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Abstract
We propose a novel broadband absorber that shows a strong absorption band much broader than that shown in previous work. In our proposed absorber, randomly arranged metal nanobumps are introduced in the incident-side metal layer of a metal/insulator / metal structure. The random structure converts broadband light into surface plasmons without any angular or polarization dependence. Using silver as the metal layer, we obtained an ultrawide region in which the absorption was higher than 50% in the wavelength region from 0.4 to 3.2 μm, which corresponds to a three-octave bandwidth.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Plasmonic structures that exhibit strong light absorption without angular or polarization dependence have been reported [1–5]. These plasmonic structures, sometimes called perfect absorbers, are based on a metal-disk / insulator / metal structure. The sandwich structure of the two metal layers strongly confines an electromagnetic field in the insulator layer due to magnetic resonance [2, 5–7]. This resonant absorption, which is generally narrowband, has been applied to bio-sensors [1, 2] and nano rulers [8]. In comparison with these narrow-band absorbers, broadband absorbers are more useful for photovoltaics [9–12], thermal-radiation devices [13,14], and optical filters. In previous reports, broadband absorbers were accomplished by spatially multiplexing perfect absorbers with different resonant wavelengths [15–18]. The reported absorption band corresponded to at most a two-octave bandwidth [12,15–22], but light absorbers operating in the range from the visible to the infrared have not been reported yet. Here, we defined the absorption band as the wavelength region where the absorption is larger than 50%. This criterion, corresponding to −3 dB, is commonly used in radio frequency region. Extension of the absorption bandwidth is required for further enhancement of the performance of several optical devices. Additionally, cost-effective fabrication methods are also desired. In most previous work, electron-beam lithography, which has a limited device area, was used for fabrication. Although self-assembling methods have also been employed to fabricate broadband absorbers, their absorption bands were still narrower than a two-octave bandwidth [10,23,24].
In this work, we propose a novel broadband absorber based on a metal / insulator / metal (MIM) structure, where randomly arranged nanobumps are introduced in the incident-side metal layer, hereafter referred to as a bumpy MIM. The bumpy MIM exhibits a strong three-octave-wide absorption band ranging from the visible to the infrared. An additional advantage of the bumpy MIM is the ease of device fabrication, because a colloidal lithography technique, which is a self-assembling process, is used for fabrication. Such a bottom-up approach is a promising one for easily enlarging the device area at low cost.
2. Structure and fabrication of the bumpy MIM
Figure 1(a) shows a schematic illustration of the bumpy MIM. A thin metal layer, a dielectric layer, and an optically thick metal layer were deposited on a glass substrate on which dielectric nanoparticles were randomly immobilized. Light was irradiated from the glass substrate side. In the experiments, we employed gold, silver, or aluminum as the metal, polymethyl methacrylate (PMMA) as the dielectric, and silica particles with a diameter of 50 nm as the dielectric nanoparticles.
Fig. 1 (a) Schematic diagram of the bumpy MIM. (b) SEM image of a bumpy MIM before depositing both a PMMA layer and a thick metal layer. (c) Cross-sectional SEM image of a bumpy MIM consisting of a 12 nm-thick thin gold layer supported on 50 nm-diameter particles and a 125 nm-thick PMMA layer.
We employed colloidal lithography for fabricating the bumpy metal layer. First, a 1 mm-thick glass substrate (38 × 25 mm2) was immersed in an aqueous solution of 3-aminopropyltrimethoxysilane (1 wt%) for 1 minute to promote the subsequent binding of silica nanospheres onto the glass surface. After rinsing with deionized water, the substrate was dried with a hotplate at 100 °C, and then immersed in a 2.5% aqueous suspension of silica nanospheres of 50 nm diameter (Polysciences) for 1 minute. After removing the excess nanospheres by rinsing with water, the substrate was dried with a hotplate at 100 °C. Then gold, silver, or aluminum was evaporated onto the substrate. We prepared a PMMA solution by dissolving PMMA (Aldrich) into 2-ethoxyethyl acetate. The mixture was stirred at 80 °C for 24 h. Then, it was spun onto the metal-coated nanospheres, followed by baking with a hotplate at 80 °C for 1 minute. Finally, an optically thick metal layer was evaporated onto the PMMA layer. The fabricated device area was 25 × 25 mm2.
Figure 1(b) shows a scanning electron microscopy (SEM) image of a 12 nm-thick bumpy gold film before depositing both the dielectric layer and the thick metal layer. Randomly immobilized particles, whose coverage was 41%, were no longer isolated, but aggregated to form metal nanobumps with various shapes. This random bumpy structure converted broadband incident light into surface plasmons without any angular or polarization dependence. Figure 1(c) shows a cross-sectional SEM image of a completed bumpy MIM made with gold. The interface between the PMMA layer and the top metal layer was fairly flat, because the PMMA layer was spun.
3. Experimental results
We investigated the optical response of the bumpy MIM. In the visible and near infrared region (0.4–2.5 μm) we measured reflection spectra including both specular reflection and diffuse reflection (scattering) using a UV-visible-NIR spectrometer (UV-3600, Shimadzu) with an integrating sphere. A 150 nm-thick silver film supported on a glass substrate was used as the reference mirror for all reflection measurements. Absorption excluding scattering was calculated by subtracting the measured reflectance from unity, because the transmission was negligible. In the infrared region (2–4 μm) we measured extinction spectra including scattering using a Fourier transform infrared (FT/IR) spectrometer (FT/IR-6300, JASCO) with a reflection attachment. In order to compensate for the extinction by the glass substrate in the infrared region, we measured the round trip extinction of a 1 mm-thick glass substrate, Eglass. The extinction of the bumpy MIMs, E, after compensating for the extinction of the glass substrate was calculated using
where Etotal is the measured extinction including the extinction of the glass substrate.Figure 2(a) shows absorption spectra of the bumpy MIMs consisting of a 12 nm-thick thin gold layer and several PMMA thicknesses. The thickness of the PMMA layer was defined as the distance between the thin metal layer on the substrate and the thick metal layer [Tp in Fig. 3(a)], which was measured with an atomic force microscope (Keyence). The obtained peak absorptions for the bumpy MIMs consisting of 84 nm, 125 nm, and 147 nm-thick PMMA layers were 94.4%, 95.3%, and 90.0%, respectively. The absorption band edge defined by 50% absorption redshifted as the PMMA thickness increased and was 3.17 μm for the device with a 147 nm-thick PMMA layer. The absorption dip around 0.5 μm also redshifted as the thickness of the PMMA layer increased. The wavelength of the absorption dip coincided with that of the absorption peak for the same structure but without particles. It is well known that the resonant frequency of an antisymmetric mode in a planar MIM structure redshifts as the thickness of the insulator layer increases [25]. Absorption in the wavelength region shorter than this dip may be caused by interband transitions in gold [26]. Figure 2(b) shows the absorption spectra for the bumpy MIMs with several particle coverages. The thicknesses of the thin gold layer and the PMMA layer were 12 nm and 125 nm, respectively. The absorption band became narrower and the band edge wavelength became shorter as the particle coverage decreased. Figure 2(c) shows the dependence of the absorption spectra on the thickness of the bumpy thin metal layer. The thickness of the PMMA layer and the particle coverage were 125 nm and 41%, respectively. The obtained maximum absorptions for the bumpy MIMs with 6 nm, 12 nm, and 25 nm-thick thin gold layers were 80.6%, 95.3%, and 86.9%, respectively. A 6-nm thick gold layer usually forms islands, whereas 12- and 25-nm thick gold films form continuous films [27]. Continuous gold layers, modulated by the particles, exhibited larger absorption compared with the island-structured gold layer. Figure 2(d) shows the absorption spectra of bumpy MIMs made of silver, aluminum, and gold. The thicknesses of the bumpy silver, aluminum, and gold layers were 19 nm, 23 nm, and 12 nm, combined with 142 nm, 138 nm, and 125 nm-thick PMMA layers, respectively. The thickness of each metal layer was adjusted to maximize the absorption peak. The PMMA layers were deposited with the same spinning conditions, but the resultant thickness highly depended on the kind of metal. Both silver and aluminum bumpy MIMs showed similar strong and ultra-broadband absorption to that of the gold one. The aluminum bumpy MIM gave the highest absorption in most of the wavelength region. The silver bumpy MIM gave the broadest absorption band, where the absorption was over 50%, ranging from 0.4 to 3.2 μm. This three-octave bandwidth is much broader than those reported previously [10,20,23].
Fig. 2 (a) Measured absorption spectra of bumpy MIMs consisting of gold layer (120 nm) / PMMA layer (84, 125, or 147 nm) / gold layer (12 nm) / silica particles (50 nm in diameter) / glass substrate. (b) Measured absorption spectra of the bumpy MIMs for particle coverages of 12%, 28%, and 41%. (c) Measured absorption spectra of the bumpy MIMs for thin gold layers with thicknesses of 6, 12, and 25 nm. (d) Measured absorption spectra of silver, aluminum, and gold bumpy MIMs. Solid curves and dashed curves correspond to the absorption measured with the UV-visible-NIR spectrophotometer and the extinction measured with the FT/IR spectrometer, respectively.
Fig. 3 (a) Typical calculation model of a bumpy MIM. The covering angle of the metal layer on the particles is denoted by θ. (b) Calculated absorption spectra of gold bumpy MIMs with a particle coverage of 41% for θ = 40°, 50°, and 60°. An experimentally obtained spectrum is also shown.
Next, we measured angular and polarization dependences of the extinction spectra by a computer-controlled system that we designed, including a visible multichannel analyzer (USB2000, Ocean Optics) and an NIR multichannel analyzer (TG-COOLED NIR-I, Hamamatsu). A xenon-based solar simulator (HAL-320, Asahi spectra) and a halogen light (Mega light 100, SCHOTT) were used as the light sources in the visible and in the near-infrared regions, respectively. Extinction was calculated by subtracting the measured reflectance from unity. Visible and NIR spectra were connected at a wavelength of 0.9 μm.
Figure 4 shows the angular and polarization dependences of a gold bumpy MIM with a 12 nm-thick thin gold layer and a 125 nm-thick PMMA layer for TM-polarized, TE-polarized, and non-polarized incidence light. For the TM polarization, the absorption monotonically increased as the angle of incidence increased, whereas for the TE polarization, the absorption monotonically decreased and was ∼70% at 70°. This polarization dependence may be caused by the Fresnel reflection at the interface between the air and the glass substrate. For the TM polarization, the surface reflection decreases as the angle of incidence increases up to Brewster’s angle, which was ∼56° in this case.
Fig. 4 Measured extinction spectra of a gold bumpy MIM as a function of the angle of incidence and wavelength for (a) TM-polarized, (b) TE-polarized, and (c) non-polarized light.
To confirm this, we performed calculations to remove the effect of the surface reflection. We calculated the surface reflectance at the glass substrate using the Cauchy dispersion equation for the dielectric function of the glass substrate:
where λ is the wavelength in micrometers, and a, b, and c are 1.51, 2.67×10−3, and 2.84×10−4, respectively. The extinction, E, compensated for the surface reflection of the glass was calculated as where E exp is the raw extinction measured in the experiments and Rglass is the calculated surface reflectance at the glass surface. The solid lines in Figs. 5(a) and 5(b) show the absorption spectra after compensating for the surface reflection at 8°, 30°, and 60° for TM-polarized and TE-polarized incident light, respectively. Figures 5(a) and 4(b) also show original absorption spectra. Almost no angular dependence was observed in the compensated absorption spectra for both polarizations. Previously reported perfect absorbers have significant angular dependence for TE-polarized incident light [2,4], whereas our proposed bumpy MIM had almost no angular dependence both for TM-polarized and TE-polarized incident light.Fig. 5 Extinction spectra of the gold bumpy MIM with (solid curves) and without (dashed curves) compensation for the surface reflection at the substrate surface for 8°, 30°, and 60° incidence for (a) TM-polarized and (b) TE-polarized light.
The glass substrate is indispensably required for the bumpy MIM, especially for the proposed fabrication procedure. However, it protects the fragile surface of the bumpy MIM and the surface reflection could be easily removed by introducing such an anti-reflection structure as a moth-eye structure into the glass surface.
4. Mechanism of the ultra-broadband absorption
To understand the mechanism of the ultra-broadband absorption of the bumpy MIM, we calculated the spectral response and the electromagnetic field distribution using the finite-difference time-domain (FDTD) method. We modeled the bumpy MIMs in a simple manner as shown in Fig. 3(a). The key point in the modeling of the structure is the geometry of the metal deposited on the particles. As seen in Fig. 1(c), the metal layer on the particles was away from the metal layer on the substrate. Thus, we modeled the metal layer on the particles as a fraction of a spherical metal shell characterized by a covering angle θ [28]. Dielectric constants of the PMMA film and glass substrate were set to 2.16 and 2.32, respectively. We employed the Drude model for the dielectric function of gold:
where ∊∞, ωp, and γ are 10.4, 1.37×1016 rad/s, and 1.18×1014 rad/s, respectively. The cell size was Δx = Δy = Δz = 2.5 nm. The perfect matching layers and periodic boundary conditions were employed in the vertical direction and the lateral direction, respectively, in the calculation of Fig. 3(b). The perfect matching layer was adopted in all directions in the calculation of Figs. 6 and 7.Fig. 6 Absorption cross-section spectra of bumpy MIMs with 1–3 particles for x-polarization (solid lines) and y-polarization (dashed lines).
Fig. 7 Calculated electromagnetic field distribution of a bumpy MIM with 3 particles, (a) Ex and (b) Hy at 1.48 μm and (c) Hx at 1.26 μm.
First, we performed calculations for the gold bumpy MIM with a 12 nm-thick gold layer and a 125 nm-thick PMMA layer. The lateral location of the particles was extracted from the SEM image whose observation area was 2.0 μm × 1.8 μm. We divided this area into four 1 μm × 1 μm areas, which were partially overlapped with each other. For each absorption spectrum, we averaged eight absorption spectra for these four areas, each of which was illuminated with two orthogonally polarized plane waves.
Figure 3(b) shows the calculated absorption spectra for several covering angles. The absorption spectrum for θ = 50° was most consistent with the experimental result. Therefore, we employed the covering angle of 50° for the following calculations. For a covering angle of θ = 40°, the thin metal layer on the particles never overlaps even if the particles contact with each other. This may result in a narrow absorption band. This result also explains that the absorption bandwidth decreases as the particle coverage decreases [see Fig. 2(b)]. For small coverage, most thin metal layers on the particle are isolated.
In order to investigate the mechanism in more detail, we employed simpler structures for the calculations. Figure 6 shows calculated absorption spectra of bumpy MIMs consisting of an isolated particle, two particles in a row, and three particles in a row for two orthogonal polarizations. One polarization had an electric field parallel to the connection axis of the particles (x-polarization), and the other had an electric field perpendicular to the connection axis (y-polarization), as shown in Fig. 6. The angle of incidence was normal to the interface. The bumpy MIMs consisting of two or three particles exhibited two distinct absorption peaks in the visible and near-infrared regions.
Then, we calculated the electromagnetic field distribution of the bumpy MIM consisting of three particles to assign these absorption peaks. Figures 7(a) and 7(b) show the calculated electric field (Ex) and magnetic field (Hy) for x-polarized incident light at the resonant wavelength of 1.48 μm. This suggests that the peak at 1.48 μm is assigned to the longitudinal mode of the localized surface plasmons (LSPs) supported by the thin metal layer on the particles. However, the magnetic resonance that was observed in the previously reported perfect absorbers was not observed. Figure 7(c) shows the calculated magnetic field (Hx) distribution at 1.26 μm excited by y-polarized incident light. This suggests that the peak is assigned to the transverse mode of the LSPs supported by the hole perforated in the thin metal layer [29,30]. In the ideal case, these two peaks should appear at the same wavelength due to Babinet’s principle [31]. The difference of the resonant wavelength for the two polarizations may be mainly caused by a difference in the shape. The thin metal layer in the bumpy MIM is corrugated due to the particles, whereas the hole is not. Other absorption peaks around the shorter wavelength region are assigned to the transverse LSP mode of the thin layer on the particles, the longitudinal LSP mode of the hole, and higher-order LSP modes. The resonant wavelength depends on the number of particles in a row, as shown in Fig. 6. Increasing the number of particles resulted in increasing the aspect ratio of both the metal layer on the particles and the hole. These results suggest that the resonant wavelength highly depends on the shape of the aggregated particles and the aggregation of the particles, resulting in broadening of the absorption bandwidth. The almost complete lack of polarization dependence in the bumpy MIM can also be explained by these results. A complementary structure, which consists of the metal layers on the particles and the holes, supports LSPs in almost the same wavelength region both for TM and TE polarizations.
5. Conclusion
We have proposed an MIM structure consisting of randomly arranged metal nanobumps, which showed a broad absorption band ranging from the visible to the infrared. The metal nanobumps were fabricated by colloidal lithography, which allowed easy preparation of large area devices at low cost. The complementary structure showed no angular or polarization dependence. All three kinds of metal used here, gold, silver, and aluminum, exhibited similar broadband absorption. The silver bumpy MIM exhibited the broadest absorption whose bandwidth corresponded to a three-octave region ranging from 0.4 to 3.2 μm. The proposed absorber is expected to find applications in various optical and optoelectronic devices, e.g. photovoltaics, thermal-radiation devices, and optical filters.
Funding
JSPS KAKENHI (JP15K04708)
Acknowledgments
A part of this work was supported by the RIKEN Junior Research Associate Program. The calculations were performed using the RIKEN HOKUSAI GreatWave Facility.
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