Elsevier

Optics Communications

Volume 458, 1 March 2020, 124772
Optics Communications

All-optical AND, OR, and XOR logic gates based on coherent perfect absorption in graphene-based metasurface at terahertz region

https://doi.org/10.1016/j.optcom.2019.124772Get rights and content

Highlights

  • Design of all optical logic gates.

  • Coherent perfect absorption (CPA) and transmission (CPT) principles.

  • Numerical results of several proposed designs.

  • Compact design and simple fabrication.

Abstract

In this paper, we propose a novel method for designing all-optical logic gates based on the coherent perfect absorption principle in graphene-based metasurface in terahertz (THz) region. The proposed structure allows us to access AND, OR, and XOR logic operations by adjusting the relative phase difference of two input signals and creating destructive and/or constructive interference. We have calculated the contrast ratio to analyze the performance of these logic gates. The working frequency of the proposed logic gates can be manipulated by varying the gate-controlled Fermi energy of graphene. These structures can be used in ultrafast all-optical signal processing and ultra-compact integrated circuits.

Introduction

All-optical logic gates provide potential applications for optical signal processing and optical networks due to their unique characteristics such as high-rate data transmission, high bandwidth, and low power consumption [1], [2], [3]. All-optical logic gates eliminate the need for conversion of optical signals to electronic ones and the existence of a nonlinear medium in electro-optical and nonlinear-optical logic functions. Therefore, it results in a decrease in power consumption [4]. A number of approaches with different schemes such as semiconductor optical amplifier (SOA) [5], electro-optical phenomena [6], [7], nonlinear photonic crystal [8], optical fibers [9], self-collimation effect in two dimensional photonic crystal (2D-PC) [10], [11], tunable saturable absorption (SA) to reverse saturable absorption (RSA) in graphene-oxide (GO) film and CuPc-doped PMMA thin films [12], [13], two-photon absorption (TPA) [14], and multichannel logic gates based on coherent perfect absorption [15] have been proposed to design all optical logic gates. Some limitations such as the need for preparing nonlinear medium, complex design, large size, high-intensity inputs, high power consumption [16], and difficulties in controlling the relative phase differences of input signals [17], increase the demand for new schemes for optical data processing.

Coherent perfect absorption (CPA), the concept of time-reversed lasing [18], has drawn great attention due to its functional features. CPA system consists of two counter-propagating waves for achieving perfect absorption, whereas in the coherent perfect transmission (CPT) system, perfect transmission occurs with minimum losses [19]. When the reflected waves from one direction cancel the transmitted waves of the other direction, perfect absorption is achieved and the scattering fields are perfectly suppressed [20]. As illustrated in Fig. 1, when an absorber film is illuminated by two incident waves, a standing wave is formed at the position of the film and or if it is placed in the node of the standing wave (Fig. 1a), the interaction of the electromagnetic field and the film is weak. Therefore, the incident waves will pass the film with low loss and the film acts as a transparent object, and thus perfect transmission occurs (CPT). On the other hand, if the absorber film is positioned at an antinode of the standing wave (Fig. 1b), the interaction would become very strong. Therefore, all the incoming energy to the system is completely absorbed by the metasurface, perfect absorption (CPA) occurs accordingly [21], [22]. Thus by manipulating the phase and the intensity of one beam, the intensity of other beams on the other direction can be modulated easily by suppressing and/or enhancing the light-matter interaction [15], [23]. In recent years, several potential applications based on the CPA system, such as absorber [24], all-optical modulators and switches [25], [26], slow-light waveguide [27], coherent perfect rotator [28], signal processing such as pulse restoration and coherence filtering [29] have been proposed and analyzed.

Graphene, a 2D metallic material with extraordinary photonic and optoelectronic properties including high conductivity, high carrier mobility, and tunable Fermi level has been intensively studied for decades [30], [31]. Graphene exhibits a strong plasmonic response in the THz region. Moreover, high confinement of graphene plasmons compared to diffraction limits results in strong light-matter interactions and compact metasurface designs [32]. Doped and patterned graphene can enhance localized plasmons, hence increasing the absorption coefficient [33]. Therefore, graphene is a promising candidate for THz optical devices such as modulators [34], [35], detectors [36], [37], sensors [38], absorbers [39], [40], antennas [41], [42], switches, and logic gates [43]. Some graphene-based logic gates such as electro-optical [6], electronic [44] and all-optical [45] have been proposed and investigated.

In this paper, we propose an ultra-compact, low power consumption, high contrast ratio THz logic gates based on CPA in graphene metasurface. The logic gates operations are based on coherent perfect transmission and absorption. Compared to the logic gates based on nonlinearity [46], [47], [48], our proposed design is fast because it profits from the linear process of coherent perfect absorption and optically linear materials and also because it is based on the interference of two lightwaves. Besides, a low-intensity input level is required. The input lightwaves can be generated by low-energy femtosecond laser pulses in the experiments [49]. Controlling the optical phase difference in our proposed design is easier than other structures which are based on photonic crystal waveguides [50]. The modulating signal in [51], [52] is the voltage applied to the graphene layer. Additionally, our structure can adapt to other structures due to its compact design, which results in the reduction of costs. Also, by introducing a proper threshold intensity for each gate, the proposed structure can be used in more compact, complicated, and cascaded devices. In addition, in our proposed device multiple logic functions could be implemented in contrast to the devices designed for only a sole logic function [53]. The working principle of the proposed logic gates is discussed in detail and their efficiencies are verified by simulations. The working frequency of the proposed logic gates can be tuned by applying a bias voltage and shifting the Fermi level of graphene.

The remaining of the paper is organized as follows: In Section 2, the theory and simulation method are described. In Section 3, the simulation results of the proposed structure are given and discussed. In Section 4, the electrical tunability of the structure is studied. The paper is concluded in Section 5.

Section snippets

Theory and simulation method

The schematic of the proposed structure and its unit cell are shown in Fig. 2. Each cell consists of a graphene square ring on a SiO2 substrate with the refractive index of n=1.45 and thickness of 1.3μm. The graphene thickness is assumed to be 0.5 nm. The values of parameters interpolated from [54] to operate in THz frequency range and then optimized to achieve the best performance and the dimensions of 26μm×26μm×1.3μm. The structure is illuminated by two coherent-counter propagating beams on

Simulation results and discussion

Simulations based on the FDTD method are performed to verify the function of proposed logic gates. As shown in Fig. 4a when the proposed structure is illuminated by a single beam, there is a plasmonic resonance centered at 4.85 THz. This strong excitation leads to the enhancement of absorption in graphene metasurface with a maximum of 49.65%. If two equal intensity light beams with zero phase difference from opposite sides impinge on the structure, the interaction of light and graphene

Electrical tunability

The frequency tunability of graphene is a remarkable property in optical devices. The CPA frequency of a graphene-based metasurface can be tuned by varying the driven voltage applied to graphene, and it can be described as follows: the Fermi energy depends on charge-carrier density as: EF=νFπ|n|. Therefore, by increasing the electrostatic doping, the charge-carrier concentration and the Fermi energy will be increased, which leads to graphene have a higher Drude weight and re-enforced

Conclusion

A method for designing all-optical AND, OR, and XOR logic gates is proposed and simulated, based on coherent perfect absorption (CPA) or transmission (CPT) principles in a graphene metasurface at terahertz (THz) region. We have shown that by adjusting the relative phase difference between the two incident beams at the absorber position, the output intensity can be manipulated, and we can switch between CPA and CPT. A variable optical attenuator and phase shifter are used for amplitude and phase

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