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Abstract
The strongly localized electric field achieved in metallic nanoparticles (NPs) and nanostructures are commonly employed to realize surface-enhanced Raman scattering. However, the heat originating from the Ohmic loss of metals may lead to the damage of the analyzed molecules, which severely limits the practical applications of pure-metallic nanostructures. Here, we propose a dielectric-metallic hybrid nanocavity placing silicon (Si) NPs onto a gold (Au) film to realize broadband Raman scattering enhancement with significantly reduced heat generation. Our results reveal that the heat generation is dramatically reduced in the hybrid nanocavity as compared with its pure-metallic counterpart while a significantly enhanced electric field is maintained. We demonstrate numerically and experimentally that the optical resonances, which arise from the coherent coupling of the electric and magnetic dipoles excited inside the Si NP with their mirror images arisen from the Au film, can be employed to enhance the excitation and radiation of Raman signals, respectively. We find that the enhancement in the radiation of Raman signals plays a crucial role in enhancing the total Raman scattering. We also show that the hybrid nanocavity acts as a nano-antenna which effectively radiates Raman signals into the far-field. These findings indicate the advantages of such hybrid nanocavities in temperature-sensitive Raman scattering characterization and supply new strategies for designing nanoscale photonic devices of other functionalities with hybrid nanocavities.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recent advances in the fields of nanophotonics and plasmonics have revolutionized our ability to enhance Raman scattering. Light can be effectively concentrated on nanoscale hot spots by using deliberately designed metallic nanostructures [1–5], making it possible to detect the Raman signal of single molecule by using the so-called surface-enhanced Raman spectroscopy (SERS) [6,7]. Benefitting from the advance in bottom-up nano-assembly, nanostructures and nanocavities in different fashions can be created in a controllable way, offering more freedoms and opportunities for the practical applications of SERS. A typical example is the so-called particle-on-film systems, in which molecules or two-dimensional materials can be easily introduced into the nanogap between the NP and the metal film, where a significantly enhanced electric field is expected [8–12]. So far, a high enhancement in Raman signal has been demonstrated in pure-metallic particle-on-film systems [13].
For many biochemical applications, especially in biochemical reactions related to living cells, the power of the excitation laser light is a critical parameter. Based on previous studies [14], the excitation laser is considered to be safe when its power density is lower than ∼2.5×107 mW/cm2. Due to the large ohmic loss inherent in metals, the heating effect caused by the absorption of laser light may lead to the degradation of the tested molecules. Besides, the surface plasmon polaritons excited in pure-metallic nanostructures may also induce photochemical reaction [15], photothermal reaction [16,17], and oxygen activation [18], which leads to low stability and repeatability of SERS data [19–21]. Therefore, additional control over the surface state and possible reaction with the molecules is necessary [22], and this will limit the practical application of SERS in biochemical detection.
As early as 1988, Hayashi published a pioneering work on small gallium phosphide (GaP) particles and demonstrated evidence of electromagnetic-enhanced Raman scattering due to Mie resonance [23]. Owing to the relatively weak electric field enhancement, SERS based on dielectric materials has received much less attention as compared with that utilizing metallic materials. In order to achieve Raman scattering enhancement while avoiding various problems caused by heating effect, it has been theoretically proposed that dielectric materials with high refractive indices, such as silicon (Si) and germanium (Ge), might be used to construct nano-antennas for enhancing both near-field electric field and far-field radiation [24,25]. Moreover, experimental efforts have been devoted to the construction of dimer-like Si-based nano-antennas [16], Si nanodisks [26], and silica microspheres [27]. Although the whisper gallery modes supported by dielectric microcavities possess high-quality factors, they are not suitable for SERS because of the difficulties in incorporating molecules into such microcavities and realizing simultaneously the enhancement in the excitation light and Raman signals. The electric field enhancement in these dielectric NPs/nanostructures originates from the Mie resonances, which exhibit relatively low-quality factors. Actually, this deficiency can be overcome by combining dielectric NPs with metal films, forming dielectric-metal nanocavities. Previously, the scattering properties of similar hybrid nanocavities and their application in optical sensing have been investigated [28,29]. In addition, the nonlinear optical responses of Si NPs supporting Mie resonances and hybrid nanocavities supporting mirror-image-induced optical resonances have also been examined [30–32]. However, a systematic study of such hybrid nanocavities in SERS remains unexplored.
In this paper, we investigate both numerically and experimentally the Raman scattering enhancement realized in a hybrid nanocavity by placing a Si NP onto a gold (Au) film. By exploiting the optical resonances formed by the coherent coupling between the electric dipole (ED) and magnetic dipole (MD) excited in the Si NP and their mirror images induced by the Au film, the enhancement in the excitation, emission and radiation of Raman signal can be achieved simultaneously, leading to a significantly enhanced Raman signal. Moreover, it is found that the heat generation in such hybrid nanocavities is dramatically reduced as compared with pure-metallic nanocavities. This unique feature ensures the stability and repeatability of this Raman enhancement strategy, which is crucial in practical applications sensitive to heat or temperature.
2. Method
2.1 Sample fabrication
In this work, we used Si NPs whose diameters ranging from 110 to 220 nm and an Au film with a thickness of 50 nm to construct dielectric/metal hybrid nanocavities. The Au film with the surface roughness of ∼0.5 nm was previously deposited on a SiO2 substrate using electron beam evaporation. Then, it was immersed into a high-concentration p-mercaptobenzoic acid (4-MBA) solution for half an hour. During this process, the Raman reporter (4-MBA) molecules were adhered on the surface of the Au film through strong Au-S bonds to form a dense self-assembled monolayer. Finally, the excess molecules were washed away with a large amount of alcohol and the surface of the Au film was blow dry by using nitrogen.
Si NPs were fabricated by femtosecond laser melting. In this case, femtosecond laser pulses with a wavelength of 800 nm, a pulse duration of 110 fs and a repetition of 1 kHz from a femtosecond amplifier (Legend, Coherent) was used to melt a monocrystalline Si wafer immersed in deionized water. After that, Si NPs with spherical shapes and diameters ranging from 110 to 220 nm were obtained by centrifugation. Finally, dielectric-metal nanocavities in which monolayer of Raman report molecules was embedded was created by dropping the aqueous solution of Si NPs onto the Au film.
2.2 Optical characterization
The scattering spectra of Si NPs (or nanocavities) were collected by a dark-field microscope, which was equipped with a spectrometer (SR-500i-D1, Andor) and an enhanced charge-coupled device (970-BVF, Andor). The Raman scattering measurements were performed by using a Raman spectrometer (Renishaw Invia). The excitation laser light at 633 nm was focused on Si NPs (i.e., nanocavities) by using a 50× objective lens (NA = 0.75) and the Raman signal was collected by using the same objective and directed to the spectrometer for analysis.
2.3 Numerical simulation
In this work, the scattering spectra of nanocavities, the electric field enhancements in nanocavities, and the temperature rises in both dielectric-metal and pure-metallic nanocavities are simulated by using the finite element method (FEM)-based numerical simulations (COMSOL Multiphysics v5.6, https://www.comsol.com). The refractive indices of Si and Au were derived from experimental data [33] or taken from previous literature [34]. The monlayer of Raman reporter molecules is assumed to be a dielectric layer with a thickness of 1.0 nm and a refractive index of 1.4 [35].
3. Results and discussion
The proposed hybrid nanocavity, which is composed Si NPs with a diameter ranging from 110–220 nm, is schematically shown in Fig. 1(a). A typical Si NPs with diameter d = 190 nm supports ED and MD resonances in the visible to near-infrared spectral range, as shown in Fig. 1(b). When a Si NP is placed on an Au film to create a dielectric-metal nanocavity, the excited ED and MD inside the Si NP will interact coherently with their own mirror images caused by the Au film, as schematically illustrated in Fig. 1(c). As a result, the scattering spectrum of the Si NP, which is determined by the interference between the ED and MD with their mirror images, is modified significantly, as shown in Fig. 1(d). Although it is still observed two resonances in the scattering spectrum of the Si NP, their resonant wavelengths and linewidths are different from those observed for the ED and MD resonances.
Fig. 1. (a) Schematic showing hybrid nanocavities obtained by placing Si NPs on a thin Au film. (b) Scattering spectrum calculated for a Si NP with d = 190 nm suspended in air. The total scattering has been decomposed into the contributions of ED and MD resonances. (c) Schematic illustrating the ED and MD excited in the Si NP placed on the Au film and their mirror images induced by the Au film. (d) Scattering spectrum calculated for a Si NP with d = 190 nm placed on an Au/SiO2 substrate. Similarly, the total scattering has been decomposed into the contributions of the two optical resonances originating from the interaction of the ED and MD with their mirror images induced by the Au film.
We first examined the temperature rise induced in a hybrid nanocavity by laser irradiation. The wavelength and power of the laser light are set as 633 nm and 1.0 mW, respectively. The laser beam possesses a Gaussian intensity distribution with a diameter of 1.0 $\mu$ m. The ambient temperature is set to be 25 $^\circ $C. The temperature distribution within a hybrid nanocavity composed of a Si NP (with a diameter of d = 195 nm) and a 50-nm-thick Au film is shown in Fig. 2(a). The highest temperature in the nanocavity is found to be 42.5°$^\circ $C. For comparison, the temperature distribution calculated for a pure-metallic nanocavity composed of an Au NP (with a diameter of d = 195 nm) and a 50-nm-thick Au film is shown in Fig. 2(b). It is noticed that the highest temperature in this nanocavity reaches 65.9$^\circ $C, which have exceeded the threshold temperature (∼50 °C) generally used in photothermal therapy. In addition, a close inspection reveals that the highest temperature appears at the contact point between the Au NP and the Au film, implying that the molecules embedded in the nanocavity can be easily damaged by the heat generated in such a nanocavity. In sharp contrast, the highest temperature appears inside the Si NP in the hybrid nanocavity and the lowest temperature is observed in the hybrid nanocavity between the Si NP and the Au film. This unique feature effectively reduces the thermal damage of the molecules inserted in the hybrid nanocavity. We also compare the temperature rises in the two types of nanocavities composed of Au or Si NPs with different diameters when the optical resonances are resonantly excited by laser light, as shown in Fig. 2(c). In all cases, the temperature rise in hybrid nanocavities are lower than those in pure-metallic nanocavities. In both cases, the highest temperature rise is observed in the nanocavity with the strongest electric field enhancement, which is highly desirable for realizing Raman scattering enhancement. For example, the pure-metallic nanocavity composed of an Au NP with d = 145 nm or the hybrid nanocavity composed of Si NP with d = 184 nm. Considering that the Raman signal enhancement can be achieved by increasing the electric field either at the excitation wavelength or at the emission wavelength, the hybrid nanocavity offers us an additional opportunity for achieving the highest Raman signal enhancement by using a Si NP with a lower heat generation (e.g., d = 195 nm), as demonstrated later.
Fig. 2. (a) Temperature distribution calculated for a hybrid nanocavity composed of a Si NP with d = 195 nm placed on an Au/SiO2 substrate. The gap between the Si NP and the Au film is 1.0 nm. The temperature distribution inside the Si NP is shown in the inset. (b) Temperature distribution calculated for a pure-metallic nanocavity composed of an Au NP with d = 195 nm placed on an Au/SiO2 substrate. The gap between the Au NP and the Au film is 1.0 nm. The temperature distribution inside the Au NP is shown in the inset. (c) Dependence of the highest temperature in the nanocavity on the diameter of the NP calculated for the hybrid and pure-metallic nanocavities. In all cases, the wavelength and power of the irradiated laser were chosen to be 633 nm and 1.0 mW
As mentioned above, a Si NP suspended in air support both ED and MD resonances (see Fig. 1(b)). Now we consider the modification of the ED and MD resonances induced by a thin Au film when the Si NP is moved to an Au/SiO2 substrate. In Fig. 3(a), we show the backward scattering spectrum calculated for a Si NP with d = 190 nm placed on the Au/SiO2 substrate. The backward scattering spectrum of the Si NP suspended in air is also provided for comparison. For the Si NP in the air, one can easily identify the ED and MD resonances which appear at 595 and 721 nm, respectively. In comparison, the hybrid nanocavity exhibits two optical resonances at 644 and 760 nm, which originate from the coherent coupling of ED and MD with their own mirror images caused by the Au film. In addition, it is noticed that the scattering intensity of the hybrid nanocavity is enhanced by nearly one order of magnitude as compared with the Si NP only, indicating that the electric and magnetic field intensities of the hybrid nanocavity are also greatly enhanced, which is shown in Fig. 3(b). In Fig. 3(c) and 3(d), we compare the electric and magnetic field distributions calculated for the hybrid nanocavity and the Si NP only at their optical resonances. Apart from the enhancements in both the electric and magnetic fields, it is noticed that the highest enhancement in the electric and magnetic fields is achieved in the gap region between the Si NP and the Au film is quite similar to that observed in pure-metallic nanocavities [11].
Fig. 3. (a) Comparison of the backward scattering spectrum calculated for a Si NP (d = 190 nm) suspended in air and for a Si NP with the same diameter placed on the Au/SiO2 substrate. The gap between the Si NP and the Au film is 1.0 nm. (b) Electric and magnetic field enhancement factors calculated for the two Si NPs shown in (a). (c)(d) Electric and magnetic field distributions calculated for the two Si NPs at the optical resonances of their scattering spectra.
The optical resonances in the hybrid nanocavity are pretty suitable for SERS. The optical resonance at the short-wavelength arises from the interference between ED and its mirror image. Since they are anti-phase, the corresponding interference leads to a reduced radiation loss (or a narrower linewidth), implying a more significant enhancement in the electric field. Thus, this optical resonance can be employed to enhance the excitation efficiency of the laser. As for the optical resonance at the long wavelength, it attributes to the coupling of MD and its mirror image which are parallel to each other. As a result, an enhanced far-field radiation (or a broadened linewidth) is expected, which facilitates the radiation of the Raman signals [36,37]. Moreover, the broad linewidth of this optical resonance greatly expands the spectral range in which the Raman signal enhancement can be achieved.
To illustrate the idea of using a hybrid nanocavity to realize Raman scattering enhancement, we decompose the total scattering of a Si NP (d = 190 nm) into the contributions of ED and MD resonances, as shown in Fig. 4(a). Similarly, the scattering of the hybrid nanocavity can also be considered as the contributions of the two optical resonances, as shown in Fig. 4(b). The original linewidths of the ED and MD resonances of the Si NP are found to be ∼98 and ∼59 nm. By placing the Si NP on an Au/SiO2 substrate, the linewidth of the ED resonance is reduced to ∼54 nm while that of the MD resonance is increased to ∼104 nm. In addition, a redshift of the resonant wavelength is observed for both the ED and the MD resonances. If the optical resonance at the short-wavelength is used to enhance the excitation laser light at 633 nm, then the enhancement in Raman stokes signals achieved by using the optical resonance at the long-wavelength can cover the wavenumber range from 0 to ∼3921 cm−1.
Fig. 4. (a) Decomposition of the scattering spectrum of a Si nanosphere with d = 190 nm suspended in air into the contributions of the ED and MD resonances. (b) Decomposition of the scattering spectrum of a Si nanosphere with d = 190 nm placed on an Au/SiO2 substrate into the contributions of the optical resonances originating from the ED and MD resonances and their mirror images. (c) Electric field enhancement factors calculated at the wavelengths of the laser light (633 nm) and the two Raman signals (678 and 703 nm) for hybrid nanocavities composed of Si NPs with different diameters. Also shown are the corresponding radiation efficiencies for hybrid nanocavities. (d) Total enhancement factors for two Raman signals at 678 and 703 nm achieved by using hybrid nanocavities composed of Si NPs with different diameters.
Now we examine the electric field enhancements achieved in such hybrid nanocavities at the wavelengths of both the laser light and the Raman signal, which determine the enhancement of the Raman signal. The wavelength of the laser light is chosen at 633 nm and the polarization direction is along with the x-axis. We focus on two Raman signals with Stokes shifts of 1049 and 1573 cm−1, corresponding to wavelengths of 678 and 703 nm. In Fig. 4(c), we present the electric field enhancement factors for the excitation of laser fex = |Eloc(λ0)/E0(λ0)|2 and the emission of Ranam signal fem = |Eloc(λR)/E0(λR)|2 for hybrid nanocavities which is consisted of Si NPs with different diameters ranging from 160–220 nm [38,39]. It is found that the highest enhancement factor fex for the laser light, which is estimated to be ∼141, is achieved in the nanocavity with d = 183 nm. In comparison, the enhancement factor for the Raman signal fem at 678 nm is only 24 for this nanocavity, implying that the traditional enhancement factor expression EF = |E/E0|4 is no longer applicable in such a hybrid nanocavity because the electric field enhancement factors for the laser light and Raman signal can be much different [40].
Based on the above discussion, it is clear that one needs to consider simultaneously the electric field enhancement factors at the wavelengths of both the laser light and the Raman signal in order to fully exploiting the advantages of such hybrid nanocavities. In addition, the collection (or radiation) efficiency of the Raman signal should also be taken into account because the nanocavity act as a nano-antenna which radiates the Raman signal into the far-field. Since the orientations of the dipole moments of reporter molecules are random, we calculate the averaged value of the radiation efficiencies for three dipoles oriented along the x, y, z directions, respectively. In the numerical simulation, the collection efficiency related to the objective (NA=0.75) used to collect the Raman signals is simulated by setting the area of the detector. Based on far-field projection [41], the collection efficiency of the Raman signal can be expressed as:
After considering the enhancement in the excitation, emission and collection efficiencies, the total gain (TG) of the Raman signal can be derived as follows:
The relationship between the TG and the diameter of the Si NP is shown in Fig. 4(d). It is noticed that the highest TG for the Raman signal at 678 nm is not achieved in the nanocavity with the largest enhancement factor for the laser light (i.e., d = 184 nm). Instead, it is realized in the nanocavity with a larger Si NP (d = 195 nm).
In experiments, we firstly examined the optical properties of the hybrid nanocavities constructed by using Si NPs with different diameters. In Fig. 5(a), we show the forward scattering spectrum measured for a nanocavity by using a dark-field microscope. The scattering spectrum calculated for the nanocavity with d = 180 nm is also provided for comparison. It can be seen that the measured scattering spectrum is in good agreement with the simulated one except for some details. In Fig. 5(b), we show the forward scattering spectra calculated for Si NPs with d = 160–220 nm. One can see a redshift of the two optical resonances, which originate from the coherent coupling of the ED and MD with their mirror images, with increasing diameter of the Si NP. If we examine the peak wavelength of the optical resonance and the diameter of the Si NP, a linear relationship is observed, as shown in Fig. 5(c). In Fig. 5(d), we show the forward scattering spectra measured for six Si NPs with different diameters. Based on the numerical simulations, the diameters of the six Si NPs are estimated to be ∼170, ∼178, ∼181, ∼193, ∼197, and ∼202 nm, respectively. The diameters obtained for Si NPs are in good agreement with those obtained based on the scanning electron microscopy (SEM), as verified in the following.
Fig. 5. (a) Forward scattering measured for a Si NP with d ∼ 180 nm placed on the Au/SiO2 substrate. The calculated scattering spectrum is also provided for comparison. (b) Forward scattering spectra calculated for Si NPs with different diameters placed on an Au/SiO2 substrate. (c) Dependence of the peak wavelength of the scattering spectrum on the diameter of the Si NP. (d) Forward scattering spectra measured for Si NPs with different diameters placed on an Au/SiO2 substrate.
In Fig. 6(a), we show the dark-field image obtained for Si NPs randomly distributed on the Au film. The diameters of Si NPs are closely related to the colors of the scattering light appearing in the image. While the green colors represent Si NPs with small diameters, the red ones correspond to Si NPs with large diameters. In Fig. 6(b), we demonstrate the Raman scattering spectra obtained from the six hybrid nanocavities consisting of Si NPs with different diameters whose forward scattering spectrum is shown in Fig. 5(d) and dark-field image in Fig. 6(a). The SEM images of the corresponding Si NPs are provided as insets. In all cases, two Raman signals with stokes shifts of 1049 and 1573 cm−1 are clearly resolved in the spectra. As expected, the weakest Raman signals existed in the nanocavity with d = 170 nm. If we compared the experimental observation with the prediction based on the numerical simulation (see Fig. 4(d)), a good agreement is observed, as discussed in detail in the following.
Fig. 6. (a) CCD image of the Si NPs distributed on the Au/SiO2 substrate obtained by using a dark-field microscope. (b) Raman scattering spectra measured for the monolayer of report molecules embedded in hybrid nanocavities composed of Si NPs with different diameters. The SEM images of the corresponding Si NPs are shown as insets. The length of the scale bar is 200 nm. (c) Comparison of the Raman spectrum of the reporter molecules embedded in a hybrid nanocavity with d = 170 nm with that of the report molecules on the Au film. The laser power used to excite the reporter molecules on the Au film was increased from 1.0 to 10 mW.
For the nanocavity with d = 170 nm (see No. 1 in Fig. 4(d) and black curve in Fig. 6(b)), the smallest TG is observed because both the laser light and the Raman signals are far away from the optical resonances (No. 1 in Fig. 4(c)). When d is increased to 178 nm, the laser light approaches the first optical resonance at the short wavelength which is induced by ED coupling, leading to a larger enhancement factor. However, the Raman signal is located at the valley between the two optical resonances, leading to a small enhancement factor and a low collection efficiency for the Raman signal (Fig. 4(c), No. 2). Consequently, only a slight increase in the TG is achieved in this nanocavity (see No. 2 in Fig. 4(d) and red curve in Fig. 6(b)). As the NP diameter continues to increases (d = 181 nm), the laser light is resonant with the first optical resonance at the short wavelength. Still, the Raman signal did not approach the second optical resonance induced by the MD coupling at the long wavelength and the wavelength range with high collection efficiency (No. 3 in Fig. 4(c)). One can see a further increase in the TG (see No. 3 in Fig. 4(d) and blue curve in Fig. 6(b)). When d is increased to 193 nm, the laser light is shifted to the long-wavelength side of the first optical resonance. At the same time, the Raman signal is close to the second optical resonance and the peak of collection efficiency (Fig. 4(c), No. 4). As a result, the strongest enhancement in the Raman signal is observed in this nanocavity (see No. 4 in Fig. 4(d) and green curve in Fig. 6(b)). Moreover, we compare the TGs obtained by using Si NPs with d = 181 and 193 nm, it is found that the latter is larger than the former, although the largest enhancement factor for the laser light is observed in the former. It indicates that the second optical resonance related to the MD resonance also plays an important role in enhancing the Raman signals. For d = 197 nm, the enhancement factor for the laser light becomes smaller while those for the emission and collection efficiencies of the Raman signals become larger (Fig. 4(c), No. 5). Consequently, the TG is reduced (see No. 5 in Fig. 4(d) and the purple curve in Fig. 6(b)). It is remarkable that the TG for the case with d = 197 nm (No. 5) is larger than that with d = 181 nm (No. 3). It suggests again that the optical resonance related to the MD resonance is more important than that related to the ED resonance in enhancing Raman signals. When d is increased to 202 nm, the optical resonances of the nanocavity are shifted to the long-wavelength side of both the laser light and the Raman signals (Fig. 4(c), No. 6). Accordingly, a further reduction in the TG is observed (see No. 6 in Fig. 4(d) and the yellow curve in Fig. 6(b)).
Specially, it is found that the TG for another Raman signal at 703 nm also exhibits a similar evolution when the diameter of the Si NP is increased, after taking into account the enhancement factors for excitation, emission and collection. For Si NPs with diameters smaller than 181 nm, the difference in the TGs for the two Raman signals is not apparent (Fig.4d, No. 1/2/3). It is noticed, however, that the TG for the Raman signal at 678 nm becomes larger than that at 703 nm when the diameter of the Si NP is increased to ∼195 nm (Fig. 4(d), No. 4). When the diameter of the Si NP is further increased, the difference between them becomes smaller again (Fig. 4(d), No. 5/6). This prediction based on numerical simulation is verified by the measurements of the Raman spectrum for hybrid nanocavities with different Si NPs diameters (Fig. 6(b)). In order to quantitatively evaluate the TG, we increase the power of the laser light from 1.0 to 10 mW and measure the Raman spectrum of 4-MBA molecules on the Au film, as shown in Fig. 6(c). It is found that the two Raman signals are not resolved in the spectrum. In sharp contrast, Raman intensity of ∼200 is obtained for the hybrid nanocavity with d = 170 nm, indicating the smallest enhancement among all nanocavities. In the case with d = 193 nm where all the three conditions for the enhancement of Raman signals are simultaneously satisfied, the Raman signal at 678 nm is enhanced by factors of 3.46, 1.78, 2.64, 6.1, and 12 as compared with the nanocavities with d = 202, 197, 181, 178, and 170 nm, respectively.
4. Conclusion
In conclusion, we propose the hybrid nanocavities composed of Si NPs and an Au film to achieve significant enhancement in Raman signals. As compared with pure-metallic nanocavities, the heat generation in such hybrid nanocavities is reduced dramatically, leading to a low-temperature rise in the nanocavities. The electric field enhancement at the optical resonances attributing to the interference of the ED and MD and their mirror images are employed to enhance the excitation, emission and radiation efficiencies of the Raman signals. It is demonstrated that the total gain for Raman signals is determined not only by the enhancement in the excitation efficiency of laser light but also by the radiation efficiency of the nanocavity which acts as a nano-antenna to radiate Raman signals into the far-field. By optimizing the size of the Si NP, a TG as large as ∼2×105 can be achieved in the hybrid nanocavity with d = 195 nm. Due to the broadband, high enhancement and low heat characteristics, the proposed hybrid nanocavities are suitable not only for the SERS of biochemical molecules but also for studying the optical properties of two-dimensional materials, such as strong plasmon-exciton interaction and photoluminescence or second harmonic generation. Therefore, our findings indicate the potential applications of these hybrid systems in temperature-sensitive situations and provide a strategy for designing repeatable and reliable photonic devices.
Funding
National Natural Science Foundation of China (61727814, 61875059).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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