The common complex architecture constitutes the major bottleneck of optical absorber operating on broadband spectrum. Here we demonstrate a super absorber consisting of tapered dielectric nanostructure coated with a thin-layer of non-noble metal chromium on flexible poly(ethylene terephthalate) substrate. The proposed device yields double-sided, omnidirectional, and polarization-independent absorption over the entire visible spectrum with an average efficiency more than 90% at normal incidence, and 80% at a tilt incident angle of 60°. It can be easily realized by nanoimprinting lithography combined with physical vapor deposition technique. Theoretical analysis demonstrates that the superior optical performance is ascribed to the non-resonant light absorption by using the metal-covered, closed-packed tapered nanostructure via adiabatic nanofocusing of the metal-dielectric-metal (MDM) guided modes excited by scattering of the gradually changing nanostructure. For the cost-effective fabrication and material strategy, the super absorber has potential applications in a wide range of passive and active photonic devices, including inkless printing, harvesting solar energy, as well as thermal emitter and optical detector.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Super absorbers have attracted enormous attentions for their potential applications in sensing [1–3], photothermovoltaics , photo detection  as well as nanostructure-assisted ablation and ionization . Independently absorptive properties of the wavelength, angle, and polarization of the incident radiation are of primary interest for fulfilling the fundamental requirement of harvesting more photons. A number of device configurations based on different working principles have been proposed to achieve these purposes. Plane film stack, which is relatively simple, has been shown to be perfect absorber but usually involving multi-layer film deposition [7–11]. Guo et. al. chose a highly absorbing material and put additional optically thin metal layer on top of the structure to improve the angular performance up to 65°, but its full-bandwidth at half maximum (FWHM) was only about 100 nm . Metamaterial absorbers (MAs) have been investigated theoretically and experimentally because they can achieve perfect absorption with robust angle-independence over a certain bandwidth [13–17]. In order to trap electromagnetic energy in small gap and consequently intensify absorption, a typical type of absorber involves building a highly dense nano-composite with nanoparticles in a dielectric spacer layer attached to a bulk metal block [18–20]. Metallic nanostructure, such as perforated metallic film and strip metallic grating, also appears as an inescapable solution for the design and realization of MAs with high optical efficiency [21–34]. Atwater et. al. demonstrated an ultrathin (260 nm) plasmonic super absorber consisting of a metal - insulator - metal stack with a nanostructured top silver ﬁlm composed of crossed trapezoidal arrays . Bozhevolnyi et. al. demonstrated experimentally that two-dimensional arrays of sharp convex grooves in bulk gold can ensure efficient (> 87%) broadband (450-850 nm) absorption of unpolarized light . Shen et. al. reported a broadband absorber with high efficiency (>90%) by an atomic layer depositing nanometer iridium film onto a porous anodic alumina template . However, metallic nanostructure based absorbers are only capable of absorbing light from one incident side and suffer from unlimited fabrication complexity, for their involving time-consuming and expensive manufacturing techniques, such as electron beam lithography, focused ion beam milling or atomic layer deposition. From these contexts, efficient absorber made by simple and low-cost fabrication procedure is highly desirable.
Here we demonstrate a double-sided, omnidirectional, and broadband absorber based on tapered nanostructure coated with a thin layer of non-noble metal. It shows the capability of absorbing light with low sensitivity to incident angle, which exceeds the structured bulk metallic surface. The value of the proposed device as super absorber needs more attention due to several facts: (i) It has much simple device configuration compared with other types of optical absorbers. (ii) It is easy to be fabricated by a combination of soft-nanoimprinting and metal deposition techniques. (iii) Capable of absorbing and trapping light from both the front- and back-sided incidence without polarization effect, the absorber can contribute to enhancing the efficiency of new exciting perspectives. (iv) High absorptive efficiency for a broad range of incident angles over broadband due to the non-resonant plasmonic effect. The possibility of the use of the tapered dielectric nanostructure coated with continuously non-noble metal as super absorber is explored, which may find potential applications in photonic devices [38–40].
2. Design and simulations
The absorber is composed of tapered nanostructure coated with a single-layer chromium (Cr), as shown in Fig. 1(a). Full-field electromagnetic wave calculation is performed using Lumerical, a commercially available finite-difference time domain method (FDTD) simulation software package. In the model, we adopted ellipsoid-shaped nanostructure. The width, height and period of the ellipsoid is denoted as w, h and p, respectively. The shape profile is fitted with the equation .The Cr thickness is d and the refractive index (RI) is from . The plasmonic effect on Cr, as non-noble metal, is not such significant as that of gold or silver, but it can be deduced from Fig. 2(b) that the real part of the Cr permittivity (it equals to the square power of the RI) is negative in 400 nm - 800 nm spectrum, which can satisfy the excitation condition of the surface plasmonic wave. The substrate is assumed to be poly(ethylene terephthalate, PET), whose RI is set to 1.65. The RI of the ultraviolet (UV) resin is set to 1.5. The optical absorption (A) is calculated by 1−T−R, where T is transmission, R is reflection and both can be obtained directly from the simulation. Since the tapered nanostructure is symmetric in x-y plane, the absorptive property is independent of the incident light polarization, the same as that in . In order to avoid any excitation of propagating wave in transverse plane, the optimized structure parameters are p = 400 nm, w = 350 nm, h = 1.0 μm and d = 50 nm. It can be seen from Fig. 1(c) that the absorption can be over 0.8 within a broad-incident-angle range up to 70° from both the front and back incidence at a fixed wavelength of 600 nm. When light is normally incident, the average absorption is greater than 0.93 over the whole visible spectrum, as shown in Fig. 1(d). The contour plots of absorption variation as functions of wavelength and incident angle, as shown in Figs. 2(a)-2(b), further verify the excellent broadband absorption behavior.
To better understand the underlying mechanism of strong absorption behavior, the electric field (, ) and magnetic field () distribution in cross-section of the tapered nanostructure is shown in Figs. 3(a) -3(f). In both cases, strong photo flux at the aperture and high electromagentic intensity towards narrow end is observed. Effective refractive index of the Cr-air-Cr waveguide for the front-sided and the Cr-dielectric-Cr waveguide for the back-sided incidence is shown in Figs. 3(g)-3(h) respectively. The effective refractive index increases as the width of the MDM waveguide becomes narrow, which contributes to the electromagnetic wave localization. The imaginary part in the metal causes strong absorptiveeffect as the wave propagating along the waveguide. The effective refractive index in the back-sided incidence is larger than that in the front-sided incident case, in accordance with the electromagnetic field distribution shown in Figs. 3(a) - 3(f).
The Poynting vector distribution is shown in Figs. 4(a)-4(b), distinctly demonstrating that the incident flow is scattered to guided-mode wave confined between the metallic sidewalls. In front-sided incident case, light is re-directed toward the air gap along the air-metal interface by scattering of the tip of the nanostructure, as shown in Fig. 4(a), then is trapped in the increasingly narrow air gap and dissipated eventually by the Cr film. In back-sided incident case, besides the portion of the light traveling in the MDM structure, partial electromagnetic energy can penetrate through the Cr film and is absorbed before escaping to free space (shown in Fig. 4(b)) due to the strongly localized electromagnetic mode. The size of the arrow is proportional to the magnitude of the Poynting vector. The time-averaged resistive heating () generated is also demonstrated in Fig. 4, which is calculated using . The fundamental TM guided-mode between two neighboring metallic sidewalls does not have a cut-off frequency, thus the electromagneticwave can squeeze into the MDM structure, instead of scattering off by the nanostructure. We have disclosed the working principle of the wide-angle band absorber composed of nano-meander . With deliberate structure parameters, the incident and reflective wave in cavity interfere to yield standing wave pattern, which is characterized by phase difference between and . Such strong resonance can effectively trap light and provide sufficient time to dissipate by the Ohmic losses within metal. In current case, however, and are almost in the same phase (front-sided incidence, as shown in Fig. 4(c)) or have a phase difference (back-sided incidence, as shown in Fig. 4(d)). The result indicates that it is the non-resonant absorption dominating the lightwave behavior. There is no fundamental difference in physics in both cases, thus the broadband and high-efficiency absorption can be achieved. The insignificant difference lies in the fact that, in the front side-sided incidence, the dielectric waveguide between two metallic walls is air which has a sharp convex profile, however, in the back-sided incidence, the dielectric waveguide between two metallic walls is UV resin which has a gentle profile and a larger effective refractive index than that in the former case. The high aspect-ratio tapered nanostructure and the utility of non-noble metal Cr leads to the adiabatic nanofocusing effect, which can strongly enhance absorptive effect. In practice, it is necessary to address the challenge of the high aspect-ratio nanostructure replication and conformal metal deposition.
3. Experimental section
To experimentally demonstrate the super absorber, the sample preparation process comprises the fabrication of an aluminum anode oxide (AAO) template with high order pore array, replication of the nanostructure, and Cr deposition, as shown in Fig. 5(a). There is no need forthe fussy fabrication of discrete metallic nano-dot array. In order to create high-order tapered nano-pore array, the AAO template is fabricated by using a well-known two-step anodization process . Then, the AAO template is fluorosilane-treated (NOVEC 7100, 3M) for demolding, which is a key process in high aspect-ratio nanostructure replication. Figure 5(b) shows the corresponding goniometer images for 1.0 μL droplet with apparent contact angle of before (top, 61°) and after (bottom, 117°) surface treatment, respectively. Large contact angle means weak adhesion to the UV resin. Then UV resin is poured onto the mold and a PET film is covered on the top. The imprinting pressure is typically less than 0.3 MPa due to the low viscosity. It takes about 20 sec to cure under a UV light-emitting diode with an intensity of 1000mW/cm2 at a distance of 1 cm. After the soft lithography process, a thin layer of metal Cr is deposited by using electron-beam evaporator (Kurt J. Lesker).
Figure 5(c) shows the side-view of the AAO template. The average distance of the neighboring nanopores is 400 nm. The height of the tapered hole is about 1.0 μm, and the diameter of the nanopore is 350 nm at top and 40 nm at bottom respectively. Figures 5(d) and 5(e) show, respectively, the side-view of the replicated nanostructure before and after Cr deposition. It can be clearly seen that, although the perfect closed-packed, ellipsoid-shaped nanostructure is difficult to obtain, the tapered nanostructure in the mold has been well replicated to the UV resin with high fidility, high filling factor and high aspect-ratio, which is helpful in promoting device performance.
Figures 6(a) and 6(b) show the measured absorption/reflection/transmission results. It can be seen that an average absorption of 90.0% and 89.3% for front- and back-sided normal incidence can be obtained in the spectrum range of 400 nm - 800 nm. There is a little difference from the simulation result due to the imperfect replication. The angle-dependent absorption spectra for bidirectional incidence are tested, as shown in Figs. 6(c)-6(d). The absorption curves show a slight dispersion as a function of wavelength, but the double-sided absorptive property is retained at the incident angle in the range of 0–60°. The average absorption still remains 79.8% and 81.5% for front- and back-sided incidence when the incident angle changes from normal to 60°. When the incident angle is greater than 60°, the performance starts to deteriorate. This is understandable because of the weak coupling of the incident light into the MDM guided-mode under oblique incidence.
It is usually difficult to economically realize structural color black. However, by using of a hollow mask, it can be easily obtained and scaled up on large-format substrate with the investigated strategy. The fabricated samples are shown in Figs. 7(a)-7(c). Figure 7(a) shows the flexibility of the sample and Fig. 7(b) shows the patterned “unto a full grown man” in Chinese on a 20 cm x 12 cm PET film. Figure 7(c) exhibits the photographs taken in natural daylight, in which the letters “SU” appear black while the letter “Z” presents a mirror-like reflection. It is because the letters “SU” are composed of the tapered nanostructure coated with Cr, however, the letter “Z” is only Cr film on bare PET. The experimental results demonstrate that the tapered nanostructure with Cr provides not only bidirectional broadband absorption but also robust angle tolerance, which may find potential application in enviromentally friendly ink-less printing.
In conclusions, we demonstrate an excellent optical absorber based on tapered nanostructure coated with one-layer non-noble metal. The investigated architecture provides great potential for manipulating light, because of the strong coupling between electromagnetic field and surface charge. Theoretical and experimental characterizations clearly demonstrate that the light absorber proposed here yields seductive double-sided omnidirectional absorption over the entire visible spectrum. Non-resonant absorption is found dominating the absorptive behavior due to the excitation of waveguide modes in MDM architecture. A very simple fabrication method, in combination nanoimprinting with film deposition techniques, is employed for creating the absorber on flexible polymer film. The method offers the advantages of high-throughput production of structural color black with low-cost and easy operation. We believe the light-matter interactions presented here can be employed in ink-less printing and energy harvesting devices.
National Natural Science Foundation of China (61575133, 61505134, 61405133, 61775076), Natural Science Foundation of Jiangsu Province (BK20140357), Key Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (14KJB140014), Science and Technology Project of Suzhou (ZXG201427), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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