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Abstract
An infrared plasmonic metamaterial absorber with a nanogap was numerically and experimentally investigated as a refractive index sensor. We experimentally demonstrated large enhancements of both sensitivity (approximately 1091 nm/refractive index unit) and figure of merit (FOM*; approximately 273) owing to the nanogap formation in the metamaterial absorber to achieve perfect absorption (99%). The refractive index sensing platform was fabricated by producible nanoimprint lithography and isotropic dry etching processes to have a large area and low cost while providing a practical solution for high-performance plasmonic biosensors.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The collective oscillations of free electrons in metal nanostructures cause localized surface plasmon resonance (LSPR) via coupling with incident light. The enhanced and confined electromagnetic near field induced by the LSPR in the vicinities of metal nanostructures enables a wide range of applications, such as surface-enhanced spectroscopy [1–6], energy harvesting [7,8], and sensing [9–12]. In particular, plasmonic nanostructures are excellent platforms for a variety of applications, such as gas sensing [13,14], thermal imaging [15,16], and biological sensing [17–20]. Moreover, the properties of the LSPR, which are influenced by the shapes, sizes, and surrounding environments of the plasmonic metal nanostructures, enable refractive index (RI) sensing for biosensors. In common plasmonic biosensors, changes in the refractive indices of the surrounding media induced by binding of the target biomolecules to the surfaces of the metal nanostructures result in shifts of the resonance wavelengths. Therefore, the sensitivity is commonly defined in terms of changes in the resonance wavelengths per refractive index unit (RIU). This sensitivity divided by the full width at half maximum (FWHM) of the resonance is used to define the figure of merit (FOM) to evaluate the performance of the plasmonic biosensor. Such FOMs require bulky spectrometers to measure the resonance wavelength shifts, which is one of the main obstacles to the widespread utilization of plasmonic biosensors in point-of-care medical diagnostics. In addition, the broadening of the resonance FWHM owing to the unavoidable ohmic losses in metals hinders the enhancement of the FOM. However, it is simpler to measure the relative intensity change ΔI (λ)/I (λ) induced by an RI change Δn at a fixed wavelength λ0 using a single-wavelength light source and detector rather than the resonance wavelength shift using a spectrometer. An alternative figure of merit is defined as FOM* = max | [ΔI (λ) / Δn (λ)] / I (λ) |, where I (λ) is the intensity at which FOM* has a maximum value. In addition, appropriate utilization of the inevitable ohmic losses in plasmonic nanostructures support the concept of a metamaterial absorber (MA) [21–23]. Because the intensity at the resonance wavelength is near zero of the reflectance intensity when using a perfect MA, MAs have been used as RI sensors for enhancing the FOM* [24–26].
The concept of minimizing the influence of a high-RI substrate beneath plasmonic nanostructure to increase the sensitivity of RI change has been reported by several research groups [27–30]. The MA is normally composed of metal–insulator–metal triple layers, which are patterned metallic antennas and thick metal mirror layers separated by thin dielectric spacer layers. Most of the enhanced near field of the MA is confined within the thin dielectric spacer between the antenna and metal mirror layer. Similarly, the sensitivity of the MA to RI changes can be increased by exposing the enhanced near field of the MA to the external environment. In our previous work [5], we demonstrated the application of an undercut profile between an Au nanoantenna and Au mirror layer to the MA to boost the surface-enhanced infrared absorption sensing signal. In addition, Bhattarai experimentally enhanced the sensitivity of the RI change using a mushroom-capped MA [31]; however, impedance matching of the mushroom-capped MA to air is required to maximize the FOM*.
In this paper, we introduce an infrared RI sensor based on an MA with a nanogap. The infrared MA is fabricated by nanoimprint lithography (NIL), which allows large-area and low-cost nanopatterning. Further, isotropic dry etching was used to form the nanogap of the MA by controlling the isotropic etching depths. We experimentally prove that both the sensitivity and FOM* can be considerably enhanced via nanogap formation on the MA and impedance matching of the MA to free space.
2. Results and discussion
Based on the Mie theory, an elliptical nanoparticle has a better sensitivity to the change in RI than a spherical one [32,33]. Thus, a rectangular metallic nanoantenna has a better sensitivity to the change in RI than a circular or a square one [34]. The cross-shaped nanoantenna, which is a combination of two perpendicularly aligned rectangles, can tune the resonance wavelength by altering the dimensions of the nanoantenna and relaxes polarization sensitivity. In addition, the cross-shape nanoantenna can induce the enhanced and confined near-field intensity at the edge of the nanoantenna. Figure 1(a) shows the schematic of a single unit of the MA containing a nanogap. The MA with the nanogap is composed of a 40 nm thick Au cross-shaped nanoantenna on the top, a 150 nm thick Au mirror layer on the bottom, and a 40 nm thick SiO2 dielectric spacer with an undercut profile between the Au layers, as shown in Fig. 1(b). The MA with the nanogap was fabricated using ultraviolet (UV) NIL and a lift-off process as described in our previous works [5,6]. Figure 1(c) shows a scanning electron microscope (SEM) image of the top view of the fabricated MA with nanogap; the dimensions of the cross-shaped nanoantenna with rounded corners (40 nm radius) are length l = 400 nm, width w = 100 nm, and array period P = 700 nm. The nanogap of the MA, as shown Fig. 1(d), was formed by isotropic dry etching of the SiO2 dielectric layer using a plasma asher with a mixture of CF4 (30 sccm) and O2 (10 sccm) gases at a power of 300 W. Numerical simulations were performed using the commercial finite-difference time domain (FDTD) software Lumerical to design the MA with the nanogap. In the lateral directions (x and y axes), a periodic boundary condition was set to describe a periodic array structure, and a perfectly matched layer was used in the vertical direction. The permittivities of the SiO2 and Au layers were as described by Palik [35] from a software database. To characterize the infrared (IR) responses of the fabricated MAs with nanogaps, the reflectance (R) spectra were measured using a Fourier-transform IR (FTIR) microscope (Bruker Hyperion 3000).
Fig. 1. (a) Schematic of a single unit of the metamaterial absorber (MA) with a nanogap. (b) Schematic cross-sectional view of a single unit of the MA with nanogap. (c) Top-view scanning electron microscope (SEM) image of the MA with nanogap. (d) Cross-sectional SEM image of the MA with nanogap.
To achieve maximum FOM*, the MA with a nanogap requires the perfect absorption via the impedance matching effect. We analyzed and optimized the impedance matching effect of the MA with a nanogap using the temporal coupled mode theory (TCMT) [36,37]. The MA with a nanogap as a coupled cavity and a single port is described by the TCMT as shown in Fig. 2(a). The absorption (A) of the MA with a nanogap is described by Eq. (1), which indicates that the absorption depends on the radiation loss rate (γrad) and the absorption loss rate (γabs):
Fig. 2. (a) Schematic of the MA as the cavity model for TCMT. (b) Radiation loss rate γrad (left y-axis) and absorption loss rate γabs (right y-axis) of the MA with varying dielectric spacer thickness. (c) Simulated reflectance spectra of the MAs with varying dielectric spacer thickness. (d) Radiation loss rate γrad (left y-axis) and absorption loss rate γabs (right y-axis) of the MA with the 40 nm dielectric spacer for different isotropic etching depths.
Fig. 3. Experimental (a) measured and (b) simulated reflectance spectra of the MAs for different dry etching times and depths. Cross-sectional views of the simulated near-field enhancement (E/E0) profiles for the (c) MA without nanogap, (d) MA with nanogap of 10 nm isotropic etching depth, (e) MA with nanogap of 20 nm isotropic etching depth, and (f) MA with nanogap of 30 nm isotropic etching depth at the resonance wavelength.
Figure 3(a) shows the measured reflectance spectra of the MAs at different dry-etching times. Even though the etching rate may not be isotropic because of the three-dimensional nanostructure, we assumed that the isotropic etching depth reduced the simulation modeling efforts. The simulated reflectance spectra of the MAs at different isotropic etching depths are shown in Fig. 3(b). The measured results were in excellent agreement with the simulated results. The wavelengths of the reflectance dips for the MAs etched for 0, 1, 2, and 3 min were located at wavelengths of 1757, 1647, 1568, and 1498 nm, respectively. As the etching time increased, the wavelength of the reflectance dip shifted toward lower values. This blue shift results from a decrease in the effective RI of the dielectric spacer partially removed within the localized electric near field. The wavelength shifts (Δλ) caused by every 1 min of increment in etching time were 110 nm, 79 nm, and 70 nm, respectively. Because the maximum electric field at resonance is distributed under the edge of the cross-shaped nanoantenna, as shown in Figs. 3(c), 3(d), 3(e), and 3(f), this area of the MA is the most sensitive to RI changes. Therefore, the wavelength shift as a function of the etching time decreases as the isotropic etching depths increase.
To maximize the sensitivity (in terms of wavelength shift per RIU) of the MA for RI changes of the surrounding medium, most of the electric near field of the MA needs to be exposed to the external environment through an increase in the etching depth of the nanogap, provided the mechanical stability of the MA with the nanogap is maintained. As the etching time increases, the simulated reflectance dip reaches a near-zero value owing to the impedance matching of the MA to free space, as shown in Fig. 3(b). The experimental reflectance intensities at the resonance wavelength for the MAs etched for 0, 1, 2, and 3 min were 4.3%, 1.9%, 1%, and 1.4%, respectively. The MA with a nanogap etched for 2 min had the lowest reflectance intensity. According to its definition, the maximum FOM* value can be obtained around the wavelength of the lowest reflectance intensity. Therefore, instead of maximizing the sensitivity of the MA with the nanogap through further etching of the nanogap, we intended to achieve a near-zero reflectance of the MA with the nanogap, which corresponds to perfect absorbance (A) (A = 1 - R), to maximize the figure of merit (FOM*).
The performance of the fabricated MA with nanogap as an RI sensor can be investigated by evaluating the FOM*. Figure 4 shows the measured reflectance spectra and FOM* values of the fabricated MAs in air (n = 1) and oil (n = 1.296). The RI change of the medium surrounding the MA induces impedance mismatches between the MA and free space and significantly increases the reflectance intensity from a near-zero to non-zero value. Therefore, the proposed MA with a nanogap enables sensitive detection of the reflectance intensity changes at a given wavelength and can thus be a promising sensing platform for biological and chemical detection. With oil as the surrounding medium, the resonance wavelengths for the MAs etched for 0, 1, 2, and 3 min were shifted by Δλ ≈ 122 nm, Δλ ≈ 205 nm, Δλ ≈ 233 nm, and Δλ ≈ 282 nm, respectively. The resonance wavelength shift for the MA etched for 3 min increased by a factor of ∼2.3 compared with that for the MA etched for 0 min. In addition, the changes in the reflectance intensities (ΔI) at the resonance wavelength for the MAs etched for 0, 1, 2, and 3 min were 52.8%, 75%, 79.7%, and 82%, respectively. The resonance wavelength shifts and changes in the reflectance intensities of the MAs increased with nanogap formation, which allows the oil medium access to the maximum electric field distributed under the edge of the cross-shaped nanoantenna. The FOM* values for the MAs etched for 0, 1, 2, and 3 min were approximately 42, 134, 273, and 196, respectively. Even though the MA with a nanogap etched for 3 min showed the greatest intensity change, the maximum value of the FOM* was around 273 for the MA with the nanogap etched for 2 min owing to the domination of the minimum reflectance intensity at the resonance wavelength with air. The FOM* for the MA etched for 2 min increased by a factor of ∼6.5 compared with that for the MA etched for 0 min. Numerous outstanding FOM* results have been reported in literature [26,40–43]. The most outstanding FOM* results due to the reflectance intensity I(λ) ≈ 0 around the wavelength of perfect absorbance are for the ideal simulated results. Liu experimentally reported an FOM* of 87 based on an IR circular nanodisk plasmonic absorber [24]. Cheng experimentally demonstrated an IR metamaterial absorber with an FOM* of 55 [25]. However, the current FOM* in this work is much higher than those of earlier experiments.
Fig. 4. Experimental measured reflectance spectra (solid curves) and FOM* (red dashed curve) for changing the surrounding medium from air (n = 1) to oil (n = 1.296) for the (a) MA etched for 0 min, (b) MA etched for 1 min, (c) MA etched for 2 min, and (d) MA etched for 3 min.
Furthermore, the bulk sensitivities of the MA with the nanogap were measured using liquid oils (Cargille Laboratories) with refractive indices of 1.296, 1.325, 1.353, and 1.382. We drop coated the oil for sensing the fabricated sample and covered it with an infrasil cover glass. Figures 5(a) and 5(b) illustrate the measured reflectance spectra for both the unetched MA and MA etched for 3 min. In addition, Figs. 5(c) and 5(d) show the corresponding numerically simulated reflectance spectra for both the unetched MA and MA etched for 3 min. We observe the expected redshift of the resonance wavelength with increasing RIs of the surrounding environment. Compared to the unetched MA, both the resonance wavelength shift per RI and intensity change per RI for the MA etched for 3 min are enhanced. Based on the linear fitting of the measured and simulated resonance wavelengths as functions of the RI of the surrounding environment, as shown in Figs. 5(e) and 5(f), the bulk sensitivities of the MAs with nanogaps are enhanced with increasing etching times. The measured and simulated bulk sensitivities obtained from the linear fitting are approximately 491 nm/RIU and 608 nm/RIU for the unetched MA and 1091 nm/RIU and 1222 nm/RIU for the MA etched for 3 min. The measured and simulated data show that the bulk sensitivity of the MA etched for 3 min increased by a factor of ∼2.2 compared with that of the unetched MA; this difference between the measured and simulated bulk sensitivities may have resulted from fabrication imperfections, such as variations in the structure sizes and presence of unetched residual SiO2 dielectric layer beneath the cross-shaped nanoantenna. We confirmed that the MA with the nanogap remained intact and damage free over dozens of measurements and cleaning processes and that it was mechanically safe for use as an RI sensor and biosensor.
Fig. 5. Experimentally measured reflectance spectra in oils with different refractive indices for the (a) unetched MA and (b) MA etched for 3 min. Simulated reflectance spectra in oils with different refractive indices for the (c) unetched MA and (d) MA etched for 3 min. Linear fittings (solid curves) of the (e) measured and (f) simulated resonance wavelengths as functions of the refractive indices of the surrounding environments.
3. Conclusions
In this study, we fabricated an infrared RI sensor based on an MA with a nanogap using a large-area and low-cost NIL process and isotropic dry etching. By reducing the effective RI of the dielectric spacer partially removed by isotropic dry etching, we confirmed the blue shift of the wavelength of the reflectance dip and near-zero reflectance dip due to impedance matches between the MA with the nanogap and air. In addition, we demonstrated the capability of the RI sensor. Because the MA with nanogap allows the sensing target to access the electric near field maximally and provides near-zero reflectance intensity at a specific wavelength, the MA with the nanogap enabled significant enhancements of the FOM* and bulk sensitivity. Compared with the MA before nanogap formation, the FOM* (273) of the MA with the nanogap increased by a factor of ∼6.5, and the bulk sensitivity (1091 nm/RIU) of the MA with the nanogap increased by a factor of ∼2.2. By altering the thickness of the nanogap in the MA based on the sizes of the target biomolecules, the proposed MA with nanogap has great potential as a high-performance plasmonic biosensor.
Funding
National Research Foundation of Korea (2014M3A6B3063700, 2014M3A6B3063707, 2019M3E6A1103999); Korea Institute of Machinery and Materials (NK232E).
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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