By means of nonlinear interactions between light and matter, nonlinear optical microscopy has become an important tool to produce sensitive optical images and spectroscopy, including coherent and incoherent processes . The coherent microscopes of coherent anti-Stokes Raman scattering (CARS) , , , second-harmonic generation , ,  and stimulated Raman scattering  generate nonlinear optical signals, whose phases are strictly stipulated by many aspects, such as the phase of excitation light and the steric distribution of target molecules. The nonlinear coherent microscopy is ground on the simultaneous scattering of two or more photons, and the coherent signal intensity is proportional to M2, where M is the concentration of radiating molecules. For incoherent microscopies, such as the two-photon excited fluorescence (TPEF) microscopy , , , they generate nonlinear optical signals, whose phases are random, and the intensity is proportional to M. Compared with the confocal Raman/fluorescence microscopy, the TPEF microscopy can penetrate deeper in thick samples. Normally, the nonlinear interactions of TPEF and CARS only occur in specific intrinsic biological molecules  or two-dimensional (2D) materials , which provide optical imaging and spectroscopy without the need for exogenous stains, especially for in vivo applications.
The optical nonlinearity originating from the material properties of interacting media are inherently weak, especially for CARS. As the coherent Raman signal is proportional to N(N−1), where N is the number of molecules , the coherence enhancement decreases with the decrease of N and vanishes for a single molecule. Thus, measuring the coherent Raman imaging at nanoscale is a highly challenging task because of the smaller number of molecules. The nanoscale confinement of optical fields using plasmonic nanostructures provides an effective strategy for enhancing the Raman/fluorescence spectroscopy . The surface plasmon arises from the collective coherent oscillation of free electrons at an optical frequency, which can significantly enhance both the incoherent and coherent nonlinear processes, including the plasmon-enhanced CARS , second-harmonic generation , , hyper-Raman scattering  and TPEF . Moreover, the plasmonic Au/Ag nanostructures are also capable of generating inherent nonlinear optical processes , including second-harmonic generation , ; excited-state absorption , multiphoton, third-harmonic generation , ; four-wave mixing  and photoluminescence (PL) . Given that just one plasmon resonance (fundamental or double) frequency can be excited for near-field enhancement, the enhancement factor is quite small for the normal plasmonic nanoparticles. Therefore, finding a special nanostructure is very important as this can enhance nonlinear optical signals at fundamental and double frequencies, i.e. multiple surface plasmon resonances (MSPRs).
In this paper, the Au@Ag nanorods are synthesized in order to demonstrate the MSPRs at the fundamental and double frequencies of 800 and 400 nm, respectively. By employing a 2D material of g-C3N4, we demonstrate that the nonlinear optical microscopy of TPEF and two-photon CARS can be significantly enhanced by the MSPRs. The simulations strongly support the experimental observations. The results also indicate that a selected Au@Ag nanorod can totally enhance the two-photon CARS of 2D materials, and provide a simple method using the MSPRs to enhance nonlinear optical microscopy.
2.1 Synthesis of the Au@Ag nanorods
All reagents were used without further purification, and deionized water was used in the experiment. HAuCl4, Hexadecyltrimethylammonium bromide (CTAB), sodium borohydride (NaBH4), silver nitrate (AgNO3) and sodium oleate (NaOL) were purchased from Sigma Aldrich (Shanghai, China). Hydrochloric acid (HCl), Sodium hydroxide (NaOH), ascorbic acid and glycine were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
The Au nanorods were synthesized using an improved seed-mediated wet-chemical method . The solution of Au seed was made by adding 0.25 ml of HAuCl4 (0.01 m) into the 9.75 ml of CTAB (0.1 m), to which 0.048 ml of NaBH4 (0.1 m) aqueous was added while stirring at 1200 rpm for 1 min. For the growth solution, 1.234 g NaOL and 0.7 g CTAB were added into 50 ml water, while stirring at 70°C, after which 0.72 ml of AgNO3 (0.01 m) was injected into the solution while the temperature was reduced to 30°C. After 15 min, 2.5 ml of HAuCl4 (0.01 m) was added to the solution while stirring for 90 min. Then, 1.8 ml of HCl (1 m) was added to adjust the pH, after which 0.08 ml of ascorbic acid (0.1 m) and 0.04 ml seeds solution were added and maintained at a temperature of 30°C for 12 h.
The Au@Ag nanorods were synthesized on the ground above the Au nanorods . The Au nanorods were re-dispersed in 20 ml of CTAB (0.1 m) solution, after which 17.8 ml of glycine buffer solution (0.4 m) and 1.2 ml of AgNO3 (0.01 m) were added while stirring. Then, 0.6 ml of ascorbic acid (0.1 m) were added into the solution while stirring for 3 hours. The prepared Au@Ag nanorods were dispersed onto the g-C3N4 layer on glass by spin coating.
2.2 Characterization and measurements
The scanning electron microscope (SEM) images were obtained by a FEI-Nova Nano SEM 450 (Hillsboro, OR, USA) operating at a voltage of 10 kV, and the transmission electron microscopy (TEM) images were collected from the JEOL 2100 (Akishima, Tokyo, Japan) operating at an acceleration voltage of 200 kV. The mappings were obtained on field-emission transmission electron microscope (FE-TEM) at a voltage of 200 kV. The UV-visible extinction spectra were measured by using a Perkin Elmer Lambda 950 spectrometer (Waltham, MA, USA). The detailed instruments and measurements are described in previous works . In short, the TPEF and CARS images are measured with the nonlinear optical microscopy. A femtosecond mode-locked Ti:Sapphire laser of 800 nm (80 fs and 80 MHz) was employed in this nonlinear optical microscopy system.
The extinction spectrum and electric field distribution were simulated by finite element method with COMSOL Multiphysics. The geometric parameters of the Au@Ag nanorods and the direction of polarization of the incident light were set. The inner nanostructure comprised an Au nanorod with a length of 100 nm and a diameter of 12 nm. A 10 nm silver layer shell was coated on each Au nanorod. The nanorods were located in the x-y plane, and a plane wave as the incident light was set in a direction with an angle of θ.
3 Results and discussion
The plasmon-enhanced nonlinear optical microscopy of g-C3N4 was studied by using the MSPRs of the Au@Ag nanorods (Figure 1A). The SEM images of the plasmonic Au@Ag nanorods shown in Figure 1B–D demonstrate that the Au@Ag nanorods are successfully synthesized, and a silver shell is coated onto each Au nanorod with a thickness of around 10 nm. Compared with the Au nanorods, the resonance peaks at 910 and 520 nm indicate a blue shift to 800 and 500 nm for the Au@Ag nanorods, and a new resonance peak at 400 nm can be obtained (Figure S1).
The plasmon-enhanced TPEF of g-C3N4 covered with the Au@Ag nanorods was first studied (Figure 2). According the absorption and PL spectra of g-C3N4 shown in Figure 2B, a strong extinction peak can be observed around 400 nm, which matches the fundamental frequency at 800 nm of the incident light. The TPEF images of g-C3N4 without (Figure 2C) and with (Figure 2D the Au@Ag nanorods were obtained by 800 nm excitation. By using strong MSPRs at 800 and 400 nm, the TPEF can be strongly enhanced via the Au@Ag nanorods covering on the g-C3N4 surface.
In order to decrease the PL intensity and promote the Raman efficiency, a defecting g-C3N4 by N vocation on tris-s-triazine was used to measure the plasmon-enhanced CARS. As shown in Figure 3, the TPEF and CARS images are obtained from a defected g-C3N4 without (A–D) and with (E–H) the Au@Ag nanorods. The TPEF images demonstrate that the PL intensity is significantly decreased (Figure 3B). However, the two-photon CARS image is too weak to be observed (Figure 3C). To enhance the intensity and resolutions of TPEF and CARS, the Au@Ag nanorods were covered on the defected g-C3N4 (Figure 3E). Due to the extremely weak PL intensity of g-C3N4, no obvious enhanced signals can be observed from the enhanced TPEF image in Figure 3F. However, the CARS signals are extremely enhanced by the Au@Ag nanorods because of a very strong nonlinear electric magnetic field enhancement (Figure 3G). Compared with the merged images (Figure 3D and H), the CARS signals are dominantly observed from the merged plasmon-enhanced TPEF and CARS images.
In order to understand the above results, the simulated electric analyses for TPEF and CARS were done by FEM (Figure 4). We found that the MSPR models can be controlled by the angle of incident light. By scanning the nanorod length and shell thickness, the geometrical parameters of the Au@Ag nanorod are well optimized to match the MSPR peaks of 800 and 400 nm, which agrees with the experimental geometry (Figure S2). As shown in the extinction spectrum in Figure 4B, the strongest SPR peaks at 800 nm and 400 nm result from the excitation along the long (0°) and short (90°) axes, respectively. When the incident angle is 0°, a longitudinal mode is excited, and the strong electric field enhancements can be found at the terminals of the Au@Ag nanorods (Figure 4C). When the incident angle is 90°, a horizontal mode is excited, and a significant enhancement of electric field on both short sides of the Au@Ag nanorod can be observed for the resonance mode at the wavelength of 400 nm (Figure 4D).
The electric field enhancement factor (EF) of TPEF and the two-photon CARS are respectively written as 
where ω and ωs are the frequencies of incident light and scattered light, respectively, and |g| is the local electric field enhancement around the probe molecules. According the experimental system, we investigated the incident angles depending on the extinction spectrum of the Au@Ag nanorods and the EF of the TPEF and CARS. Figure 4B shows the extinction spectra of the Au@Ag nanorod structure with incident angles varying from 0° to 90°. The intensity of the resonance peak at 800 nm is increased with the decrease of the resonance peak at 400 nm. The extinction spectrum well matches the experimental extinction spectrum when the incident angle is 30°, which can be attributed to the average effect in experiments. According to Equations (1) and (2), we calculated the EF of TPEF and CARS of the Au@Ag nanorod by the electric field enhancement at 800 nm and 400 nm. The incident angle-dependent electric field enhancement for plasmon-enhanced TPEF and CARS demonstrate that the angle of the strongest plasmon enhancements is 30°, which is up to 104 and 1016, respectively (Figure 4E and F). As can be seen on the distribution for the TPEF and CARS enhancements shown in Figure 4G and H, a strong electric field enhancement can be observed at the end and both sides of the Au@Ag nanorods, because the symmetry is broken by the oblique incidence and some multipolar resonance mode can be excited. The electric field distributions of other incident angles are shown in the supporting information (Figures S3–5). Therefore, the EF of the TPEF and two-photon CARS can reach the maximum of 1.6×104 and 6×1016 with an incident angle of 30°, respectively, which are larger than those of the normal fluorescence and CARS, respectively.
Using the chemically synthesized Au@Ag nanorods, the plasmon-enhanced nonlinear optical microscopy of the TPEF and two-photon CARS were revealed by the MSPRs. The nonlinear optical signals of g-C3N4 is significantly enhanced due to the double-enhanced frequencies at 800 and 400 nm. Moreover, the CARS signals can be clearly distinguished from the merge nonlinear optical images. The simulation results strongly support the experimental observations, and the calculated enhancements reach up to 104 and 1016 for the TPEF and two-photon CARS, respectively. The new method of using the MSPRs not only enhance the nonlinear optical signal intensity but also has the potential to improve imaging accuracy and resolution in future applications.
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The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2018-0231).
About the article
Published Online: 2019-02-07
Funding Source: National Natural Science Foundation of China
Award identifier / Grant number: 91436102
Award identifier / Grant number: 11374353
Funding Source: Central Universities in USTB and SNNU
Award identifier / Grant number: GK201701008
Funding Source: National Basic Research Program of China
Award identifier / Grant number: 2016YFA0200802
Funding Source: 111 Project
Award identifier / Grant number: B170003
This work was supported by the National Natural Science Foundation of China (Grant Nos. 91436102 and 11374353), the Fundamental Research Funds for the Central Universities in USTB and SNNU (GK201701008), the National Basic Research Program of China (Grant No. 2016YFA0200802) and the 111 Project (Grant No. B170003).
Citation Information: Nanophotonics, Volume 8, Issue 3, Pages 487–493, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2018-0231.
©2019 Mengtao Sun, Zhenglong Zhang et al., published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution 4.0 Public License. BY 4.0