Shuyuan Xiao

Tunable light trapping and absorption enhancement with graphene ring arrays

Surface plasmon resonance (SPR) has been intensively studied and widely employed for light trapping and absorption enhancement. In the mid-infrared and terahertz (THz) regime, graphene supports tunable SPR via manipulating its Fermi energy and enhances light-matter interaction at the selected wavelength. Most previous studies have concentrated on the absorption enhancement in graphene itself while little attention has been paid to trapping light and enhancing the light absorption in other light-absorbing materials with graphene SPR. In this work, periodic arrays of graphene rings are proposed to introduce tunable light trapping with good angle polarization tolerance and enhance the absorption in the surrounding light-absorbing materials by more than one order of magnitude. Moreover, the design principle here could be set as a template to achieve multi-band plasmonic absorption enhancement by introducing more graphene concentric rings into each unit cell. This work not only opens up new ways of employing graphene SPR, but also leads to practical applications in high-performance simultaneous multi-color photodetection with high efficiency and tunable spectral selectivity.

Dynamically tunable plasmon induced transparency in a graphene-based nanoribbon waveguide coupled with graphene rectangular resonators structure on sapphire substrate

In this paper, we propose dynamically tunable plasmon induced transparency (PIT) in a graphene-based nanoribbon waveguide coupled with graphene rectangular resonators structure on sapphire substrate by shifting the Fermi energy level of the graphene. Two different methods are employed to obtain the PIT effect: one is based on the direct destructive interference between a radiative state and a dark state, the other is based on the indirect coupling through a graphene nanoribbon waveguide. Our numerical results reveal that high tunability in the PIT transparency window can be obtained by altering the Fermi energy levels of the graphene rectangular resonators. Moreover, double PITs are also numerically predicted in this ultracompact structure, comprising series of graphene rectangular resonators. Compared with previously proposed graphene-based PIT effects, our proposed scheme is much easier to design and fabricate. This work not only paves a new way towards the realization of graphene-based integrated nanophotonic devices, but also has important applications in multi-channel-selective filters, sensors, and slow light.

Active modulation of electromagnetically induced transparency analogue in terahertz hybrid metal-graphene metamaterials

Abstract Metamaterial analogues of electromagnetically induced transparency (EIT) have been intensively studied and widely employed for slow light and enhanced nonlinear effects. In particular, the active modulation of the EIT analogue and well-controlled group delay in metamaterials have shown great prospects in optical communication networks. Here we integrate a monolayer graphene into metal-based terahertz (THz) metamaterials, and realize a complete modulation in the resonance strength of the EIT analogue via manipulating the Fermi level of graphene. The physical mechanism lies in the active tuning of the damping rate of the dark mode resonator through the recombination effect of the conductive graphene. This work presents a novel modulation strategy of the EIT analogue in the hybrid metamaterials, and paves the way towards designing very compact slow light devices to meet the future demand of ultrafast optical signal processing.

High Sensitivity Nanoplasmonic Sensor Based on Plasmon-Induced Transparency in a Graphene Nanoribbon Waveguide Coupled with Detuned Graphene Square-Nanoring Resonators

A novel nanoscale structure for high sensitivity sensing which consists of a graphene nanoribbon waveguide coupled with detuned graphene square-nanoring resonators (GSNR) based on edge mode is investigated in detail. By altering the Fermi energy level of the graphene, the plasmon-induced transparency (PIT) window from the destructive interference between a radiative square-nanoring resonator and a dark square-nanoring resonator can be easily tailored. The coupled mode theory (CMT) is used to show that the theoretical results agree well with the finite difference time domain (FDTD) simulations. This nanosensor yields a ultrahigh sensitivity of ∼2600 nm/refractive index unit (RIU) and a figure of merit (FOM) of ∼54 in the mid-infrared (MIR) spectrum. The revealed results indicate that the Fermi energy level of the graphene and the coupling distance play important roles in optimizing the sensing properties. Our proposed structure exerts a peculiar fascination on the realization of ultra-compact graphene plasmonic nanosensor in the future.

An Ultrasensitive and Multispectral Refractive Index Sensor Design Based on Quad-Supercell Metamaterials

Plasmonic metamaterials support the localized surface plasmon resonance (LSPR), which is sensitive to the change in the dielectric environment and highly desirable for ultrasensitive biochemical sensing. In this work, a novel design of supercell metamaterials of four mutually rotating split ring resonators (SRRs) is proposed, where simultaneous excitations of odd ( N = 1 and N = 3) and even ( N = 2) resonance modes are realized due to additional asymmetry from the rotation and show insensitivity to two orthogonal polarizations. The full utilization of these three resonance dips show bright prospects for multispectral application. As a refractive index (RI) sensor, ultrahigh sensitivities ∼1000 nm/RIU for LC mode ( N = 1) and ∼500 nm/RIU for plasmon mode ( N = 2) are obtained in the near infrared (NIR) spectrum.

Dynamically controllable plasmon induced transparency based on hybrid metal-graphene metamaterials

Novel hybrid metal-graphene metamaterials featuring dynamically controllable single, double and multiple plasmon induced transparency (PIT) windows are numerically explored in the terahertz (THz) regime. The designed plasmonic metamaterials composed of a strip and a ring with graphene integration generate a novel PIT window. Once the ring is divided into pairs of asymmetrical arcs, double PIT windows both with the spectral contrast ratio 100% are obtained, where one originates from the destructive interference between bright-dark modes, and the other is based on the interaction of bright-bright modes. Just because the double PIT windows are induced by two different mechanisms, the continuously controllable conductivity and damping of graphene are employed to appropriately interpret the high tunability in double transparency peaks at the resonant frequency, respectively. Moreover, multiple PIT windows can be achieved by introducing an additional bright mode to form the other bright-bright modes coupling. At the PIT transparent windows, the dispersions undergo tremendous modifications and the group delays reach up to 43 ps, 22 ps, and 25 ps, correspondingly. Our results suggest the existence of strong interaction between the monolayer graphene layer and metal-based resonant plasmonic metamaterials, which may hold widely applications in filters, modulators, switching, sensors and optical buffers.

Tunable ultra-high-efficiency light absorption of monolayer graphene using critical coupling with guided resonance

We numerically demonstrate a novel monolayer graphene-based perfect absorption multi-layer photonic structure by the mechanism of critical coupling with guided resonance, in which the absorption of graphene can significantly reach 99% at telecommunication wavelengths. The highly efficient absorption and spectral selectivity can be obtained with designing structural parameters in the near-infrared region. Compared to previous works, we achieve the complete absorption of single-atomic-layer graphene in the perfect absorber with a lossless dielectric Bragg mirror, which not only opens up new methods of enhancing the light-graphene interaction, but also makes for practical applications in high-performance optoelectronic devices, such as modulators and sensors.

A Spectrally Tunable Plasmonic Photosensor with an Ultrathin Semiconductor Region

Surface plasmon resonance (SPR) has been widely utilized to improve the absorption performance in the photosensors. Graphene has emerged as a promising plasmonic material, which supports tunable SPR and shows significant flexibility over metals. In this letter, a hybrid photosensor based on the integration of periodic cross-shaped graphene arrays with an ultrathin light-absorbing semiconductor is proposed. A tenfold absorption enhancement over a large range of the incidence angle for both light polarizations as well as a considerably high photogeneration rate (∼ 1037) is demonstrated at the resonance. Compared with traditional metal-based plasmon-enhanced photosensors, the absorption enhancement here can be expediently tuned with manipulating the Fermi energy of graphene. The proposed photosensor can amplify the photoresponse to the incidence light at the selected wavelength and thus be utilized in photosensing with high efficiency and tunable spectral selectivity in the mid-infrared (mid-IR) and terahertz (THz) regime.

Approaching perfect absorption of monolayer molybdenum disulfide at visible wavelengths using critical coupling

A simple perfect absorption structure is proposed to achieve the high efficiency light absorption of monolayer molybdenum disulfide (MoS2) by the critical coupling mechanism of guided resonances. The results of numerical simulation and theoretical analysis show that the light absorption in this atomically thin layer can be as high as 98.3% at the visible wavelengths, which is over 12 times more than that of a bare monolayer MoS2. In addition, the operating wavelength can be tuned flexibly by adjusting the radius of the air hole and the thickness of the dielectric layers, which is of great practical significance to improve the efficiency and selectivity of the absorption in monolayer MoS2. The novel idea of using critical coupling to enhance the light-MoS2 interaction can be also adopted in other atomically thin materials. The meaningful improvement and tunability of the absorption in monolayer MoS2 provides a good prospect for the realization of high-performance MoS2-based optoelectronic applications, such as photodetection and photoluminescence.

Independently tunable dual-spectral electromagnetically induced transparency in a terahertz metal–graphene metamaterial

We theoretically investigate the interaction between the conductive graphene layer with the dual-spectral electromagnetically induced transparency (EIT) metamaterial and achieve independent amplitude modulation of the transmission peaks in terahertz (THz) regime. The dual-spectral EIT resonance results from the strong near field coupling effects between the bright cut wire resonator (CWR) in the middle and two dark double-split ring resonators (DSRRs) on the two sides. By integrating monolayer graphene under the dark mode resonators, the two transmission peaks of the EIT resonance can exhibit independent amplitude modulation via shifting the Fermi level of the corresponding graphene layer. The physical mechanism of the modulation can be attributed to the variation of damping factors of the dark mode resonators arising from the tunable conductivity of graphene. This work shows great prospects in designing multiple-spectral THz functional devices with highly flexible tunability and implies promising applications in multi-channel selective switching, modulation and slow light.