Open Access
Add to BibSonomy
Abstract
In this paper, the absorptivity of a tungsten ($W$) based metamaterial absorber has been studied. The paper revolves around the spectral characteristics of the nano-holed $W$ film coated over silicon oxide (SiO${_2}$) substrate. The anisotropic effective permittivity of the nanoholed W film has been deduced by employing the effective medium theory. The light-plasmon coupling at $W$-$SiO_{2}$ interface has been investigated by the eigenvalue equation. The effect of the nanoholed radii on coupling and absorptivity has been analyzed. As such, absorption features of the absorber have been studied in the visible and near-infrared (NIR) regimes by finite difference time domain(FDTD) simulation under the excitation of fundamental transverse electric(TE)- and transverse magnetic (TM)-mode. It has been observed that absorptivity can be altered by tailoring the holes radii of tungsten nanolayer. Further, the effects of the incidence angle of the light on the absorptivity have been studied. Observations reveal that absorptivity depends on the nanohole radius of $W$, and angle of incidence of excited light. Also, wideband absorptivity has been attained using $W$ thin film. Such an absorber would be useful for solar cell, solar heating and integrated optics related applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic metamaterials are known due to their unnatural exotic features which cannot be found in naturally occurring materials [1,2]. These have attained unprecedent properties that are not found in naturally existing media, such as negative refractive index, backward waves, reversal of Doppler shift, Cherenkov radiations, etc [2–6]. These unusual properties manoeuvre the multifarious fantastic applications, like perfect lens, wideband perfect absorber, narrow- and wide-band antenna, photo-detector, slow-light, efficient solar cell, ultra-sensitive biosensor and so on [7–15]. Investigations reveal that metamaterials are used to tune and control the electromagnetic induced transparency and plasmon induced transparency [15–17]. Recently, hyperbolic metamaterials have demonstrated very large sensitivity due to their strong anisotropic nature [10,18]. Within the context, metamaterials are composed of periodically arranged subwavelength-sized unit cells — unit cell of the metamaterial is composed of metallic-dielectric partner. The overall properties of the metamaterial are deduced from its unit cell [19]. As such, the unit cell properties depends on its shape, geometry and dielectric properties of its constituents [11,20].
One of the outstanding applications of the metamaterial is perfect absorption. Landy et al. firstly proposed the metamaterial-based absorber to attain the perfect absorption [21]. To date, metamaterial based absorbers have been investigated in the microwave, terahertz, infrared, visible and UV regimes of the electromagnetic spectra [22–26]. Investigations reveal that wideband absorption has been attained in the UV, visible and near infrared (NIR) regimes due to strong anisotropic nature of the hyperbolic metamaterials[27,28]. References [7,29–31] demonstrate that metamaterials are found for broadband absorption in the UV, visible and NIR regimes of light. The anisotropy plays a very important role to tailor the electromagnetic waves coupling with metasurface. Generally, metamaterial-based absorbers work on localized surface plasmon resonance (LSPR). It is notable that at LSPR, impedance of the metasurface match with the free space impedance, therefore, all the incidence light gets absorbed. Investigations report that wideband absorptivity has been achieved by the metamaterials absorber made of gold ($Au$), silver ($Ag$), platinum ($Pt$), titanium ($Ti$), chromium ($Cr$) and indium antimonides ($InSb$) [27,32–39]. Recently, it has been demonstrated that wideband absorptivity can be achieved with metamaterial composed of tungsten ($W$) [24,25,40]. Anisotropic metamaterial comprised of alternate multi-layered slab with alternating tungsten ($W$) and germanium (Ge) thin films has shown wideband absorptivity in the visible and near-infrared (NIR) regimes [25]. Usually, $Au, Pt, Cr, Ag, In,Ti$ are expensive metals have been used to achieve the large absorptivity in the visible and NIR regimes of light. To cope with this problem, $W$ metal is used as it is cost-effective as compared to other metals. The aim of this communication is to develop a cost-effective metamaterial-based absorber that operate in the visible and near-infrared regime of the light. Therefore, in this case, $Au$ metal is replaced with $W$. To the best of author knowledge, nanoholed W thin film based metamaterial absorber is the first investigation. As such, it is a cost-effective as compared to the previously used metals in the metamaterial absorbers. Nanoholed $W$-based thin film depicts strong coupling with the light in the visible and NIR regimes. Further, nanoholed arrays are easy to fabricate in the thin film.
In this communication, the absorption through nanoholed tungsten ($W$)-based absorber has been analyzed. The proposed absorber has been composed of three layers; top nanoholed $W$ layer fabricated over $SiO_2$ substrate glass and a bottom $Ag$ act as a mirror to the excited light. The middle $SiO_{2}$ substrate glass keeps trapping the incidence light. The absorptivity through the proposed absorber has been investigated in the visible and near-infrared regime of light, i.e., 300 nm to 1000 nm in the present case. Further, the absorptivity has been taken up corresponding to the different incidence angles of light. As such, absorptivity has been analyzed by varying the unit cell nanohole radius. Such an absorber would be useful for solar heating and solar cell related applications.
2. Analytical formulation and method
Figure 1 shows the schematic of the unit cell of wideband visible and near-infrared frequency absorber. The proposed absorber is composed of nanosized $W$ layer (with air holes) which is fabricated over $SiO_{2}$, the bottom side of the $SiO_{2}$ is coated with a silver ($Ag$) layer of thickness as $d_{1}$= 50 nm. The thin layer of the $Ag$ acts as a perfect reflector the incident light. The nanoholed $W$ film has the fixed thickness which is kept as $d_{3}$ = 100 nm, $SiO_{2}$ as $d_2$ = 1 $um$. The nanoholes in the $W$ have variable radii as r. The cross-section area of the unit cell is kept as $0.4\times 0.4$ $um^2$. The unit cell of the proposed metamaterial absorber structure has been simulated by using Lumerical FDTD simulator. The periodic boundary conditions are employed in the xy-plane. The fundamental transverse electric (TE)- and transverse magnetic (TM)- modes are excited along the z-axis. Also, the perfectly matched layer (PML) are employed above the reflection monitor and below the transmission monitor.
Both $Ag$ and $W$ are dispersive in nature, the frequency-dependent permittivity of the bulk $Ag$ and $W$ have been determined by Lorentz-Drude (LD) model. However, the effective permittivity of the HMM-layer has been deduced by the use of effective medium theory [25]. According to LD model of dielectric constant, permittivity of $Ag$ and $W$ can be expressed as [41]
It is notable that metals and dielectrics are nonmagnetic in nature at optical frequencies, therefore, the permeability of $W$, $Ag$ and $SiO_2$ will be unity [25]. The effective dielectric permittivity of the top metasurface, in terms of the longitudinal $\epsilon _{\parallel }$ and transverse $\epsilon _{\perp }$ components, can be expressed as
Figure 2 shows the effective permittivities of the metamaterial deduced by employing the mixing theory described in the set of Eq. (3). In case of Fig. 2(a) as the volume fraction of metal fraction is kept as $f$ = 0.2, the tangential component of the permittivity $\Re (\epsilon _{\parallel })$ has its value as 1.25 (as shown by solid black line) for $\lambda$ = 300 nm, and it increases with the increase of the operating wavelength to reach at 1.8 for $\lambda$ = 450 nm. As such, a further increase in the operating wavelength lowers the $\Re (\epsilon _{\parallel })$ to reach at 1.7 for $\lambda$ = 550 nm. However, a further increase in the operating wavelength causes the increase in $\Re (\epsilon _{\parallel })$, and it reaches to its value as 2 corresponding to $\lambda$ = 730 nm. As the operating wavelength further increases, $\Re (\epsilon _{\parallel })$ lowers to reach at 1.1 for $\lambda$ = 1000 nm. Considering the transverse component of the permittivity (as shown by the dashed black line), it is observed that real part of the transverse permittivity $\Re (\epsilon _{\perp })$ has its value as 1.25, and it remains constant for the entire operating wavelength. As the volume fraction ($f$) is increased to 0.44, in this case the permittivity has a similar pattern to the previous case. The $\Re (\epsilon _{\parallel })$ has its maximum value as 3.4 for $\lambda$ = 720 nm, and lowest value as 1.3 for for $\lambda$ = 1000 nm. As such, $\Re (\epsilon _{\perp })$ has its value as 1.75 for the entire operating wavelength, i.e., 300 nm to 1000 nm. Considering the imaginary part of the permittivities, Fig. 2(b) corresponds to this situation. It is noticed that tangential component of the imaginary permittivity $\Im (\epsilon _{\parallel })$ has its values as 3 for $\lambda$ = 300 nm, and it continuously increases to reach at 4.2 corresponding to $\lambda$ = 680 nm (as obvious in Fig. 2(b) by the solid black line). However, a further increase in the operating wavelength lowers the $\Im (\epsilon _{\parallel })$ to reach its value as 4 for $\lambda$ = 1000 nm. As the volume fraction is increased to 0.44, $\Im (\epsilon _{\parallel })$ has a similar pattern to the previous case (for $f$ as 0.2), however, its value is considerably increased as shown by solid red line. The $\Im (\epsilon _{\parallel })$ has maximum value as 9.5 for $\lambda$ = 680 nm, and the lowest value as 6.2 for $\lambda$ = 300 nm. Taking into account the transverse component of the imaginary permittivity $\Im (\epsilon _{\perp })$, it is obvious that it has near zero value for both values of $f$ as 0.2 and 0.44. From aforesaid discussion, it is obvious that the effective permittivity of the top metasurface depends on the geometry of the metasurface.
3. Results and discussion
Now, it is necessary to study the light-plasmon coupling. For this purpose, considering the $SiO2-W$ anisotropic waveguide, dispersion relation has been deduced by employing the continuity boundary conditions at the interface of SiO2 (core)-W anisotropic. The dispersion relation is used to deduced the propagation constant $\beta$ along z-axis. The propagation constant of TM-mode obtained solving the equation [42].
For even mode as
for odd mode as whereas $\gamma _{1}$= $\sqrt {\beta ^2-k^2\mu \epsilon _1}$, $\gamma _{2}$ = $\sqrt {k^{2}\mu \epsilon _{2z}-\epsilon _{2z}\beta ^2\over {\epsilon _{2x}}}$ and $k={\omega \over c}$ is the wavevector, $d_{3}$ is the thickness of the $W$ in this case.The allowed values of the $\beta$ for zeroth order modes are deduced from the dispersion relation described in Eq. (4).
Figure 3 shows the dispersion relation corresponding to different thickness values of the $W$ layer, keeping the nanohole radius as 100 nm. The dispersion curve shows the low order mode of the 2D $W/ SiO_{2}$ anisotropic waveguide for different thickness values of the top metasurface. The black line corresponds to the dispersion of the light. The red, blue, green and magenta lines correspond to the thickness values of the top layer as 50 nm, 100 nm, 150 nm and 200 nm, respectively. From the figure, it is obvious that strong light-matter coupling is achieved the anisotropic $W/SiO_{2}$ waveguide interface. It is noticed that for each dispersion curve, there is a cut-off mode for a certain frequency that reduces group velocity due to surface plasmon–thereby excite the slow-light modes in the anisotropic waveguide. The slow-light modes enhance the absorption in the given structure. In this way, the incident light remains trape in $SiO_{2}$ glass. As such, the bottom $Ag$ layer block the transmission of light. Hence, the absorptivity is enhanced in the wideband. The results show that light-matter coupling of anisotropic waveguide altered by changing the thickness of the $W$ film in the guide.
Fig. 3. Dispersion curves of the 2D $W/SiO_{2}$ waveguide for different thickness values of the $W$.
Now the absorptivity of the proposed metamaterial-based structure has been analyzed by varying the nanoholes radius, taking into account both TE- and TM-modes. Further, four different incidence angles are chosen namely 0$^\circ$, 20$^\circ$, 40$^\circ$ and 60$^\circ$. The absorption characteristic of the proposed structure has been taken up by exploiting the FDTD simulation technique in the 300-400nm wavelength range. The wavelength-dependent absorptivity is defined as $A(\lambda )=1-R(\lambda )-T(\lambda )$, where $R(\lambda )$ is the reflection from top metasurface and $T(\lambda )$ is the transmission through the proposed structure. The pupose of the thin $Ag$ layer is to keep block the transmission through $SiO_{2}$ substrate glass, therefore all the incidence light remains trapped in the glass and transmission is almost near zero, $T(\lambda ) \approx$ 0.
Firstly, considering the situation with nanohole radii as 50 $nm$, the absorptivity is investigated corresponding to different incidence angles (for both TE- and TM-modes) as shown in Fig. 4. In this case, the filling factor of W is attained as $f$ = 0.95. Fig. 4(a) demonstrates the absorptivity of the absorber for TE-mode of excitation. Considering the incidence angle as $\theta$ = 0$^\circ$, the absorptivity is approximately 50$\%$ for the entire operating wavelength regimes as shown by the solid black line. For $\theta$ = 20$^\circ$ (as shown by red solid line), it is noticed that absorption is considerably increased as compared to the absorptivity attained for $\theta$ = 0$^\circ$. As such a small absorption dip is observed in the wavelength range from 300 $nm$ to 400 $nm$, however, a further increase in the operating wavelength cause increase in the absorptivity. It is noticed that absorptivity is almost 60$\%$ for the operating wavelength range from 400 $nm$ to 1000 $nm$ with a small ripples$-$these ripples attribute due to scattering through the nanoholes. For $\theta$ = 40$^\circ$ and 60$^\circ$, absorptivity is greatly increased, however absorption dips are observed at 400 $nm$ for both incidence angles. As the operating wavelength is increased from 400 $nm$, for $\theta$ = 40$^\circ$, absorptivity is initially $\approx$ 85$\%$ and it gets lowered from 420 $nm$ to 550 $nm$, whereas further increase in operating wavelength, absorptivity continuously increases, and it is obvious that absorptivity is above 85 $\%$ in the entire operating wavelgnth 700 $nm$ to 1000 $nm$. For the incidence angle $\theta$ = 60$^\circ$, similar absorption trend has been noticed (as in case of $\theta$ = 40$^\circ$). As operating wavelength is sligthly greater than 400 $nm$ absorptivity is almost 100$\%$, whereas, a further increase in the operating wavelength, absorptivity is above 90$\%$ for 400 $nm$ to 700 $nm$, and absorptivity is above 95$\%$ from 700 $nm$ to 1000 $nm$. Considering the Fig. 4(b) that corresponds the TM-polarization case, it is noticed that for the incidence angle $\theta$ = 0$^\circ$, the absorption is similar to TE-mode case (as observed in Fig. 4(a)). However, for incidence angle $\theta$ = 20$^\circ$, 40$^\circ$ and 60$^\circ$, dips with zero absorption have been observed around $\approx$ 400 nm for all cases. Comparing with Fig. 4a (TE-mode situation), it is noticed that absorptivity is considerably increased in the regime 500 nm to 700 nm, and it is lowered for the wavelength range 700 nm to 1000 nm. It is obvious that polarization of the excited light has great effect on the absorptivity. It is noteworthy that the profound effect of the alterations in the absorptivity by changing the incidence angle is merely due to the anisotropic nature of the top metasurface. It is obvious from the results that the absorptivity has been increased with the increase of the incidence angle. Therefore, it can be inferred that the LSPR effects are more prominent for the larger incidence angle. The following reports illustrate that surface plasmon resonance is sensitive to the incidence angle of the excited light [39,43].
Fig. 4. Absorptivity corresponding to different incidence angle for nanohole radius of $W$ as 50 nm for (a) TE-Mode and (b) TM-mode.
Secondly, nanohole radius is increased to 100 nm keeping all other parametric values fixed (the filling factor of free space is attained as $f$ = 0.8), Fig. 5 corresponds to the current situation. It is noticed that compared to the previous situation (when the hole radius is 50 nm). The absorptivity has shown a modest increment compared to the previous case. Examining Fig. 5(a) that corresponds to the TE-mode, it is noticed that absorptivity is increased for 0$^\circ$ as compared to the previous case. As such, for larger incidence angles, absorptivity is considerably increased in the visible regime of light, however, it is similar to the Fig. 4(a) in the near-infrared regime. The increase in the absorptivity is due to the fact that with the increase of the nanohole radius causes increase in LSPR for TE mode. Considering Fig. 5(b) that corresponds to the TM-mode case, it is obvious that absorptivity is almost similar to Fig. 4(b), except for $\theta$ = 0$^\circ$.
Fig. 5. Absorptivity corresponding to different incidence angle for nanohole radius of $W$ as 100 nm for (a) TE-Mode and (b) TM-mode.
Thirdly, Fig. 6 shows the absorptivity keeping the nanohole radius as 150 nm, keeping all other parametric values fixed (filling factor of the metal attained as $f$ = 0.55). In this case, similar to the previous situations discussed in the Figs. 3 and 4, absorptivity is significantly lowered in the range 300 nm to 400 nm. As such, absorption dips are observed at 400 nm, and these are prominent for the TM mode (as shown in Fig. 6(b)). Also, the large number of dips have been noticed for wavelength 400 nm to 1000 nm corresponding to TM mode. Similarly to the previous discussion, it is obvious that absorptivity increases with the increase of the incidence angle. Comparing the Figs. 6(a) and (b), it is noticed that absorptivity is large (for TE-mode) in the NIR regime of incidence light. however, absorptivity is larger (for TE-mode) in the visible regime of the light as obvious in Fig. 6. In the present situation, absorptivity is very much dependent on the polarization of the incident light. For TE-mode (shown in Fig. 6(a)) absorptivity is almost 100 $\%$ in the operating wavelength range $\lambda$ $\approx$ 800 nm to 950 nm for $\theta$ = 60$^\circ$. Whereas, for TM-mode (as shown in Fig. 6(b)) the 100 $\%$ absorptivity has been attained for operating wavelength 530 nm to 670 nm corresponding to $\theta$ = 60$^\circ$.
Fig. 6. Absorptivity corresponding to different incidence angle for nanohole radius of $W$ as 150 nm for (a) TE-Mode and (b) TM-mode.
Lastly, the nanohole radius has been kept as 200 nm, as shown in Fig. 7. With the increase of the nanohole radius, the filling factor of the $W$ metal has been reduced to $f$ = 0.2. In the present case, absorption peaks have been noticeably increased with the increase of the nanohole radius for both TE- and TM-modes. As such, a large dips magnitude have been increased, as obvious in Fig. 7. Comparing with the previous results and discussion, it is obvious that ripples in the absorption generally increase with the increase of the nanohole radius that reduces the volume fraction ($f$) of the $W$ metal. A large dip in the absorptivity has been observed at $\approx$ 440 nm for both TE- and TM-modes. Observations reveal that the ripples generally with the increase of the nanohole radius of the unit cell. The increase in the ripples corresponds to the scattering from the nanoholes. In the case of TM-mode, absorptivity is almost around $\approx 430 nm$ that shows the incident light is completely reflected from the top metasurface. From Fig. 7, it is depicted that larger absorption dips are attained for TM-mode. However, the absorptivity has similar pattern corresponding to TE- and TM-modes for higher wavelength values as obvious in the NIR regime of light. The larger number of absorption peaks corresponds to the multiple LSPR.The purpose of the top metasurface is to allow the light to penetrate from top surface. It is notable that at LSPR the impedance of the top metasurface match with the impedance of free-space, therefore, reflection is minimized.
Fig. 7. Absorptivity corresponding to different incidence angle for nanohole radius of $W$ as 200 nm for (a) TE-Mode and (b) TM-mode.
Comparing the Figs. 4, 5, 6 and 7, it is obvious that absorptivity of the nanoholed $W$ thin film based absorber merely depends on the radius of the nanohole in the thin film. It is noteworthy that as the nanohole radius is lower the volume fraction ($f$) of the $W$ is larger and the absorptivity is lower. As the nanohole radius gets increased the volume fraction $f$ of the metal becomes low, and the absorptivity has been increased as obvious for nanohole radii as 100 nm and 150 nm. As the nanohole radius becomes very large as in the case of r = 200 nm, the volume fraction of the metal becomes very low. In this case, multiple absorption peaks are attained and absorptivity has becomes low, and a lot of absorption dips have been noticed. From aforesaid discussion, it can be deduced that volume fraction ($f$) and polarization have great effects on the absorptivity of the proposed absorber.
4. Conclusion
The absorptivity of the nanoholed tungsten thin film based absorber has been analyzed. The anisotropic permittivities of the nanoholed tungsten thin film are deduced by employing the effective medium theory. Further, the frequency-dependent permittivity of the tungsten and silver are deduced by using the Lorentz-Drude model. The top (nanoholed tungsten) layer allows the light to penetrate in $SiO_2$ glass due to LSPR. The bottom silver layer act as a mirror to the light trapped in the substrate glass of $SiO_2$, thereby, the light remains trapped inside the $SiO_{2}$. Also, light-matter coupling through proposed structure has been investigated by dispersion equation. Observations reveal that the angle of incidence plays a vital role for the absorption light in the proposed metamaterial absorber. As such, from aforesaid results and discussion, it is obvious that absorption is very much dependent on filling factor ($f$) of the tungsten metal in the top layer and polarization of incidence light. It can be concluded that absorptivity of the metamaterial depends on the anisotropy of the top metamaterial layer. Also, the proposed absorber is cost effective, therefore, it would be useful for solar heating and energy harvesting applications.
Funding
Higher Education Commission, Pakistan (21-1811/SRGP/R&D/HEC/2017).
Acknowledgments
The author is thankful to the three anonymous reviewers for making constructive criticism on the manuscript. The comments made by the reviewers have greatly improved the content of the manuscript.
References
1. V. G. Veselago, “Electrodynamics of materials with negative index of refraction,” Phys.-Usp. 46(7), 764–768 (2003). [CrossRef]
2. J. B. Pendry, “Negative refraction,” Contemp. Phys. 45(3), 191–202 (2004). [CrossRef]
3. A. Fang, T. Koschny, and C. M. Soukoulis, “Optical anisotropic metamaterials: Negative refraction and focusing,” Phys. Rev. B 79(24), 245127 (2009). [CrossRef]
4. S. Zhai, X. Zhao, S. Liu, F. Shen, L. Li, and C. Luo, “Inverse doppler effects in broadband acoustic metamaterials,” Sci. Rep. 6(1), 32388 (2016). [CrossRef]
5. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85(18), 3966–3969 (2000). [CrossRef]
6. S. Tretyakov, A. Sihvola, and L. Jylhä, “Backward-wave regime and negative refraction in chiral composites,” Photonics Nanostructures-Fundamentals Appl. 3(2-3), 107–115 (2005). [CrossRef]
7. H. Gao, W. Peng, S. Chu, W. Cui, Z. Liu, L. Yu, and Z. Jing, “Refractory ultra-broadband perfect absorber from visible to near-infrared,” Nanomaterials 8(12), 1038 (2018). [CrossRef]
8. R. Li, D. Wu, Y. Liu, L. Yu, Z. Yu, and H. Ye, “Infrared plasmonic refractive index sensor with ultra-high figure of merit based on the optimized all-metal grating,” Nanoscale Res. Lett. 12(1), 1 (2017). [CrossRef]
9. E. Ünal, M. Bağmancı, M. Karaaslan, O. Akgol, H. T. Arat, and C. Sabah, “Zinc oxide–tungsten-based pyramids in construction of ultra-broadband metamaterial absorber for solar energy harvesting,” IET Optoelectron. 11(3), 114–120 (2017). [CrossRef]
10. M. A. Baqir, A. Farmani, T. Fatima, M. Raza, S. Shaukat, and A. Mir, “Nanoscale, tunable, and highly sensitive biosensor utilizing hyperbolic metamaterials in the near-infrared range,” Appl. Opt. 57(31), 9447–9454 (2018). [CrossRef]
11. Y. Wang, T. Sun, T. Paudel, Y. Zhang, Z. Ren, and K. Kempa, “Metamaterial-plasmonic absorber structure for high efficiency amorphous silicon solar cells,” Nano Lett. 12(1), 440–445 (2012). [CrossRef]
12. R. W. Ziolkowski and A. Erentok, “Metamaterial-based efficient electrically small antennas,” IEEE Trans. Antennas Propag. 54(7), 2113–2130 (2006). [CrossRef]
13. I. J. Luxmoore, P. Q. Liu, P. Li, J. Faist, and G. R. Nash, “Graphene–metamaterial photodetectors for integrated infrared sensing,” ACS Photonics 3(6), 936–941 (2016). [CrossRef]
14. A. Farmani, “Three-dimensional fdtd analysis of a nanostructured plasmonic sensor in the near-infrared range,” J. Opt. Soc. Am. B 36(2), 401–407 (2019). [CrossRef]
15. A. Alipour, A. Farmani, and A. Mir, “High sensitivity and tunable nanoscale sensor based on plasmon-induced transparency in plasmonic metasurface,” IEEE Sens. J. 18(17), 7047–7054 (2018). [CrossRef]
16. S. Xiao, T. Wang, T. Liu, X. Yan, Z. Li, and C. Xu, “Active modulation of electromagnetically induced transparency analogue in terahertz hybrid metal-graphene metamaterials,” Carbon 126, 271–278 (2018). [CrossRef]
17. T. Liu, H. Wang, Y. Liu, L. Xiao, C. Zhou, C. Xu, and S. Xiao, “Dynamically tunable electromagnetically induced transparency in a terahertz hybrid metamaterial,” Phys. E 104, 229–232 (2018). [CrossRef]
18. K. V. Sreekanth, Y. Alapan, M. ElKabbash, E. Ilker, M. Hinczewski, U. A. Gurkan, A. De Luca, and G. Strangi, “Extreme sensitivity biosensing platform based on hyperbolic metamaterials,” Nat. Mater. 15(6), 621–627 (2016). [CrossRef]
19. H. Jeong, T. T. Nguyen, and S. Lim, “Subwavelength metamaterial unit cell for low-frequency electromagnetic absorber applications,” Sci. Rep. 8(1), 16774 (2018). [CrossRef]
20. L. Meng, D. Zhao, Q. Li, and M. Qiu, “Polarization-sensitive perfect absorbers at near-infrared wavelengths,” Opt. Express 21(S1), A111–A122 (2013). [CrossRef]
21. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef]
22. Y. Zhang, Y. Feng, B. Zhu, J. Zhao, and T. Jiang, “Graphene based tunable metamaterial absorber and polarization modulation in terahertz frequency,” Opt. Express 22(19), 22743–22752 (2014). [CrossRef]
23. K. Zhou, Q. Cheng, J. Song, L. Lu, Z. Jia, and J. Li, “Broadband perfect infrared absorption by tuning epsilon-near-zero and epsilon-near-pole resonances of multilayer ito nanowires,” Appl. Opt. 57(1), 102–111 (2018). [CrossRef]
24. A. S. Rana, M. Q. Mehmood, H. Jeong, I. Kim, and J. Rho, “Tungsten-based ultrathin absorber for visible regime,” Sci. Rep. 8(1), 2443 (2018). [CrossRef]
25. Y. Lin, Y. Cui, F. Ding, K. H. Fung, T. Ji, D. Li, and Y. Hao, “Tungsten based anisotropic metamaterial as an ultra-broadband absorber,” Opt. Mater. Express 7(2), 606–617 (2017). [CrossRef]
26. Z. Li, L. Stan, D. A. Czaplewski, X. Yang, and J. Gao, “Wavelength-selective mid-infrared metamaterial absorbers with multiple tungsten cross resonators,” Opt. Express 26(5), 5616–5631 (2018). [CrossRef]
27. M. A. Baqir, P. K. Choudhury, and M. Mughal, “Gold nanowires-based hyperbolic metamaterial multiband absorber operating in the visible and near-infrared regimes,” Plasmonics 14(2), 485–492 (2019). [CrossRef]
28. M. A. Baqir and P. Choudhury, “Hyperbolic metamaterial-based uv absorber,” IEEE Photonics Technol. Lett. 29(18), 1548–1551 (2017). [CrossRef]
29. M. Ghasemi, P. K. Choudhury, M. A. Baqir, M. A. Mohamed, A. R. M. Zain, and B. Y. Majlis, “Metamaterial absorber comprising chromium–gold nanorods-based columnar thin films,” J. Nanophotonics 11(4), 043505 (2017). [CrossRef]
30. H. Gao, D. Zhou, W. Cui, Z. Liu, Y. Liu, Z. Jing, and W. Peng, “Ultraviolet broadband plasmonic absorber with dual visible and near-infrared narrow bands,” J. Opt. Soc. Am. A 36(2), 264–269 (2019). [CrossRef]
31. M. Ghasemi, M. A. Baqir, and P. K. Choudhury, “On the metasurface-based comb filters,” IEEE Photonics Technol. Lett. 28(10), 1100–1103 (2016). [CrossRef]
32. B. Zhang, J. Hendrickson, and J. Guo, “Multispectral near-perfect metamaterial absorbers using spatially multiplexed plasmon resonance metal square structures,” J. Opt. Soc. Am. B 30(3), 656–662 (2013). [CrossRef]
33. Y. Bai, L. Zhao, D. Ju, Y. Jiang, and L. Liu, “Wide-angle, polarization-independent and dual-band infrared perfect absorber based on l-shaped metamaterial,” Opt. Express 23(7), 8670–8680 (2015). [CrossRef]
34. H. Jiao, X. Niu, X. Zhang, J. Zhang, X. Cheng, and Z. Wang, “Ultra-broadband perfect absorber based on successive nano-cr-film,” in Advances in Optical Thin Films VI, Vol. 10691 (International Society for Optics and Photonics, 2018), p. 106911S.
35. X. Zhang, Y. Fan, L. Qi, and H. Li, “Broadband plasmonic metamaterial absorber with fish-scale structure at visible frequencies,” Opt. Mater. Express 6(7), 2448–2457 (2016). [CrossRef]
36. H. Luo and Y. Z. Cheng, “Design of an ultrabroadband visible metamaterial absorber based on three-dimensional metallic nanostructures,” Mod. Phys. Lett. B 31(25), 1750231 (2017). [CrossRef]
37. X. Ling, Z. Xiao, and X. Zheng, “An ultra-broadband metamaterial absorber based on the hybrid materials in the visible region,” Opt. Quantum Electron. 49(7), 248 (2017). [CrossRef]
38. J. Yuan, H. Mu, L. Li, Y. Chen, W. Yu, K. Zhang, B. Sun, S. Lin, S. Li, and Q. Bao, “Few-layer platinum diselenide as a new saturable absorber for ultrafast fiber lasers,” ACS Appl. Mater. Interfaces 10(25), 21534–21540 (2018). [CrossRef]
39. M. A. Baqir and P. K. Choudhury, “Design of hyperbolic metamaterial-based absorber comprised of ti nanospheres,” IEEE Photonics Technol. Lett.1 (2019).
40. X. Han, K. He, Z. He, and Z. Zhang, “Tungsten-based highly selective solar absorber using simple nanodisk array,” Opt. Express 25(24), A1072–A1078 (2017). [CrossRef]
41. A. D. Rakić, A. B. Djurišić, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 37(22), 5271–5283 (1998). [CrossRef]
42. H. Hu, D. Ji, X. Zeng, K. Liu, and Q. Gan, “Rainbow trapping in hyperbolic metamaterial waveguide,” Sci. Rep. 3(1), 1249 (2013). [CrossRef]
43. C. Liu, Z. Yao, Y. Huang, W. Xu, Y. Tian, H. Wang, Y. Jin, and X. Xu, “Angular dependent strong coupling between localized waveguide resonance and surface plasmon resonance in complementary metamaterials,” J. Phys.: Condens. Matter 31(8), 085301 (2019). [CrossRef]
