorod, [37–39] and triangular nanoprism, [40] etc. However, to the best of our knowledge, for these different shapes of metal nanostructures in a solution environment synthesized by chemical methods, their magnitude of third-order nonlinear optical susceptibility (χ(3)) is usually on the order of 10−20∼10−18 m2/V2 (10−13∼10−11 esu) that is not strong enough to generate an applicable nonlinear effect at low power consumption. [17,34] Also, the optical response time of plasmonic nanostructures includes the dephasing time of localized surface plasmons (∼10 fs), [1,30] the decay time of electron-electron scattering (∼100 fs), electron-phonon scattering (∼1 ps) and phonon-phonon scattering (∼100 ps). [34] For most of the plasmonic nanostructures in a solution environment, their optical response time measured by the optical Kerr effect (OKE) technique is usually on the timescale of several hundreds of femtoseconds. Such fast response time of plasmonic nanostructures in the OKE measurements is attributed to the electron transitions contribution (intraband or interband) and the change in dielectric constant due to the excitation of hot electrons.
In this paper, we prepared a high-planeness and thin Au plasmonic nanodisk to generate large optical nonlinearity and ultrafast light response. The sample was synthesized by the chemical method with the advantages of low-cost, easy fabrication and controllable. [41–44] The thin Au nanodisk has a large contact area and a very flat two-dimensional (2D) plane at the interface, which can confine more light at the interface leading to a larger local field than other metal nanostructures. A huge third-order optical nonlinearity with χ(3) on the order of 10−18 m2/V2 (∼10−10 esu) is observed, together with optical response time of ∼100 fs. Our findings demonstrate that the thin Au nanodisk is a promising candidate for application in optical-switch devices and plasmonic optoelectronic devices.
2. Methods and materials
2.1 Sample preparation
The synthesizing processes of thin Au nanodisks can be divided into three steps. First, Au triangular nanoprisms were fabricated following a procedure modified from the reported work. [45] Specifically, the Au seeds were prepared by the reduction reaction in a solution with the presence of HAuCl4 aqueous solution (0.01 M, 1 mL), NaBH4 (0.1 M, 1 mL), and deionized water (36 mL). The mixture solution was aged at a temperature of 25 °C for 4 hours to allow the hydrolysis of unreacted NaBH4. After that three groups of growth solutions (A, B C) were prepared for the seed-mediated growth steps. The solutions A and B were the same containing NaOH (0.1 M, 0.05 mL), ascorbic acid (0.1 M, 0.05 mL), KI (0.1 M, 4.5 µL), CTAB (0.05 M, 9.0 mL) and HAuCl4 (10 mM, 0.25 mL). The solution C contained NaOH (0.1 M,0.25 mL), ascorbic acid (0.1 M, 0.25 mL), KI (0.1 M, 23 µL), CTAB (0.05 M, 45 mL) and HAuCl4 (10 mM, 1.25 mL). The growth of Au triangular nanoprisms began from adding 1 mL of seed solution to the solution A with a gently shaken. The mixture of 1 mL was then added to the solution B following a gently shaken. The whole mixture was finally added to the solution C resulting in the formation of the Au triangular nanoprisms. Second, the prepared Au triangular nanoprisms were purified by using the surface area-based purification. [46] Specifically, NaCl solution (4.0 M, 2.0 mL) was added into the mixture that was left undisturbed for 12 h. Then, the supernatant solution was gently transferred to another cleaned beaker. The Au triangular nanoprisms with larger surface areas were first adhered at the bottom of the beaker. After dropping 10 mL CTAB (50 mM) into the container and repeated pipetting with a straw, the purified Au triangular nanoprisms were achieved. Third, the Au nanodisks can be obtained by using an Au conproportionation reaction to converts purified Au triangular nanoprisms into similarly sized Au nanodisks. [44] Specifically, an appropriate amount of HAuCl4 solution was added into the purified Au triangular nanoprisms solution under vigorous stirring. The mixture solution was then placed in a temperature-controlled water bath at 28°C for 4 hours to result in the formation of Au nanodisks.
2.2 Sample characterization and measurement
The morphology of the sample is characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM graphs were recorded by using a Zeiss Auriga-39-34 operated at 20.0 kV. The TEM graphs were recorded by using a JEOL 2010HT TEM machine operated at 200 kV. The extinction spectra of the samples were measured by using an ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometer (PerkinElmer Lambda950). The nonlinear optical measurements were performed through the Z-scan and OKE techniques. A Ti:sapphire laser (Mira 900 Coherent) with a pulse duration of 130 fs and a repetition rate of 76 MHz was used as the laser source covering from 700 to 1000 nm. During the Z-scan and OKE measurements, a 1 mm thick quartz cuvette (volume ∼0.35 mL) was used to hold the Au nanodisks solution with a concentration of about 24 nmol/mL. For the Z-scan measurements, the laser beam was focused on the sample. And then the transmitted laser beam was divided into two beams that were recorded by two powermeter detectors. The movement of sample was controlled by a linear motor stage. The whole measuring processes were controlled through a LabVIEW program. While for the OKE measurement, the laser beam was divided into two parts with an intensity ratio of 10:1. The polarization angle between the two laser beams was 45°. The strong part was used as the pump light that was passed through a delay line, the other part was used as the probe light. Then the two laser beams were through the chopper and focused on the sample at the same point. The orthogonal optical Kerr signal was detected by a photodiode following by accessing in a lock-in amplifier that was used to improve the signal-to-noise ratio of the measuring system.
2.3 Computational Simulations
The whole measuring processes were also controlled by a LabVIEW program written by ourselves. The computational simulations were performed by using the finite-difference time-domain (FDTD) simulations method (a commercial software) with perfectly matched layers (PML) boundary condition. For the simulations of scattering spectra, the thickness of Au nanodisk is set to be 7 nm, while the diameter of the sample changes from 49 to 126 nm. For the simulations of electric field distribution, the diameter of Au nanodisk is set to be 60 nm, while the thickness of the sample changes from 7 to 30 nm.
3. Results and discussion
Here, we adopted an Au conproportionation reaction to etch a collection of Au triangular nanoprisms into a uniform Au nanodisks product. This method is both low-cost and useful to transform the structure with irregular initial shape into the structure with a single well-defined shape. By using this synthetic method, a high-planeness Au nanodisk can be prepared as shown in Figs. 1(a) and 1(b). The transmission electron microscopy (TEM) and corresponding high-resolution TEM micrographs of Au nanodisks are presented in Figs. 1(a) and 1(b). It is seen that the Au nanodisk is a regular circle with (422) and (220) fringes separated by the space of 0.79 Å and 1.40 Å, respectively. The high resolution TEM result indicates that the prepared Au nanodisk is a good crystallographic structure, making the seldom reported (422) fringe with the space of 0.79 A can be observed. In addition, the thickness of Au nanodisks are about 7 nm as shown in the side view of the samples in Fig. 2d, which are thin comparing with the thickness of other shapes of Au nanostructures. These thin Au nanodisks possess a 2D shape with high aspect ratio, and can only support one in-plane dipolar mode due to higher symmetry. [44] The 2D characteristic of the thin Au nanodisks makes more excitation energy concentrate in a smaller mode volume, which leads to a stronger oscillator strength, a narrower linewidth and a larger local electric field. It is reasonable to forecast that the Au nanodisk may be an excellent nonlinear material due to its 2D shape with thin thickness. As can be seen in Fig. 1(c), we measured the third-order optical nonlinearity and ultrafast optical response characteristics of Au nanodisks by using Z-scan and OKE techniques. The experimental results show that the magnitude of third-order nonlinear susceptibility (χ(3)) for Au nanodisks in a solution environment is on the order of 10−18 m2/V2 and their ultrafast optical response time is on the order of 100 fs. The magnitude of χ(3) for other plasmonic nanostructures in a solution environment is on the order of 10−20∼10−18 m2/V2 from the results of other reported works. [34–40] This result indicates that the magnitude of χ(3) of Au nanodisks is several orders of magnitude larger than that of other Au nanostructures. It is notice that our previous works as reported in the Ref. [36,37] have measured the χ(3) values of Au nanorods and Au triangular nanoprisms that are less than that of the thin Au nanodisks in a solution environment. These works were performed by the same Z-scan and OKE measuring system that indicated the measuring χ(3) values between different shapes of Au nanostructures can be comparable, which further demonstrated the thin Au nanodisks possesses a better third-order optical nonlinearity. Furthermore, the ultrafast optical response time of other plasmonic nanostructures reported by other works that also has been demonstrated by our previous works is generally a few hundreds of femtoseconds, which implies the ultrafast optical response time of thin Au nanodisks is quickened several times than that of others. These results suggest that the thin Au nanodisk is not only a promising nonlinear material for application in optical-switch devices, but also an optoelectronic device material for plasmonic optoelectronic devices.
Fig. 1. A high-planeness and thin Au plasmonic nanodisk to generate large optical nonlinearity of 10−18 m2/V2 and ultrafast optical response as fast as of ∼100 fs. (a) and (b) TEM image and corresponding high-resolution TEM micrograph of Au nanodisks which confirm the high-planeness of Au nanodisks. (c) Schematic diagram of the third-order optical nonlinearity and ultrafast optical response characteristics of Au nanodisks measured by Z-scan and OKE techniques. The magnitude of χ(3) for Au nanodisks is on the order of 10−18 m2/V2 and their ultrafast optical response time is on the order of 100 fs.
Fig. 2. Morphology characterization and absorption spectra of thin au nanodisks. (a) TEM image of thin Au nanodisks that presents their average diameter is about 78 nm. (b) Selected area electron diffraction (SEAD) patterns of thin Au nanodisks. the spot marked by a square can be indexed to the allowed (220) reflection, the spot marked by a triangle can be indexed to the allowed (422) reflection, and the inner spot marked by a circle can be assigned to the formally forbidden (1/3)(422) reflection. (c) The experimental measurements (top) and theoretical simulations (down) of extinction spectra of thin Au nanodisks with various diameter sizes. (d) The SEM side view of the Au nanodisks on a Si substrate.
In order to confirm the uniform feature of the prepared Au nanodisks, their TEM image is presented in Fig. 2(a). The diameter of Au nanodisks is measured to be 78 ± 8 nm and it can be seen that their shapes are relatively regular circles and they also have a good particle homogeneity. To further prove the single crystal properties and the surface smoothness of the Au nanodisks, the selected area electron diffraction (SEAD) pattern of the samples is shown in Fig. 2(b). It is clearly seen that the SEAD pattern exhibits the hexagonal symmetrical diffraction spots, which strongly confirm the single-crystalline structure of Au nanodisks. As shown in Fig. 2(b), there are identified three sets of spots based on d-spacing. Two sets of reflection are allowed by a face-centered cubic (fcc) Au lattice including the (220) reflection and (422) reflection of fcc Au. One with a spacing of 1.40 Å is the (220) reflection of fcc Au, which confirms the prepared Au nanodisks are single-crystalline with (111) lattice planes as a basal plane. The other with a lattice spacing of 0.79 Å corresponds to the (422) Bragg reflection. The another set with a lattice spacing of 2.41 Å can be attributed to the forbidden 1/3(422) reflection, which indicates the atomically flat surfaces of the Au nanodisks that is also observed in other reported works. [47] Fig. 2(c) is the diameter-dependent extinction spectra of Au nanodisks including the results of experimental measurements (top) and theoretical simulations (down). The experimental data are measured by a UV-vis-NIR spectrophotometer, while the theoretical data were performed out by using the FDTD method with commercial software of FDTD Solutions 8.0. It is seen that the extinction peak position of Au nanodisks is redshifted with their diameter increases. From Fig. 2(c), one can notice that the results of theoretical simulation agree well with that of experimental measurements. It should also be noted that the full width at half-maximum (FWHM) of extinction peak is overall relatively narrow, especially when their diameters are reduced to 50 nm, the FWHM of extinction peak between the experimental data and theoretical data is almost the same. These results not only demonstrate the uniformity of Au nanodisks size distribution, but also indicate the high-planeness of the prepared Au nanodisks. The thickness of the Au nanodisks is measured by the SEM side view of the samples that is shown in Fig. 2d. The results indicate that the average thickness for the prepared Au nanodisks is about 7 nm. Such thin characteristic makes it possible to consider the Au nanodisk as a two-dimensional material that can confine more light energy on a small mode volume.
In order to further prove the thin characteristics of the prepared Au nanodisks, we measured the scattering spectra of a single Au nanodisk with various diameters as shown in Fig. 3. Figures 3(a)–3(d) are the comparison of scattering spectra between experimental measurements and theoretical simulations with a diameter of 49 nm, 70 nm, 92 nm and 126 nm. It can be seen that the measured experimental curves are perfect matching to the theoretical simulated curves. The insets in Figs. 3(a)–3(d) are the electric field distributions images (left) and SEM images (right) of the corresponding Au nanodisk, respectively. The insets show that the maximum points of field distribution are located at the edges of the Au nanodisks attributing to the presence of dipole plasmon mode, which suggests the large field enhancements appear at its edges. Figure 3(e) is the diameter-dependent plasmon peaks of experiment and calculation. From Fig. 3(e), it can be seen the plasmon peaks of experimental measurement are basically consistent with that of theoretical calculation as the diameters of Au nanodisks change from 45 nm to 126 nm. It should be noticed that the thickness of Au nanodisks is fixed at 7 nm when their scattering spectra are theoretical simulated. While there is a deviation between experimental data and simulated data at both ends of the curve. When the diameter of Au nanodisk is larger than 100 nm, the plasmon peaks of experimental measurements slightly blueshift. Only if the thickness of Au nanodisk increases, the simulated results can agree well with the experimental data that implies its actual thickness is larger than 7 nm. When its diameter is less than 60 nm, the plasmon peaks of experimental measurements slightly redshift. This result is quite the opposite as above that implies its actual thickness is less than 7 nm. These results indicate that 7 nm is an average thickness for the prepared Au nanodisks.
Fig. 3. Scattering spectra of a single thin Au nanodisk with various diameters. The experimental measurements and theoretical simulations of scattering spectra of a single thin Au nanodisk with a diameter of (a) 49 nm, (b) 70 nm, (c) 92 nm and (d) 126 nm. The dots are measured experimental data and the solid lines are theoretical simulation data by using the FDTD method. The insets in Figs. 3(a)–3(d) are the electric field distributions images (left) and SEM images (right) of the corresponding thin Au nanodisk, respectively. (e) The dependence of plasmon peak on diameter including experiment data (blue hollow circle) and calculation data (red solid square). Thickness of Au nanodisks is fixed at 7 nm.
Since the thin Au nanodisks display large electric field confinement at their edges, which may lead to large third-order optical nonlinearities. Thus, the measurements of third-order optical nonlinear absorption and refraction of Au nanodisks were carried out by the Z-scan technique. The Schematic diagram of Z-scan experimental setup is shown in Fig. 4(a). Detailed introduction of the Z-scan measuring system can be found in the experiment section. Figure 4(b) presents third-order optical nonlinear absorption and refraction curves of thin Au nanodisks measured by the Z-scan technique. As can be seen from Fig. 4(b), a typical saturable absorption at surface plasmon resonance in thin Au nanodisks have been observed. For the saturable absorption, the absorption coefficient has the form α(I) = α0/(1 + I/Is). When the pump fluence is at a low excitation level (I/Is ≪1), this expression can be approximated to α(I)≈α0−(α0/Is)I, and then the second term in the expression is defined as the nonlinear absorption coefficient β = −α0/Is, which is proportional to the imaginary part of third-order nonlinear susceptibility Imχ(3). [48,49] The excitation wavelength for Z-scan measurements is 750 nm with a peak irradiance of 0.1 GW/cm2. The peak irradiance is calculated by the formula of I0=P/fτπr2. Where P is the laser power on the surface of the sample measured by the powermeter, f is the laser pulse duration (∼130 fs), τ is the laser repetition rate (∼76 MHz) and r is the waist radius of laser beam measured by the cutting blade method. By using the formula in Ref. [40] to fit the curves in Fig. 4(b), we can obtain the values of nonlinear absorption and refraction coefficients. And then the Im χ(3) and Re χ(3) of the Au nanodisks can be calculate to be −4.65◊10−20 m2/V2 and −1.61◊10−18 m2/V2, respectively. Figure 4(c) exhibits the dependence of Im χ(3) and Re χ(3) on excitation wavelength at a peak irradiance of 0.1 GW/cm2. As shown in this graph, the absolute values of Im χ(3) reach a peak of 4.65◊10−20 m2/V2 at 750 nm, while the absolute values of Re χ(3) almost remain unchanged as the excitation wavelength increases from 720 nm to 780 nm. Through the measurements of third-order optical nonlinearity of thin Au nanodisks, we can get the nonlinear absorption and refraction coefficients. And then, according to the relationship between the nonlinear coefficients and Figures of merits, one-photon Figures of merit (W) and two-photon Figures of merit (T) can be calculated. [50] The two parameters are essential to evaluate the performance of optical nonlinear materials for optical-switching application that demands W > 1 and T < 1. The excitation wavelength dependent W and T at a peak irradiance of 0.1 GW/cm2 is shown in Fig. 4(d). As can be seen from graph, the two curves reach a high point at 750 nm. Figure 4(e) is the excitation wavelength dependent total χ(3) of thin Au nanodisks and the linear absorption spectrum of the corresponding sample. As shown in Fig. 4(e), we can see clearly that the magnitude of total χ(3) of thin Au nanodisks is dominated by their plasmon resonance peak, which stem from large local electric field enhancement of the thin Au nanodisks when plasmon resonance occurs. It should notice that the value of total χ(3) is one or two orders of magnitude larger than that of the other shapes of Au nanostructures, such as Au nanosphere, Au nanorods, and Au triangle nanoprisms in a solution environment etc. It should be notice that when the Au nanostructures are suspended in a solution as long as several hours, the Au nanostructures may start to precipitate, leading to the decrease of the magnitude of the third-order optical susceptibility. [51] In this work, in order to avoid the influence of precipitation of the Au nanodisks, the measuring time of the samples was usually controlled within a few minutes, and the samples were processed by ultrasonic treatments to ensure their dispersion before measurement. These results indicate that the high-planeness and thin Au nanodisk can induce the generation of large third-order optical nonlinearities, which makes it be a suitable nonlinear material for application in nonlinear plasmonic devices. For the thin Au nanodisks, their size and homogeneity are easily regulated in a solution environment, and they also have extremely large third-order nonlinearities especially when they are suspended or self-assembled on different substrates to form nanoscale metal films. However, their dispersion and orientation on the substrates are difficult to control, and the stability caused by the film forming process is also poor, which also challenges the practical application of devices. For the plasmonic nanoparticles embedded in a substrate, their dispersion, orientation and stability are controllable which also possesses large third-order nonlinearities. However, the size and homogeneity of the nanoparticles are hard to control. Moreover, the density of doped nanoparticles is also difficult to control and hard to reach a high value. Generally, the two methods of preparing metal nanostructured films have their own advantages and disadvantages. [49]
Fig. 4. The measurements of third-order optical nonlinearity for thin Au nanodisks by the Z-scan technique. (a) Schematic diagram of the Z-Scan experimental setup. D1 and D2 are the photodetectors used to convert the light into electrical signals and input them to the powermeter. (b) Third-order optical nonlinear absorption and refraction curves of thin Au nanodisks measured by the Z-scan technique. (c) The dependence of Im χ(3) and Re χ(3) on excitation wavelength with a peak irradiance of 0.1 GW/cm2. (d) The excitation wavelength dependent W and T at a peak irradiance of 0.1 GW/cm2. (e) The excitation wavelength dependent total χ(3) of thin Au nanodisks (red solid sphere) and the linear absorption spectrum of the corresponding sample (black solid line). The peak irradiance is fixed at 0.1 GW/cm2.
To prove the enhancement of local field induced by the thin characteristics of Au nanodisks, the dependence of electric field distribution on the thickness of Au nanodisks is shown in Fig. 5. Figures 5(a)–5(d) are the electric field distributions of the upper plane of the sample, Figs. 5(e)–5(h) correspond to that of the middle plane and Figs. 5(i)–5(l) correspond to that of the bottom plane. As can be seen that the electric field intensity of the Au nanodisks is greatly enhanced as their thicknesses decrease. On the whole, the electric field intensity of the Au nanodisks is enhanced more than two-fold when the thickness of the sample is decreased from 30 nm to 7 nm. Especially, the electric field intensity at the interface between the Au nanodisk and the aqueous solution is enhanced more than three-fold as the thickness of the Au nanodisks decreases from 30 nm to 7 nm. The simulation results indicate that the thin Au nanodisk can confined more light at the interface, which indicates the collision probability of hot carriers for the thin Au nanodisk becomes greater than that of the thick Au nanodisk. This result suggests that the thin Au nanodisk may have a faster optical response time compared with other shapes of Au nanostructures, which makes it to be a suitable optoelectronic device material for plasmonic optoelectronic devices.
Fig. 5. Simulation of the electric field intensity distribution. The dependence of electric field distributions on the thickness of the Au nanodisk at upper plane (a)–(d), middle plane (e)–(h) and bottom plane (i)–(l). The diameter of the Au nanodisks is fixed at 60 nm and the dielectric environment is an aqueous solution. The inset in these figures are the schematic diagrams of the corresponding plane for the sample in an aqueous solution.
It is well known that metallic nanostructures are considered to have enormous potential applications in all-optical manipulator, fast light switch, and plasmonic devices due to their excellent third-order optical nonlinearities and ultrafast optical response properties. [17,34–37] It is reasonable to forecast that there is also true for the Au nanodisk especially its 2D characteristics make it possible to response faster to light. The ultrafast optical response properties of Au nanodisks were measured by the OKE technique. Figure 6(a) is the schematic diagram of OKE experimental setup that a detailed description is presented in the experiment section. Figure 6(b) presents the time-resolved OKE signal of thin Au nanodisks in solution and CS2 solution with an excitation wavelength of 750 nm and a peak irradiance of 0.7 GW/cm2. From Fig. 6(b), we can see clearly that a typical temporal behavior of the OKE signal for Au nanodisks. By using a single-exponential decay function to fit the curve. the decay time (response time) of Au nanodisks is about 105 fs. Such ultrafast response time of the thin Au nanodisks is faster than that of the other Au nanostructures reported in most of the previous works in a solution environment. [17,34,39,40] This is because the existence of thin characteristics of Au nanodisks that leads to the increase of local density of states at the edges of the samples. This result gives rise to the increase of scattering probability including electron-electron scattering and electron-surface scattering, which can make the response time of Au nanodisk faster. In addition, the value of χ(3) for Au nanodisks can also be obtained by the OKE technique, which is calculated by using the following equation:
Fig. 6. The measurements of ultrafast optical response for thin Au nanodisks by the OKE technique. (a) Schematic diagram of OKE experimental setup. (b) Time-resolved OKE signal of thin Au nanodisks in solution (blue solid sphere) and CS2 solution (gray solid sphere) with an excitation wavelength of 750 nm and a peak irradiance of 0.7 GW/cm2. The red solid lines are fitting curves by using single exponential decay function.
According to the Eq. (3), the χm(3) value of the thin Au nanodisks can be calculated to be ∼7.1×10−16 m2/V2 at 750 nm. It is notice that the χm(3) value of the sample is a theoretical estimate value that is dominate by the conduction electron intraband transition and the change in dielectric constant due to the excitation of hot electrons. The two mechanisms are the major contributions to the Kerr nonlinearity for the thin Au nanodisks. [52,53] In our OKE measurements, the pump and probe wavelength are at 750 nm corresponding to the peak position of local surface plasmon resonance (LSPR), which means that the intraband transition probability of conduction electron and the hot electrons-induced the change of dielectric constant are enhanced by the LSPR. While the thermal effect induced by the excitation of hot electrons is unavoidable in the OKE measurements that means the theoretical estimate value of the χm(3) is usually larger than its actual value. The OKE technique is not suitable to measure the magnitude of χ(3) of the thin Au nanodisks compared with the Z-Scan technique.
It should be notice that the pulse duration greatly affects the magnitude of optical nonlinearity because the long pulse duration can lead to the thermal effects. When the pulse duration of the laser is increased from 0.1 ps to 5.8 ps, the effective nonlinear optical absorption coefficient can be increased by ∼100 times. [54] In our experiment, the pulse duration of the laser is only 100 fs and the Au nanodisks were measured in a solution environment, which means the thermal effects can basically be neglected. It also notices that the thickness of Au nanodisks is about 7 nm that indicates the quantum size effects may play an important role in enhancing the nonlinear response. This effect has been observed in a nanometer scale gold quantum well, which proved the ultrathin characteristic of the structures induced quantum size effects can lead to a giant nonlinear response. [55] It should be also noticed that plasmon resonance peaks of Au nanodisks are easily modulated to near-infrared range by only increasing their diameter, which suggests that Au nanodisks could be a good near-infrared nonlinear material due to their huge third-order optical nonlinearity and ultrafast optical response characteristics.
4. Conclusion
In summary, a high-planeness and thin Au nanodisk is synthetized by an Au conproportionation reaction method, which provides a way to prepare structurally uniform and tailorable 2D metal nanostructures with broadly tunable plasmon resonances. In particular, the plasmon mode of this structure has an effectively 2D characteristics, which provides access to confine the absorbed light energy at the interface of a heterostructure that would be difficult to be replicated in other nanostructures. The third-order optical nonlinearity and ultrafast optical response characteristics of Au nanodisks in a solution environment are measured by using Z-scan and OKE techniques. The Z-scan experimental results show that the magnitude of third-order nonlinear susceptibility (χ(3)) for Au nanodisks is on the order of 10−18 m2/V2, which is one or two orders of magnitude larger than that of other shapes of plasmonic nanostructures in a solution environment. The OKE experimental results present that their ultrafast optical response time is on the order of 100 fs, which is quickened several times than that of other plasmonic nanostructures in previous reports. These Au nanodisks with a high-planeness and thin characteristics are demonstrated to have an ultrafast third-order optical nonlinearity, which not only broaden our fundamental understanding of photon-matter interaction in nanoscale, but also help to their applications in ultrafast all-optical switching and plasmonic optoelectronic devices.
Funding
National Key R&D Program of China (2016YFA0301300); National Natural Science Foundation of China (NSFC) (11761141015, 11804407, 11804408, 61675237, 91750207); Guangdong Natural Science Funds for Distinguished Young Scholars (2017B030306007); Guangdong Special Support Program (2017TQ04C487); Natural Science Foundation of Guangdong Province (2016A030312012, 2018A030313333); Pearl River S&T Nova Program of Guangzhou (201806010033); Guangzhou Science and Technology Projects (201607020023, 201805010004).
Acknowledgements
Z.Z. supervised the project. Y.Y. and Z.Z. performed the experiments. All authors discussed the results and wrote the manuscript.
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