1. Introduction

Nonlinear optical devices are pronouncing for the next generation of information technology. Optical materials with large third-order optical nonlinearity and fast response time are key to the all-optical applications. Plasmonic nanostructures [1–3] and dielectric materials [4–6] are two promising materials which have gained much more attentions than others. For plasmonic nanostructures, surface plasmon resonance induced large local field enhancement make the third-order optical susceptibility as large as 10⁻⁶ esu under the excitation power from several microwatt to tens of milliwatt [7–10]. Relaxation of electrons due to the electron-electron scattering and electron-phonon scattering enable the materials to have the response time from several picosecond to hundreds of femtosecond [11]. However, the strong absorption induced by the surface plasmon resonance increases the linear loss of the materials and the manufactured devices, which limited its applications. Therefore, dielectric optical materials with high optical transmittance and large third-order optical susceptibility becomes substitutes of the plasmonic nanostructures [12–20]. For examples, large third-order optical nonlinearities have been achieved in ZnO [12,13], indium tin oxide [14–19], etc. At the wavelength where the real part of the permittivity vanishes, maximum third-optical nonlinear could be obtained. However, due to the relative small nonlinear optical coefficients, the required excitation power are much larger than those of plasmonic nanostructures [14– 19].

To obtain nonlinear optical materials with large optical nonlinearity, low excitation power and low linear optical absorption loss, a possible way is to borrow the advantages from both plasmonic materials and dielectric materials through combine them together. An example is that Boyd et al. achieved three orders of nonlinear refractive index enhancement in ITO thin film by fabrication good antenna onto the surface of ITO thin film [12].¹² Besides the local field enhancement generated by the distributed plasmonic nanostructures, cavity could also confine the electric field inside a small volume [20,21]. Therefore, constructing cavities by using plasmonic nanostructures and dielectric nonlinear optical materials as functional blocks may further improve the nonlinear optical properties of the individual materials. Based on this idea, this work will construct a plasmonic cavity composed of plasmonic nanostructures and dielectric thin films for the first time, where further local field enhancement would be achieved inside the cavity besides the nanostructures themselves.

Our previous work demonstrated that random distributed gold nanorods (AuNRs) has large third-order optical nonlinearities under ultralow excitation laser powers. So, it’s a good candidate to be used as a plasmonic nanostructures in plamonic cavities. In this letter, a sandwich plasmonic cavity composed of random distributed AuNRs, ITO, and gold film was designed and fabricated by using sputtering method. Through confining the light and the local electric field inside the ITO space layer and the gap of Au nanorods, large third-order optical susceptibility has been obtained. Compared to the bare Au nanorods film which already has large optical nonlinearity, 3∼15 times of further enhancement has been observed within the wavelength range of 720-900 nm.

2. Experimental

The random distributed gold nanorods on glass was obtained from a commercial company (NanoSeedz). The synthetic process of Au nanorods can be found in previous articles [3]. ITO and Au thin films were prepared by using sputtering methods. The thickness of ITO thin film is about 5 nm, within the interaction length of local electric field of AuNRs. The thickness of Au film is controlled to be 16 ± 1 nm, which allows the transmission of the excitation laser and the confinement of local electric field. Optical absorption spectra were measured on a UV-Vis-NIR spectrophotometer (PE Lambda 950). The third-order optical nonlinearities were performed by using a home-made standard Z-scan setup [22]. A tuned Ti: sapphire laser (Coherent, Mira 900) with the output central wavelength range from 720 nm to 900 nm was used as a laser source, the pulse duration is nearly 130 fs and the repetition rate is about 76 MHz. To minimize the possible contributing of heat effect, ultralow excitation powers from 0.1 mW (0.96 MW/cm²) to 5 mW were selected. Finite-difference time domain (FDTD) solutions were used to calculate the electric field distribution of the nanorods and the corresponding plasmonic nanostructures.

3. Results and discussion

Figure 1 shows the morphology, linear and third-order optical nonlinear properties of AuNRs/ITO/Au nanostructure. Figure 1(a) shows the Scanning electron microscopy (SEM) image of AuNRs thin film, AuNRs distributed randomly on the whole glass substrate with the area of 75mm×25 mm, the AuNR density is nearly 64 AuNRs per square micrometer. Figure 1(b) presents the length distribution of AuNRs, the average length (L_average) is about 85.4 nm. Figure 1(c) is the absorbance spectrum of AuNRs/ITO/Au nanostructure, together with that of AuNRs/ITO and AuNRs for comparison. When ITO film was deposited onto the surface of AuNRs, the absorption peak red-shift slightly due to the change of dielectrics. When Au film was further deposited, the absorbance increase dramatically and the absorption peak shift to the longer wavelength side. The third-order optical nonlinear absorption and refraction were measured by using Z-scan method, where the scheme of the setup was shown in Fig. 1(d). A Ti: Sappire laser was used as a laser source, by tuning the birefringent filter (BRF) knob together with the Brewster prism lens position, the central wavelength of the laser pulse could be adjusted continuously within 720-900 nm. For a fixed laser pulse with certain central wavelength, the full width of half maximum (FWHM) of the laser pulse is around 10 nm. The sample was put onto a moving stage which can sweep along the laser beam directions (± z direction). The focus length of the focus lens before the sample is 50 mm, the measured radius of the laser beam at the focus position is around 17 µm. Two power detectors (D1 and D2) were used to monitor and record the laser powers. The transmission T was extracted from the ratio between power value obtained by detector D2 and that at the front of the sample calculated from detector D1.

 

Fig. 1. Linear and nonlinear optical properties of AuNRs/ITO/Au nanostructure. (a) SEM image of AuNRs. (b) Length distribution of AuNRs. (c) Absorption spectra of AuNRs-ITO-Au, AuNRs, and AuNRs-ITO. (d) Scheme of Z-scan setup used for characterization of nonlinear absorption and nonlinear refraction. (e) Typical third-order optical absorption curve, and (f) typical nonlinear optical refraction curve of AuNRs/ITO/Au nanostructure.

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Figure 1(e) and (f) present the typical nonlinear absorption and nonlinear refraction curves of AuNRs/ITO/Au nanostructure. The hollow dots are experimental data, the solid lines are fitting curves. For nonlinear absorption, two-photon absorption, three-photon absorption [23] and hot electrons may have contributions. In our work, the power intensity is several orders lower than those used to observe three-photon absorption, the contribution of three-photon absorption should not be the dominant effect. Instead, it originates from the absorption of intra-band electrons [24]. By fitting the curve using $T = \sum {({ - {q_0}} )^m}/{({1 + {z^2}/z_0^2} )^m}{({1 + m} )^{3/2}}{\; }({m = 0,1,2,\ldots } )$, ${q_0}$ could be obtained, where z₀ is Rayleigh range and equals to $\textrm{n}{\pi _\omega }_0^2/\lambda $, $\omega_0$ is the Gaussian beam spot radius at focus ($z = 0$), ${q_0}{ = _{\textrm{eff}}}{I_0}{L_{\textrm{eff}}}$, and ${L_{\textrm{eff}}}$ is he effective thickness of the samples [25]. While for nonlinear refraction, the theoretical relation between T and z is expressed as $T = 1 + 4{{\Delta }_{\Phi }}_0\left( {z/{z_0}} \right)/\left\{ {\left[ {{{\left( {z/{z_0}} \right)}^2} + 9} \right]\left[ {{{\left( {z/{z_0}} \right)}^2} + 1} \right]} \right\}$ [25], where ${{\Delta }_{\Phi }}_0 = {n_2}k{I_0}{L_{\textrm{eff}}}$, ${n_2}$ is the nonlinear refraction index, and k is the wavenumber. The peak-valley type curves means that the third-order optical nonlinear refractive indexes of the sample is self-defocusing effect. Besides, spatial phase modulation (SPM) and group velocity dispersion (GVD) may also have contributions to nonlinear refraction. For SPM, multiple diffraction rings should be observed, this doesn’t happens in our experiment. Wile for GVD, it is related to the laser beam travelling path length. The thickness of our sample is about 83 nm, the GVD effect could be neglected.

From the nonlinear absorption coefficient $\beta_{\textrm{eff}}$ and the nonlinear refraction index ${n_2}$ extracted from the experimental data, both image part (Imχ⁽³⁾) and real part (Reχ⁽³⁾) of χ⁽³⁾ could be calculated by using equations of $\textrm{Im}$χ⁽³⁾=$n{_0^2{_{\varepsilon 0}}}c{n_2}$, Reχ⁽³⁾=$2n_0^2{\varepsilon ^{_0}}c{n_2}$. Then, χ⁽³⁾ could be calculated using $|{\chi ^{\left( 3 \right)}}| = \sqrt {{{\left( {\textrm{I}{\textrm{m}_\chi }^{\left( 3 \right)}} \right)}^2} + {{\left( {\textrm{R}{\textrm{e}_\chi }^{\left( 3 \right)}} \right)}^2}} $. Figure 2 presents the wavelength dependent third-order optical susceptibility of AuNRs/ITO/Au nanostructure under different excitation powers. Figure 2(a) shows the Imχ⁽³⁾ data. As the excitation power increases from 0.1 mW to 5 mW, the absolute value of Imχ⁽³⁾ decreases monotonously, and this can be explained to be the saturation effect of AuNRs.¹⁹ For the wavelength dependent Imχ⁽³⁾ taken at a fixed excitation power of 0.1 mW, the Imχ⁽³⁾ value varies between the 9.4 × 10⁻⁵ esu and 3.7 × 10⁻⁴ esu, and the maxima occurs at 880 nm. The changing trend of Imχ⁽³⁾ follows that of linear absorbance. However, due to experimental errors and the variation of lasers and the change sample positions, the experimental data curves are not as smooth as linear absorption curves. Figure 2(b) gives the corresponding wavelength dependent Reχ⁽³⁾, for the data obtained at a fixed wavelength, the absolute value of Reχ⁽³⁾ also decreases with the increasing of excitation power. While for wavelength dependent Reχ⁽³⁾, the maximum value is about 2.0 × 10⁻⁴ esu around 860 nm. A sign changes of Reχ⁽³⁾ could be observed around 880 nm, this could be attributed to changes of the dielectric constant induced by the creation of hot electrons [26].

 

Fig. 2. Third-order optical susceptibility of AuNRs/ITO/Au nanostructure. (a), (b) and (c) are wavelength dependent Imχ(3), Reχ(3) and χ(3), respectively.

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Figure 2(c) demonstrates the absolute value of χ^(3)­, the maximum value is 3.9 × 10⁻⁴ esu at 880 nm, which is quite large compared to those of other nonlinear optical materials or nanostructures under similar excitation conditions. For Z-scan measurements under high repetition pulse rate, thermal effect might have contributions in nonlinear refraction index [27,28]. To minimize thermal effect, the main data was taken under the laser power of 0.1 mW, accurate elimination of thermal effect could be performed by using the methods by Mohamed et al. [27]. Here, we will focus on the further enhancement of optical nonlinearities induced by cavity confinement.

To demonstrate the significance of AuNRs/ITO/Au nanostructure, wavelength and power dependent χ⁽³⁾ are given together with those of AuNRs/ITO and AuNRs. Figure 3(a) shows the wavelength dependent of χ⁽³⁾ for the three kinds of nanostructures under 0.1 mW. The χ⁽³⁾ of AuNRs/ITO is comparable to that of AuNRs, while χ⁽³⁾ in AuNRs/ITO/Au is tens of times larger. And the maximum value in AuNRs/ITO/Au is about 3.9 × 10⁻⁴ esu, nearly 15 times to that of the maxima of bare AuNRs at 880 nm, which is about 1.14 × 10⁻⁵ esu. Figure 3(b) is the power dependent χ⁽³⁾ of AuNRs/ITO/Au and AuNRs/ITO nanostructures. As the excitation laser power increases from 0.1 mW to 5 mW, χ⁽³⁾ of AuNRs/ITO/Au decreases accordingly. This is similar to the saturation effect of AuNRs reported previously [19]. To confirm the origin of this further enhancement of AuNRs/ITO/Au relative to AuNRs and AuNRs/ITO, electrical field distribution maps of the three kinds of nanostructures were calculated by using FDTD software under 880 nm. As shown in Fig. 3(c)-(h), stronger electrical filed could be observed in AuNRs/ITO/Au, which lead to the further enhancement of χ⁽³⁾ than pure AuNRs and AuNRs/ITO. Table 1 presents the χ⁽³⁾ values of ITO and some plasmonic nanostructures, the results show that χ⁽³⁾ in AuNRs/ITO/Au is much larger than others. Therefore, construction AuNRs/ITO/Au cavities is a powerful method to obtain large χ⁽³⁾ under low excitation powers in plasmonic nanostructures.

 

Fig. 3. Comparison of χ(3) between AuNRs/ITO/Au nanostructure, AuNRs/ITO and AuNRs. (a) Wavelength dependent χ(3) for the three kinds of nanostructures with the excitation power fixed at 0.1 mW. (b) Power dependent of χ(3) for AuNRs-ITO-Au nanostructure and AuNRs-ITO. (c) Schematic diagram of AuNRs thin film, (d) Electrical field distribution maps of AuNRs thin film. (e) Schematic diagram of AuNRs/ITO, (f) Electrical field distribution maps of AuNRs/ITO. (g) Schematic diagram of AuNRs/ITO/Au film, (h) Electrical field distribution maps of AuNRs/ITO/Au film.

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Table 1. Comparison of χ(3) in dielectric nonlinear optical materials and some plasmonic nanostructures.

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4. Conclusion

AuNRs/ITO/Au composite plasmonic nanostructure with an ITO space layer of 5 nm has been prepared. Cavity-induced further enhancement of third-order optical nonlinearity has been obtained. The results show that the χ⁽³⁾ of AuNRs/ITO/Au are several to tens of times larger than those of AuNRs and AuNRs/ITO in the wavelength range of 720-880 nm. The maxima of χ⁽³⁾ in AuNRs/ITO/Au is 3.92 × 10⁻⁴ esu under the irradiation intensity of 0.96 MW/cm² (0.1 mW). The huge large χ⁽³⁾ obtained under ultralow irradiation intensity enable AuNRs/ITO/Au have promising applications in ultralow-power all-optical switches and plasmonic waveguides.