Letter pubs.acs.org/NanoLett
Plasmonic Optical Interference Dukhyun Choi,*,† Chang Kyun Shin,‡ Daesung Yoon,† Deuk Seok Chung,‡ Yong Wan Jin,*,‡ and Luke P. Lee*,§ †
Department of Mechanical Engineering, School of Engineering, Kyung Hee University, Yongin, 446-701, Republic of Korea Samsung Advanced Institute of Technology, Yongin, Gyeonggi 446-712, Republic of Korea § Biomolecular Nanotechnology Center, Berkeley Sensor and Actuator Center, Department of Bioengineering, University of California at Berkeley, Berkeley, California 94720, United States ‡
S Supporting Information *
ABSTRACT: Understanding optical interference is of great importance in fundamental and analytical optical design for next-generation personal, industrial, and military applications. So far, various researches have been performed for optical interference phenomena, but there have been no reports on plasmonic optical interference. Here, we report that optical interference could be effectively coupled with surface plasmons, resulting in enhanced optical absorption. We prepared a three-dimensional (3D) plasmonic nanostructure that consists of a plasmonic layer at the top, a nanoporous dielectric layer at the center, and a mirror layer at the bottom. The plasmonic layer mediates strong plasmonic absorption when the constructive interference pattern is matched with the plasmonic component. By tailoring the thickness of the dielectric layer, the strong plasmonic absorption can facilely be controlled and covers the full visible range. The plasmonic interference in the 3D nanostructure thus creates brilliant structural colors. We develop a design equation to determine the thickness of the dielectric layer in a 3D plasmonic nanostructure that could create the maximum absorption at a given wavelength. It is further demonstrated that the 3D plasmonic nanostructure can be realized on a flexible substrate. Our 3D plasmonic nanostructures will have a huge impact on the fields of optoelectronic systems, biochemical optical sensors, and spectral imaging. KEYWORDS: Plasmon, optical interference, absorption, structural color, anodic aluminum oxide
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characteristics of reflected light. Since optical interference results from the superimposition of light (i.e., electromagnetic) waves, constructive interference provides the maximum intensity of superimposed light wave and thus could enhance the plasmonic effect. Previously, surface plasmon interference nanolithography (SPIN)14,15 has been introduced, but this is a totally different concept with plasmonic optical interference. SPIN uses the interference of surface plasmon waves, but plasmonic optical interference considers coupling between optical interference from light waves and surface plasmon. In this study, we introduce plasmonic optical interference from multilayered 3D plasmonic nanostructures that consist of three elements; a plasmonic layer at the top, a dielectric layer at the center, and a mirror layer at the bottom. In our 3D plasmonic nanostructure, the optical behavior of reflected light is controlled by coupling optical interference with surface plasmon. We systematically characterize the functional role of each layer in a 3D plasmonic nanostructure. Maximum reflection and plasmonic absorption are analyzed by 3D
hen two light waves are superimposed, their interaction leads to constructive and destructive interference, resulting in interference fringes.1−4 Particularly, interference fringes on thin structured layers could provide brilliant colors called structural color, and examples of this can be found in nature in creatures such as butterflies, hummingbirds, beetles, and snakes.5−8 Therefore, thin-film interference could provide a great potential for industrial, commercial, and military applications, with biomimetic surfaces that could provide adaptive camouflage, efficient optical switches, and antireflection coating. Interference can also greatly enhance Raman scattering by absorbing all of the intensity of an incident beam (i.e., leaving no reflected beam), which is known as interference-enhanced Raman scattering (IERS).9,10 Thus, optical interference is a powerful optical tool to effectively manipulate light at the nanoscale and has many applications in electronics and biophotonics. Optical interference mainly depends on the thickness of the film or the refractive index of the film medium. 2−4 Furthermore, periodic patterns and photonic lens structures could change optical interference.11−13 However, there have been no reports that optical interference can be effectively coupled by surface plasmon to manipulate the optical © 2014 American Chemical Society
Received: March 7, 2014 Revised: May 5, 2014 Published: May 7, 2014 3374
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Figure 1. Plasmonic optical interference in a multilayered 3D plasmonic nanostructure. (a) SEM image of gold (Au)-coated AAO as the experimental model and schematic illustration of each layer. The structure consists of a plasmonic layer at the top, a dielectric layer at the center, and a mirror layer at the bottom. The thickness (t) of a dielectric layer can be tuned. (b) Role of plasmonic and mirror layers. High reflection without a plasmonic layer occurs due to the mirror layer. Without a mirror layer at the bottom, the structure is semitransparent and creates plasmonic absorption from the nanoporous gold layer at the top. (c) Coupling of surface plasmons and interference. Strong plasmonic absorption takes places when constructive interference (red color lines) is matched with the plasmonic layer. On the other hand, reflection occurs when deconstructive interference is located on the plasmonic layer. (d) Reflection behavior of our 3D plasmonic nanostructure. Due to the strong plasmonic absorption and high reflection, high peak-to-peak amplitude could be created.
employing the nanoporous plasmonic layer at the top, the collective excitation of the electron gas is confined near the interfaces of the plasmonic nanopores, and the EM field of the light at the interface is greatly enhanced. This leads to a significant enhancement of the incident and reflected radiation around the plasmonic nanopores. Without a mirror layer at the bottom, plasmonic absorption is created from the nanoporous gold layer at the top, but the peak-to-peak amplitude (from the reflection to the absorption) is very small due to low reflection and interference (Figure 1b). The porous dielectric layer at the middle plays a role to control interference mode matching with the top plasmonic layer by changing its thickness or the wavelength of the incident light. Theoretically, the dielectric layer should not be porous in the 3D layered structure. However, the porous dielectric layer at the middle in our system provides a great pathway to create a large-area, uniform nanopore plasmonic layer by simply depositing a metal layer. Clear interference patterns (Figure 1c, red lines denote constructive interference) were produced by the three-layer 3D nanostructure, based on thin-film interference theory. Generally, interference conditions (i.e., constructive or deconstructive) for thin films are determined by
electrodynamic calculations and experiments in terms of the coupling of surface plasmon and interference in the 3D plasmonic nanostructure. Depending on the wavelength of an incident beam and the thickness of the dielectric layer, the wavelengths of the maximum absorption and reflection are significantly changed. Based on the plasmonic interference, clear structural colors could be observed. We exploit the design equation to determine the thickness of the dielectric layer that produces the strong plasmonic absorption at a given wavelength. The flexible form of the 3D plasmonic nanostructures is also demonstrated. Finally, we investigate the angle-dependent optical behaviors of the 3D nanostructure. Figure 1a shows a scanning electron microscopy (SEM) image of our 3D plasmonic nanostructure design using a goldcoated anodic aluminum oxide (AAO) template. An evaporated gold thin film at the top serves as a plasmonic layer, an AAO layer (i.e., nanoporous alumina) at the center is the dielectric layer, and aluminum (Al) at the bottom acts as a reflecting mirror layer. Incident white light (Iwhite(λ)) results in a reflected wave (Ir(λ)) from the 3D plasmonic nanostructure. The spectrum of the reflected wave is critically dependent on each layer of the 3D nanostructure. Without a plasmonic layer, high reflection from the nanostructure is dominant due to the mirror layer and the transparent dielectric layer (Figure 1b). By 3375
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Figure 2. Coupling behavior of surface plasmon and interference in the 3D plasmonic nanostructure according to the wavelength of an incident beam. (a) Local field distribution and interference fringe patterns in the 3D plasmonic nanostructure with the same thickness (tAAO = 300 nm) of a dielectric layer under different wavelengths. When the constructive interference is positioned on the plasmonic component, plasmonic absorption (case (i) and (iii)) is created. On the other hand, reflection (case (ii)) occurs when deconstructive interference is located on the plasmonic layer. Reflection spectra (b) from FDTD simulation and (c) from experiments. The critical role of a plasmonic layer is demonstrated by the reflectance spectrum of the 3D nanostructure with and without a plasmonic gold (Au) layer at the top. The scale bar of the SEM image is 100 nm. (d) Function of the Al mirror layer at the bottom. Photographs of regions with and without the Al layer in the 3D plasmonic nanostructure. The corresponding reflectance spectra for each region are shown. 3376
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Nano Letters m × λ = 2nd cos θ 2
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based on our experimental model (Figure S1a). As shown in the SEM image, the top of the AAO template was not flat. We fabricated AAO by two-step anodization in oxalic acid where the initial pore size (Dp) of AAO was 32 nm and the unit cell size (ρ) was 100 nm.24−26 To maintain the open pores in the plasmonic layer as well as the AAO, the thickness of evaporated gold was set to 20 nm.26 We monitored the optical coupling behavior of surface plasmon and interference in 3D plasmonic nanostructures by changing the wavelength (λ) of an incident beam. Figure 2a shows the local field distribution and interference fringe patterns in the 3D plasmonic nanostructure with a 300 nm-thick dielectric layer (i.e., tAAO = 300 nm) according to the wavelength of an incident beam in the visible spectrum. Interference patterns occurred at different positions in the 3D plasmonic nanostructure, depending on the wavelength, with bright fringes indicating constructive interference and dark fringes showing destructive interference. The enhanced light wave of constructive interference created a strong nanoplasmonic local field around the plasmonic nanopore (Figures 2a(i) and 2a(iii)), while no nanoplasmonic field distribution was detected around the plasmonic nanopore with deconstructive interference (Figure 2a(ii)). Figure 2b shows the corresponding simulated reflectance spectrum from the 3D plasmonic nanostructure. Clear absorption minima were generated at λ1 and λ3 (Figure 2a(i) and (iii)), whereas strong reflection occurred at λ2 (Figure 2a(ii)). The same optical behavior was demonstrated with the gold-coated AAO with tAu = 20 nm and tAAO = 300 nm (inset SEM image; Figure 2c). The importance of the top plasmonic gold layer was demonstrated by reflection spectra with significantly reduced absorption (red lines in Figures 2b and c) in its absence, in both the simulation and the experiment. We also experimentally examined the role of the mirror layer at the bottom of our 3D nanostructure. Figure 2d shows a photograph of a goldcoated AAO template with tAAO = 220 nm. Before removing the Al mirror layer, a yellow color was observed, but a red color was displayed after removing the mirror layer, thus clearly demonstrating the change of optical properties of the 3D nanostructure by the mirror layer. The optical behavior was completely different in the presence or absence of the mirror layer, as shown by the reflection spectra of the 3D nanostructure. Without the mirror layer, the 3D nanostructure showed plasmonic absorption at about 500 nm, which is intrinsically created by a nanoporous gold layer. With the mirror layer, the 3D nanostructure formed interference patterns inside the structure, and the interaction between surface plasmon and interference created a different plasmonic absorption position (here, at about 430 nm) based on the thickness of the dielectric layer. Thus, constructive interference at a wavelength of 430 nm was well matched with the plasmonic layer at the top in the 3D plasmonic nanostructure with tAAO = 220 nm. We concluded that the optical coupling of surface plasmon and interference could indeed determine the optical response (i.e., absorption or reflectance) at a specific wavelength in the 3D plasmonic nanostructure. Our experimental model for the 3D plasmonic nanostructure allowed us to easily tailor the thickness of a nanoporous dielectric layer (i.e., tAAO). The coupling of surface plasmon and interference could be controlled by changing the thickness of the dielectric layer at given wavelengths. When the thickness of the dielectric layer was 100 nm under the incident beam of 380 nm, destructive interference was coupled with the plasmonic layer, and no nanoplasmonic local field was produced, thus
(1)
Where m is the interference order as an integer larger than zero, λ is the incident wavelength, n is the refractive index of a thin film, d is the film thickness, and θ is the angle of light incident to the thin film. When reflected light experiences a 180° phase change due to the reflection from a medium of higher refractive index based on Fresnel conditions, constructive interference occurs when m is odd, whereas deconstructive interference takes place when m is even. In the 3D plasmonic nanostructure, we can imagine two cases: constructive interference is matched with the plasmonic layer (Figure 1c(i)), and destructive interference is positioned with the plasmonic layer (Figure 1c(ii)). The enhanced light wave in constructive interference may generate strong plasmonic absorption at a certain wavelength, as shown in the yellow color of Figure 1c(i). When the wavelength is changed, destructive interference is formed at the plasmonic layer and may prevent the plasmonic absorption due to minimized light at the plasmonic layer, as shown in Figure 1c(ii). Thus, a reflected wave Ir(λ) with strong absorption and high reflection peaks can be produced according to the wavelength (Figure 1d). Finally, we could understand that optical behaviors by interference could be controlled by surface plasmons. Since an extraordinary transmission from plasmonic nanohole arrays16 has been reported, many studies regarding nanopore plasmonic structures have been extensively investigated.17−20 However, most of these studies focused on the nanofabrication and the characterization of hole arrays such as hole periodicity, hole dimensions, and hole shapes. Our 3D layered nanostructure includes a nanopore plasmonic layer, but we focus on the coupling behaviors between the plasmonic layer and the interference modes by adopting a dielectric spacer and a mirror layer. Of course, a thin plasmonic layer on a dielectric spacer with a back reflector has been also introduced and showed color patterns, but the light absorption was controlled by the environmental dielectric materials deposited on plasmonic layer (i.e., nanocomposite layer on a fixed dielectric spacer) by atomic layer deposition (ALD).21 Our main control parameter is the thickness of the dielectric spacer below the nanopore plasmonic layer, thus controlling interference modes (i.e., destructive or constructive interference). Previously, the coupling effects between the plasmonic layer and the dielectric film/cavities have been also investigated by considering Fabry−Perot modes, but their structure dimensions and material configurations are different with our 3D layered structure, and the Fabry−Perot modes (i.e., multiple interference) are different in concept than our interference modes (i.e., construction and destruction modes).22,23 Generally, the Fabry−Perot interferometer is made of a transparent plate with two reflecting surfaces, but our interference modes can occur from a dielectric layer (i.e., AAO) on one reflecting surface (i.e., Al). By adopting a plasmonic layer on the dielectric layer with a mirror, we could highly enhance the plasmonic absorption due to the coupling between the constructive interference and the plasmonic layer. To determine the optical coupling characteristics of surface plasmon and interference in a 3D plasmonic nanostructure, 3D electrodynamic calculations were performed by employing a finite difference time domain (FDTD) simulation (see Methods and Supporting Information). The geometry of the 3D plasmonic nanostructure for the 3D FDTD simulation was 3377
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Figure 3. Tunable optical behavior according to the thickness of the dielectric layer at the middle of the 3D plasmonic nanostructure. (a) Local field distribution according to the thickness of the dielectric layer at the middle at a wavelength of 380 nm. Reflectance spectra for different thicknesses (tAAO = 100−300 nm) of a dielectric layer (b) from FDTD simulation and (c) from experiments. (d) Color gamut for structural color display based on the reflectance spectra for 3D plasmonic nanostructures. The photographs show each color according to the thickness of the dielectric layer. The structural color covers the full visible range as shown.
leading to no absorption (i.e., strong reflectance) as shown in Figure 3a(i). On the other hand, when tAAO was 140 or 260 nm under the same wavelength of 380 nm, the constructive interference was matched with the plasmonic layer, so that a strong plasmonic local field was produced (Figure 3a(ii) and (iii)). Finally, strong absorption could be created at a specific wavelength as shown in positions (ii) and (iii) of Figure 3b. Thus, we concluded that the thickness (tAAO) of the dielectric
layer was a critical parameter to control the absorption and reflectance at a given wavelength. From the 3D simulations, we could determine the optical reflectance spectra for different AAO thicknesses (Figure 3b). To experimentally verify the optical behavior of our 3D plasmonic nanostructures, we measured the reflectance of 3D plasmonic nanostructures fabricated by the deposition of gold on AAO templates with different thicknesses. The AAO thickness (tAAO) increased linearly with anodization time, 3378
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Figure 4. 3D plasmonic nanostructure applications. (a) Design equation to determine the thickness of a dielectric layer where plasmonic absorption could be created at a given wavelength. (b) Structural color display of “KHU” capitals for “RGB” colors by tailoring the thickness of the AAO dielectric layer. The scale bar of the SEM images is 200 nm. (c) Flexible 3D plasmonic nanostructure. Angle dependency of 3D plasmonic nanostructures for “RGB” colored samples. (d) Color gamut and (e) percent reflectance.
shifted up to approximately 100 nm due to the lower effective dielectric constant of the AAO with larger pores.27,28 Our findings clearly demonstrated that the optical behavior of 3D plasmonic structures could be controlled by tailoring the geometry of the AAO dielectric layer. We calculated the thickness of the dielectric layer in the 3D plasmonic structure needed to create strong absorption at a given wavelength based on eq 1 and the simulation results in Figure S4. The effective refractive index of the AAO dielectric layer can be determined by the Maxwell-Garnett equation29 in which the volume fraction of air and alumina is taken into account. For an AAO pore size of 32 nm, the effective refractive index was determined to be 1.7 by using the refractive index of alumina of 1.77.30−32 When the Al thickness is 20 nm (tAl) at the central position (Figure S2) and the light wave is vertically incident to the structure, eq 1 can be modified as follows:
allowing it to be monitored (Figure S2). AAO templates with thicknesses of 100, 120, 140, 260, and 300 nm were created. The evaporated gold thickness was 20 nm in order to produce a nanoporous plasmonic layer. The thicker gold blocked the AAO pores, and the thinner gold formed a discontinuous film. As shown in Figure 3c, the absorption minimum was redshifted, and the secondary absorption minimum appeared as the AAO thickness increased, showing good agreement with the theoretically simulated spectra (Figure 3b). Interestingly, the 3D plasmonic nanostructures produced distinct structural colors from red to purple, covering the full visible range (Figure 3d). The colors were very well matched with the reflectance spectra of the 3D plasmonic structures and could be tuned by tailoring the AAO thickness (tAAO). Without a plasmonic layer in the 3D plasmonic structures, such a brilliant color display could not be produced. We also explored the effect of pore size of AAO on optical behavior (Figure S3). As the pore size increased from 32 to 80 nm, the absorption peak was blue3379
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Nano Letters ⎛ m λ − 400 ⎞ ⎟ × λ = 2 × 1.7 × ⎜tAAO + tAl − ⎝ ⎠ 2 4
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angle of ±20° due to the blue-shift of the reflected light at 700−800 nm. On the other hand, the percent reflectance of the “G” sample was greatly reduced from the viewing angle of ±10° since the major reflected light at 450−550 nm escaped from the visible range by the blue-shift of the reflection spectrum. Therefore, we concluded that the total reflection intensity and color display were critically dependent on the viewing angle. In conclusion, we have theoretically and experimentally demonstrated plasmonic optical interference in layered 3D plasmonic nanostructures. The plasmonic layer at the top played a critical role in creating plasmonic absorption, and the mirror layer at the bottom provided a clear interference pattern in the nanostructure. Plasmonic absorption and reflection were controlled by the thickness of the dielectric layer at a given wavelength. We developed a design equation to determine the thickness of a dielectric layer which can create strong absorption at a given wavelength. Furthermore, we demonstrated that plasmon-enhanced structural colors could be obtained from the 3D plasmonic structures, and we explored their flexible design and angle dependency. Such 3D plasmonic nanostructures based on the optical coupling of surface plasmon and interference offer a promising platform for nextgeneration nano/biophotonic devices.
(2)
The third term in the parentheses is the calibration factor, which may arise from the geometry of our 3D nanostructure as the wavelength increases. The maximum absorption is determined when the interference order, m, is odd. Figure 4a shows the critical thickness (tAAO)c of the AAO dielectric layer that yields the maximum absorption at a given wavelength. The critical AAO thicknesses determined by eq 2 showed good agreement with the experimental results which were determined by the absorption positions (Figure 3c). By using eq 2, 3D plasmonic structures could thus be designed where a strong nanoplasmonic local field can be obtained at a given wavelength, to produce highly tunable nanoplasmonic structures. Based on the coupling of surface plasmons and interference from the 3D plasmonic structures, we designed RGB patterns as shown in Figure 4b. A clear red (R) color was found for the 3D plasmonic structure with an AAO thickness of 60 nm, and blue (B) and green (G) colors were produced by the 3D plasmonic structures with tAAO = 120 and 140 nm, respectively, as shown in the reflectance spectrum (Figure S5b). Thus, the capitals “KHU” could be produced with RGB colors. The detailed fabrication process is described in Figure S5a. To date, most color tuning has relied on color filters or sophisticated methods for controlling the pattern geometry at the micro/ nanoscale.33−36 In contrast, our 3D plasmonic structure provides a remarkably simple way of tuning the color display in the full visible range, simply by tailoring the thickness of a dielectric layer. Nanoporous AAO templates can be formed on a flexible substrate. After deposition of 200 nm thick Al on a polyethylene terephthalate (PET) substrate, the first-step anodization of Al provides a nanoporous AAO with an Al layer at the bottom. Since the uniformity of the nanopore array and the pore size of AAO on a flexible PET substrate are slightly different from the AAO templates fabricated by twostep anodization, the resonance wavelengths were different, and this is under investigation. However, we were able to find the same coupling behaviors of surface plasmon and interference as well as the brilliant color display from the flexible 3D plasmonic nanostructure. Figure 4c shows a photograph of the goldcoated AAO on PET that exhibits the flexibility with a structural color. The size was 3 × 3 cm2 (Figure S6), but it could be easily scaled up to over a 4 in. wafer size. We examined the angle dependency of color display of the 3D plasmonic structure. Figure 4d shows the color gamut of “R”, “G”, and “B” samples according to the viewing angle. To clearly follow the color change of each sample when the viewing angle was varied, we used the counter color dots (i.e., blue for “R” sample, red for “G” sample, and green for “B” sample) in Figure 4d. The RGB colors were converged to white via different color domains. Figure 4e shows the percent reflectance (defined as the intensity ratio of the reflected light to the incident light) in the full visible range as a function of a viewing angle. Generally, as the viewing angle increased, the reflectance spectrum was blue-shifted based on eq 1. The percent reflectance of the “B” sample rarely changed up to a view angle of ±40°, but it increased significantly due to the reflected light at 500−600 nm which stems from the blue-shift of the reflected light at 800−900 nm. Similarly, the percent reflectance of the “R” sample was augmented from the viewing
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ASSOCIATED CONTENT
* Supporting Information S
Simulation and fabrication details, pore size effect, theoretical approach for governing equation, and other supporting images. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
D. Choi and C. K. Shin contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Energy International Collaboration Research & Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE) (2011-8520010050), by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013R1A1A2063798), and by a grant from the Kyung Hee University in 2013 (KHU-20130693).
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