Effective Electro-Optical Modulation with High Extinction Ratio by a Graphene–Silicon Microring Resonator

Abstract

Graphene opens up for novel optoelectronic applications thanks to its high carrier mobility, ultralarge absorption bandwidth, and extremely fast material response. In particular, the opportunity to control optoelectronic properties through tuning of Fermi level enables electro-optical modulation, optical-optical switching, and other optoelectronics applications. However, achieving a high modulation depth remains a challenge because of the modest graphene-light interaction in the graphene-silicon devices, typically, utilizing only a monolayer or few layers of graphene. Here, we comprehensively study the interaction between graphene and a microring resonator, and its influence on the optical modulation depth. We demonstrate graphene-silicon microring devices showing a high modulation depth of 12.5 dB with a relatively low bias voltage of 8.8 V. On-off electro-optical switching with an extinction ratio of 3.8 dB is successfully demonstrated by applying a square-waveform with a 4 V peak-to-peak voltage.Key words: graphene photonics, silicon microring resonator, electro-optical modulation, high modulation depth In addition to holding novel electronic properties, graphene is now also emerging as a material of interest in the area of optoelectronics. Graphene has many unique properties, such as zero-band gap and tunable Fermi level [1, 2], ultra-broad absorption bandwidth [3,4], high carrier mobility around 200000 cm 2 V -1 s -1 at room temperature [5,6], and a super high Kerr coefficient for high nonlinearity applications [7,8]. Those interesting properties give rise to many potential applications [9,10], such as wafer-scale integrated circuits [11], solar cells [12,13], high-speed graphene-silicon electro-optical modulators [14][15][16], optical-optical switches [17,18], saturation absorbers [19][20][21], photodetectors [22][23][24], and nonlinear media for four-wave mixing (FWM) [25,26].The deployment of graphene on top of a silicon waveguide is an efficient mean to make graphene-silicon hybrid devices. In order to electrostatically tune the Fermi level of graphene, there is a need to sandwich a thin layer of material with a high dielectric constant (e.g. Al 2 O 3 [27,28] or Si 3 N 4 [29]) between the silicon layer and the added layer of graphene. When this graphene-silicon capacitor is biased, carriers can be either accumulated on the graphene sheet, or swept out from the graphene sheet, resulting in a convenient tuning of the Fermi level and, thus, optical absorption [14]. This technology has enabled highspeed electro-optical modulators [14][15][16]. However, the strong light confinement in the high-index silicon gives a modest optical field overlap with the graphene layer. Hence, the graphene-light interaction is consequently too low to obtain a significant modulation depth. In order to enhance the modulation depth,

Graphene-plasmon polaritons: From fundamental properties to potential applications

“…Compared to plasmon polaritons in noble metals [31], graphene-plasmon polaritons (GPPs) exhibit even stronger mode confinement and relatively longer propagation distance, with an additional unique ability of being electrically or chemi- [24,[32][33][34]. These extraordinary features of graphene plasmons have stimulated intense lines of investigation into both the fundamental properties of graphene plasmons [35][36][37] and potential applications in metamaterials [30,38], modulators [39,40], photodetectors [41], and sensors [42,43].…”

Photonic Structure-Integrated Two-Dimensional Material Optoelectronics

“…Under this circumstance, a pre-designed break in periodicity will introduce confinement of light. Photonic crystal/2D material hybrid systems are also reported in previous literatures [108][109][110][111][112][113]. Gan et al have employed linear three-hole defect cavities in air-suspended 2D photonic crystals with graphene thin films transferred to the top [109].…”

“…First, the introduction of photonic structures into 2D materials can significantly enhance their photoresponsivity [93][94][95][96][97][98][99][100][101][102][103][104][105][106]. On the other hand, by electrostatically tuning the Fermi level of 2D materials, with which photonic structures like waveguides, resonators and cavities are integrated, their modulation performance can be improved [107][108][109][110][111][112][113]. Table 1 summarizes different photonic structure-integrated 2D material optoelectronics.…”

“…Furthermore, modulation of the guided light at frequencies over 1 GHz is demonstrated by electrically tuning the Fermi level of the graphene. Ring-shaped silicon microring resonators have also been reported to be combined with the graphene transistor as displayed in Figure 5b [108]. By tuning coverage lengths and electrostatic biases of graphene, a high extinction ratio of 12.8 dB is achieved.…”

Graphene‐based THz modulator analysed by equivalent circuit model

“…In recent years, graphene is found to have widespread application prospects in microwave and terahertz (THz) devices, and a series of novel microwave and THz devices based on graphene are proposed [4][5][6][7][8][9][10]. Terahertz (THz) modulator is one of the key components of wireless communication.…”