We present an ultra-broadband perfect absorber composed of metal-insulator composite multilayer (MICM) stacks by placing the insulator-metal-insulator (IMI) grating on the metal-insulator-metal (MIM) film stacks. The absorber shows over 90% absorption spanning between 570 nm and 3539 nm, with an average absorption of 97% under normal incidence. The ultra-broadband perfect absorption characteristics are achieved by the synergy of guided mode resonances (GMRs), localized surface plasmons (LSPs), propagating surface plasmons (PSPs), and cavity modes. The polarization insensitivity is demonstrated by analyzing the absorption performance over arbitrary polarization angles. The ultra-broadband absorption remains more than 80% over a wide incident angle up to 50°, for both transverse electric (TE) and transverse magnetic (TM) modes. The ultra-broadband perfect absorber has tremendous potential for various applications, such as solar thermal energy harvesting, thermoelectrics, and imaging.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Light absorption is one of the most fundamental properties of blackbody absorbers and plays an important role in modern optoelectronics such as photodetectors, solar cells and imaging [1–5]. The ideal blackbody absorbers possess high absorption, large bandwidth and polarization insensitivity [1–4,6–11]. With the advent of plasmonics, significant progresses have been made over the past decade to engineer perfect absorbers with superior absorption performance [6–11]. For instance, Landy et al. proposed a perfect absorber with a single resonant frequency based on the metallic split ring resonators ; Li et al. reported the shape-dependent broadband absorption in the metallic nanoparticles . However, the absorption capability of single-layer plasmonic structures was not efficient due to the lack of trapping and confinement mechanisms. Blending various strong resonators with neighboring resonant frequencies together is an effective way to increase the absorption bandwidth [12,13]. However, the relative fabrication methods and techniques are mostly time-consuming, costly and only suitable over small areas (usually less than 1 mm2).
Many reports were obsessed with the metal-insulator-metal (MIM) configurations. It is because that the MIM structures could amend the functionality of devices compared to the conventional bulk absorbers and give the flexibility of geometry alterations to tune the bands and bandwidths [14–16]. Typical MIM architectures include: 1) MIM continuous film stacks  and 2) metal meta-surfaces separated by a thin dielectric layer from a thick metal ground layer . The MIM continuous film stacks need not involve any complex fabrication techniques. However, most conventional three-layer MIM continuous film stacks only have finite absorption bandwidths in the visible region, e.g., 400-800 nm . Recently, it is discovered that the absorption bandwidth can be extended to the near-infrared (NIR) range by increasing the layer number of metal-insulator continuous film stacks. For example, Hu et al. proposed the eight-layer continuous film stacks with the 98.3% absorption in the region of 250-2000 nm , greatly wider than that found in the common three-layer MIM continuous film stacks [11–14,17]. Up to now, different metal-insulator multilayer continuous film stacks were utilized to obtain the ultra-broadband light absorption [15,20]. For the second type, the absorption behavior is mainly governed by the excitation and coupling of surface plasmons between the upper metal meta-surface and the lower metal-insulator two-layer film stacks . Aydin et al. designed a MIM structure with a crossed trapezoidal array and obtained the polarization-independent light absorption over the entire visible spectrum (400-700 nm) . Lei et al. theoretically put forth the MIM Fabry-Pérot (FP) cavity with the 90% absorption bandwidth over 712 nm owing to the excitation of surface plasmon resonance and FP cavity resonance . Ding et al. demonstrated an average absorption of 90% within the 900 nm to 1825 nm wavelength range in the MIM absorber with a metal nano-disk array . In addition, metal grating structures were also integrated into the metal-insulator two-layer film stacks to acquire rather high light absorption [18,22]. However, most of the above mentioned designs with meta-surfaces or grating structures only have narrow bands .
Here, we present an ultra-broadband and polarization-insensitive perfect absorber. The absorber consists of metal-insulator composite multilayer stacks (MICM stacks) by placing the insulator-metal-insulator (IMI) grating on the MIM continuous film stacks. Our proposed absorber shows the absorption bandwidth over 2800 nm (absorption >90%) in the visible and middle-infrared regions due to the synergy of guided mode resonances (GMRs), localized surface plasmons (LSPs), propagating surface plasmons (PSPs) and cavity modes. The ultra-broadband absorption of the proposed absorber also displays greatly insensitive to the angle and polarization of incident light. The angular invariance is up to 50° of incident light for both transverse electric (TE) mode (magnetic field perpendicular to the grating slits) and transverse magnetic (TM) mode (magnetic field parallel to the grating slits) with the 80% absorption bandwidth over 2500 nm. In addition, the ultra-broadband absorption is with excellent thermal stability due to the employed refractory materials of titanium (Ti), chromium (Cr), tungsten (W) and alumina (Al2O3). Therefore, the presented absorber has tremendous potential for various applications, such as solar thermal energy harvesting, thermoelectrics and imaging.
2. Structure design and simulations
Figure 1 depicts the schematic graphs of the proposed ultra-broadband perfect absorber based on the MICM stacks. The proposed absorber can be treated as a three-layer IMI ridge grating structure (enclosed by the green dashed rectangle) standing on the three-layer MIM continuous film stacks (enclosed by the blue dashed rectangle). As depicted in Fig. 1(a), the grating period and the ridge width are denoted as p and w, respectively. The separation distance (s) between the two adjacent ridges, i.e., slit width, can be calculated by s = p –w. The bottom metal film with 200 nm in thickness is opaque to the incident light. The thicknesses of the insulator layers and the other two metal layers can be tuned artificially and are respectively marked with t1, t2, t3, d1 and d2 as displayed in Fig. 1(b). Ti, Cr, W and Al2O3 (with melting points of 1668, 1907, 3422 and 2072 °C at room temperature, respectively) are chosen to build the MICM stacks due to their extraordinarily resistance to heat [15,23,24]. Except where otherwise stated, the metal material used in this work is Ti. The optical performance of the proposed absorber is calculated by using the finite-difference time-domain (FDTD) method . Since the device can be treated infinite long in y direction, a simplified 2D simulation model is applied. The periodic boundary conditions are applied in the x direction and perfectly matched layers are used in the z direction. The dielectric constants of Ti, Cr and W are taken from Palik  and the refractive index of Al2O3 is n = 1.76. The transmission (T) in this absorber is equal to zero due to the opaque metal film used at the bottom. The absorption (A) of this absorber can be calculated by A = 1- R, where R denotes the reflection.
The sample structure can be prepared by combining the magnetron sputtering technique  and the electron beam lithography . Firstly, the MIM film stacks can be fabricated by sputtering the related materials on the silica substrates in turn. Then, a layer of photoresist would be coated on the MIM film stacks and etched by the electron beam lithography to form one-dimensionally periodic strips. After that, the IMI grating can be fabricated by successively sputtering insulator, metal and insulator materials on the photoresist strip array. Lastly, the lift-off method would be used to remove the photoresist strips coated with the insulator and metal layers. The MICM stacks are thus formed.
3. Results and discussion
Figure 2(a) shows the calculated absorption spectra of the designed MICM stacks (black), IMI grating (purple), three-layer MIM film stacks (pink), and six-layer metal-insulator film stacks (blue) with the same structural parameters at normal incidence. The thicknesses of metal layers and insulator layers are 20 nm (d1, d2) and 120 nm (t1, t2, t3), respectively. The period p is equal to 260 nm and the ridge width w is 220 nm. Two shallow and narrow absorption peaks at 895 nm and 3200 nm are found in the IMI grating with the maximum absorption less than 60%. The MIM and six-layer film stacks both present broad absorption bands and the absorption in the six-layer film stacks is stronger than that in the MIM film stacks, in accordance with the previous reports [15,19,20]. However, only a 90% absorption bandwidth of 570 nm is achieved even that the layer number of film stacks increases to six. Interestingly, by integrating the IMI grating and the MIM film stacks, an ultra-broadband with the absorption over 90% is observed in the wavelength region from 570 nm to 3539 nm, much wider than that of the state-of-the-art solar thermal absorber reported before . The average absorption in this range is up to 97% with the maximum absorption reaching 100% in the range of 700 nm to 850 nm. Furthermore, the absorption performance of the MICM stacks is distinctly superior to those observed in the IMI grating, three-layer MIM film stacks, and six-layer metal-insulator film stacks as shown in Fig. 2(a). Figure 2(b) displays the color plot of the absorption as a function of the polarization angle, where 0° corresponds to the TM polarization and 90° corresponds to the TE polarization. Obviously, the absorption above 90%, over a broad wavelength range in visible and infrared regime (570 nm −3200 nm), is entirely kept when the polarization angle increases from 0° to 90°. This indicates the polarization independence of the ultra-broadband perfect absorber.
To reveal the physical mechanism of the proposed ultra-broadband perfect absorber, we calculated the electric (|E|) and magnetic field (|H|) distributions of absorption spectrum at 895 nm for the MICM stacks. Figure 3(a) and (b) shows the distributions of electric and magnetic field at 895 nm for the TM polarization, respectively. It is found that the electric field is with three order magnitudes larger than that of magnetic field, indicating the electric resonances dominate the broadband absorption. Strong electric field mainly exists in the grating slits, especially at the corners of metal strips between adjacent unit cells. The electric field distribution indicates the appearance of cavity modes [21,28,29] supported by the grating slits and the excitation and near-field coupling of LSPs of metal strips [21,29–31]. Incident light is coupled into the IMI grating layer by the excited cavity modes and LSPs and further excites the PSPs in the MIM film layer, which can be observed from the electromagnetic field distributions at the dielectric-metal interfaces [32–34]. Therefore, the cavity modes and LSPs are responsible for the high absorption and the excited PSPs further strengthen the light absorption . For the TE polarization, the electromagnetic field mainly locates in the top and middle dielectric strips and the grating slits as shown in Fig. 3(c) and (d), revealing the role of GMRs  and their hybridized coupling effect to the excited surface plasmons. Therefore, the perfect absorption observed at 895 nm can be ascribed to the synergy of GMRs, PSPs, LSPs and cavity modes.
In order to further understand the physical mechanism, we also calculated the electromagnetic distributions of absorption at 2400 nm [Fig. 4(a) and (b)] and 3300 nm [Fig. 4(c) and (d)] under the normal TM polarized light. In Fig. 4(a) and (b), strong electromagnetic field energy concentrates in the grating slits and at the corners of metal strips due to the existence of cavity modes [28,29] and the excitation and coupling of LSPs [32–34]. The enhanced electromagnetic field energy in the middle and bottom dielectric layers suggests the excited PSPs and cavity modes in the FP cavity formed by the two metal reflectors with the dielectric spacer . In Fig. 4(c) and (d), strong electromagnetic field energy distributions at the four corners of metal strips are mainly due to the excitation and strongly hybridized coupling of LSPs and PSPs [32–34]. The cavity modes in the grating slits and FP cavity and GMRs in the dielectric strips also contribute to the strong light absorption at 3300 nm. In general, the synergy of LSPs, PSPs, cavity modes, GMRs and their hybridized coupling effect lead to the broadband perfect absorption in the visible and infrared regions. That is, the ultra-broad perfect absorption mainly originates from the blending various strong resonators with neighboring resonant frequencies together, which is an effective way to increase the absorption bandwidth [12,13].
To provide insight into the impact of the geometrical parameters on the performance of the MICM stacks, we study the light absorption characteristics by tuning or fixing the grating slit width (s = p -w) as shown in Fig. 5(a) and (b), respectively. The period of grating here increases from 200 nm to 310 nm. The thicknesses of metal layers and insulator layers were respectively set to 20 nm and 120 nm. A better qualitative comparison can be made by taking 90% absorption as the threshold of perfect light absorption. In Fig. 5(a), the grating slit width changes from 10 nm to 120 nm due to that the grating width w is fixed to 190 nm. Although extremely strong near-field coupling of LSPs appears when the grating slit width is only 10 nm, two dips with the absorption below 90% are observed in the wavelength ranges of 1320 nm-2084 nm and 3047 nm-3719 nm. It is because that light coupled into the IMI layer by the cavity modes and LSPs can only excite the PSPs near the top metal strips. Partial energy is reflected into the top air by the metal film, which leads to the reduced absorption. Interestingly, the absorption bandwidth greatly broadened when s = 20 nm and then becomes narrow and narrow with the slit width s increases from 20 nm to 120 nm. The increased s results in the reduced near-field coupling of LSPs , which leads to the blue-shift of resonant band and the decreased absorption in the long wavelength region (i.e., the narrowed absorption band). The largest absorption (> 90%) bandwidth is up to 3231 nm (from 589 nm to 3820 nm) with s = 20 nm. Such excellent characteristics are essential for absorptive devices those need to be integrated into industrial applications. Figure 5(b) shows the absorption spectra of the proposed absorber with fixed slit width (s = 40 nm) by simultaneously changing the period P and width w of grating. Here, P increases from 200 nm to 310 nm and w increases from 160 nm to 270 nm. It can be clearly observed that the 90% bandwidth changes slightly, i.e., slightly increases with P increasing from 200 nm to 260 and then slightly decreases with P increasing to 310 nm. As reported before, GMRs are closely related to the thickness or the refractive index of the waveguide strips . That is, the effect of width change of waveguide strips on GMRs is not obvious. Therefore, the slight change observed in Fig. 5(b) mainly originates from the excited PSPs and its coupling with other resonances.
Figure 6 shows the absorption properties of the proposed MICM stacks with the top- and middle-layer Ti of different thicknesses. The thicknesses of another Ti layer and all insulator layers are 20 nm and 120 nm, respectively (P = 260 nm and w = 220 nm). As the thickness of top-layer Ti d1 increases from 10 nm to 60 nm, the absorption gradually increases in the longer wavelength region (λ > 2690 nm) and decreases in the short wavelength range (570 nm < λ < 2690 nm) as shown in Fig. 6(a). The enhanced absorption in the longer wavelength region with d1 mainly results from the excitation and strong near-field coupling effect of LSPs (see the inset in Fig. 6(a)) [33,34]. For the decreased absorption in the short wavelength range, electric field energy in the IMI grating layer (except near to the top-layer metal strips) decreases largely as compared with that in the longer wavelength range (see the insets in Fig. 6(a)). This suggests the weakened coupling effect of cavity modes and GMRs to the incident light, leading to the reduced absorption in the short wavelength range. Considering the bandwidth with the absorption above 90%, the optimal thickness range of top-layer Ti is 20-35 nm. For the middle-layer Ti, as shown in Fig. 6(b), the absorption bandwidth of 90% absorption decreases with the increased d2 except for the 10 nm thickness. The decreased absorption in the longer wavelength region results from the deteriorated charge distribution on the middle metal strip’ surface due to the finite skin depth . Ultra-broadband perfect light absorption mainly occurs in the model with 15-25 nm thick middle-layer Ti.
The absorption properties of the MICM stacks with the top-, middle- and bottom-layer Al2O3 of different thicknesses are also considered here. The thicknesses of Ti layers and other Al2O3 layers are 20 nm and 120 nm, respectively. The period P and width w of grating are respectively equal to 260 nm and 220 nm. The absorption properties also display obvious changes with the thickness t1 of the top-layer Al2O3 as plotted in Fig. 7(a). When the top-layer Al2O3 is moved away (i.e, five-layer metal-insulator composite stacks), the bandwidth with absorption above 90% is only equal to 1096 nm. While for the absorber with the top-layer Al2O3, the 90% absorption bandwidth is greatly broadened. The widest perfect absorption bandwidth is found when the thickness of the top-layer insulator is equal to 120 nm. Similar optimized thickness t2 of middle-layer Al2O3 is found in Fig. 7(b). While for the bottom-layer Al2O3, the increased thickness t3 leads to the broadened absorption band as shown in Fig. 7(c). The largest bandwidth of 3378 nm (from 620 nm to 4000 nm) with perfect absorption is achieved when the thickness is equal to 220 nm. However, as the curves shown in Fig. 7(c), a dip near 2300 nm becomes gradually apparent with the increased thickness of the bottom-layer Al2O3, which leads the decrease of average absorption of the absorber (only 95.9%) in this wavelength range. As reported before, GMRs are closely related to the thickness or the refractive index of the waveguide strips . Therefore, the resonant red-shift with the thickness of dielectric layers in Fig. 7 is closely associated with GMRs [30,36].
In the practical applications, such as photovoltaic solar, thermal power generation and blackbody thermal emitters, the absorption should be less sensitive to the incident angle . We performed calculations to verify the angle dependence for both TM polarization and TE polarization and the results are depicted in Fig. 8. The incident angle of light varies from 0° to 60°. The thicknesses of Ti layers and Al2O3 layers are 20 nm and 120 nm, respectively. The period P and grating width w is 260 nm and 220 nm, respectively. Obviously, the absorption effect is nearly robust for a relatively wide range of incident angles. For the TM polarization, it can be seen that the ultra-broadband perfect absorption (>90%) response can be achieved even when the angle is up to 60° in the wavelength range of 570 nm to 3200 nm [Fig. 8(a)]. For the TE polarization, over 90% and 80% absorption in the range of 570 nm to 3200 nm are also observed when the incident angle of light is up to 40° and 50°, respectively [Fig. 8(b)]. It is because that the effective permittivity of the waveguide layer gradually decreases with the increased incident angle [30,37], which leads to the blue-shift of absorption band. These demonstrate that the broadband perfect absorber is with large incident angle-insensitivity.
We also analyzed the effect of metal materials on the absorption performances of the proposed absorber by replacing the Ti layer by Cr or W while keeping the geometry parameters constant. The absorption results are illustrated in Fig. 9. It can be inferred from the results that when using Ti or Cr, the absorption spectrum exhibits superior performance with an ultra-broadband perfect absorption (over 90% absorption bandwidth is larger than 2300 nm). When using W, though a relatively narrow band is obtained in the region of 600 nm to 1680 nm with over 90% absorption, it is comparable with those obtained in metal-insulator multilayer film stacks [15,19,20]. The discrepancy observed here is decided by the metal’s intrinsic dispersion property. For the lossy metal of Ti or Cr with an appropriate thickness satisfies the impedance match conditions and thus offers a low quality factor cavity in an ultra-broad band wavelength range [14,38].
We present an ultra-broadband perfect absorber composed of MICM stacks by placing the IMI optical grating on the MIM film stacks. The proposed structure shows over 90% absorption in the wavelength range from 570 nm to 3539 nm with an average absorption of 97%. The ultra-broadband absorption characteristics are achieved by blending various strong resonators such as cavity modes, GMRs, LSPs and PSPs with neighboring resonant frequencies together. The ultra-broadband perfect absorption of the proposed absorber also displays greatly insensitive to the angle and polarization of incident light. The 80% absorption ultra-broadband more than 2500 nm is still maintained over a wide incidence angle up to 50° for both TE and TM polarizations. The ultra-broadband perfect absorber has tremendous potential for various applications in solar thermal energy harvesting, thermoelectrics and imaging.
National Natural Science Foundation of China (51761015, 11564017, 11804134, 11464019), Natural Science Foundation (20181BAB201015), Major Academic and Technical Leaders (20182BCB22002) and Outstanding Youth Program (2018ACB21005) of Jiangxi province.
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