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Abstract
Bowtie nano-apertures can confine light into deep subwavelength volumes with extreme field enhancement, making them a useful tool for various applications such as optical trapping, deep subwavelength imaging, nanolithography, and sensors. However, the correlation between the near- and far-field properties of bowtie nano-aperture arrays has yet to be fully explored. In this study, we experimentally investigated the polarization-dependent surface plasmon resonance in bowtie nano-aperture arrays using both optical transmission spectroscopy and photoemission electron microscopy. The experimental results reveal a nonlinear redshift in the transmission spectra as the gap size of the bowtie nanoaperture decreases for vertically polarized light, while the transmission spectra remain unchanged with different gap sizes for horizontally polarized light. To elucidate the underlying mechanisms, we present simulated charge and current distributions, revealing how the electrons respond to light and generate the plasmonic fields. These near-field distributions were verified by photoemission electron microscopy. This study provides a comprehensive understanding of the plasmonic properties of bowtie nano-aperture, enabling their further applications, one of which is the optical switching of the resonance wavelength in the widely used visible spectral region without changing the geometry of the nanostructure.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
When light passes through an array of subwavelength apertures in a metal film, its transmission is several orders of magnitude higher than that expected by classical aperture theory [1,2]. This optical phenomenon is called extraordinary optical transmission (EOT) and is known to be caused by surface plasmons [3–6]. Surface plasmons are collective charge oscillations created by the resonant interaction of light and free electrons at the metal-dielectric interface [7–9]. Surface plasmons enable the strong confinement of electromagnetic fields in nanoscale regions. This unique characteristic has led to many applications in surface-enhanced Raman scattering [10], fluorescence enhancement [11], biological sensing [12], nanocircuits [13], photovoltaic devices [14], and nanoscale light source [15]. Since the properties of surface plasmons strongly depend on the shape of the nanostructure, various plasmonic nanostructures have been designed and studied to manipulate the electromagnetic field on the nanoscale [16–20]. Bowtie nano-apertures have attracted significant attention due to their capabilities of squeezing light into subwavelength volumes to greatly enhance the localized electromagnetic fields in the gaps [21]. These strong near-field gap modes are particularly useful for advanced applications in optical trapping [22] and deep-subwavelength imaging [23]. In addition, the EOT characteristics of bowtie nano-apertures for dependence on the geometry [24,25] and polarization state of light [26,27] have been studied and applied to nanolithography [28] and sensors [29]. However, the correlation between the near- and far-fields of a bowtie nano-aperture has not yet been explored and experimental verification of its near-field distribution is still lacking. A recent theoretical study [27] revealed polarization-dependent surface plasmon resonance (SPR), which can be utilized for wavelength-tunable optical filters without mechanical deformation of the film. This work experimentally demonstrates polarization-dependent SPR through far- and near-field studies, using optical transmission spectroscopy and photoemission electron microscopy (PEEM), respectively. This study demonstrates a way to switch the resonance wavelength in the visible spectral region without changing the nanostructure geometry.
In this study, we investigated the polarization-dependent SPR of bowtie nano-aperture arrays (BNAs) with different gap sizes. The BNAs were fabricated using focused ion beam (FIB) milling. The EOT was measured using optical transmission spectroscopy, and the near-field distributions were measured by PEEM, as shown in Fig. 1(a). Different transmission spectra are observed for irradiating horizontally (along the x-axis) and vertically (along the y-axis) polarized light on BNAs with different gap sizes. This polarization-dependent far-field behavior can be explained by different near-field distributions. In addition, the simulated charge and current distributions show how the free electrons respond to the different polarization states of light to form plasmonic near fields. This work not only deepens the understanding of the plasmonic response of a BNA but also proposes potential applications such as polarization-tunable optical switching and polarimetric imaging.
Fig. 1. (a) Schematic layout of the experimental setup. Polarization-dependent near- and far-field properties of BNAs with different gap sizes are investigated, using PEEM and optical spectroscopy. Transmission spectra through the BNAs are obtained with horizontally (along the x-axis) and vertically (along the y-axis) polarized light denoted by the arrows in the inset. Photoelectrons emitted from the sample by illuminating light via multiphoton photoemission are imaged by PEEM to map out the near-field distribution. (b) Geometry of a bowtie nano-aperture. (c) SEM image taken at an angle of 40°, showing a representative BNA with a gap size of 40 nm.
2. Results & discussion
The geometry of the BNAs investigated in this study is shown in Fig. 1(b). Arrays of two equilateral triangle-shaped apertures with a height of 180 nm facing each other with varying gap sizes were fabricated on a 100 nm-thick gold film on a glass substrate. The bowtie nano-apertures are periodically arranged every 700 nm horizontally and 300 nm vertically. Arrays with a dimension of 50 µm × 50 µm (70 × 165 apertures) for each gap size were milled using FIB (30 kV acceleration voltage, 40 nA ion beam current). A 40-degree tilted scanning electron microscope (SEM) image of a 40 nm gap BNA (Fig. 1(c)) shows that the top surface of the gold film is etched more than the bottom surface, and the size of the triangle gradually decreases from the top surface to the bottom as depicted in Fig. 1(b). The difference in the etched size between the top (dotted line) and the bottom (solid line) is 18 nm which is obtained by analyzing the intensity profile of the SEM image shown in Fig. 4(a). The gap size, that is, the distance between the facing apices of two equilateral triangles, therefore varies between the top and the bottom surface. The gap size in this work refers to the gap size on the bottom surface. BNAs for five different gap sizes of 23, 40, 50, 64, and 100 nm were studied in this work. A 75 MHz broadband (630–1100 nm) femtosecond Ti:sapphire laser (∼2.5 nJ, pulse duration of <6 fs, and center wavelength of about 770 nm) was used as the light source for both far-field and near-field measurements. Far field transmission spectra of the BNAs were measured using a conventional optical spectrometer. The normalized transmission spectra for BNAs with different gap sizes are shown in Fig. 2(a) for horizontally polarized light. The dip around 710 nm is attributed to Wood’s anomaly [30,31]. Note that there is almost no change in the peak positions at around 750 nm. On the other hand, for vertically polarized light, as shown in Fig. 2(b), a significant redshift from 700 nm (100 nm gap size) to 950 nm (23 nm gap size) is observed as the gap size decreases. Figure 2(c) shows the change of the resonance wavelength with respect to the change of the gap size. The resonant wavelength refers to the wavelength at the maximum transmission. The fabricated bowtie nano-apertures are not identical because of the fabrication limits of FIB milling, thus giving rise to slight variations of the gap sizes. The average values of the measured gap sizes (red circles) from nine representative bowties for each gap size array and their corresponding standard deviations (red bars) are also shown.
Fig. 2. Experimental (solid lines) and simulated (dashed lines) normalized transmission spectra for BNAs of different gap sizes for (a) horizontally and (b) vertically polarized light. (c) Measured (red) and simulated (black) resonance wavelengths of the far-fields for different gap sizes in the case of vertically polarized light. The horizontal red bars indicate the typical standard deviations of the gap sizes within each BNA due to limited fabrication accuracy, while the measured resonance wavelengths are effective values for the entire BNA within the laser focus area.
The nanoplasmonic simulations were performed using a finite-difference time-domain (FDTD) solver (Lumerical, Inc.). Periodic boundary conditions were employed to simulate infinitely-extending arrays. The optical properties of Au were obtained using the data from Yakubovsky et al. [32], and the substrate was modeled with a refractive index of n = 1.45 to represent glass. The experimental spectra are broader than the simulated spectra because the fabricated bowtie nano-apertures are not identical, resulting in slightly different resonant wavelengths. In addition, applying a smoothing filter to the spectra in order to increase the signal-to-noise ratio causes some broadening. The simulated resonance wavelengths (solid black dots) are plotted in Fig. 2(c) for a comparison with the measurement (red circles). Note the excellent agreement between the experiment and simulation results. Furthermore, we observed a nonlinear relationship between the spectral shift and the decreasing gap size of the bowtie nano-aperture. For example, a spectral shift of 45 nm occurs as the gap size changes from 100 nm to 64 nm, whereas a larger spectral shift of 120 nm occurs as the gap size changes from 40 nm to 23 nm. The increasing redshift with decreasing gap size indicates that the transmission peak primarily arises from plasmonic resonance rather than Fabry-Pérot resonance since the latter is more sensitive to film thickness rather than the geometry [24,25]. As the gap size decreases, the gap structure undergoes a significant change of the geometry in the vertical direction. For a gap size of 100 nm, the area between the apices on the top surface is connected (Fig. 3(a)). However, for a gap size of 40 nm, such a connection on the top surface is broken and the gap becomes open in the vertical direction (along the y-axis) as well (Fig. 3(b)). Hence, the plasmonic response to vertically polarized light may be significantly different for this geometry. This difference in the response is manifested as a large shift in the resonance peak.
Fig. 3. Effective charge and current distributions. Top and side views of a bowtie nano-aperture with gap sizes of (a) 100 nm and (b) 40 nm, respectively. (c) The y-z cross-sections at the center of the bowtie nano-aperture with gap sizes of 100 nm and 40 nm. Simulated charge distributions for gap sizes of 100 nm (d, f, h) and 40 nm (e, g, i) in the case of horizontally (d, e) and vertically (f, g, h, i) polarized light, showing the top (d, e, f, g) and bottom (h, i) surfaces, respectively, at their resonance wavelengths. Simulated current density distributions along the y-z cross-section for gap sizes of 100 nm (j) and 40 nm (k) in the case of vertically polarized light at the respective resonance wavelengths. White arrows indicate the direction of the current. The amplitudes in each plot are normalized to their maxima.
The FDTD simulations provide insights to understand the strong sensitivity of the plasmonic response to the bowtie’s gap size for vertically polarized illumination by revealing the charge distribution as well as the field distribution in a BNA. The effective charge distributions of the BNAs with gap sizes of 100 nm and 40 nm for excitation by horizontally polarized broadband femtosecond pulses are shown in Fig. 3(d) and (e), respectively. As shown by the color scale, yellow-red and blue hues indicate opposite charges. Note that the two opposing triangles exhibit an opposite charge distribution around the bases but not at the apices of the triangles, implying that a stronger electric field is formed near the bases parallel to the y-axis compared to that at the apexes. This observation is supported by the simulated electric field enhancement result depicted in Fig. 4(e). Interestingly, the overall charge distributions of the 100 nm gap and 40 nm gap BNA are very similar, indicating that surface plasmon is insensitive to the geometric details of the nanostructure in this case. However, the situation becomes quite different for vertically polarized light, as charges accumulate at the vertices of the triangles near the gap. Figure 3(c) shows the cross-sections in the y-z plane at the center of the gap for both gap sizes, indicating that electron paths are different between the top and bottom surfaces for different gap sizes. In the case of a gap size of 100 nm, the electron paths are connected at both the top and bottom surfaces. Thus, electrons can move parallel to the surfaces at the top and bottom, with more electrons flowing at the top surface, as shown in Fig. 3(j). In the case of a gap of 40 nm, only the lower part along the z-axis is connected across the gap, while the path at the top surface is no longer connected. Electrons now have to move through a narrow gap area near the bottom surface, as shown in Fig. 3(k), which limits the rate of charges passing through. Since large charge densities with opposite signs accumulate at the vertices of the triangles for vertical polarization, a change in the gap size can cause a significant change in the bowtie’s charge distribution, as shown in Fig. 3(f)–(i) where Fig. 3(f) and (h) show charge distributions on the top and bottom surface, respectively, for the BNA with 100 nm gap size, and Fig. 3(g) and (i) for the BNA with 40 nm gap size.
The observed redshift with a decrease in the gap size can be attributed to two factors. Firstly, we consider the effective oscillation path length of the electrons. As shown in Fig. 3(k), conduction electrons flow through the connected gap area located near the bottom surface. As the gap size decreases, the disconnected gap area increases, which increases the effective oscillation path length of the electrons, resulting in an increase in the resonance wavelength because the resonance wavelength is proportional to the effective oscillation path length of the conduction electrons [24]. This factor only needs to be considered if the gap size (g) < 72 nm because the path at the top surface is disconnected if g < 72 nm. The resonance wavelength, considering this factor, is expressed as ${\mathrm{\lambda }_{\textrm{res}}}{ = \; }{\textrm{a}_\textrm{1}}{\; + \; }{\textrm{a}_{\textrm{2}\; }}\textrm{L}$, where L is the length of v-shaped path shown in Fig. 3(c), and L = 2.8 ${\times} $ (72 nm – g), ${\textrm{a}_\textrm{1}}$ and ${\textrm{a}_\textrm{2}}$ being the coefficients which are determined by geometry and the properties of the metal and surrounding medium. Secondly, the width of the path through which the electrons pass affects the resonance wavelength. Duan et al. demonstrated the redshift of plasmonic resonance as the path width of bridged bowtie nanostructures decreases and explained it by using an LC circuit model [33]. The resonance wavelength, considering this width factor, can be expressed as ${\mathrm{\lambda }_{\textrm{res}}}{ = \; }{\textrm{a}_\textrm{3}}{\; /\; }\sqrt {{\textrm{a}_\textrm{4}}\; \textrm{g}\; + \; 1} $, where g is the gap size, ${\textrm{a}_\textrm{3}}$ and ${\textrm{a}_\textrm{4}}$ are the coefficients which are determined by geometry and the properties of the metal and surrounding medium. Hence in the case of the illumination with vertically polarized light, the electrons’ response is sensitive to the geometric details of the nanostructure at the gap, the overall effect of which manifests itself as a red shift of the resonance wavelength.
To confirm our findings from the simulations, multiphoton PEEM experiments were performed in order to map out the near-field distribution around the bowtie nano-apertures. Prior to laser excitation, an ultraviolet (UV) mercury lamp was used as an excitation light source for linear (single-photon) PEEM imaging and to locate the BNAs of interest. After that, the same sample area was illuminated with a near-normal angle of incidence (4$\mathrm{^\circ }$ angle of incidence) for plasmonic excitation using the same broadband Ti:sapphire femtosecond laser employed for the far-field measurements. The laser peak intensity at the sample is ∼2 × $\textrm{1}{\textrm{0}^{\textrm{10}}}\; \textrm{W/c}{\textrm{m}^\textrm{2}}$. Photoelectrons emitted from the gold BNAs under femtosecond laser excitation are due to multiphoton photoemission since 3-4 photons are required at the photon energies of our laser source (1.1–1.9 eV) to overcome the work function of gold. Figure 4(a) shows a SEM image (top) and a PEEM image of a BNA with a gap size of 40 nm under UV light excitation (bottom). The comparison between the PEEM images obtained with UV light (Fig. 4(a)) and linearly polarized lasers (Fig. 4(b) and (c)), using the same PEEM parameters, allows us to determine the exact positions of localized hotspots within the structure. In the case of vertically polarized light (Fig. 4(b)), strong hotspots where most photoelectrons are emitted are seen to be around the gap area in which the vertices of the two equilateral triangles of a bowtie nano-aperture face each other, indicating that most photoelectrons are emitted from the gap area due to the concentration of the plasmonic near-fields at that location. In contrast, in the case of horizontally polarized light (Fig. 4(c)), hotspots, even though weak, are observed around the edges of the triangles parallel to the y-axis but not around the gap area. These experimental results are in excellent agreement with the simulated distributions of the electric field intensity for the BNA by FDTD, as shown in Fig. 4(d) and (e). The simulations show the distributions of the electric field intensity relative to the excitation field intensity at wavelengths of 825 nm and 750 nm, which are the resonance wavelengths for the vertical and horizontal polarization of illuminating light, respectively. These polarization-dependent near-field distributions explain the polarization-dependent far-field properties. In the case of horizontally polarized light, the near fields are induced at the sides of the triangles which are located relatively far from the gap, thus the plasmon resonance wavelength remains unchanged when changing the gap size. However, for vertically polarized light, the near-field is localized in the vicinity of the gap. Consequently, the plasmon resonance is sensitive to the geometric details of the structure at the gap due to the variation in the gap size. These remarkable characteristics may be utilized for the on-demand switching of resonance wavelength by polarization. For example, taking a BNA with a gap size of 23 nm, the resonance can be shifted from 950 nm to 750 nm by simply changing the polarization from the vertical direction to the horizontal direction.
Fig. 4. (a) SEM image (top) and UV-PEEM image (bottom) of a BNA with a gap size of 40 nm. (b) and (c) PEEM images of multiphoton photoemission by ultrashort broadband laser pulses with vertical and horizontal polarization, respectively, for the same BNA as shown in (a). Yellow dashed lines outline bowtie nano-apertures as a guide to the eye. (d) and (e) Simulated spatial distributions of electric field intensity enhancement (logarithmic color scale) in a 40-nm gap BNA for illumination by vertically and horizontally polarized light at 825 nm and 750 nm resonance wavelengths, respectively.
3. Conclusion
In summary, we investigated polarization-dependent far-field and near-field characteristics of BNAs with varying gap sizes through FDTD simulations and experimentally by using optical transmission spectroscopy and PEEM. The optical transmission spectroscopy result reveals a nonlinear redshift in the transmission spectra as the gap size of the bowtie nanoaperture decreases for vertically polarized light. In contrast, the transmission spectra remain unchanged with different gap sizes for horizontally polarized light. Simulated charge and current distributions provided insight into how the oscillation of electrons is influenced by the polarization of incident light. The observed redshift with decreasing gap size is attributed to changes in both the width and length of the electron oscillation paths. Additionally, the polarization-dependent charge oscillations create distinct near-field distributions that were demonstrated through FDTD simulation and confirmed experimentally using PEEM. This study provides a comprehensive understanding of the plasmonic properties of bowtie nano-apertures, enabling their further use in a wide range of applications, one of which is the optical switching of the resonance wavelength without altering nanostructure geometry.
Funding
National Research Foundation of Korea (2022M3H4A1A04074153, 2020R1A2C2103181, 2020R1C1C1007691, RS-2022-00154676).
Acknowledgments
This work has been supported in part by the National Research Foundation of Korea (NRF) grants (grant No. 2022M3H4A1A04074153, No. 2020R1A2C2103181, 2020R1C1C1007691 and RS-2022-00154676) funded by the Ministry of Science, and ICT, and by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea government (MOTIE) (P0008763, HRD Program for Industrial Innovation)
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.
Supplemental document
See Supplement 1 for supporting content.
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