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Abstract
Near-field scanning optical microscopy (NSOM) is a powerful tool for study of the nanoscale information of objects by measuring their near-field electric field distributions. The near-field probe, which determines NSOM system performance, can be either a scattering-type or an aperture-type. Both types have strengths and weaknesses. Here we propose and study a surface plasmon-coupled type nano-probe, which works as a hybrid scheme and could potentially combine the advantages of the two NSOM probe types. The key element of the proposed probe is a nanoparticle-on-film structure designed on a tapered fiber tip. On the one hand, the probe can yield the signals scattered in the near field by a nanoparticle with a scattering mechanism; on the other hand, the scattered signals can be transmitted by the metal film and coupled into the fiber via surface plasmon coupled emission, thus providing a collection mode similar to an aperture-type NSOM. This will lead to signal enhancement, while greatly suppressing background noise. This surface plasmon-coupled nano-probe thus has great potential for near-field optical microscopy applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The near-field scanning optical microscope (NSOM) is an important technology in modern nano-optics that enables probing of the complex electromagnetic fields in the near-field region with ultra-high resolution. The concept and the initial model of the NSOM were first proposed by Synge in 1928, although it was not until the 1990s that the technique entered into a new era following the rapid development of nano-fabrication and nano-manipulation technologies [1–4]. To achieve improved resolution, higher collection efficiency and a remarkable signal-to-noise ratio, various types of near-field nano-probe have been designed and demonstrated, including probes based on tapered fiber tips [5–7], metallic/silicon cantilever tips, [8] nano-apertures [9–18], and nanoparticles [19–21].
NSOMs are classified into two categories based on the collection mode of the tip used to perform the near-field mapping operation: scattering-type (s-NSOM) or aperture-type NSOM (a-NSOM) [22]. The s-NSOM is equipped with an apertureless tip that scatters the near-field high-spatial-frequency signal to the far field. A low numerical aperture (NA) objective lens with a long working distance is commonly used to collect the weak scattering signals. The s-NSOM is generally sensitive to the longitudinal (out-of-plane) component of the electric field because of the special alignment of the probe. The a-NSOM, in contrast, collects signals directly by coupling the near-field electromagnetic fields to a fiber via the aperture; the signals are then directed to a photomultiplier to be characterized. The aperture-type probes are generally sensitive to the transverse (in-plane) field components in the visible spectral range because they have a higher coupling efficiency than the longitudinal components [22]. One inevitable factor associated with a-NSOMs is that the collected signal contains background radiation induced by the incident light. This will lead to reduction of both the signal-to-noise ratio and the precision of the mapping results. The s-NSOM can reduce the effects of the background light to a certain degree by using an oblique collection configuration and introducing a lock-in amplifier. However, this usually requires an objective lens with a low NA, which greatly reduces the collection efficiency. In addition, the introduction of the lock-in amplifier will also increase the system complexity.
Here, we propose and numerically study a near-field probe that could potentially combine the advantages associated with the two NSOM probe types. The key element of the proposed near-field probe is a nanoparticle-on-film (NPOF) structure (Fig. 1(a)), which is deposited and fabricated on a tapered fiber tip (Fig. 2(a)). Use of this type of NPOF structure as a probe could, on the one hand, yield the scattered signals in the near field using a nanoparticle with a scattering mechanism; on the other hand, the scattered signals could be transmitted via the metal film and coupled into the fiber via surface plasmon-coupled emission (SPCE), thus enabling a collection mode similar to that of an a-NSOM. SPCE is an unusual phenomenon where the scattered radiation from the NP can pass through the metal film and be re-radiated at the resonance angle of the surface plasmon polaritons (SPPs) with a major enhancement [20, 23,24]. Simultaneously, the presence of the metal film could block the incident radiation to a great extent. Both aspects will lead to an improvement in the signal-to-noise ratio of the detection system with the proposed probe. Furthermore, the surface plasmon gap mode, which is supported by the NPOF structure, is polarization-dependent [25,26]. This allows us to tune the probe’s polarization sensitivity by appropriate design of its structural parameters. This type of surface plasmon-coupled nano-probe has major potential for applications in near-field optical microscopy.
2. Design of the probe
Before designing the nano-probe, we begin by studying the SPCE phenomenon when it occurs on an NPOF structure. A schematic diagram of the structure is shown in Fig. 1(a), where an Au nanoparticle with radius r0 is attached to a thin Au film. This thin film was deposited on a silica substrate. Three-dimensional finite-difference time-domain (FDTD) simulations (Lumerical, FDTD Solution) were performed to investigate the optical response of the structure. In the simulation, the Au film thickness was chosen to be 50 nm. The NP radius was varied from 20 nm to 70 nm in 10 nm increments. The refractive index of silica was set at 1.5, while that of the Au film was taken from the database within the software. An incident beam with a total field scattered field (TFSF) mode (mode area: 200nm×240 nm) was used as the illumination source, with a spectrum ranging from 400 nm to 800 nm. The beam illuminated the structure from the top (NP) side to the bottom (substrate) side. Perfectly matched layer (PML) boundary conditions were set for all of the three directions.
Fig. 1. (a) Schematic diagram of NPOF structure used to realize SPCE. (b) Far-field Fourier domain pattern of the scattered radiation from the NP on the metal film, where the two bright arcs demonstrate the SPCE feature. (c) Transmission (T) spectra of SPCE of the NPOF structure at various NP sizes. (d) The spectra of isolated NPs of different sizes for comparison purposes. The curved arrows in (c) and (d) indicate the variation of the NP size from 20nm to 70nm with a 10nm increment
Figure 1(b) shows the far-field Fourier domain pattern of the scattered radiation from the NP that is transmitted along the Au film when the incident beam is x-polarized. Two bright arcs (the SPCE signals) aligned along the x-axis are clearly shown. It can be seen that the intensity of the SPCE from the NP is far greater than that of the central incident light that is transmitted directly through the Au film. The SPCE transmission spectra for several different NP sizes are shown in Fig. 1(c). For comparison, the spectra of the individual NPs in the absence of the metal film and the substrate were also calculated and the results are shown in Fig. 1(d). It was found that despite the presence of the metal film in the NPOF structure, which would cause a dramatic decline in the light transmission (normally by one order of magnitude), the SPCE intensity remains comparable to the radiation that is scattered directly from the NP. This clearly indicates the major signal enhancement caused by the surface plasmon coupling.
A surface plasmon-coupled nano-probe was then designed by combining the NPOF structure with a fiber tip, as illustrated in Fig. 2. The NP and the Au film are placed on the core of a tapered cone fiber to realize SPCE. The function of the cone is to increase the NA of the fiber. The vertical height of the cone is denoted by h, and the radii of the cone and the fiber are denoted by r and R, respectively. A monitor is placed in the fiber at a distance H below the NP to analyze the SPCE signals that are emitted into the fiber. In the subsequent 3D-FDTD simulations, the Au film thickness was fixed at 50 nm. The incident beam was also the TFSF mode with the same size as above. The mesh grids in the NP and in the Au film were set at 2 nm and 5 nm, respectively, while the grid size in other areas was set at 40 nm. Perfectly matched layer (PML) boundary condition was set for all of the three directions.
Fig. 2. (a) Schematic diagram of surface plasmon-coupled nano-probe and the NSOM system. (b) The cross-section view of the probe where a NPOF structure was designed on a tapered cone fiber.
The cone angle is important to guarantee stable transmission of the SPCE signal within the fiber. According to transmission theory, the angle of incidence after the Nth total reflection (θn) in the fiber is given by:
where θ is the angle of incidence of the first reflection and α is half the cone angle. If it is assumed that the SPCE is only reflected once in the cone, then according to the transmission theory of cones and fibers, the conditions for favorable transmission of SPCE in the fiber should satisfy the following:The curve of the SPCE angles over the spectral range from 400 nm to 800 nm is presented in Fig. 3; this curve was calculated using transfer matrix theory on a multilayer system. Based on the SPCE angle data, the two cone angles were designed as shown in the following to investigate the SPCE transmission properties of the fiber, i.e., α = arctan3/5 and α = arctan3, where the former satisfies the stable transmission condition and the latter does not.
Fig. 3. SPCE angles over the wavelength range from 400 nm to 800 nm when the Au film is 50 nm thick and is deposited on a silica substrate.
Figure 4(a) shows the SPCE spectra obtained in the fiber at various values of H when r0 = 40 nm, r = 1 µm, R = 4 µm and h = 5 µm. This fiber shape corresponds to a cone angle of α = arctan3/5. To provide a clear demonstration of the transmission variations, the peak intensities were detected and plotted versus H, as shown in Fig. 4(b) (square dots). The signal intensity is shown to decrease initially when the monitor moves away from the NP, but tends to become stable when the monitor is sufficiently far away. For comparison, the curve for α = arctan3 (r = 1 µm, R = 4 µm, h = 1 µm) was plotted along with the curve for α = arctan3/5 in Fig. 4(b) (circular dots). The continuous reduction in the intensity can be observed when the monitor is moved away, thus indicating the unstable transmission of the SPCE signal under these circumstances. Figure 4(c) and (d) show the electric field distributions in the x-y plane of the fiber when H is 6 µm for α = arctan3 and α = arctan3/5, respectively, while Fig. 4(e) and (f) show the corresponding distributions in the x-z plane. Figure 4(d) and Fig. 4(f) clearly show that the SPCE is well confined within the fiber when α = arctan3/5, while the SPCE energy leaks out of the fiber when α = arctan3, as illustrated in both Fig. 4(c) and Fig. 4(e). These results further verify the preceding analysis above.
Fig. 4. (a) SPCE transmission spectra of the fiber probe obtained in the fiber at various distances to the NP when the cone angle of the probe α = arctan3/5. (b) The transmission at resonance (Tres) as obtained from (a) plotted against the monitor distance for both cone angles of arctan3/5 and arctan3. (c) and (d) Electric field distributions in the horizontal plane in the fiber when α = arctan3 and arctan3/5, respectively. (e) and (f) Corresponding distributions in the vertical plane of the probe.
3. Results and discussion
Under the stable transmission condition, we further investigated the NP size effect on the transmission efficiency and the calculation results are shown in Fig. 5(a). The transmission efficiency generally shows a growth trend as the NP size increases. The resonance wavelengths and the peak intensities were obtained from Fig. 5(a) and were then plotted against the NP size, as shown in Fig. 5(b) and Fig. 5(c), respectively. For comparison purposes, the same curves for the SPCE from an NPOF structure on a silica substrate (Fig. 1(a)) and for the scattered radiation from an isolated Au nanoparticle were calculated and are shown together in Fig. 5(b)–(c). The NP can be regarded as a resonator, which means that the resonance wavelength is red-shifted as the NP size increases. However, the resonance wavelengths of the NPOF structure and the fiber probe show larger shifts due to the stronger interaction of the nanoparticle with the substrate. The transmission efficiency of the scattered signal for the fiber structure is comparable to that of the SPCE in the NPOF structure, as shown in Fig. 5(c). This verifies that the SPCE effect also applies when the NPOF structure is designed on a fiber probe.
Fig. 5. (a) SPCE transmission spectra of the fiber probe for different NP sizes. The curved arrow indicates the variation of the NP size from 20 nm to 70 nm with a 10 nm increment. (b) Resonance wavelengths plotted versus the NP sizes for the fiber probe structures, the bare NPOF structure and an isolated Au nanoparticle. (c) Corresponding resonant transmission efficiencies for the structures shown in (b).
The fiber probe transmission characteristics are also affected by the size of the gap between the NP and the metal film. Figure 6 shows the variations in the resonance wavelengths and the peak efficiencies of the probe when the NP-to-film gap is varied from 1 nm to 10 nm. In this case, the resonance wavelength is blue-shifted as the gap increases. This is expected, because the increased gap will cause weaker interaction between the NP and the metal film and thus lead to a shorter resonance wavelength [27]. The SPCE efficiency reaches its highest value when the gap between the NP and the film is 4 nm. The insets of Fig. 6(a) and (b) show the peak wavelength and SPCE efficiency characteristics versus the gap between the NP and the Au film on the glass substrate for comparison purposes. Nearly consistent changes appear in the two different structures.
Fig. 6. (a) Resonance wavelengths of the SPCE of the fiber probe when varying the gap size between the NP and the Au film. (b) Resonant transmission efficiencies of the fiber probe at various gap sizes. The insets show the corresponding calculation results for the bare NPOF structure on a silica substrate. The size of NP is set at 40 nm and remaining the same for the simulation.
4. Summary
In summary, a type of surface plasmon-coupled nano-probe is designed by combining an NPOF structure with a fiber tip. The SPCE effect in the fiber probe is investigated and verified using FDTD simulations. The cone angle of the fiber probe must be designed carefully to meet the required conditions for stable transmission of the SPCE in the fiber. The effects of both the NP size and the NP-to-film gap on the SPCE characteristics were studied and compared with those of a bare NPOF structure and an isolated Au nanoparticle. The simulation results presented in this work will provide the basis for experimental study of the probe in future work. This type of surface plasmon-coupled nano-probe has considerable potential for applications in near-field optical microscopy.
Funding
National Natural Science Foundation of China (61427819, 61490712, 61622504, 61705135, 61905163); Leading Talents Program of Guangdong Province (00201505); Natural Science Foundation of Guangdong Province (2016A030312010); Science, Technology and Innovation Commission of Shenzhen Municipality (KQTD2015071016560101, KQTD2017033011044403, ZDSYS201703031605029); China Postdoctoral Science Foundation (2017M622765, 2018M643161).
Acknowledgments
L. Du acknowledges the support given by Guangdong Special Support Program.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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