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Abstract
Grating-coupled surface plasmon resonance (SPR) is demonstrated with one-dimensional gratings fabricated on the surface of bulk stainless steel using imprinting combined with electrochemical etching. The extent of light coupling and the wavelengths of SPR peaks were characterized with respect to the incident angle and polarization states of the light. When the plane of incidence was orthogonal to the grating grooves, only TM polarization was absorbed at two different wavelengths. In the plane of incidence parallel to the grooves, a single resonance peak was observed only when the incident light was TE-polarized. The dependence of SPR wavelengths on the incident angle was in good agreement with theoretical consideration.
© 2017 Optical Society of America
1. Introduction
Metals have dominantly been used as structural materials because of their superior mechanical properties. As the application fields of metals are nowadays expanded to consumer electronics, art, decoration, accessory, and building interior, their esthetic functions become more and more significant. The generation of vivid colors is a crucial factor involving the esthetic functions. Although metals exhibit metallic colors that arise from the electron transition between orbitals of different energy, each metal has a fixed metallic color. A simple way of producing colors in a metal is to coat its surface with pigment-based chemicals. However, chemical colors are environment-sensitive and can be easily faded. An alternative method is to anodize the metal surface to create an oxidation layer. But this method is applicable only to certain metals such as Al and Ti [1,2]. In this respect, the production of structural colors, by controlling the optical properties of metals, has drawn a great deal of research attention over the past decade. Unlike traditional color, which comes from pigments or dyes that absorb light, structural color can be made resistant to fading.
Plasmonic colors are structural colors that emerge from resonant interactions between light and metallic nanostructures. Apart from their utilization for color generation [3–6], plasmonic nanostructures have a wide range of applications from bio-sensors to optical devices such as filters, polarizers, phase retarders, and surface holograms [7–12]. These plasmonic devices are based on localized surface plasmon resonance (LSPR) and thus require structures smaller than the wavelength of light. Since subwavelength-scale patterns need to be fabricated either by e-beam lithography or focused ion beam, LSPR-based plasmonic devices are highly expensive and not scalable. Noble metals such as Au and Ag have suitable properties for plasmonics. Therefore, relevant research works have mainly been focused on these two materials. Recently, Al draws increasing attention as a plasmonic material for applications in the ultraviolet (UV) range [13–15].
Meanwhile, stainless steel (STS) is widely used in our daily life as well as in industry owing to its excellent resistance to corrosion. Colors and decorations are important for the value improvement of real consumer products. Several methods have thus been suggested to make STS appear colored. One is to form a porous anodic film on its surface by direct current anodizing in electrolyte at elevated temperatures [16]. While the anodic films formed by this approach resulted in different colors depending on the anodizing conditions, they were always cracked upon drying in air. A different method is to induce an oxide passivation layer on the surface by chemical reaction or laser irradiation [17–21]. It remains a challenge to form a uniform layer with the given methods. Laser-induced surface structuring can also be utilized to colorize metals [22]. Several groups independently fabricated aperiodic, ripple structures on STS with femtosecond lasers, generating specific color patterns due to the diffraction effect [23–25]. Luo et al. [26] produced micro/nanostructures on the STS surfaces by a nanosecond laser in different gaseous environments. Oxygen-rich environment was found to boost the formation of nanostructures and the appearance of colors. However, whether the color effect results from plasmon resonance or oxidation is still unclear. Laser-induced surface structuring generally require a long processing time and the obtained structures are only statistically reproducible.
While plasmonic resonant absorption provides an effective means of producing colors, little has been known about the plasmonic behavior of STS. In this article, we demonstrate surface plasmon resonance (SPR) on bulk STS. Surface plasmons, which are charge oscillations propagating on a metal-dielectric interface, cannot be excited by incident light on a planar metal surface due to the momentum mismatch. A way of coupling the incident light into surface plasmons is through the use of metal gratings [27–34]. In the present study, one-dimensional (1D) gratings of 500 nm period were fabricated on STS surfaces by imprinting and electrochemical etching. The extent of light coupling and the wavelength of SPR peaks were then characterized as functions of the incident direction and angle of light and its polarization states. The fabricated grating structure led to conspicuous SPR absorption and the observed peak positions were in good agreement with theory. Grating-coupled surface plasmon resonance in stainless steel may find many applications including color display, product identification, anti-counterfeiting, and decoration.
2. Experimental section
The samples used for this study were prepared from commercially available stainless steel plates (STS 316L, thickness = 1 mm, one side mirror polished). They were cut into dimensions of 20 mm x 20 mm. A central region (~10 mm x 10 mm) of the sample was patterned. The gratings were fabricated by imprinting and electrochemical etching. The fabrication process is schematically illustrated in Fig. 1. A thin photoresist (PR) layer of 160 nm thickness was spin-coated onto the surface of STS and the PR layer (negative photoresist SU 8) was imprinted using a polyurethane acrylate (PUA) stamp. The PUA stamp was casted from a Si master pattern (tapered 1D line pattern of 500 nm period), which was fabricated by photolithography and anisotropic wet etching. The imprinted PR layer was cured by UV light and then baked at 150°C. The top portion of the imprinted PR layer was eliminated by reactive ion etching (RIE) to make the surface of STS partially exposed. Then, STS was electrochemically etched with an aqueous solution of oxalic acid (C2H2O4), where the sample and a Cu block were used as the anode and cathode, respectively [35]. Finally, the residual PR layer was removed with a piranha solution. In terms of the quality and uniformity of fabricated patterns, the electrochemical etching gave much better results than typical acid etching using HCl or H2SO4. In the latter case, the PR layer was easily damaged by the etchant. This resulted in nonuniform line widths, making some lines disconnected in the middle.
In the current work, the period of the pattern was fixed to 500 nm, which is the minimum scale achievable with photolithography. The width and height of the line pattern could be varied by controlling RIE and electrochemical etching conditions. Figure 2 shows scanning electron microscopy (SEM) images for line patterns of different widths and heights. Insets are magnified images. The STS sample is too thick to cut. Therefore, the pattern height was estimated by casting a polydimethylsiloxane mold from the sample and taking its cross-sectional SEM image. The cross-sections of the line patterns were not rectangular but a little tapered. The width and height given in Fig. 2 represent the values measured at the top and bottom center of the pattern, respectively. Diffuse reflectance measurements were performed using a spectrophotometer equipped with an integration sphere (NIR Cary 5000), in which unpolarized light was incident at a fixed angle of 7°. Specular reflectance spectra were measured by a spectrophotometer (model: photon RT, Essent optics) in an incident angle range of 10 - 50°. The polarization of incident light was controlled by an equipped polarization filter. The dielectric constant of STS 316L was derived from the real and imaginary parts of refractive indices measured by ellipsometry using a polished sample.
Fig. 2 SEM images of gratings. (a) Pattern width = 260 nm, height = 100 nm, (b) width = 180 nm, height = 155 nm, (c) width = 60 nm, height = 170 nm.
3. Theoretical background
For light travelling in free space, the relation between wave vector Κ and angular frequency ω is linear, i.e., K = ω/c, where c is the speed of light in vacuum. The dispersion relation for surface plasmons propagating on a planar metal surface can be obtained by solving Maxwell’s equations and applying proper boundary conditions [27,36]. The dispersion relation of surface plasmons at a dielectric-metal interface is written as
Here εm is the frequency-dependent dielectric function for the metal and εd, the dielectric constant for the medium surrounding the metal (εd = 1 on an air-metal interface). Surface plasmons cannot be excited by direct illumination of light because energy and momentum conservation (ωlight = ωsp and Klight = Ksp) cannot be fulfilled at the same time. In order to excite surface plasmons, a momentum (i.e., wave vector) transfer process should thus be established. A prominent way of achieving this is to use diffraction effect. When incident light strikes a diffraction grating, multiple integers of the grating vector can be added to or subtracted from the wave vector of the light via Bragg scattering. Figure 3 depicts a 1D grating of period “d”, where the grating grooves run along the y-direction and the grating vector whose magnitude is given by 2π/d is parallel to the x-direction. Suppose that a plane wave of wave vector K impinges on the grating at an incident angle θ with respect to the surface normal, in the plane of incidence that makes an azimuthal angle α with the grating grooves. Then the x-component of the wave vector iswhere m is an integer representing the diffraction order (m = ± 1, ± 2, ..). The second term on the right-hand side of Eq. (2), an integral multiple of the grating vector, arises from the diffraction grating. The y-component of the wave vector isThe conservation of energy and momentum can be simultaneously achieved if the surface plasmon wave vector in Eq. (1) equals the wave vector of the incident light on the metal surface, that is., Ksp = (Kx2 + Ky2)1/2. This leads to the following relation.Here λ is the vacuum wavelength of the incident light. The direction of the incident light is described by two angles θ and α. The resonant incident angle θ depends on the azimuthal angle α of the plane of incidence. Equation (1) is widely used in literature to bring up discussions related to the surface plasmons. However, whether it is still valid for the grating structure remains as a question. There is no report on the grating-coupled SPR of STS. Therefore, to find out whether it is applicable to textured STS surface is also the objective of the current work. Equation (4) means that for a given plane of incidence, light of specific wavelength can be absorbed at specific incident angles. A case of special interest occurs when the plane of incidence is perpendicular to the grating grooves, i.e., α = 90°. Equation (4) then reduces toAt α = 90°, Ky does not exist. Therefore, surface plasmons can propagate only in the x-direction. For a 1D grating, the m = ± 1 terms cause a strong diffraction effect. Therefore, surface plasmons can be excited at two different wavelengths for a given incident angle. In another case when α = 0°, both Kx and Ky have a non-zero value. Equation (4) becomesThen, only one resonance wavelength is possible for a given incident angle. It is to be noted that the above equations mention nothing on the effect of polarization of the incident light. As will be seen later, the incident light should have a polarization component perpendicular to the grooves in order for the grating to support SPR [27,31].4. Results and discussion
4.1 Diffuse reflectance
Figure 4(a) shows digital-camera images of patterned samples taken at near-normal viewing angles. The arrows represent the orientation of the grating vector. The patterned STS surface revealed orange and greenish colors at α = 0° and 90°, respectively. As the viewing angle was tilted (30°, 45°, and 75°), more vivid colors appeared due to diffraction (Fig. 4(b)). Since the polished surface of STS was highly reflective, the presented images were taken with white papers placed near the sample to minimize light coming from the surrounding regions. It is to be noted that the observed colors, which were very sensitive to the viewing angle, result from the combined effect of diffraction and plasmon resonance. Diffuse reflectance was first measured using unpolarized light to find out whether SPR absorption actually occurs or not. The results are shown in Fig. 5, where the reflectance spectra were taken at a fixed incident angle of θ = 7°. The orientation of the plane of incidence was also illustrated. When the plane of incidence was orthogonal to the grating grooves (i.e., α = 90°), the reflectance spectrum exhibited two strong absorption peaks at 442 nm and 592 nm. On the contrary, when the plane of incidence was parallel to the grooves (α = 0°), a single peak was observed at 523 nm. The positions of the absorption peaks are consistent with the colors shown in Fig. 4(a). According to Eq. (5), SPR can be supported at two different wavelengths when α = 90°. In this case, the grating vector lies in the plane of incidence. This grating vector can thus be added to or subtracted from the tangential component of the incident wave vector, depending on whether m is 1 or −1. In each case, there exists a wavelength satisfying Eq. (5), i.e., the momentum matching condition. As a result, SPR can be supported at two different wavelengths for a given incident angle. Meanwhile, when α = 0°, the grating vector is perpendicular to the plane of incidence. Since the magnitude of the total tangential component of the wave vector does not change with the direction of the grating vector, only a single resonance mode can be supported. In other words, the θ value in Eq. (6) is independent of whether m is 1 or −1. Therefore, when θ is fixed, the resonance wavelength is also fixed. While the surface of STS may be oxidized during etching, elemental analysis showed no appreciable oxide layer on the patterned surface. An oxide layer would increase or decrease the reflectance in a specific wavelength range owing to interference. However, no such a peak was observed in diffuse reflectance measurement. When polished STS sample was electrochemically etched without patterning, the overall reflectance was slightly decreased over the whole visible range, probably due to roughened surface. This implies that the peaks in Fig. 5 arise from SPR absorption.
Fig. 4 (a) Digital-camera images of patterned samples. The arrow represents the orientation of the grating vector. (b) Obliquely viewed samples. The images were taken with white papers placed near the sample to minimize light coming from the surrounding regions. The oblique angle in (b) was approximately 30° (left), 45° (middle), and 75° (right).
Fig. 5 Diffuse reflectance from the sample. Solid lines represent measured data points. Squares, triangles, and circles put on the lines are just to differentiate these data lines from one another.
4.2 Specular reflectance
Since the diffuse reflectance spectra were taken at a fixed incident angle using unpolarized light, specular reflectance was also measured using a different spectrophotometer to investigate the dependence of SPR on the incident angle and polarization state of the light. At α = 90°, SPR didn’t occur regardless of the incident angle when the light was TE-polarized (Fig. 6(a)). For TM-polarized light, SPR absorption was observed at two different wavelengths. The spectral separation between two absorption peaks increased with an increasing incident angle, as shown in Fig. 6(b). At α = 0°, a single resonance peak was observed when the incident light was TE-polarized, while no SPR occurred for TM polarization, as shown in Fig. 7. The resonance peak blue-shifted as the incident angle increased. Figure 6 and Fig. 7 show that in order for SPR absorption to occur, the incident light should have a polarization component perpendicular to the grating grooves. This is consistent with literature [27,31]. The incident angle-dependence of the peak position was stronger at α = 90° than at α = 0°. When the plane of incidence is orthogonal to the grating grooves (i.e., α = 90°), the tangential component of the incident wave vector and the grating vector are oriented in the same direction. Therefore, the total wave vector of light on the metal surface is more sensitive to the incident angle, in comparison to the case when the plane of incidence is parallel to the grooves. Compared with the diffuse reflectance, the specular reflectance rather abruptly dropped at wavelengths below 500 nm, as manifested from Fig. 7(b). This is likely due to the spread of light by diffraction, which is not detected by the specular measurement. In fact, diffraction spots were observed when a light beam of λ = 405 nm from a laser pointer was made to strike the grating. For instance, when α = 90° and θ = 10°, two spots were produced with diffraction angles of θd = 78° and −39°. Equation (2) originally comes from the diffraction relation of d(sinθd – sinθ) = mλ. The two spots corresponded to m = ± 1. Specular reflectance was also measured from an unpatterned reference sample for both polarizations. Such an abrupt decrease was not observed from the reference sample. The reflectance slowly decreased as the wavelength decreased, regardless of the polarization and incident angle of light.
Fig. 6 Specular reflectance at α = 90°: (a) TE polarization, (b) TM polarization. Solid lines represent measured data points. Squares, triangles, and circles put on the lines are just to differentiate these data lines from one another.
The frequency-dependent dielectric functions of STS were derived from the refractive indices measured by ellipsometry. The derived real dielectric constant εm was inserted into Eqs. (5) and (6) to calculate the theoretical positions of SPR peaks. Data for the refractive indices and dielectric constant are provided as a supplementary material (see Dataset 1) [37]. The experimental results are compared with theory in Fig. 8, where the orientation of the plane of incidence is given as an inset. For both azimuthal angles of α = 90° and 0°, the experimental values were in good agreement with the theoretical calculations. The theoretical curves given in Fig. 8 correspond to m = ± 1. They start from 529 nm at θ = 0. According to Eq. (4), there is another set of theoretical branches starting at 306 nm, which correspond to m = ± 2. However, no conspicuous SPR peaks for these branches were observed, possibly because they are inherently too weak to be detected.
Fig. 8 Dependence of resonance wavelength on the incident angle of light, when (a) α = 90° and (b) α = 0°.
4.3 Effect of grating morphology
The results of Fig. 5 to Fig. 8 were obtained for the line pattern whose width and height were 260 nm and 100 nm, respectively. The corresponding morphology is shown in Fig. 2(a), which was obtained after RIE etching for 15 s at 300 W, followed by electrochemical etching for 10 s at 12 V. For this fixed geometry, the experimental results seem predictable by Eq. (4). It is important to note, however, that since the theoretical calculations given here are based purely on the mechanism of grating-assisted light coupling, they may not be applicable for gratings with periods smaller than d = 500 nm. As the period of the grating decreases, the scale of its features (such as trench width) will also diminish. Nanoscale features themselves can absorb light strongly due to LSPR. For gratings of small period, LSPR may thus be dominant over the grating-coupled SPR [33,38,39]. Even when the grating has a period comparable to visible wavelengths, LSPR may be profound depending on the geometric details. The morphologies shown in Fig. 2(b) and 2(c) were obtained when the electrochemical etching time was increased to 20 s and 30 s, respectively, where the RIE time was fixed at 15 s. Figure 9 compares diffuse reflectance for the three different morphologies. The pattern with a line width of 60 nm exhibited slightly smaller absorption strengths compared to the other structures. But any noticeable shift in the peak position was not detected. This looks rather contradictory to the results of other materials. A previous work on silver gratings of 400 nm period [34] showed that many factors including film properties (e.g., grain size and roughness) have an influence on SPR generation. The grating width and height also had a substantial effect on the SPR wavelength and coupling strength. In Fig. 9, all three samples have different widths and heights from one another. This makes it difficult to separate the effect of pattern width from that of height. The two structural parameters should be independently controlled in order to discuss the effect of grating morphology on the SPR characteristics. In the current work, the 1D line patterns were fabricated by etching the STS surface pre-exposed by RIE. Therefore, the final morphology is influenced by both processes. A fine tuning of the process conditions is required to make the two structural parameters independently variable. The STS gratings used in the present study have a fixed period of 500 nm. Due to the limited geometric variations, it is hard to judge whether there is LSPR in the STS structure. To find out the validity range of the grating-coupling theory in STS, it is also necessary to prepare gratings with various periods, along with the characterization of their resonance behaviors. This remains for the future work. Manufacturing subwavelength-scale gratings commonly involves the use of e-beam lithography or focused ion beam when preparing the master stamp. It will thus be more expensive than the conventional lithographic process employed in the present work. Here we obtained uniform patterns over an area of ~1 cm2. As the gratings are scaled down, it may also be more difficult to fabricate uniform patterns by imprinting.
Fig. 9 Diffuse reflectance from three samples of different pattern morphology, when (a) α = 90° and (b) α = 0°. Solid lines represent measured data points. Squares, triangles, and circles put on the lines are just to differentiate these data lines from one another.
Funding
R and D Convergence Program of National Research Council of Science and Technology of Korea (NO. CAP-16-10-KIMS).
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