In the ultraviolet range, it is still a critical challenge to enhance and engineer light absorption inside graphene for optoelectronic applications. Here, we propose a metal-dielectric-metal plasmonic structure to achieve a high absorption ratio of ultraviolet incident light inside graphene. The absorption of ultraviolet light in single layer graphene is enhanced up to 44%, while the absorption spectrum can be tuned by optimizing the dimensions of the integrated structure. Furthermore, the structure can tolerate a wide range of incident angles, while the improved structure with aluminum nanoparticles also shows polarization-independent feature. Besides, the effect of surface oxidation on this structure is also revealed. Our research provides an important theoretical guide for designing novel optoelectronic devices based on graphene in the ultraviolet region.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Graphene, a single layer of carbon atoms with a honeycomb lattice, is the first two-dimensional material to be observed and isolated in nature [1,2]. Numerous graphene-based devices have been developed in the past few years due to its exceptional properties, including fast carrier mobility, high optical transparency, unique mechanical flexibility and strength [3–5]. The light-trapping capability inside the ultrathin graphene is a critical factor for the performance of graphene-based photonic devices. In order to enhance the light-matter interaction, various structures have been proposed to design the graphene-based absorber for novel photodetectors, photovoltaics and light modulators [6–14]. Xiao et al. has demonstrated a polarization-insensitive and angle-independent hybrid photosensor based on the integration of periodic cross-shaped graphene arrays with an ultrathin light-absorbing semiconductor . Zhu et al. has introduced a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances . Ye et al. has proposed a broadband terahertz absorber with near-unity absorption using a net-shaped periodically sinusoidally-patterned graphene sheet, which is placed on a dielectric spacer supported by a metallic reflecting plate . We have utilized the gap-plasmon mode to develop a spatial modulator in the previous work, whose modulation capability is highly enhanced due to the concentration of the optical electric field in the deep sub-wavelength scale . However, all of the above configurations work in the terahertz, infrared and visible ranges. Only a few efforts have been made on achieving high ultraviolet (UV) absorption in single layer graphene, which have great promise for numerous applications, including UV detection, Raman microscopy, chemical sensing, UV photoluminescence, flame monitoring, and space communication [15–17].
Recently, wide band-gap semiconductors such as ZnO  and β-Ga2O3  have been incorporated with graphene for high sensitive UV detection devices, but they do not allow the light-graphene interaction to be enhanced and manipulated for a specific spectral region in the UV range. Zhu et al. is inspired by the single-channel coherent perfect absorption to enhance the UV interaction with single layer graphene using a simple and practical structure configuration without any patterning on graphene . Zhou et al. adopts the configuration of an all-dielectric nanostructure to achieve perfect ultraviolet trapping on monolayer graphene, in which the magnetic resonance of optical field is combined with a UV mirror . However, the high UV absorption of graphene in these two configurations is highly dependent on the polarization and angle of incident light, which limits the further applications in high-performance optoelectronic devices. Nanostructures based on plasmonic effects might be utilized to overcome these drawbacks , but the dedicated research on the structural design is still quite in demand.
In this work, we propose a graphene-based metal-dielectric-metal (GBMDM) plasmonic structure to enhance and manipulate the light-matter interaction between the incident UV light and the single layer graphene.
2. Modeling and methods
The GBMDM nanostructure consists of graphene layer sandwiched between an aluminum layer and aluminum nanoribbons. To avoid direct electrical contact and carrier transport between aluminum and graphene, two Al2O3 layers with a relative permittivity of ~3.2 are inserted between them, as shown in Fig. 1. The thickness of the substrate aluminum layer is 50 nm, which is thick enough to block the transmission of the incident UV light. The optical constants of Al and Al2O3 are obtained from Ref. 23. The enhancement and tunability of the light-graphene interaction are investigated using finite element method (FEM) conducted by the commercial software COMSOL Multiphysics, which numerically solves Maxwell’s equations under the incident port and Floquet periodic boundary conditions. Adaptive inhomogeneous triangle (2D modeling) or tetrahedral (3D modeling) mesh is used to discretize the unit cells according to different material properties. The UV plane wave is incident downward on the top surface of the absorber with the incident plane in the x-z plane.
In FEM simulations, we assume that graphene is an anisotropic dispersive dielectric material with an effective relative permittivity tensor E as24], εxx(ω) and εyy(ω) are in-plane components of permittivity, which can be represented by the surface conductivity of graphene σ(ω) as25]. Therefore, the surface conductivity of graphene σ(ω) can be expressed by the following equations based on the Fano model :27]. The Fano parameter q = −1, which influences the strength of the excitonic transition to the unperturbed band transitions and the asymmetry of the conductivity line shape. As can be seen from the comparison in the literature , the theoretical modeling above indicates good agreement with the experimental measurement from the literature .
The optical loss inside graphene can be evaluated by the following equation:24]. The imaginary part for the out-of-plane component of refractive index is zero in the spectral range. Therefore, only the in-plane component of refractive index contributes to the optical loss inside graphene. The effects of optical saturation and non-linear response are not taken into account in the physical model.
3. Results and discussions
In order to elucidate the enhancement effect of the proposed structure, we first consider the influence of changing the height of Al nanoribbons on the UV light absorption of graphene as shown in Fig. 2 (a). When h = 20 nm, the absorption of graphene is significantly enhanced up to 44% at the resonant wavelength λ = 270 nm. As h becomes larger, the absorption of graphene slightly decreases, while the resonant wavelength remains unchanged. In comparison, the metallic loss in Al is also plotted in Fig. 2 (b). Similarly, the absorption is almost constant as the thickness of Al increases.
The absorption enhancement can be explained by analyzing the electric and magnetic field distributions, which is demonstrated in Fig. 3. As shown in Fig. 3(a), the enhanced electric field is mainly located around the bottom of Al nanoribbons. Therefore, the height of the Al nanoribbons has little impact on the absorption effect of graphene and Al as presented in Fig. 2.
The complex refractive index of Al is nAl = 0.21643 + 3.1891i at the graphene absorption peak of 270 nm, so the skin depth of Al δ ≈λ/[2πIm(nAl)] ≈13 nm is smaller than the thickness of Al nanoribbons. Thus, this lead to the weak electromagnetic coupling of optical fields at the air/nanoribbon interface and Al2O3/nanoribbon interface. Furthermore, it can be seen clearly that there is little interaction between adjacent Al nanoribbons. Therefore, the electric field surrounding the corners of Al nanoribbons is concentrated and strengthened as observed from Fig. 3(a), and magnetic field is distributed between Al nanoribbon layer and the Al substrate as shown in Fig. 3(c). The incident UV light is trapped surrounding the graphene layer as the plasmon edge mode , which induced the effects of near field enhancement and strong energy concentration. The enhanced UV energy penetrating graphene is consumed in the lossy dielectric and leads to the enhanced UV absorption inside graphene. On the contrary, for λ = 700 nm as presented in Figs. 3(b) and 3(d), there is no enhanced plasmon near field for absorption enhancement in graphene because this wavelength is far away from the resonance wavelength on the spectrum.
We next analyze the influence of the thickness of insulator spacers as shown in Fig. 4. When the upper Al2O3 layer is 1 nm, the local electric field surrounding graphene has the strongest enhancement, while the UV incident light absorption of graphene reaches 44%. As the upper Al2O3 layer gets thicker, the near field enhancement effect becomes much weaker, leading to a decrease of the absorption in graphene. However, even when the Al2O3 layer is 2 nm, the maximum absorption can still reach up to 38%.
In the application of graphene-based photonic devices, the proposed device structures should ensure high stability working in different circumstances for a long period. With regard of this purpose, we reveal the effect of surface oxidation of Al nanoribbons in Fig. 5. As Al2O3 coating around the Al nanoribbons increase from 0 nm to 1 nm, which means the surface oxidation emerges, the absorption of graphene drops from 44% to 41.5% at the resonant wavelength. While the thickness of Al2O3 coating increases from 1 nm to 2 nm, which means the surface oxidation becomes notable, the absorption of graphene slightly decreases from 41.5% to 40.5%. Normally, the Al2O3 cannot be thicker than 2 nm in reality. These results show that the proposed structure can be utilized as a stable photonic device without chemical oxidation harmness, which is due to the utilization of Al.
As a critical factor in an MDM plasmonic nanostructure, the thickness of dielectric spacer layers influences the resonance properties of the proposed nanostructures sensitively, which facilitates tuning the absorption band of graphene in a GBMDM configuration by slightly varying the spacer thickness. This property is demonstrated in Fig. 6(a). As the thickness of lower bottom Al2O3 increases from 10 nm to 30 nm, the plasmon coupling strength gets stronger until a maximum absorption rate is achieved at an optimum value and becomes weaker subsequently. Thus, there is an optimal thickness of the dielectric layer, at which the absorption peak of graphene reaches the maximum. When t1 = 20 nm, the absorption of graphene can be up to 44% at the wavelength of 270 nm. This resonance property is attributed to the coupling of near fields between each Al nanoribbon and the underlying Al substrate, which form an optimized light-trapping configuration. When t1>30 nm, each Al nanoribbon is far away from the Al substrate, then the entire GBMDM structure cannot be regarded as a homogeneous effective medium for perfect absorption, because the conditions as a sub-wavelength structure are broken . As a result, the interaction between incident light and such a configuration is mainly determined by the Al substrate. The absorption of the Al substrate dominates the total absorption, leading to a broadband absorption spectrum with lower absorbing ratios for graphene. This phenomenon can be utilized for designing both broadband graphene absorbers and frequency selective absorbers from visible to UV spectrum for novel optoelectronic devices.
While Al nanoribbons is illuminated under UV light, the incident electric field will excites electrons of Al nanoribbons to vibrate in the finite width w. The nanoribbons act as a Fabry-Perot resonator for the horizontal plasmon edge mode. Therefore, the resonant wavelength is significantly sensitive to the width of nanoribbons. As shown in Fig. 6(b), the center wavelength of the graphene absorption band is determined by w. As w rises up, the absorption spectra of graphene are red-shifted attributed to the increment of the effective resonance wavelength of the localized surface plasmon. Meanwhile, the absorption efficiency of graphene is also related to the filling factor of Al nanoribbons, which can be defined as F = w/p. When w increases, F becomes larger, and the intensity of the field concentration and enhancement between neighboring nanoribbons and inside graphene are significantly strengthened, so the absorption efficiency remarkably rises. According to the definition of F, the period of nanoribbon arrays is also critical in the absorption spectra of graphene. Thus, graphene absorption spectra versus different p are plotted in Fig. 6(c). As can be seen, when p increases from 50 nm to 130 nm, the absorption peak rises up dramatically. When p becomes larger than 130 nm, the absorption spectra almost remain unchanged. This is mainly attributed to the fact that the w is only 20 nm and p (>130 nm) is much larger than w, therefore, the continuing increase of p has little influence on the absorption enhancement in graphene, which mainly occurs surrounding the corners of nanoribbons.
The discussion above is based on normal incident UV light. However, in the application of graphene-based optical devices, the proposed structure should tolerate a wide range of incident angles. Therefore, we plot the absorption of graphene as a function of the free space wavelength and angle of incidence, as shown in Fig. 7. The result indicates that the maximum absorption of graphene can maintain at a high value larger than 30% under the incident angle below 50 degree, which is consistent with the common features of MDM absorbers. These features originate from the fact that the direction of the magnetic field for the incident light can be sufficiently kept while the angle of incidence is changed, so the intensity of magnetic resonance remains almost constant and further ensures the high loss inside graphene for a wide range of incident angles.
4. Further evaluation for the three-dimensional enhancement structure
The interaction between incident light and nanoribbons pattern is highly sensitive to the polarization of incidence . When the incident electric field is parallel to the nanoribbons, localized plasmonic resonance in nanoribbons is poorly excited. On the contrary, if the incident electric field is parallel to a finite length, the UV light will induce the localized plasmonic resonance. In order to further overcome the limitations of polarization and incidence, we investigate the integrated structure as shown in Fig. 8. Figure 8(a) indicates the perspective view of the three-dimensional enhancement structure consisting of aluminum nanoparticles, Al2O3 layers, graphene layer and aluminum substrate. Figure 8(b) shows the unit cell in the nanoparticle pattern. The cross-section view is shown in Fig. 8(c). The geometry of the 3D structure is described by w, h, t, t1 and p as shown in Fig. 8(c).
As we know, the optical properties of MDM structure with nanoparticle pattern are highly sensitive to the geometry dimensions of the structure [30,31], including nanoparticle height, width, period and the dielectric thickness. Thus, we investigate the relationship between the absorption spectrum of graphene and the geometric dimensions of nanostructure to illustrate how to design such a high-performance graphene UV light absorber in Fig. 9.
The absorption of UV light in graphene with different heights of aluminum nanoparticles is illustrated in Fig. 9(a). As h increases from 10 nm to 60 nm, the resonant wavelength remains almost unchanged, and the absorption peak gradually decreases first then increases. This is attributed to the result of the diffraction enhancement of Al nanoparticles with the increase of the thickness, which causes the absorption to decrease first. Nevertheless the enhancement of the plasmon coupling strength between the thicker Al nanoparticles and the Al substrate contributes to the increase of the absorption.
Figure 9(b) and (c) demonstrates the effects of p and w on the absorption of graphene, respectively. As p decreases or w increases, the filling factor F becomes larger, which leads to the reinforcement of the intensity of the field concentration and enhancement between neighboring nanoparticles and graphene. Besides, the effective resonance wavelength of the localized surface plasmon is increased when w rises up. As a result, the absorption spectra are red-shifted and the band becomes wider. Therefore, the resonance wavelength and bandwidth can be changed over a wide spectral range by changing w and p. These properties are promising for designing both broadband absorbers and frequency selective absorbers based on graphene in the UV region for novel photonic devices.
Figure 9(d) shows the relationship between the thickness of lower bottom Al2O3 layer and the absorption of graphene. As t1 increases from 10 nm to 30 nm, the absorption peak rises up. When t1 continues to increase, the absorption peak has a dramatic drop. Thus, there is an optimum thickness of the dielectric layer, at which the absorption peak of graphene reaches the maximum. When t1 = 30 nm, the absorption of graphene can be up to 37.6% at the wavelength of 268 nm. These properties can be explained similarly as the explanation in Fig. 6(a), which is a common characteristic for the MDM configuration.
Finally, we perform numerical simulations to investigate the absorption dependence on the polarization and angle of the incident UV light in Fig. 10. As shown in Fig. 10(a), the absorption of graphene maintains above 25% when the proposed structure is illuminated with TE mode incident light with incident angle under 60°. In comparison, Fig. 10(b) shows the absorption of graphene illuminated with TM mode incident light of corresponding angles. Similarly, the result implies that when the incident angle is under 60°, the absorption of graphene keeps at a high value above 25%, although the bandwidth becomes slightly narrower. This can be explained by that the lengths of nanoparticle in both directions are finite, in which the incident electric field can both excite the localized surface plasmonic resonance with different polarizations. Besides, the direction of magnetic field remains unchanged when the incident angle is altered, which can maintain the intensity of magnetic response effectively and further guarantee the high optical loss in graphene. Thus, the proposed structure can tolerate a wide range of incident angles, and has a polarization-independent property.
In summary, we have theoretically and numerically studied the optical properties of UV light energy trapping in graphene utilizing a plasmonic structure. Up to 44% UV light can be absorbed inside single layer graphene in the GBMDM structure. The geometric parameters have been investigated. Furthermore, three-dimensional integrated plasmonic UV light absorber has also been discussed, which shows polarization-independent property. These proposed structures can tolerate a wide range of incident angles. Our investigation provides a guide for designing novel graphene-based photonic devices in the UV region.
NSAF (No. U1830116); the Young and Middle-aged Teachers Education and Scientific Research Foundation of Fujian Province (No. JAT170405); the High Level Talent Project of Xiamen University of Technology (No. YKJ16011R); National Natural Science Foundation of China (NSFC) (No. 61601393).
Technical advice from the program managers Dr. Zhiping Cai and Dr. Qing Huo Liu are greatly appreciated. Extra supports are acknowledged for Dr. Y. Zhou. The authors also thank Mr. Liu for language check.
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