We propose an ultraviolet perfect ultranarrow band absorber by coating a dielectric grating on the monolayer graphene-dielectric-metal structure. The absorber presents an ultranarrow Fano lineshape with quality (Q) factor of 70 and a nearly perfect absorption of over 99.9% in the ultraviolet region, which is ascribed to the near field coupling of the optical dissipation of graphene and guide mode resonance of the dielectric grating. Structure parameters to the influence of the performance are investigated. The structure exhibits the high optical sensitivity (S = 150 nm/RIU, S* = 48/RIU) and figure of merit (FOM = 50, FOM* = 25374) and can also be used to detect the nanoscale analyte layer of sub-nanometer thickness, suggesting great potential applications in ultra-compact efficient biosensors for a much more sensitive detection of small refractive index changes.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Graphene, a monolayer carbon atom arranged in a honeycomb lattice, has been drawing increasing attentions in recent years, thanks to its outstanding electrical, mechanical and chemical properties . In the past decades, a number of graphene-based nanoelectronic devices and optoelectronic devices have been reported, including photodetectors , photovoltaics , optical modulators  and optical sensors [5–7]. In these investigations, graphene has shown poor optical absorption and further poor light-matter interaction for its extremely small thickness, which limits the applications of graphene in optoelectronics. Recently, there have been many efforts to enhance the optical absorption of graphene in a wide spectra range from to terahertz (THz) to visible frequency [8–13]. Among these applications, perfect absorbers have proven promising for optical sensing because of the near-unity absorption, which corresponds to high signal-to-noise ratio [14–17]. In spite of these reports, there is seldom research about the schemes to achieve ultraviolet (UV) perfect absorption (PA) in monolayer graphene based nanostructure, which is also potentially promising for many applications, including UV photoluminescence , UV Raman microscopy , chemical sensing  and flame monitoring . Zhu group firstly reported that the UV absorption of a monolayer graphene-dielectric-metal structure can be enhanced up to 71.4% and 20% under transverse electric (TE) and transverse magnetic (TM) polarizations, respectively . Afterwards, Zhu group further achieved the UV PA in a monolayer graphene integrated with one dimensional (1D) photonic nanostructure .
Ultranarrow optical resonance linewidths supported by photonics nanostructures have proven promising for ultrasensitive optical sensing because of the high figure of merit (FOM) of refractive index sensing [20,24,25]. An efficient approach to achieve an extremely narrow linewidth is to utilize the coupling between two or more optical modes to form electromagnetically induced transparency or sharp Fano resonance mode [26,27]. On the other hand, the other approach to obtain ultranarrow linewidth is to substitute conventional plasmonic systems by the lossless all-dielectric nanostructure [28,29]. But, it is still challenging to realize ultra-narrow band perfect absorption in the UV range of graphene based hybrid nanostructures, which is in urgent need for UV ultrasensitive biosensing.
In this work, we design and numerically investigate an ultranarrow Fano lineshape with a perfect absorption of over 99.9% and quality (Q) factor of 70 working at the ultraviolet range based on a dielectric grating loaded on a monolayer graphene-dielectric-metal film (DGGDM) structure. The asymmetric ultranarrow band phenomenon can be resulted from the synergy of the optical dissipation of graphene and the guide mode resonance of the dielectric grating. The optical properties of the proposed the structure can be conveniently tailored by structural parameters. Our designed structure has very high sensitivity (S = 150 nm/RIU, S* = 48/RIU) and figure of merit (FOM = 50, FOM* = 25374). The proposed structure can be used in biomedical applications.
2. Structural design and numerical simulations
The proposed DGGDM structure composed of 1D Al2O3 grating (DG) array standing on a three-layer graphene-dielectric-Aluminium (Al) film (GDM) structure is schematically depicted in Fig. 1. The period, width and height of the Al2O3 grating and the thicknesses of the dielectric Al2O3 films are denoted as P, wg, hg and td as shown in Figs. 1(a) and 1(b), respectively. The thicknesses of the bottom layer of Al film is fixed at 50 nm. The optical constants of Al and Al2O3 are taken from the experimental data [30,31]. In our simulations, the graphene is treated as an ultrathin film with a thickness (tg) of 0.5 nm. Unlike the traditional study of graphene from visible to terahertz, the many-body effects should be taken into consideration for UV excitation. Therefore, a monolayer graphene is considered as a two-dimensional conductive surface with a wavelength-dependent conductivity σg, described by the equations of Fano model [22,23,32]. Then, the dielectric constant of monolayer graphene is obtained by ɛg = 1 + iσg/(ɛ0ωtg). Here, ɛ0 is the permittivity of free space. Numerical simulations in this paper are performed by using a finite-difference time-domain (FDTD) method based software package (EastFDTD, version 5.0). In our numerical calculations, periodic boundary conditions are applied to the boundaries. A plane wave of TM polarization used as the light source impinges along the negative z-axis direction from the top of the structure. Although our work mainly focuses on theoretical results, the designed structure can be fabricated by the following process: the bottom Al layer and the Al2O3 spacer are firstly deposit on glass substrate by the electronic beam evaporation method. Then, a monolayer graphene is transferred onto the Al2O3 spacer. After that, the Al2O3 grating is fabricated by the electron beam lithography to obtain the DGGDM structure.
3. Results and discussion
3.1 Generating ultranarrow ultraviolet perfect absorption
Figure 2(a) presents the normal-incidence absorption spectra of the proposed DGGDM structure (red line), three-layer GDM structure (blue line) and Al2O3 grating (DG) (black line). For the three-layer GDM, only a low broadband absorption (< 30%) under TM polarization is observed, which has also been reported in . For the Al2O3grating on Al2O3 spacer-Al film, a narrowband absorption peak at 283 nm is obtained. This narrowband optical mode is the guide mode resonance of dielectric Al2O3 grating on Al2O3–Al film substrate, which contributes to the coupling of incident light into the Al2O3 spacer and further excites the SPPs of Al film . The wavevector of the guide mode is given by k = m(2π/P)+ k0sinθ, where k0,θand P are the wavevector of the incident light, the incident angle of the light and the period of the Al2O3 grating, respectively . Interestingly, by placing the Al2O3 grating on the GDM structure (i.e., DGGDM structure), one asymmetric narrowband absorption peak is achieved at 283 nm with absorption ratio of 99.9% and Q factor of 70. We further investigate normalized optical the electric field (E) and magnetic field (H) from front view for the absorption peak (λ = 283 nm), as shown in Figs. 2(b) and 2(c). The electric field mainly distributes on the surface of graphene around the two bottom corners of the grating strips and in the dielectric Al2O3 layer [Fig. 2(b)]. The magnetic field is mainly localized in the dielectric Al2O3layer shown in Fig. 2(c). These indicate that the ultranarrow Fano lineshape with a perfect absorption at UV range results from the near field coupling between the optical dissipation of graphene and the guide mode of the Al2O3 grating. The guide mode resonance of Al2O3 grating contributes to the coupling of light into the monolayer graphene and further enhances the absorption. The normalized the electric field and magnetic field of the GDM structure without the dielectric grating is also shown in Figs. 2(d) and 2(e) for comparison. It is found that our proposed DGGDM structure has much higher electromagnetic field intensities than the GDM structure. We further calculated the optical absorption of the DGGDM structure as a function of the incident angle and wavelength shown in Fig. 2(f). As the incident angle increases, the guide resonant mode (m = -1) of Al2O3 grating on Al2O3–Al film substrate is red-shifted, which further causes the ultranarrow absorption peak to be red-shifted from UV to visible range. This is important for practical tunable photonic applications in authentic nanostructures and devices based on graphene. Meanwhile, as the incident angle increases from 0° to 15°, the guide resonant mode (m = 1) is blue-shifted, which further causes the ultranarrow absorption peak to be blue-shifted from 280 nm to 240 nm. The low broad absorption band at the top left corner in Fig. 2(f) is the absorption of the three-layer GDM structure without coupling with the dielectric Al2O3 grating.
For further insight into the phenomenon, we investigate the influence of the period (P) of the Al2O3 grating and the thicknesses (td) of the dielectric (Al2O3) spacer on the position of the ultranarrow absorption band as well as its optimized (maximum) absorption of the DGGDM structure under normal-incidence as shown in Fig. 3(a). Pronounced red-shifts are observed with the increased P. It is because that the increased P will lead to the red-shift of excited guide mode of the Al2O3 grating, which further couples to the broadband optical dissipation of monolayer graphene. Meanwhile, the maximum absorption of the tunable ultranarrow absorption band of the DGGDM structure ranged from UV to visible light can be optimized over 80% by choosing the suitable td for each P. Interestingly, as P is increased to 260 nm, a secondary absorption peak is generated at around 270 nm with td of 70 nm, which corresponds to the second order (m = 2) guide resonant mode of the Al2O3 grating on Al2O3–Al film substrate. Figure 3(b) shows the light absorption of the DGGDM structure as a function of refractive index (nd) of the dielectric spacer, where the geometrical parameters are as the same as shown in Fig. 2(a). When nd increases, the position of the ultranarrow absorption band is red-shifted. The results in Fig. 3 demonstrate that the engineering on the period of the dielectric grating on the top and the thickness and refractive index of the dielectric layer of GDM can be performed, in order to tune the absorption band of DGGDM ranged from UV to visible light.
We next investigate the effects of height hg, width wg and refractive index ng of the dielectric grating on UV perfect absorption of DGGDM shown in Fig. 4. It is found that the central wavelength of UV absorption band has a steady red-shift for all the three parameters hg, wg and ng of the dielectric grating increase, as shown in Figs. 4(a)–4(c), respectively, which is due to the increased effective refractive index of the two layers of the dielectric grating and the Al2O3 spacer. Meanwhile, for the size of hg and wg increases, the maximum UV absorption is maintained above 95%. For the refractive index ng range from 1 to 2.2 and range from 2.7 to 3.3, the maximum UV absorption is also maintained above 95%.
3.2 Ultraviolet sensing behaviors
We now proceed to explore the performance of the DGGDM structure for UV sensing applications. To evaluate the sensing performance of the designed structure, the quantities of sensitivity (S) and figure of merit (FOM) are calculated, whose definitions are expressed in the following equations [35–38]:
The bio-sensing performance was further carried out by trapping the small molecules as an analyte layer on top surface of the grating in the DGGDM structure as the inset shown in Fig. 5(c). The molecular film is assumed to have a refractive index of 1.5 in the UV range. Based on the strong near field coupling effect between graphene and the Al2O3 grating, noticeable redshift of the reflection dip is obtained as the thickness of the analyte layer increases from 0 to 20 nm. Such a good sensing capability of our proposed DGGDM structure is beneficial to a high sensitive detection of small changes of refractive index and ultrathin thickness of analyte layer, which may have potential applications in biosensing.
In conclusion, we have theoretically investigated an ultranarrow ultraviolet perfect absorption with quality factor of 70 based on a dielectric grating loaded on a monolayer graphene-dielectric-metal film (DGGDM) structure. The asymmetric ultranarrow perfect absorption is ascribed to near field coupling between the optical dissipation of graphene and guide mode of the dielectric grating. Thanks to the narrow bandwidth, our designed structure displays superior sensitivity (S = 150 nm/RIU, S* = 48/RIU) and figure of merit (FOM = 50, FOM* = 25374). In addition, our DGGDM structure can also be used for the measure of the sub-nanometer thickness of bio-molecule layer adsorbed onto the top surface of the structure. The proposed structure has potential applications in biosensing.
National Natural Science Foundation of China (11674166, 11704184, 11904344, 51701176); Natural Science Foundation of Jiangsu Province (BK20150870).
The authors declare no conflicts of interest.
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]
2. F. Xia, T. Mueller, Y. M. Lin, A. Valdes-Garcia, and P. Avouris, “Ultrafast graphene photodetector,” Nat. Nanotechnol. 4(12), 839–843 (2009). [CrossRef]
3. S. Das, D. Pandey, J. Thomas, and T. Roy, “The role of graphene and other 2D materials in solar photovoltaics,” Adv. Mater. 31(1), 1802722 (2019). [CrossRef]
4. X. Liu, G. Liu, P. Tang, G. Fu, G. Du, Q. Chen, and Z. Liu, “Quantitatively optical and electrical-adjusting high-performance switch by graphene plasmonic perfect absorbers,” Carbon 140, 362–367 (2018). [CrossRef]
5. T. Wenger, G. Viola, J. Kinaret, M. Fogelström, and P. Tassin, “High-sensitivity plasmonic refractive index sensing using graphene,” 2D Mater. 4(2), 025103 (2017). [CrossRef]
6. Y. Cheng, H. Zhang, X. S. Mao, and R. Z. Gong, “Dual-band plasmonic perfect absorber based on all-metal nanostructure for refractive index sensing application,” Mater. Lett. 219, 123–126 (2018). [CrossRef]
7. Y. Cheng, H. Luo, F. Chen, and R. Z. Gong, “Triple narrow-band plasmonic perfect absorber for refractive index sensing applications of optical frequency,” OSA Continuum 2(7), 2113 (2019). [CrossRef]
8. M. L. Huang, Y. Z. Cheng, Z. Z. Cheng, H. R. Chen, X. S. Mao, and R. Z. Gong, “Design of a Broadband Tunable Terahertz Metamaterial Absorber Based on Complementary Structural Graphene,” Materials 11(4), 540 (2018). [CrossRef]
9. M. L. Huang, Y. Z. Cheng, Z. Z. Cheng, H. R. Chen, X. S. Mao, and R. Z. Gong, “Based on graphene tunable dual-band terahertz metamaterial absorber with wide-angle,” Opt. Commun. 415, 194–201 (2018). [CrossRef]
10. F. Koppens, T. Mueller, P. Avouris, A. Ferrari, M. Vitiello, and M. Polini, “Photodetectors based on graphene, other two-dimensional materials and hybrid systems,” Nat. Nanotechnol. 9(10), 780–793 (2014). [CrossRef]
11. J. Zhu, Q. H. Liu, and T. Lin, “Manipulating light absorption of graphene using plasmonic nanoparticles,” Nanoscale 5(17), 7785–7789 (2013). [CrossRef]
12. 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]
13. S. Thongrattanasiri, F. H. L. Koppens, and F. Javier García de Abajo, “Complete optical absorption in periodically patterned graphene,” Phys. Rev. Lett. 108(4), 047401 (2012). [CrossRef]
14. C. Chen, G. Wang, Z. Zhang, and K. Zhang, “Dual narrow-band absorber based on metal–insulator–metal configuration for refractive index sensing,” Opt. Lett. 43(15), 3630–3633 (2018). [CrossRef]
15. P. Wang, N. Chen, C. Tang, J. Chen, F. Liu, S. Sheng, B. Yan, and C. Sui, “Engineering the complex-valued constitutive parameters of metamaterials for perfect absorption,” Nanoscale Res. Lett. 12(1), 276 (2017). [CrossRef]
16. L. Lei, S. Li, H. Huang, K. Tao, and P. Xu, “Ultra-broadband absorber from visible to near-infrared using plasmonic metamaterial,” Opt. Express 26(5), 5686–5693 (2018). [CrossRef]
17. G. D. Liu, X. Zhai, H. Y. Meng, Q. Lin, Y. Huang, C. J. Zhao, and L. L. Wang, “Dirac semimetals based tunable narrowband absorber at terahertz frequencies,” Opt. Express 26(9), 11471–11480 (2018). [CrossRef]
18. L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K. S. Teng, C. M. Luk, S. Zeng, and J. Hao, “Deep ultraviolet photoluminescence of water-soluble self-passivated graphene quantum dots,” ACS Nano 6(6), 5102–5110 (2012). [CrossRef]
19. I. Calizo, I. Bejenari, M. Rahman, G. Liu, and A. A. Balandin, “Ultraviolet Raman microscopy of single and multilayer graphene,” J. Appl. Phys. 106(4), 043509 (2009). [CrossRef]
20. S. Luo, J. Zhao, D. Zuo, and X. Wang, “Perfect narrow band absorber for sensing applications,” Opt. Express 24(9), 9288–9294 (2016). [CrossRef]
21. V. Q. Dang, T. Q. Trung, D. I. Kim, L. T. Duy, B. U. Hwang, D. W. Lee, B. Y. Kim, L. D. Toan, and N. E. Lee, “Ultrahigh responsivity in graphene–ZnO nanorod hybrid UV photodetector,” Small 11(25), 3054–3065 (2015). [CrossRef]
22. J. Zhu, S. Yan, N. Feng, L. Ye, J. Y. Ou, and Q. H. Liu, “Near unity ultraviolet absorption in graphene without patterning,” Appl. Phys. Lett. 112(15), 153106 (2018). [CrossRef]
23. J. Zhou, S. Yan, C. Li, J. Zhu, and Q. H. Liu, “Perfect ultraviolet absorption in graphene using the magnetic resonance of an all-dielectric nanostructure,” Opt. Express 26(14), 18155–18163 (2018). [CrossRef]
24. Z. Yan, X. Wen, P. Gu, H. Zhong, P. Zhan, Z. Chen, and Z. Wang, “Double Fano resonances in an individual metallic nanostructure for high sensing sensitivity,” Nanotechnology 28(47), 475203 (2017). [CrossRef]
25. A. Feng, Z. Yu, and X. Sun, “Ultranarrow-band metagrating absorbers for sensing and modulation,” Opt. Express 26(22), 28197–28205 (2018). [CrossRef]
26. B. Liu, C. Tang, J. Chen, M. Zhu, M. Pei, and X. Zhu, “Electrically Tunable Fano Resonance from the Coupling between Interband Transition in Monolayer Graphene and Magnetic Dipole in Metamaterials,” Sci. Rep. 7(1), 17117 (2017). [CrossRef]
27. B. Luk’yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen, and C. T. Chong, “The Fano resonance in plasmonic nanostructures and metamaterials,” Nat. Mater. 9(9), 707–715 (2010). [CrossRef]
28. Z. Yan, L. Qian, P. Zhan, and Z. Wang, “Generation of tunable double Fano resonances by plasmon hybridization in graphene–metal metamaterial,” Appl. Phys. Express 11(7), 072001 (2018). [CrossRef]
29. X. Guo, H. Hu, X. Zhu, X. Yang, and Q. Dai, “Generation of tunable double Fano resonances by plasmon hybridization in graphene–metal metamaterial,” Nanoscale 9(39), 14998–15004 (2017). [CrossRef]
30. D. Aspnes and E. Palik, Handbook of Optical Constants of Solids (Academic, 1985).
31. M. J. Weber, Handbook of Optical Materials [(CRC, 2003), Chap. 1.
32. K. F. Mak, J. Shan, and T. F. Heinz, “Seeing many-body effects in single-and few-layer graphene: observation of two-dimensional saddle-point excitons,” Phys. Rev. Lett. 106(4), 046401 (2011). [CrossRef]
33. Z. Zhang, Z. Yu, Y. Liang, and T. Xu, “Dual-band nearly perfect absorber at visible frequencies,” Opt. Mater. Express 8(2), 463–468 (2018). [CrossRef]
34. H. Lu, X. Gan, D. Mao, B. Jia, and J. Zhao, “Flexibly tunable high-quality-factor induced transparency in plasmonic systems,” Sci. Rep. 8(1), 1558 (2018). [CrossRef]
35. Z. Yong, S. Zhang, C. Gong, and S. He, “Narrow band perfect absorber for maximum localized magnetic and electric field enhancement and sensing applications,” Sci. Rep. 6(1), 24063 (2016). [CrossRef]
36. R. Ameling, L. Langguth, M. Hentschel, M. Mesch, P. V. Braun, and H. Giessen, “Cavity-enhanced localized plasmon resonance sensing,” Appl. Phys. Lett. 97(25), 253116 (2010). [CrossRef]
37. A. E. Cetin and H. Altug, “Fano resonant ring/disk plasmonic nanocavities on conducting substrates for advanced biosensing,” ACS Nano 6(11), 9989–9995 (2012). [CrossRef]
38. Y. Zhu, H. Zhang, D. Li, Z. Zhang, S. Zhang, J. Yi, and W. Wang, “Magnetic plasmons in a simple metallic nanogroove array for refractive index sensing,” Opt. Express 26(7), 9148–9154 (2018). [CrossRef]