2.1 Design of plasmonic nanostructure coupled with dipole and grating nanostructures
To enhance image resolution, the optical signal must be amplified, and the noise must be canceled. As shown in Fig. 1, we designed and synthesized a metal plasmonic nanostructure (MPN) using the dipole nanoantenna (DN), bowtie nanostructure (BN), and grating nanostructure (GN) theories to determine the suitability of these nanostructures for amplifying an optical signal in the near-infrared (NIR) range to enhance the image resolution. The nanostructure proposed in this paper was first simulated to confirm the enhancement and coupling of the electric field to amplify the optical signal of an image. The MPN was designed in three parts: plasmonic (PP), horizontal (HP), and vertical (VP) parts. The PP enhances the electric field in the nanostructure. As mentioned earlier, the plasmonic effect, which involves the oscillation of free electrons in the metal in resonance with the incident light, can enhance the electric field by resonance matching the vibration of electrons and the incident light wave vector (k). Momentum matching is achieved by various methods, such as prism-coupled SPR with a Kretschmann geometry via a subwavelength grating (which reduces the size and complexity of the Kretschmann geometry). [12, 17, 23–26] In this method, the surface plasmon polariton (SPP) wave vectors must satisfy the plasmonic equation to ensure the coupling of the incident light photons and SPP:
$${k}_{SPP}=\frac{\omega }{c}sin{\theta }_{\alpha }+n\frac{2\pi }{a}$$
where \(\omega\) is the wave frequency of the incident light, c is the speed of light, \({\theta }_{\alpha }\)is the angle of light incidence, n is an integer, and a is the grating period. Thus, the grating pitch is also a crucial parameter for achieving SPR, which results in the concentration of the electric field at the center.
The VP was designed to receive a specific wavelength, e.g., in the form of a DN. A DN can enhance EM waves owing to the occurrence of resonance in the structure for signal processing and the transmission of radiation. Unlike in the antenna theory, [18, 19, 27, 28] the shape and length of the DN need to be designed depending on the wavelength because the refractive index differs depending on the structure (called the effective refractive index), such as in a BN and rectangular nanoantenna. [13, 24, 25, 29–31]
The HP can concentrate the electric field at the center of the unit cell of the nanostructure. In addition, it has a plasmonic effect on the surface. The HP was also designed as a DN but with different lengths for each nanostructure to concentrate the electric field at the center of the unit cell, similar to the BN structure.
The finite-difference time-domain (FDTD) simulation tool was used to maximize the optical characteristics, such as the electric field enhancement and far-field gain of the MPN structure along with the resonance of each structure. The key parameters for amplifying optical signals using the MPN are its dimensions (which determine the wavelength required to trigger the required resonance), the period of the structure, and the angle of light incidence required to generate a plasmonic surface.
As confirmed in Fig. 1, based on the characteristics of the DN, we optimized the length of the VP to 1177.25 nm, and its period was taken to be 196 nm, which is λ⁄4, where λ is the excitation wavelength. The dimensions of the HP were fixed at 392.5 nm, 588.75 nm, and 785 nm (corresponding to λ⁄2, 3λ⁄4, and λ, based on the nanoantenna theory; in Fig. 1(c), they are denoted correspondingly as \({L}_{1},{L}_{2}, \text{a}\text{n}\text{d}{L}_{3})\), and its period was fixed at 196 nm (λ⁄4, in Fig. 1(c), P).
As shown in Fig. 2, each nanostructure has a dipole momentum that occurs owing to the oscillation of free electrons of silver atoms on the surface. However, the HP of the nanostructure alone has a small plasmonic effect through which the electric field transmits the forward incident light, which can concentrate the electric field according to the grating theory [32], as shown in Figs. 2(a,b). The VP receives a specific wavelength depending on its length and period, as shown in Fig. 2(c). Combining both structures, the electric field of the MPN was concentrated in the middle of the structure, confirming that such a dipole can be created at the surface of an Ag-based plasmonic nanostructure. Additionally, the electric field was enhanced at the center of the structure owing to the plasmonic effect of each grating, as shown in Fig. 2.
For the VP with the dimension of 1177.25 nm, the generated electric field was confirmed as 31.826 V/m which was 15 times higher than other DN and GN structure as shown in Fig. 3(a). The magnitude of the electric field is also affected by the angle of light incidence, owing to the plasmonic effects mentioned above. When the plasmonic nanostructure resonated with the incident light, the extinction, scattering, and absorption cross-sections changed.
Figures 3(c, d, e) confirm that the resonance of the MPN depends on the angle of incident light. As confirmed Fig. 3(c), when the angle of incident light was 30° for the MPN, the extinction cross-section was 1.33\(\times {10}^{-11} {m}^{2}\), whereas the corresponding cross-sections were 3.97\(\times {10}^{-13} {m}^{2}\)and 4.716\(\times {10}^{-14} {m}^{2}\), 7.873\(\times {10}^{-16} {m}^{2}\) respectively, for the other structures(GN,BN,DN). Thus, for an angle of light incidence of 30° (Fig. 3(b)), the far-field gain was determined to be 31.826 V/m and 1137.8, indicating that the generated electric field was 600 and 1100 times higher than that of the DN and GN, respectively.
As confirmed by the simulation data, the proposed plasmonic nanoantenna can enhance the electric field concentrated in the middle of the nanostructure owing to coupling with the grating structure and dipole antenna, which have plasmonic characteristics. Figure 3(f) shows the enhancement of the optical intensity (\(I/{I}_{0})\), which indicates that the plasmonic nanostructure enhances the optical signal by more than 226 times at a distance of 3 µm.
2.2 Selective deposition and fabrication
The MPN was fabricated by combining tape lithography and nanoimprint lithography (NIL). A schematic of each process step is shown in Supplementary 2. The Si material was fabricated using electron beam (e-beam) lithography. An e-beam resist (AR-P 6200, Allresist GmbH, Germany) was coated onto the Si wafer; subsequently, arrays of square nanopatterns were patterned using e-beam lithography (JBX-9300FS, JEOL Ltd., USA). The pattern width W was 392.5 nm; the period P was 196 nm; and the lengths \({\text{L}}_{\text{1}}\text{, }{\text{L}}_{\text{2}}\text{, }{\text{L}}_{\text{3}}\text{, and }{\text{L}}_{\text{4}}\) were 392.5 nm, 588.75 nm, 785 nm, and 1177.25 nm, respectively. The height was 230 nm, and the patterns were square arrays. Figs. 4(a, b) show a scanning electron microscopy (SEM, SNE-3200M, SEC, Korea) image of the fabricated Si stamp. Before the hybrid nanoimprinting process, the Si master was subjected to surface treatment by dipping it into a fluoric release agent solution (OPTOOL DSX, Daikin Industries Ltd., Japan, 147 μL and FC-3283, 3M, USA, 50 g) for 10 min. This treatment decreased the surface energy of the Si mold, facilitating demolding during the nanoimprinting process to enable stronger van-der-Waals-force-based bond formation between the Si mold and Ag atoms. After the surface treatment, the contact angle of the Si master was measured using a contact angle meter (Phoenix, SEO, Korea). The increase in the contact angle from 55.10° to 115.14° confirmed that the release agent was bonded to the surface of the Si master, and that the surface treatment had been properly performed, as shown in Supplementary 3.
A 50-nm-thick Ag film (3–5 mm granules, 99.99%, Taewon Scientific Co., Ltd.) was deposited onto the Si master using an e-evaporator (MEP5000, SNTEK Co. Ltd., Korea). The deposition rate determines the roughness and (more importantly) grain size of the resulting thin metal film. As light scattering at the grain boundaries in a metal has been reported to lead to losses, the deposition rate was varied from 1 to 20 Å/s. After metal deposition, an adhesive tape (3M; St. Paul, MN, USA) was used to remove the metal protruding from the surface of the Si master. The adhesion of the metal varied according to the substrate used: Ag showed poor adhesion with Si and was therefore easily transferred to the adhesive tape. Figures 4(c, d) show an SEM image of the Ag nanopatterns stripped off the adhesive tape, confirming that the protruding Ag film was the only material removed by the adhesive tape.
A thermal NIL process (NIL-6, Obducat, Germany) was used to fabricate noble-metal plasmonic nanostructures. Prior to this process, a one-step process was conducted to fabricate nanostructures on a polymethyl methacrylate (PMMA) substrate and transfer the Ag layer to this nanostructure simultaneously. This process was conducted at 145°C and a maximum pressure of 4 MPa over 300 s, with a demolding temperature of 75°C.
Figures 4(e, f) show the final nanostructure fabricated using thermal imprint lithography. As confirmed in Fig. 4(f), silver was selectively transferred from the mold to the substrate according to the difference in surface energy. Fidelity analysis was conducted on the final structure. The ideal length was 1177.25 nm, where the length of the fabricated Si master was 1144.9 nm, and that of the fabricated plasmonic nanostructure was 1130.2 nm. Accordingly, the fidelity was determined to be 2.43%, which was lower than the simulated tolerance value of 3.8%. This confirms that the proposed hybrid imprint process is effective and accurate for the fabrication of MPNs.
2.3 Plasmonic effects of the nanostructures
To confirm that the MPN can enhance the optical signal, a 785 nm laser was used with a laser–detector (fiber) distance of 3 cm, as shown in Supplementary 1. Fig. 5 shows the results, which demonstrate that the intensity of light is enhanced by using a plasmonic structure. Fig. 5(a) shows the amplification of the optical intensity with the plasmonic nanostructure depending on the angle of incident light. In this graph, the input power passing through the collimator was 0.1791 mW. However, after the light passed through the plasmonic nanostructure, the power increased to 0.4022 mW at an incidence angle of 30°. By calculating the normalized intensity by comparing the reference intensity and the intensity after amplification with the plasmonic nanostructure, we confirmed that the light intensity increased 2.24 times when the laser passed through the plasmonic nanostructure compared with that of the original laser. This is because light that would be otherwise scattered in the air and by the surface was concentrated in the middle of the nanostructure. When the difference between the orientation of the plasmonic nanostructure and the laser direction was 0°, the detected power decreased slightly. When the plasmonic effect occurred, which resulted in resonance with the incident light wave vector, the intensity of the output power was observed to be the highest at an angle of incident light of 30°. This indicates that the EM wave from the incident light was reinforced on the surface of the MPN by the EM wave of the latter (comparing Fig. 3(f) and Fig. 5(b)).
Figure 5(b) shows the amplification of signal intensity depending on the distance between the source and the detector. This figure demonstrates that the plasmonic nanostructure can enhance the optical signal at an angle of incident light of 30°. We also confirmed that the lower the input power, the more the output power is enhanced. Comparing the MPN and the bare film (PMMA), the MPN amplifies the optical signal up to 2.61 times when the distance is 9 cm, because of the coupling and enhancement of the electric field in the unit cell. However, the amplification obtained by the bare film and the nanostructure without silver is less than 0.92 owing to the transmittance of ~ 90%.
Figure 5(c) shows atomic force microscopy (AFM) images and power spectral density (PSD) spectra of the MPN without and with a laser, which can provide insights into the amplification of the optical electric field by an external electric field. The AFM topography was characterized using cantilever tips in the tapping mode. The AFM surface topography image without the laser agrees with the results of the SEM image, in which the film morphology feature of the MPN and P are 230 nm and 193 nm, respectively. When the laser beam is incident at 30° on the MPN structure, the height of the MPN structure increases to 280 nm. Moreover, the height difference of the MPN with the laser, detected using AFM cantilever tips, is comparable to the electric field difference in the simulation. This indicates that the atomic force interaction between the cantilever tips and the MPN structure was affected by the electric field difference, which was attributed to the incident laser. This atomic force interaction owing to the electric field was quantified through PSD calculations as a series of fast Fourier transforms. PSD spectra offer periodic signals as peaks, which are functions of the frequency. Compared with the PSD results of the MPN structure without a laser, we observed a new peak at approximately 0.001 nm-1 in the PSD spectrum of the MPN structure with a laser, where the amplitude of the peak is comparable to that of other reference peaks. The new periodic signal appeared similar to the signal obtained with the structure, suggesting a high correlation between the laser incidence (control variable) and the MPN structure. Because of the plasmonic effects and coupling of the electric field in each structure, we confirmed that the electric field was enhanced in the MPN from the AFM data.
Figure 5(d) shows the enhanced focused NIR spectroscopy (fNIRS) spatial resolution for brain activation imaging owing to brain signal enhancement. fNIRS is a brain activation imaging system that observes changes in oxyhemoglobin and deoxyhemoglobin using NIR light, which can penetrate the brain. Figure 5(d) shows the amplitudes of the signals of detectors 1 and 2 depending on the frequency for the normal control experiment with brain activation (circle) and the MPN (square). The signal amplitudes of oxyhemoglobin and deoxyhemoglobin are nearly the same at \(1.19\times {10}^{-8}\) and\(6.9\times {10}^{-9}\) at 0 Hz, respectively, in the normal control and \(1.07\times {10}^{-8}\) and \(9.55\times {10}^{-9}\), respectively, in the MPN. However, above a frequency of 0.02 Hz, the signals that change in the brain but are not detected in the normal control are enhanced as compared with the control values of \(5.6\times {10}^{-10}\) and\(3.32\times {10}^{-10}\). Brain activation can be detected owing to the amplification of the signals by more than 76 and 106 times, respectively, to \(4.26\times {10}^{-8}\) and\(3.52\times {10}^{-8}\). In Fig. 5(d), the brain images show the brain activation region and the extent of brain activation, as indicated by the changes in oxyhemoglobin and deoxyhemoglobin.
Figure 5(e) shows a fluorescence image indicating the enhancement of a specific wavelength. Compared with the fluorescence image of 660 nm, laser signals of 785 nm were amplified up to 1.6 times, whereas the amplitude of the fluorescence signal of 660 nm was decreased by 0.87. The MPN can enhance the specific wavelength signal owing to the dipole nanoantenna and plasmonic effect.