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Abstract
The Au/SiO2-SiO2 multilayer periodic structure was fabricated by ion implantation and radio-frequency magnetron sputtering. We proposed a scheme to change the refractive index of the nanocomposite layer based on the important influence of the annealing temperatures on the Au nanoparticles (NPs), and analyzed the changes in the size, volume fraction and dipolar interaction factor of the Au NPs at different temperatures. As a result, the reflectivity of the sample with four periods increased from 65.0% to 82.6% at 800 °C. This method is promising as an application to prepare large-scale photonic integrated circuits as a small size but efficient Bragg reflector.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As the simplest one-dimensional photonic crystal, distributed Bragg reflectors (DBRs) are usually formed by stacking two materials with different dielectric constants [1]. Recently, the DBRs combined with many special technologies have been widely applied in various fields, such as the electric field structural color for electro-optical switches [2], angle-insensitive infrared stealth functional material [3], and tunable specific wavelengths and wavebands through ion beam technology [4]. It is often necessary to achieve a high reflectivity for the applications. The main factors affecting reflectivity are the number of stacked periods and the refractive index difference (Δn). Multiple interferences of light between the stacks increase the reflectivity at certain wavelengths ranges, and the peak position is related to the selected period thickness. The traditional preparation methods are mainly molecular beam epitaxy [5,6], chemical vapor deposition [7], thermal evaporation [8] and sol-gel method [9], etc. Usually, it is necessary to consider close lattice matching to avoid dislocations that adversely affecting the quality of the entire structure. Peiris et al. prepared ZnMgSe/ZnSeTe (Δn=0.35) samples to achieve 85% reflectivity in 10 periods. Besides, the sample of ZnMgSe/ZnCdSe (Δn=0.2) matched well, but the refractive index difference is relatively low, it is usually necessary to prepare a large number of periods to obtain high reflectivity [5]. This is unfavorable to produce large-scale integrated optical circuits. Therefore, it is still a huge challenge to fabricate a Bragg reflector based on dielectric materials with a small number of pairs but high reflectivity and lattice matching.
It is well known that metal NPs have a strong local surface plasmon resonance (LSPR) effect, which largely depends on different species, size, shape and surrounding dielectric environment [10,11]. Many works have shown that various metal NPs can be embedded in the substrate at a relatively high fluence through ion implantation [12,13]. Nanocomposite layer and substrate will produce a difference in refractive index, which provides us with an idea for preparing DBR by setting appropriate parameters [14]. In addition, the scattering effect of metal NPs also have a positive impact on improving reflectivity [15,16]. Applying this effect to the construction of DBR is expected to reduce the number of stacked layers [17]. Furthermore, the study of quantum interference between surface plasmons of metal NPs has also aroused great interest in the coherent properties of plasmon polaritons optically excited on periodic nanostructures. [18,19]. The interaction between the plasmon resonance of metal NPs and the Bloch mode of the photonic crystal broadened the reflection band width [20]. It is worth noting that when the average sizes (r) of metal NPs are small, the absorption cross section changes to r6, while the scattering cross section is only r3. Meanwhile, as the NPs gradually increase, the scattering effect becomes more obvious [21]. However, the size distribution of metal NPs is often wide, and the size of NPs is small by ion implantation. These factors have always restricted the specific applications of these nanocomposites. Combined with the subsequent thermal annealing, the diffusion of the implant in the substrate can be enhanced, and the size distribution and optical properties of the metal NP can be modified [22,23]. Especially, after the sample is annealed, the diameter of the Au NPs gradually becomes large, leading to a significant scattering effect. Besides, different annealing atmosphere also has an impact effect on size distribution and the growth of NPs. Mattei et al. found that O2 can promote the diffusion of Au atoms, and Au NPs agglomerate more easily than in other atmospheres, thereby forming larger NPs [24].
In this work, SiO2 substrate was implanted with 30 keV Au ions at a fluence of 6 × 106 ions/cm2, and SiO2 films were prepared by radio frequency (RF) magnetron sputtering. The Bragg peak of the sample was coupled with the LSPR scattering peak to enhance the reflectivity at normal incidence. Subsequent annealing is an important method to modify metal NPs, we analyzed the competition of different parameters on the refractive index of the Au NPs/SiO2 nanocomposite layer at different annealing temperatures. As a result, the reflectivity increased from 65.0% to 82.6% at 800 °C, which can be attributed to the change in refractive index of the nanocomposite layer and the enhancement of the scattering effect of Au NPs. This finding provides a feasible method for the preparation of high reflectivity DBR, which has potential applications in large-scale photonic integrated circuits.
2. Experimental
The four periodic structure of Au NPs/SiO2 nanocomposites and SiO2 films was prepared by ion implantation and RF magnetron sputtering. The substrate was optically polished SiO2 with a thickness of 1.0 mm, which were implanted with 30 keV Au ions at a fluence of 6.0 × 1016 ions/cm2 by using metal vapor vacuum arc (MEVVA) implanter. During implantation, the beam current density was kept below 4 μA/cm2 and the target plate rotated at a constant speed. Then the RF magnetron sputtering was used to continue growing SiO2 films, the thickness was about 215.0 nm. The sputtering was carried out in an argon flow of 30 sccm, the pressure of 0.75 Pa and the RF power of 120 W. Here we formed the Au NPs/SiO2 nanocomposite layers (for simplicity, Au/SiO2 is named below) and the SiO2 films are used as high and low refractive index materials, respectively. Finally, we prepared four periodic structures and used it as an example to study the change of angular reflectance spectra and the influence of subsequent annealing. The conditions of the subsequent annealing were carried out at the temperature of 400 to 900 °C for 1 hour in air atmosphere.
The periodic structure and size and distribution of the NPs were characterized by cross-sectional transmission electron microscopy (XTEM) by using a Tecnai G2 F20 S-Twin operating at an acceleration voltage of 200 kV. The absorption and reflection spectra were measured with a fiber optic spectrometer (Maya2000 Pro, Ocean Optics), and the angle between the incident light and the normal direction of the sample was changed to measure the angular reflection spectra.
3. Results and discussion
3.1 Characterization of the multilayer structure and size distribution of nanoparticles
Figure 1(a) shows the prepared multilayer structure. The dark part is the Au/SiO2, and the light part is the SiO2 films. The details of each nanocomposite layer are shown in Figs. 1(b-e), the large Au NPs are mainly distributed in shallow layers, while the small NPs are in deep layers. The insert image shows that the lattice spacing of one NP in agreement with Au (111) plane. The subsequent implantation subjects the previously prepared NPs layers to thermal effects [25], which increases the size of the NPs, causing the diameter of the NPs to gradually decrease from the bottom to the top layer, as shown in Figs. 1(f-i). In general, the periods of the prepared samples are 183.6, 182.7 and 185.9 nm. The selected parameters are the same, but the results are slightly different due to the sputtering effect of ion implantation and the error of magnetron sputtering.
Fig. 1. (a) XTEM image of the multilayer structure, (b)-(e) detail of each nanocomposite layer, the insert image shows the high-resolution TEM (HRTEM) of individual NP and (f)-(i) size distribution of NPs.
3.2 Optical properties of the multilayer structure
The central wavelength is calculated by the Bragg equation [26]:
Where, d is the period, neff represents the effective refractive index, nc and ns are the refractive indexes of the Au/SiO2 and SiO2 film, respectively, p is the ratio of the depth of the nanocomposite layer to the period, λmax is the Bragg peak, and θ is the incident angle. As shown in Fig. 2, the central wavelength of the prepared sample is about 555.3 nm, which deviated from the absorption peak at 525.0 nm caused by LSPR of Au NPs, and the reflectivity at the central wavelength reached about 65.0%.
In order to reveal the mechanism of high reflectivity, we measured the angular reflection spectra in the range of 0-45° in Fig. 3(a). The dielectric function of the Au/SiO2 can be obtained by using the following equations [27]:
Where, the effective dielectric function εeff = ε1,eff + i · ε2,eff was calculated by M-G theory [28,29], nc and ns represent the refractive and absorption coefficient of the Au/SiO2 nanocomposite layer, respectively. These parameters were substituted into the transfer matrix [30], and other parameters were consistent with the experiment in Fig. 1. The transfer matrix method is a traditional method used to simulate the reflection spectra of one-dimensional photonic crystals, and the simulation results are shown in Fig. 3(b). The small deviations between the simulated and experimental results are due to the thickness detection error in the magnetron sputtering process and the difference in the refractive index of the material. With increasing incident angle, the spectra are broadened and separated into two peaks, the reflectance spectra of the Bragg peak shifted to the short wavelength, which is a typical feature of Bragg reflector [31]. It can be calculated by the Bragg equation that as the incident angle increases, the Bragg peak decreases when other parameters are constant. The reflection peak at about 555 nm remains unchanged, which can be attributed to the LSPR of Au NPs. Therefore, the coupling of the two factors enhances the reflectivity at normal incidence.
3.3 Results of annealing at different temperatures
Figure 4 shows the change of reflectance with annealing temperatures. Since the diffusion of gold is weak [32], the influence of Au NPs is small before the annealing temperature of 600 °C, and thus the change of the reflection spectra is not obvious. The reflectivity of central wavelength is improved from 600 to 800 °C, and reaches 82.6% at 800 °C, which is greatly improved compared with implanted sample. However, the reflectivity decreases at 900 °C, which is the result of competition among multiple factors, mainly including NP diameter D, volume fraction f and dipolar interaction factor K. We combine these three parameters to analyze the influence of the change of Au NPs on the reflectivity during the annealing process.
Fig. 4. Reflectance spectra at normal incidence annealed at 400 to 900 °C, the inserted figure shows the reflectivity at the central wavelength varies with annealing temperatures.
In actual calculations, the NP diameters can be obtained from the statistical results of XTEM and the optical absorption method [33,34], while f and K need to be fitted to the absorption spectra. Figure 5(a) is the result of the NP diameters and volume fraction varying with annealing temperatures. Besides, the dipolar interaction factor K increases slightly before 800 °C, and decreases at 900 °C. In fact, the combination of these factors leads to the change of the refractive index of the Au/SiO2, as shown in Fig. 5(b), this trend is consistent with the reflectance of the sample.
Fig. 5. Simulation results based on M-G theory at different annealing temperatures (a) NP diameter and volume fraction (b) Refractive index varies with wavelength.
The growth of Au NPs can be explained by Ostwald’s ripening mechanism [35,36]. Appropriate annealing treatment can enhance the diffusion of implanted ions in the substrate, thereby promoting the growth of NPs and improving the distribution of NPs. The Ostwald process emphasizes that small-sized NPs gradually fuse to produce large-sized NPs, and the small-sized NPs gradually disappear. When the annealing temperature is high enough, the implanted ions can be dissolved, and the NPs will reassemble into nuclei and grow during the cooling process to form large-sized NPs. Moreover, annealing can also cause ions to overflow from the surface or side of the sample, thereby reducing the volume fraction of implanted ions. XTEM images with annealing temperatures of 800 °C and 900 °C are shown in Fig. 6, and the changes of Au NPs can be visually observed. The surface layer is affected by the air atmosphere, and thus the diameter of the Au NPs is large. Before the annealing temperature is 800 °C, the increase of the parameters D and K is dominant, and their influence is more significant than the volume fraction f, and thus the reflectivity gradually increases. However, when the annealing temperature reaches 900 °C, the influence of diameter D on the reflectivity becomes weak, and both f and K decrease, leading to a low reflectivity. Furthermore, from the XTEM images shown in Fig. 6, we can see that the distribution of Au NPs become narrow, the period we designed is destroyed, thus the LSPR scattering and the Bragg peak cannot be well coupled.
3.4 Discussion on the modification of nanoparticles
We can effectively improve the reflectivity by increasing the volume fraction of NPs. In the case of surface sputtering and size saturation effects [37], it is difficult to improve the volume fraction of NPs by increasing the fluence. Sequential ion implantation may be a feasible method, such as pre-implantation of Zn ions, then the sputtering loss of implantation can be effectively suppressed [38], and the volume fraction of NPs can be increased, which is expected to further increase the reflectivity of the DBR. In addition, subsequent ion irradiation is also an important method to modify nanoparticles. The refractive index and the thickness of the material can be modified simultaneously [4], and its optical properties can be adjusted more flexibility.
4. Conclusion
We prepared a Bragg reflector with four periodic structures by ion implantation of Au NPs and preparation of SiO2 films by RF magnetron sputtering. The average period of this sample is 184.1 nm, which corresponds to the Bragg peak at the center wavelength of 555.3 nm. This peak is close to the LSPR scattering peak caused by Au NPs, and the coupling of these two peaks enhances the reflectivity. We have verified the important role of Au NPs through experiments and simulations of the reflection spectra at different incident angles. Moreover, the subsequent annealing improved the refractive index of the nanocomposite layer to expand the refractive index difference with the substrate. At the same time, the scattering effect of NPs was also continuously enhanced as the size increased, which also had a positive effect on improving the reflectivity. Our findings indicated that modulating the NPs could be very helpful to prepare efficient Bragg reflectors.
Funding
National Natural Science Foundation of China (11675120, 11535008).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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