## Abstract

We investigate THz absorption properties of graphene-based heterostructures by using characteristics matrix method based on conductivity. We demonstrate that the proposed structure can lead to perfect THz absorption because of strong photon localization in the defect layer of the heterostructure. The THz absorption may be tuned continuously from 0 to 100% by controlling the chemical potential through a gate voltage. By adjusting the incident angle or the period number of the two PCs with respect to the graphene layer, one can tailor the maximum THz absorption value. The position of the THz absorption peaks can be tuned by changing either the center wavelength or the thicknesses ratio of the layers constituting the heterostructure. Our proposal may have potentially important applications in optoelectronic devices.

© 2014 Optical Society of America

## 1. Introduction

As a two-dimension substance composed of a single layer carbon atom arranged in a honey comb lattice, graphene has attracted significant attention in recent years due to its unique optical, electronic, and mechanical properties [1–3]. It has had a notable impact on the fields of photonics and optoelectronics [3–5]. In the photonics context, graphene is a two-dimensional conductor that graphene has strong plasmonic effects which can be modified by gating, doping, and so on [4, 5].

Optical absorption plays an important role in a variety of applications such as photodetectors, saturable absorbers and photovoltaics. The concept of perfect absorbers has initiated a new research area and has important applications in optoelectronics [6–8]. Various microstructures have been proposed to get complete absorption in graphene, *e.g.*, the periodically patterned graphene, the microcavity, the graphene negative permittivity metamaterials, and the attenuated total reflectance, graphene-based hyperbolic metamaterials, etc [9–13]. In recent years, various types of THz absorbers have attracted an explosion of research interest in many scopes, like spectroscopy, medical imaging, communications and so on [14]. By changing the various bias voltages, the absorption is controlled in the graphene fishnet metamaterials [15] and the biperiodic graphene metasurfaces [16].

In this paper, we theoretically investigate THz absorption properties of graphene-based one-dimensional photonic crystals (1DPC) heterostructure as shown in Fig. 1. We demonstrate that the proposed structure can lead to perfect THz absorption because of strong photon localization in the defect layer of the heterostructure. The THz absorption may be tuned continuously from 0 to 100% by controlling the chemical potentials${\mu}_{c}$. By adjusting the incident angle or the period number of the two PCs with respect to the graphene layer, one can tailor the maximum THz absorption value. The position of the THz absorption peaks can be tuned by changing either the center wavelength${\lambda}_{0}$or the thicknesses ratio of the layers constituting the heterostructure. Our proposal is very easy to implement using the existing technology and may have potentialsly important applications in optoelectronic devices.

## 2. Theoretical model and numerical method

Previous research has shown that the conductivity of graphene came from the contribution of intraband and interband [17–19]. The interband conductivity tends to be ignorable for the THz frequencies because of the photon energy$\hslash \omega \ll {\mu}_{c},$ where ${\mu}_{c}$is chemical potential [20]. Therefore, in the THz range graphene is well described by the Drude-like conductivity [21]

To determine the THz absorption we have to compute the amount of light reflected and transmitted by and through the heterostructure, respectively. A light beam is incident at theangle of $\theta $ onto the heterostructure ${\text{(}ab\text{)}}^{M}Gr{\text{(}ba\text{)}}^{N}$ from the left along the *z*-direction in Fig. 1. Based on Maxwell’s equations, the electric field and the magnetic field of the light beam in the *j*th layer is given by

*z*component of the wave vector in the

*j*th layer, ${\epsilon}_{l}$ is the dielectric constant of the

*j*th layer. The transverse wave vector component, ${k}_{x}={k}_{0}\mathrm{sin}\theta $, is preserved across all interfaces, ${k}_{0}$is the wave vector in incident medium. The dynamic matrix referring to the transfer through the boundary of two dielectrics

*i*and

*j*and the propagation matrix characterizing the free propagation through dielectric

*j*are given respectively by

*L*(

*R*) stands for the fields to the left (right) of graphene. From Eqs. (2), (3), (6) and (7), we can get

## 3. Results and discussion

In the following calculations, we choose the center wavelength ${\lambda}_{0}$ = 80$\mu \text{m,}$ the absolute temperature *T* = 300 K, and the relaxation rate $\Gamma $ = 2.5 meV/$\hslash .$ In the THz range we approximate the dielectric constants of Si and SiO_{2} by their static values, ${\epsilon}_{Si}$ = 11.9 and ${\epsilon}_{Si{O}_{2}}$ = 3.9. The thicknesses of the Si and SiO_{2} layers in the periodic structure are given by${d}_{Si{O}_{2}}={\lambda}_{0}/4{({\epsilon}_{Si{O}_{2}})}^{1/2}$ and ${d}_{Si}={\lambda}_{0}/4{({\epsilon}_{Si})}^{1/2},$ respectively.In what follows, the THz absorption of graphene on a system such as the represented in Fig. 1 is computed. In Fig. 2(a) we represent the THz absorption of graphene-based heterostructures as a function of the light frequency$\omega .$ We compare the THz absorption at normal incidence for bare graphene [red dashed curve in Fig. 2(a)], and graphene within the defect layer of the1DPC heterostructure (Si/SiO_{2})^{M}Gr(SiO_{2}/Si)^{N} in Fig. 1 [black solid curve for M = 2 and N = 5, olive dash dotted for M = 3 and N = 5, and blue dotted curve for M = 4 and N = 4]. Since the dielectric constants we have used for SiO_{2} and Si are real, all the THz absorption comes of graphene alone. Based on the presented results, one observes that the THz absorption of the graphene monolayer without the 1DPC decreases form 16.7% to 5% when the light frequency $\omega $increases from 10 meV to 22 meV. In contrast, the graphene is introduced in the defect state of the PC heterostructure, a more dramatic enhancement is obtained, THz absorptions near the center of the photonic band gap are nearly 100% for M = 2 and N = 5, 73.8% for M = 3 and N = 5, and 32% for M = 4 and N = 4, respectively. To get a perfect matching of M and N to achieve the maximum absorption value, we consider more {M, N} case, in Fig. 2(b), we represent the absorption peak value with respect to M and N, we can observe the presence of an optimal number of periods M and N that maximize absorption. As is shown in Fig. 2(b) M = 2, N has little or no influence when N$\ge $5. Therefore, by varying the period number M and N, one can tailor the maximum THz absorption value in the heterostructure.

To understand the almost 100% absorption of incident energy by the heterostructure, we show the electric field distribution in the heterostructure (Si/SiO_{2})^{M}Gr(SiO_{2}/Si)^{N} in the inset of Fig. 2(a), the spectra reveal the electric field dramatic enhanced in the defect layer, a peak of the electric field appears at the position of graphene in the 1DPC heterostructure. The PC heterostructure [(Si/SiO_{2})^{M}Gr(SiO_{2}/Si)^{N}] formed a $\lambda /2$SiO_{2} defect layer, thus the graphene layer and the SiO_{2} defect act as the defect layer of the heterostructure, the presence of a localized state in the defect layer improves field localization at the graphene position, thanks to strong photon localization in the defect layer, there is a dramatic enhancement of THz absorption near the center of the gap of the PC heterostructure. Why {M = 2, N$\ge $5} case is the best in terms of absorption, it can be understood from the following explanation. PC1 and PC2 form a Bragg cavity, PC1 acts as a Bragg reflector, and also as the medium which introduces incident light into the Bragg cavity, while PC2 simply act as a Bragg mirror, when M and N are relatively large, the light inside the cavity can be sufficiently reflected and localized in the cavity, however, when M is relatively small, the incident light outside the cavity can be sufficiently introduced into the cavity, so when M = 2 and N$\ge $5, light is fully localized and absorbed in the cavity shown in Fig. 2(b).

To examine how a finite THz absorption in SiO_{2} will affect the performance of the proposed structure, in Fig. 2(c), we represent the THz absorption and the electric field distribution of the proposed structure for ${\epsilon}_{Si{O}_{2}}\text{=}3.9\text{+}0.025i$ [22], we can see that, when taking into account the absorption loss of SiO_{2}, the near perfect THz absorption and the electric field distribution are almost unchanged, while for higher frequencies ($\omega >$18 THz), the THz absorption slightly fluctuates, strong photon localization in SiO_{2}-filled region of the heterostructure causes broadening and suppression of the resonance.

In Fig. 3, we plot the THz absorption maps of a graphene-based heterostructure as a function of incident frequency and angle of incidence for TE and TM polarized incident lights. It is shown that the THz absorption peak moves toward higher frequencies with increasing incident angle $\theta $ both for the TE and TM polarizations. It can be understood from the relationship between the propagation angle${\theta}^{\text{'}}$and the resonant frequency$\omega $in a microcavity$\omega \propto 1/\mathrm{cos}{\theta}^{\text{'}}$ [23], when the incident angle increases, $\mathrm{cos}{\theta}^{\text{'}}$can be decreased, the resonant peak moves toward higher frequencies. Consequently, the THz absorption of graphene can be tuned by varying incident angle $\theta .$

In order to better control THz absorption, in Fig. 4(a) we represent the absorption as a function of chemical potentials ${\mu}_{c}$ for graphene within the defect layer of the heterostructure (Si/SiO_{2})^{2}Gr(SiO_{2}/Si)^{5} with $\omega $ = 16 meV, it is shown that the absorption increases from 0 to 100% when chemical potentials ${\mu}_{c}$ increases from 0 to 0.5 eV, while the absorption decreases from 100% to 12% when chemical potentials ${\mu}_{c}$ increases from 0.5 eV to 1 eV. In Fig. 4(b) we plot intensity of the absorption as a function of the chemical potentials ${\mu}_{c}$ and the light frequencies$\omega .$ It is shown that the THz absorption peak moves toward higher frequencies with increasing chemical potentials${\mu}_{c}.$ We note that, since chemical potentials ${\mu}_{c}$ can be tuned by varying the density of charge carriers through a gate voltage, the conductivity $\sigma $ of graphene can also be changed. So we can control conveniently the optical absorption of the proposed structure in the THz spectral range by varying chemical potentials ${\mu}_{c}.$

In Fig. 5(a) we represent the absorption as function of the parameter ${d}_{Si{O}_{2}}/{d}_{Si}$. Recall that the parameter ${d}_{Si{O}_{2}}/{d}_{Si}$ controls the thickness of the SiO_{2} layer,when changing ${d}_{Si{O}_{2}}/{d}_{Si},$ the optical path of the SiO_{2} layer can be changed, leading to a resonant wavelength altered, and absorption peaks moved. We see that the absorption arrive a maximum value almost 100% when ${d}_{Si{O}_{2}}/{d}_{Si}$ = 1.8, 5.2, and 8.5, respectively. In Fig. 5(b) we represent the absorption as a function of the parameter ${\lambda}_{0}$ and $\omega .$ Since${\lambda}_{0}$influences the thicknesses of the layers constituting the periodic structure, when varying${\lambda}_{0},$ leading to resonant frequencies $\omega $ altered, and the position of absorption peaks can be changed. As seen in Fig. 5(b), the position of the absorption peaks is sensitive to the parameter${\lambda}_{0},$the absorption peaks move to higher frequencies with decreasing${\lambda}_{0}.$These type of plots allow the optimization of the heterostructure in order to maximize the absorption.

## 4. Conclusion

In conclusion, we investigated THz absorption properties of graphene-based heterostructures by using characteristics matrix method based on conductivity. We demonstrated that the proposed structure can lead to perfect THz absorption because of strong photon localization in the defect layer of the heterostructure. The THz absorption may be tuned continuously from 0 to 100%. Our proposal is very easy to implement using the existing technology and may have potentially important applications in optoelectronic devices.

## Acknowledgments

This work was supported by the NSFC Grant Nos. 61464007, 11364033, 11264030 and 11264029, the Open Research Fund of State Key Laboratory of Millimeter Waves (NO. K201216), and the Postdoctoral Science Foundation of Jiangxi Province No. 2014KY32.

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