Open Access
Add to BibSonomy
Abstract
We numerically demonstrate a novel monolayer graphene-based perfect absorption multi-layer photonic structure by the mechanism of critical coupling with guided resonance, in which the absorption of graphene can significantly reach 99% at telecommunication wavelengths. The highly efficient absorption and spectral selectivity can be obtained with designing structural parameters in the near-infrared region. Compared to previous works, we achieve the complete absorption of single-atomic-layer graphene in the perfect absorber with a lossless dielectric Bragg mirror, which not only opens up new methods of enhancing the light-graphene interaction, but also makes for practical applications in high-performance optoelectronic devices, such as modulators and sensors.
© 2017 Optical Society of America
1. Introduction
Graphene, a novel two-dimensional material, has attracted particular attention recently due to its exceptional optical and electronic properties [1]. The ultra-broad spectral response, the ultra-thin atomic layer thickness and the ultra-high carrier mobility of graphene make it an ideal material for optoelectronic devices such as photodetectors [2–6], biosensors [7–11], absorbers [12–14], and modulators [15–22]. However, for monolayer graphene, there are two inherent defects that hinder its high-performance on optical devices. First, the absorption of monolayer graphene is only 2.3% in the visible and near-infrared ranges, which limits the quantum efficiency and results in low photoresponsivity [23]. Second, monolayer graphene does not display spectral selectivity because of its ultra-wide absorption spectrum range from the ultraviolet to the terahertz. Over the past few years, various photonic technologies have been presented to improve the absorption of the monolayer graphene by enhancing the light-graphene interaction. On one hand, in the visible and near-infrared, one can place monolayer graphene inside various nano- or micro-cavities [24, 25] or use an attenuated total reflectance (ATR) configuration [26, 27] to achieve the perfect absorption of graphene but the devices are quite complex. On the other hand, Tamm plasmon polaritons (TPPs) [28, 29], magnetic resonances in deep metal gratings [30] and localized plasmons of metallic nanostructures [31] have been used for light trapping to enhance the absorption of the monolayer graphene in the visible and near-infrared ranges. However, the metal attenuation and surface reflection lead to a failure to achieve total absorption in monolayer graphene. Therefore, the perfect absorption of the monolayer-graphene is still rare and in urgent need for graphene functional design, especially in the visible and near infrared bands.
In this work, we theoretically investigate a graphene-based perfect absorption structure by using critical coupling with guided resonance theory, in which the absorption of monolayer graphene can reach almost 99% at telecommunication wavelengths. These results originate from the electric field distributions surrounding monolayer graphene can be significantly enhanced by coupling mode with guided resonance of lossless multilayer dielectric combinations. Compared to the previous devices with a metallic reflector, we choice a dielectric Bragg mirrors with fewer layers as back reflector, because the metal parasitic absorption and its own attenuation reduce the light absorption of graphene [32, 33]. In addition, the proposed structure is simple and ultra-high-efficiency light absorption of graphene can be achieved by the mechanism of critical coupling. Meanwhile, the selectivity of the spectrum also can be obtained by adjusting the parameters of the structure.
2. The geometric structure and numerical model
The schematic image of perfect absorption system with monolayer graphene is shown in Fig. 1, a monolayer graphene is sandwiched between a 2D polynethy 1-methacrylate (PMMA) layer and a silicon dioxide (SiO2) layer with an array of cross-shaped groove air waveguide, and a dielectric Bragg mirror with 5.5-pair alternately stacked silicon (Si) and SiO2 layers is deposited at the back side of SiO2 layer to prevent the transmission of the incident light [34]. Numerical simulations are analyzed by utilizing the finite-difference time-domain (FDTD) method. In the simulation, the monolayer graphene with a thin thickness of dG = 1 nm can be viewed as a conductive surface with a light conductivity of G0 ≈ 6.08 × 10−5Ω−1, which corresponds to free standing graphene absorbs 2.3% of the incident light at the same wavelength [35]. The refractive indices of PMMA, air, SiO2 and Si are taken to be 1.48, 1, 1.45 and 3.48, respectively. The relative geometrical parameters are labeled on Fig. 1.
Fig. 1 (a) Schematic diagram of the proposed monolayer-graphene-based perfect absorption structure. dp and ds stand for the thickness of PMMA and SiO2, respectively. da is the depth of the cross-shaped groove air resonator. d1 (d2) represents the thickness of Si (SiO2) layer in the Bragg mirror with a period number N. The direction of incident light is indicated by the yellow arrow. (b) A top view of the designed structure. W and P stand for the width of the cross-shaped groove air resonator and the lattice period. The yellow cross-shaped dotted lines stand for symmetrical positions.
3. Results and discussions
First of all, in the FDTD simulations, the normal incidence light is supposed to be TM-polarized (the electric field parallel to the X-axis). Figure 2(a) shows that the absorption of the entire structure and monolayer graphene are compared to that of bare graphene in air, when the parameters are assumed as dp = 440 nm, ds = 560 nm, da = 280 nm, d1 = 100 nm, d2 = 260 nm, w = 560 nm and P = 1250 nm. We set up 5.5 pairs of dielectric layers in the Bragg mirror in order to make the system more stable and efficient. As depicted in Fig. 2(a), it can be seen that the incident light is almost completely absorbed by the entire device (black line) and monolayer graphene with being inserted in the structure (red line) at resonance wavelength (at communication wavelength of 1550 nm), compared to the absorption of bare graphene (blue line) standing in air (about 2.3% of the incident light) at the same band. As can be seen from the diagram, the red line and the black line are almost coincident, which indicates that the incident light is totally absorbed by the graphene in the structure. In order to account for this phenomenon, we will use the coupled mode theory with guided resonance formalism. The coupling mode theory is used to explain the input and output performance of a resonator, which affects coherence directly and indirectly. Since we chose subwavelength structure, only the zero-order mode will propagate, indicating that the incident light will excite a guided resonance at normal incidence, which corresponds to only one absorption peak in Fig. 2(a). We consider a resonator with a single resonance at ω0, whose input and output waves of amplitudes are u and y, respectively. The external leakage rate of the resonant cavity is γe, and the intrinsic loss rate of monolayer graphene is δ, the reflectivity coefficient of the system can be calculated by the equation [36],
and the absorption can be defined by the equation, From Eq. (1) and Eq. (2), it can be seen that when the system is in the resonance state (ω = ω0), and the external leakage rate is equal to the intrinsic loss rate of graphene (γe = δ), the whole system satisfies the critical coupling condition at which the reflection coefficient vanishes and all incident energies are absorbed. In addition, monolayer graphene has low single-pass and high transmittance at communication wavelengths, making it a minimum disruption underlying the behaviour of the resonator, thus, we can use the guided resonance to obtain the critical coupling of graphene to enhance its absorption rate. In other words, when the system meets the critical coupling condition (γe = δ) and the guided resonance is excited in the cross-shaped air groove with the incident light at resonance wavelength, the electric field intensity around the monolayer graphene is enhanced by the guided resonance of a cross-shaped groove resonator, which reinforces the graphene-light interaction and boosts the absorption of graphene. As can be seen in Fig. 2(b), when the system is in the critically coupled resonance state, the numerical simulation of the light absorption in monolayer graphene agrees well with the theoretical calculation using Eq. (2). To further verify the highly efficient absorption response of the graphene, we plot the electric field intensity distribution at on-resonant (1550 nm) wavelength and off-resonant (1600 nm) wavelength in Fig. 3(a) and 3(b). As shown in Fig. 3(a) and 3(b), when the resonant cavity is excited (on-resonant) and satisfies the critical coupling condition, corresponding to the peak absorption (1550 nm) in Fig. 2(a), the electric field intensity distribution at this time is shown in Fig. 3(a), and the electric field intensity around the graphene is obviously enhanced. In contrast, when the resonant cavity is not excited (off-resonant), the reflection coefficient of the system can be equivalent to 1, corresponding to the low absorption value (1600 nm) in Fig. 2(a), and the electric field intensity distribution is shown in Fig. 3(b).Fig. 2 The absorption spectra of the whole designed devices (black line) and the monolayer graphene in the structure (red line) are compared with bare graphene monolayer standing in air (blue line) at the same wavelengths range are shown in Fig. 2(a), and in the proposed hybrid multilayer system with dp = 440 nm, ds = 560 nm, da = 280 nm, d1 = 100 nm, d2 = 260 nm, w = 560 nm, P = 1250 nm and N = 5.5. (b) The black curve (red circle) is the numerical (theoretical) result achieved by the FDTD (coupled mode theory) method when the system is in the critically coupled resonance state.
Fig. 3 Simulated electric field amplitude distributions (|E|) of the proposed a graphene-based structure under normal incidence at on-resonant (1550 nm) wavelength (a) and off-resonant (1600 nm) wavelength (b). The location of the solid lines stand for the vicinity of monolayer graphene and the dotted lines represent the air guide cavity, while under the dashed-dotted lines represent Bragg mirror.
Considering that the absorption of monolayer graphene within the band range of our study is largely independent of frequency, and it has a relatively fixed intrinsic loss rate(δ), therefore, controlling the external leakage rate (γe) of the structure is the key to realize the perfect absorption of graphene. Here, we investigate the relationship between the external leakage rate (γe) and various related structural parameters, and effect of changing parameters on graphene absorption. As shown in Fig. 4(a) and 4(b), when the width (w) and depth (da) of the cross-shaped groove air resonator are adjusted, the corresponding absorption spectrum of the monolayer graphene undergoes a significant blue shift. Meanwhile, the peaks magnitude of the absorption are also altered. The main reason is that the external leakage rate (γe) of the resonator increases continuously, and the system experience three states, namely, undercoupling, critical coupling and overcoupling. Monolayer-graphene perfect absorption can only occur in the critical coupling state, corresponding to the spectral lines of w = 560 nm and da = 280 nm in the figure. At the same time, we also consider the effect of SiO2 (ds) thickness in the system as shown in Fig. 4(c). We can find that the thickness of SiO2 has a small effect on the absorption of graphene and resonance wavelength compared to the previous two parameters (w and da), this is because the external leakage rate (γe) is not sensitive to its minor changes. Fig. 4(d) shows that the almost perfect absorption of monolayer graphene and the absorption wavelengths are linearly tuned by the thickness of PMMA (dp). The spectra lines are red-shifted from 1532 nm to 1568 nm as dp increases from 400 nm to 480 nm. The spectral selectivity of the structure is improved by adjusting the thickness of PMMA (dp), and the feasibility of the experiment is also provided for the designed in the paper [37, 38]. In addition, the influence of the thickness of Si (d1) and SiO2 (d2) layers in the Bragg mirror are also investigated, as shown in Fig. 5(a) and 5(b), respectively. The peaks of the absorption spectra of monolayer graphene are linearly red-shifted with the increase of the thickness of Si and SiO2 due to the change of the gap position, in which the effect of Si are relatively obvious [39], as can be seen in Fig. 5(a). Theoretically, the peaks wavelength of the absorption spectra of graphene can be approximately calculated by the equation λ0 = 2(n1d1 + n2d2), where n1 and n2 are the refractive indices of Si and SiO2 in the Bragg mirror, respectively. From the formula above, we can see that the thickness of Si has a greater influence on the peaks wavelength (λ0) than the SiO2 layers thickness. But they have minimal effect on the peaks magnitude of the absorption spectra of graphene. As depicted in Fig. 6, we simulate the absorption of monolyer-graphene (black line) and the whole structure (red line) with increasing the period number (N) of the Bragg mirror. It can be found that when the number of period N > 3, the highly efficient absorption of the graphene layer can be achieved, and then the system almost tends to be stable when the number of period N > 5.5.
Fig. 4 The absorption spectra of monolayer graphene with various structural parameters when P = 1250 nm, d1 = 100 nm, d2 = 260 nm and 5.5 pairs of dielectric layers. Using different widths (a) and depths (b) of the cross-shaped groove air resonators for ds = 560 nm, dp = 440 nm, and using different SiO2 layer thickness (c) and PMMA layer thickness (d) for w = 560 nm and da = 280 nm.
Fig. 5 Influence of Si layers thickness (a) and SiO2 layers thickness (b) in the Bragg mirror on light absorption of monolayer graphene with dp = 440 nm, ds = 560 nm, da = 280 nm, w = 560 nm, P = 1250 nm and N = 5.5.
Fig. 6 Light absorption in the whole structure (red line) and the graphene monolayer (black line) using different period number (N) of the Bragg mirror at normal incidence. Other geometric parameters are assumed as dp = 440 nm, ds = 560 nm, da = 280 nm, d1 = 100 nm, d2 = 260 nm, w = 560 nm and P = 1250 nm.
Until now, the characteristics of the designed structure have been investigated at normal incidence. Subsequently, in Fig. 7 we show the absorption of the graphene layer as functions of the incident angle and wavelength for TM polarization and TE polarization. As shown in Fig. 7(a), the wavelengths of the major absorption peaks are almost unchanged with the increase of the incident angle continuously for TM-polarized, mainly due to the insensitivity of the guided mode resonance to the incident angle [33, 35], it can be valuable in applications on integrated optoelectronic devices. In contrast, as for the TE-polarized, when the incident light is tilted at a certain angle, another resonant mode is stimulated by the incident light, resulting in an additional graphene absorption peak appearing on the absorption spectrum in Fig. 7(b). Meanwhile, the wavelengths of two graphene absorption peaks are also changing with increasing the tilt angle of incident light. Therefore, we have proved that the designed structure can simultaneously achieve the critical coupling of multiple resonances, which is a major technical index of multispectral optical detection. And these angular characteristics of the structure have potential applications in the field of space optical measurement [40].
Fig. 7 Absorption spectra of the monolayer-graphene layer as functions of the wavelengths and incident angle for TM-polarization (a) and TE-polarization (b). (a) spectrum for TM-polarization, where the electric field is parallel to the plane of incidence (or the X-axis), and (b) Spectrum for TE-polarization, where the electric field is perpendicular to the plane of incidence (or the Y-axis).
4. Conclusions
In summary, we investigate a monolayer graphene-based perfect absorption structure with a cross-shaped groove air resonator, in which the absorption of graphene can achieve total absorption (about 99% of the incident light at normal incidence) at telecommunication wavelengths through the mechanism of critical coupling with guided resonance. The modeling work implies that the absorption wavelength of monolayer graphene can be tuned by adjusting the parameters of structure. In other words, the perfect absorption efficiency and spectral selectivity are obtained with attaining critical coupling condition. The results of research work also show that the different polarization modes (TM or TE) have different sensitivity to the incident angle, which lead to their different incident angular tolerance. In addition, the proposed graphene-based perfect absorption structure with a dielectric Bragg mirror together with its design principle can be extended to enhance the absorption of other two-dimensional materials.
Funding
National Natural Science Foundation of China (NSFC) (Grant NoS. 61376055 and 61775064); Fundamental Research Funds for the Central Universities (HUST: 2016YXMS024).
References and links
1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). [CrossRef] [PubMed]
2. C. H. Liu, Y. C. Chang, T. B. Norris, and Z. Zhong, “Graphene photodetectors with ultra-broadband and high responsivity at room temperature,” Nat. Nanotechnol. 9(4), 273–278 (2014). [CrossRef] [PubMed]
3. S. Song, Q. Chen, L. Jin, and F. Sun, “Great light absorption enhancement in a graphene photodetector integrated with a metamaterial perfect absorber,” Nanoscale 5(20), 9615–9619 (2013). [CrossRef] [PubMed]
4. J. Zhang, Z. Zhu, W. Liu, X. Yuan, and S. Qin, “Towards photodetection with high efficiency and tunable spectral selectivity: graphene plasmonics for light trapping and absorption engineering,” Nanoscale 7(32), 13530–13536 (2015). [CrossRef] [PubMed]
5. S. Xiao, T. Wang, Y. Liu, C. Xu, X. Han, and X. Yan, “Tunable light trapping and absorption enhancement with graphene ring arrays,” Phys. Chem. Chem. Phys. 18(38), 26661–26669 (2016). [CrossRef] [PubMed]
6. S. Thongrattanasiri, F. H. Koppens, and F. J. García de Abajo, “Complete optical absorption in periodically patterned graphene,” Phys. Rev. Lett. 108(4), 047401 (2012). [CrossRef] [PubMed]
7. D. Rodrigo, O. Limaj, D. Janner, D. Etezadi, F. J. García de Abajo, V. Pruneri, and H. Altug, “Mid-infrared plasmonic biosensing with graphene,” Science 349(6244), 165–168 (2015). [CrossRef] [PubMed]
8. M. Grande, M. A. Vincenti, T. Stomeo, G. V. Bianco, D. de Ceglia, N. Aközbek, V. Petruzzelli, G. Bruno, M. De Vittorio, M. Scalora, and A. D’Orazio, “Graphene-based absorber exploiting guided mode resonances in one-dimensional gratings,” Opt. Express 22(25), 31511–31519 (2014). [CrossRef]
9. Q. Li, L. Cong, R. Singh, N. Xu, W. Cao, X. Zhang, Z. Tian, L. Du, J. Han, and W. Zhang, “Monolayer graphene sensing enabled by the strong Fano-resonant metasurface,” Nanoscale 8(39), 17278–17284 (2016). [CrossRef] [PubMed]
10. S. Xiao, T. Wang, X. Jiang, X. Yan, L. Cheng, B. Wang, and C. Xu, “Strong interaction between graphene layer and Fano resonance in terahertz metamaterials,” J. Phys. D: Appl. Phys. 50(19), 195101 (2017). [CrossRef]
11. X. Yan, T. Wang, X. Han, S. Xiao, Y. Zhu, and Y. Wang, “High Sensitivity Nanoplasmonic Sensor Based on Plasmon-Induced Transparency in a Graphene Nanoribbon Waveguide Coupled with Detuned Graphene Square-Nanoring Resonators,” Plasmonics 12(5), 1449–1455 (2017). [CrossRef]
12. R. Alaee, M. Farhat, C. Rockstuhl, and F. Lederer, “A perfect absorber made of a graphene micro-ribbon metamaterial,” Opt. Express 20(27), 28017–28024 (2012). [CrossRef] [PubMed]
13. P. Y. Chen, M. Farhat, and H. Bağcı, “Graphene metascreen for designing compact infrared absorbers with enhanced bandwidth,” Nanotechnology 26(16), 164002 (2015). [CrossRef] [PubMed]
14. I. S. Nefedov, C. A. Valaginnopoulos, and L. A. Melnikov, “Perfect absorption in graphene multilayers,” J. Opt. 15(11), 114003 (2013). [CrossRef]
15. M. Liu, X. Yin, E. Ulin-Avila, B. Geng, T. Zentgraf, L. Ju, F. Wang, and X. Zhang, “A graphene-based broadband optical modulator,” Nature 474(7349), 64–67 (2011). [CrossRef] [PubMed]
16. Q. Bao and K. P. Loh, “Graphene photonics, plasmonics, and broadband optoelectronic devices,” ACS nano 6(5), 3677–3694 (2012). [CrossRef] [PubMed]
17. X. Y. He, “Tunable terahertz graphene metamaterials,” Carbon 82, 229–237 (2015). [CrossRef]
18. X. He, Z. Zhao, and W. Shi, “Graphene-supported tunable near-IR metamaterials,” Opt. Lett. 40(2), 178–181 (2015). [CrossRef] [PubMed]
19. X. Zhao, C. Yuan, L. Zhu, and J. Yao, “Graphene-based tunable terahertz plasmon-induced transparency metamaterial,” Nanoscale 8(33), 15273–15280 (2016). [CrossRef] [PubMed]
20. H. Lu, X. Gan, D. Mao, and J. Zhao, “Graphene-supported manipulation of surface plasmon polaritons in metallic nanowaveguides,” Photon. Res. 5(3), 162–167 (2017). [CrossRef]
21. X. He, P. Gao, and W. Shi, “A further comparison of graphene and thin metal layers for plasmonics,” Nanoscale 8, 10388–10397 (2016). [CrossRef] [PubMed]
22. X. He, F. Lin, F. Liu, and W. Shi, “Terahertz tunable graphene Fano resonance,” Nanotechnology 27, 485202 (2016). [CrossRef] [PubMed]
23. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008). [CrossRef] [PubMed]
24. X. Gan, R. J. Shiue, Y. Gao, I. Meric, T. F. Heinz, K. Shepard, J. Hone, S. Assefa, and D. Englund, “Chip-integrated ultrafast graphene photodetector with high responsivity,” Nat. Photonics 7(11), 883–887 (2013). [CrossRef]
25. R. Shiue, X. Gan, Y. Gao, L. Li, X. Yao, A. Szep, D. Walker, J. Hone, and D. Englund, “Enhanced photodetection in graphene-integrated photonic crystal cavity,” Appl. Phys. Lett. 103(24), 241109 (2013). [CrossRef]
26. G. Pirruccio, L. Moreno, G. Lozano, and J. Rivas, “Coherent and broadband enhanced optical absorption in graphene,” ACS nano 7(6), 4810–4817 (2013). [CrossRef] [PubMed]
27. G. Zheng, J. Cong, Y. Chen, L. Xu, and S. Xiao, “Angularly dense comb-like enhanced absorption of graphene monolayer with attenuated-total-reflection configuration,” Opt. Lett. 42(15), 2984–2987 (2017). [CrossRef] [PubMed]
28. H. Lu, X. Gan, D. Mao, Y. Fan, D. Yang, and J. Zhao, “Nearly perfect absorption of light in monolayer molybdenum disulfide supported by multilayer structures,” Opt. Express 25(18), 21630 (2017). [CrossRef]
29. H. Lu, X. Gan, B. Jia, D. Mao, and J. Zhao, “Tunable high-efficiency light absorption of monolayer graphene via Tamm plasmon polaritons,” Opt. Lett. 41(20), 4743–4746 (2016). [CrossRef] [PubMed]
30. B. Zhao, J. M. Zhao, and Z. M. Zhang, “Enhancement of near-infrared absorption in graphene with metal gratings,” Appl. Phys. Lett. 105(3), 031905 (2014). [CrossRef]
31. H. Lu, B. P. Cumming, and M. Gu, “Highly efficient plasmonic enhancement of graphene absorption at telecommunication wavelengths,” Opt. Lett. 40(15), 3647–3650 (2015). [CrossRef] [PubMed]
32. M. Grande, M. A. Vincenti, T. Stomeo, G. V. Bianco, D. de Ceglia, N. Aközbek, V. Petruzzelli, G. Bruno, M. De Vittorio, M. Scalora, and A. D’Orazio, “Graphene-based perfect optical absorbers harnessing guided mode resonances,” Opt. Express 23(16), 21032–21042 (2015). [CrossRef] [PubMed]
33. C. C. Guo, Z. H. Zhu, X. D. Yuan, W. M. Ye, K. Liu, J. F. Zhang, W. Xu, and S. Q. Qin, “Experimental Demonstration of Total Absorption over 99% in the Near Infrared for Monolayer-Graphene-Based Subwavelength Structures,” Adv. Optical Mater. 4(12), 1955–1960 (2016). [CrossRef]
34. C. A. Barrios, V. R. Almeida, R. R. Panepucci, B. S. Schmidt, and M. Lipson, “Compact silicon tunable Fabry-Perot resonator with low power consumption,” IEEE Photon. Technol. Lett. 16(2), 506–508 (2004). [CrossRef]
35. Y. S. Fan, C. C. Guo, Z. H. Zhu, W. Xu, F. Wu, X. D. Yuan, and S. Q. Qin, “Monolayer-graphene-based perfect absorption structures in the near infrared,” Opt. Express 25(12), 13079–13086 (2017). [CrossRef] [PubMed]
36. J. R. Piper and S. Fan, “Total absorption in a graphene monolayer in the optical regime by critical coupling with a photonic crystal guided resonance,” ACS Photonics 1(4), 347–353 (2014). [CrossRef]
37. J. W. Suk, A. Kitt, C. W. Magnuson, Y. Hao, S. Ahmed, J. An, A. K. Swan, B. B. Goldberg, and R. S. Ruoff, “Transfer of CVD-grown monolayer graphene onto arbitrary substrates,” ACS nano 5(9), 6916–6924 (2011). [CrossRef] [PubMed]
38. L. A. Eldada, A. Nahata, and J. T. Yardley, “Robust photopolymers for MCM, board, and backplane optical interconnects,” Proc. SPIE 3288, 175 (1998). [CrossRef]
39. M. A. Duguay, Y. Kukubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2-Si multilayer structures,” Appl. Phys. Lett. 49(1), 13–15 (1986). [CrossRef]
40. Y. Qu, Q. Li, H. Gong, K. Du, S. Bai, D. Zhao, H. Ye, and M. Qiu, “Spatially and spectrally resolved narrowband optical absorber based on 2D grating nanostructures on metallic films,” Adv. Optical Mater. 4(3), 480–486 (2016). [CrossRef]
