Dual-band perfect graphene absorber with an all-dielectric zero-contrast grating-based resonant cavity

Graphene has become an ideal two-dimensional material for high-performance optoelectronic devices due to its broad-spectrum and ultra-high carrier rate [1]. However, the poor absorption efficiency of monolayer graphene, which has been demonstrated to be only 2.3% in the visible and near-infrared regions [2], hinders its potential application in optoelectronic devices, including photodetectors [3], [4], [5] and modulators [6], [7]. Surface plasmon resonance has been utilized to enhance the absorption efficiency of monolayer graphene in the mid-infrared and terahertz bands [8]. However, it is not easy to support surface plasmon polaritons in the near-infrared range for graphene. Graphene-based absorbers have been reported using different kinds of resonant structures, including Fabry–Perot (F–P) cavity [9], [10], dielectric grating structure [11], [12], [13], photonic crystal slab [14], metal–dielectric–metal​ structure [15], and photonics structures with bound states in the continuum (BIC) [16], [17]. Recently, multi-band graphene-based absorbers with excellent spectral selectivity have attracted significant attention [18], [19], [20], [21], [22]. For example, Liu et al. presented a silver nano-disk to realize a dual-band high efficiency absorption of monolayer graphene using the surface plasmon polaritons and magnetic dipole resonances [23]. Qing et al. proposed a hybrid Tamm plasmonic system with guided-mode resonance (GMR) and Tamm plasmon polaritons (TPPs) to achieve multichannel absorption peaks [24]. In addition, with the combination of F–P resonance and guided-mode resonance in the F–P structure based on the Ag grating, the proposed system yields a dual-band absorption peak under TM polarization [25]. One can see from the above that metal also contributes to the total absorption efficiency of the absorber. However, the intrinsic ohmic loss of metal and thermal effect is not conducive to a device. Thus, it is highly desirable to enhance near-infrared multi-band absorption of monolayer graphene by employing all-dielectric resonant structures.

Recently, multi-band perfect absorption of two-dimensional materials, including graphene and MoS2, has been achieved using the critical coupling induced by guided mode resonances (GMRs) in all-dielectric resonant structures [26], [27], [28], [29]. Here, the critical coupling has been theoretically confirmed using the coupled-mode theory (CMT) by Piper and Fan [14]. Multi-band absorption of graphene is realized using GMR of one-dimensional photonic crystal excited by subwavelength grating [26]. A graphene-based dual-band metamaterial absorber has been demonstrated by exciting two different orders guided resonance modes of a single-layer dielectric grating [27]. However, absorption efficiency is sensitive to the change of grating period in the single-layer grating due to the excitation of the multi-order GMR modes in the structure. Nie et al. proposed a zero-contrast grating (ZCG) to achieve ultra-narrowband perfect absorption of a monolayer of MoS2 [28], where the local reflection and phase changes on enhanced absorption are both eliminated by the zero-contrast grating, which is a new concept of subwavelength grating and named by Magnusson in 2014 [30]. However, it is challenging to maintain robustly the critical coupling of these systems using an all-dielectric resonant grating structure with multi-order GMRs.

In this work, we presented a dual-band graphene-based absorber with an all-dielectric grating cavity structure consisting of a ZCG, a SiO2 spacer layer, and a distributed Bragg reflector (DBR), where the monolayer graphene is placed between the slab waveguide layer of ZCG and the spacer layer. It is shown that dual-band perfect absorption of monolayer graphene is achieved at the near-infrared range by exciting the F–P cavity resonance and guided-mode resonance. The critical coupling of the proposed absorber can be controlled robustly by changing the grating period of ZCG and the thickness of the grating layer. In addition, absorption peaks caused by the F–P​ cavity-dominated mode can be adjusted through the varied structural parameter, such as the thickness of the waveguide layer, spacer layer, and the number of periods in DBR. This proposed structure may have potential application in high-performance graphene-based optoelectronic devices.

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