1
Absorption Enhancement in Graphene-based Photodetector with Plasmonic Structure
Elham Khosravian1, Hamid Reza Mashayekhi1
1Department of Physics, Shahid Bahonar University of Kerman, Kerman, Iran
E-mail: [email protected], [email protected]
1. Abstract- Graphene has demonstrated as a good candidate for optoelectronic devices such as ultrafast photodetectors. In this paper, a graphene based-photodetector with plasmonic structure consisting of Si and Sio2 substrate and graphene layer over a metal nanograting is proposed. The optical absorption of graphene layer in this structure increases through excitation of surface plasmon polaritons in Metal and graphene interface. The optical response of the proposed structure is numerically simulated using the finite-difference time-domain (FDTD) method. Based on numerical results, deep metal nanograting due to the occurrence of plasmonic effects can improve the graphene absorption from 2.3% to 70%, even to a maximum value of 80% in three-layer graphene at telecommunication wavelength (1.55 µm). Parameters like nanograting height and duty cycle (DC) are optimized for absorption enhancement in this spectral region. Also, the effects of the number of graphene layers on the absorption spectrum have been investigated.
2. Keywords: Graphene, Metal nanograting, Optoelectronic, Photodetector, Plasmonic
2
Graphene is a two-dimensional (2D) material composed of carbon atoms arranged into a hexagonal lattice. The unique electrical and optical properties of graphene make it suitable material for photonic and optoelectronic applications [1,7]. Additionally, graphene has a unique band structure with zero bandgap, enabling optical absorption and charge carrier generationin a broad frequency range from ultraviolet to terahertz [5].
The electrons in graphene behave as massless Dirac fermions with a linear relationship between energy and momentum, and remarkably ultrahigh charge-carrier mobility of graphene can reach up to 200000 cm2V-1s-1 for both electrons and holes at room temperature [1,5]. As a result, the use of graphene has been extensively studied for ultrafast optoelectronic devices, such as photodetectors [4]. In the visible and near-infrared (NIR) region, graphene has no plasmonic response but has a frequency- independent absorptivity of about 2.3%, which can be related to the fine structure constant [5]. The low value of absorption for graphene has limited its application in photodetection. Recently, several photonic technologies were proposed to improve the light absorption of graphene [2,3]
.
Echtermeyeret al.combined a plasmonic nanostructure and a graphene layer to enhance theefficiency of graphene photodetection [8]. Abajo et al. proposed the periodically patternedgraphene to achieve complete optical absorption in the infraredrange [9]. Zhu et al. found that plasmonic coupling of a nanovoid array in the visible range could result in 30% enhancement of absolute light absorption in graphene [6]. The reports above show that the optical absorption value can be increased by a combination of graphene with plasmonic nanostructures. In this paper, we designed a graphene-based photodetector with high optical response at telecommunication wavelength. The proposed photodetector consists of substrate, gold (Au) nanograting and graphene layer. In this proposed structure, most of the optical absorption is due to excitation of surface plasmon polaritons (SPPs) in metal nanograting and graphene layer interface. The photonic response of the graphene photodetector is numerically simulated using the finite-difference time-domain (FDTD) method [2].2. Optical properties of materials
The optical characteristic of graphene is described by the surface conductivity
, which can be obtained from the Kubo formula [7]. The frequency-dependent surface conductivity is modelled as a sum of an interband term and a Drude (intraband) term, viz.,inter intra
(1)
2 1
int 1
ln 2
4 2
c er
c
e i
i i
2
int 2 1
2 ln 1
c
K TB
c B
ra
B
e K T
i e
i K T
where
e
,
,T
and
are the electron charge, angular frequency, temperature and momentum relaxation time, respectively. andK
B are the reduced Planck’s constant and Boltzmann’s constant, respectively. The chemical potential (Femi-level) is represented by
c, which depends on the charge carrier density. The graphene absorption of light can be tuned by controlling chemical potential.
The relative permittivity of graphene can be equivalently expressed as [6,7]3
0
1 i
(2)where
0and
are the vacuum permittivity and graphene layer thickness, respectively.The relative permittivity of Au can be characterized by the Drude model [2]:
2 2p
i
(3) Here
is the relative permittivity at the infinity frequency.
P and
are the bulk plasma frequency and electron collision frequency, respectively.3. Proposed structure and simulation results
The schematic view of the proposed graphene-based photodetector is shown in Fig.1(a).
The structure is composed of SiO2 (thickness=300nm)/Si substrate, graphene layer and deep metal nanogratings. The graphene layer is covered on the deep grating. As shown in Fig.
1(b), The nanograting (extended to infinite in the y-direction) is made of Au and the geometric parameters are period Λ, height h, and trench width b. The plane of incidence is the x–z, and the electric field (E) is along the x-axis.
(a)
(b)
Fig. 1. (a) Schematic view of the proposed graphene-based photodetector. (b) Cross-section of the graphene-covered Au nanograting.
The photodetector is illuminated by a p-polarized plane wavesource perpendicular to the structure at the central wavelength of 1.55 µm. To study the optical response of structure, we adopt two-dimensional (2D) FDTD method. The periodic boundary condition is applied along the x-axes, and the perfectly matched layer (PML) is used at the z-axes. In the simulation, the graphene layer is modelled as a thin sheet of thickness Δ=0.34nm. The parameters used in the calculations are: relaxation time τ=10-13s, chemical potential µc=0.3ev and temperature T=300 K. The high optical absorption in graphene photodetector at telecommunication wavelength (1.55 µm) can be tuned by the geometry of the grating.
Fig. 2 displays the absorption contour as a function of wavelength and nanograting height h, where Λ=400nm, b=30nm and µc=0.3ev are fixed. By increasing h, the SPPs resonance wavelength is shifted to a higher value.
4
Fig. 2. Absorption contour for different values of the nanograting height assisted graphene photodetector.
Fig. 3. Absorption contour for different values of duty cycles in nanograting assisted graphene photodetector.
The duty cycle of the nanograting can be equivalently expressed as DC= Λ-b/Λ. Fig. 3 displays the absorption contour for TM waves as a function of wavelength and duty cycle DC, where Λ=400nm, h=210nm and µc=0.3ev are fixed. For nanogratings, since a strong field coupling between the two walls of the trench widthb can only happen when the trench width is very small. It can be inferred that the peak wavelength is different for each specific duty cycle and the maximum absorption occurs when DC >0.9.
Fig. 4. Field distribution at the cross- section of structure.
5
Fig. 5. Absorption spectra of graphene in the structure with different numbers of graphene layer.
The optical field is strongly enhanced in the graphene close to the trenchdue to excitation of SPPs in Metal and graphene interface. Fig. 4 shows the normalized electric field distributions (|E|) at the absorption peak for normal incidence of a p-polarized plane wave source.
Fig. 6. Absorption spectra of the optimal structure with h=210nm, DC=0.925, Λ=400nm, b=30nm
By increasing the number of graphene layers, we find that the absorption of light in graphene is further enhanced from 70% to 80% for three layers’ graphene, as shown in Fig. 5. The absorption spectra for optimal graphene-based photodetector is shown in Fig.
6. It can be seen the optical absorption is increased to the value of 0.98 at telecommunication wavelength (1.55 µm).
4. Conclusion
We have designed a graphene-based photodetector with plasmonic structure to improve light absorption at telecommunication wavelength. Light absorption in proposed structure optimized by varying geometric parameters of nanograting.It has been shown that the absorption of the optimal structure is almost complete using metal nanograting along with graphene due to occurrence of plasmonic effects.
REFERENCES
6
with metal gratings”, Applied Physics Letters, vol. 105, p.031905, (2014), pp.1-5.
[2] Narottam. Das, Farzaneh. Fadakar. Masouleh, Hamid. Reza. Mashayekhi, “A Comprehensive Analysis of Plasmonics-Based GaAs MSM-Photodetector for High Bandwidth-Product Responsivity”, Advances in OptoElectronics, vol.2013, 793253, (2013), pp.1-10.
[3] Yue. Su, Zhongxun. Guo, Wen. Huang, Zhiwei. Liu, Tianxun. Gong, Yiwen. He and Bin.
Yu, “Ultra-sensitive graphene photodetector with plasmonic structure”, Applied Physics Letters, vol.109, 173107, (2016), pp.1-4.
[4] Thomas. Mueller, Fengnian. Xia, and Phaedon. Avouris, “Graphene photodetectors for high-speed Optical communications”, NaturePhotonics, vol.4, (2010), pp. 297–301.
[5] Tim. Echtermeyer, Silvia. Milana, Ugo. Sassi, Anna. Eiden, Menglin. Wu, Elefterios.
Lidorikis and Andrea C. Ferrar, “Surface plasmon polariton graphene photodetectors”, Nano Lett, vol.16 (1), (2016), pp. 8-20
[6] H. Lu, B. P. Cumming, and M. Gu, “Highly efficient plasmonic enhancement of graphene absorption at telecommunication wavelengths”, Optics letters, vol. 40, (2015), pp. 3647- 3650.
[7] G. Zhen, H. Zhang, L. Xu, Y. Liu, “Enhanced absorption of graphene monolayer with a single-layer resonant grating at the Brewster angle in the visible range”, Optics letters, Vol.41, No.10, (2016), pp.2274-2277.
[8] T. Echtermeyer, L. Britnell, P. Jasnos, A. Lombardo, R. Gorbachev, A. Grigorenko, A. Geim, A. Ferrari, and K. Novoselov, “Strong plasmonic enhancement of photovoltage in graphene”, Nat. Commun.2, (2011), 458.
[9] F. Abajo, “Complete Optical Absorption in Periodically Patterned Graphene”, Phys.
Rev. Lett.108, (2012), 047401.
