Yuanjiang Xiang

Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction

By coupling few-layer Molybdenum Disulfide (MoS2) with fiber-taper evanescent light field, a new type of MoS2 based nonlinear optical modulating element had been successfully fabricated as a two-dimensional layered saturable absorber with strong light-matter interaction. This MoS2-taper-fiber device is not only capable of passively mode-locking an all-normal-dispersion ytterbium-doped fiber laser and enduring high power laser excitation (up to 1 W), but also functions as a polarization sensitive optical modulating component (that is, different polarized light can induce different nonlinear optical response). Thanks to the combined advantages from the strong nonlinear optical response in MoS2 together with the sufficiently-long-range interaction between light and MoS2, this device allows for the generation of high power stable dissipative solitons at 1042.6 nm with pulse duration of 656 ps and a repetition rate of 6.74 MHz at a pump power of 210 mW. Our work may also constitute the first example of MoS2-enabled wave-guiding photonic device, and potentially give some new insights into two-dimensional layered materials related photonics.

Critical coupling with graphene-based hyperbolic metamaterials

In order to effectively realize and control the critical coupling, a graphene-based hyperbolic metamaterial has been proposed to replace the absorbing thin film in the critically coupled resonance structure. Our calculations demonstrate that the critical coupling effect (near-perfect light absorption) can be achieved at the near-infrared wavelength by using this layered structure, while the critical coupling frequency can be tuned by varying the Fermi energy level of graphene sheets via electrostatic biasing. Moreover, we show that the critical coupling frequency can be tuned by changing the thickness of the dielectric or layer number of the graphene sheets in the unit cell of the graphene-dielectric HMM. The optimization performance has also been indicated, which may offer an opportunity towards the experimental designs of high efficient graphene based critical coupling devices.

Low threshold optical bistability at terahertz frequencies with graphene surface plasmons

We propose a modified Kretschmann-Raether configuration to realize the low threshold optical bistable devices at the terahertz frequencies. The metal layer is replaced by the dielectric sandwich structure with the insertion of graphene, and this configuration can support TM-polarization surface electromagnetic wave. The surface plasmon resonance is strongly dependent on the Fermi-level of graphene and the thickness of the sandwich structure. It is found that the switching-up and switching-down intensities required to observe the optical bistable behavior are lowered markedly due to the excitation of the graphene surface plasmons, thus making this configuration a prime candidate for experimental investigation at the terahertz range. And the switching threshold value can be further reduced by decreasing the Fermi-level or increasing the thickness of sandwich structure, hence providing a new way for realizing tunable optical bistable devices. Finally, the optical bistability at higher terahertz frequency and the influence of relaxation time under the actual experimental condition on Fermi-level are discussed.

Tunable optical bistability of dielectric/nonlinear graphene/dielectric heterostructures

We have established the theoretical relation of nonlinear optical response with respect to the dielectric/nonlinear graphene/dielectric heterostructures and further demonstrated the tunable optical bistability at terahertz frequencies. It is shown that the hysteretic behavior is strongly dependent on the Fermi energy of graphene, and the threshold electric fields could be correspondingly adjusted with the continuous tuning of Fermi Energy level. It is clear that the bistable thresholds can be lowered dramatically by decreasing the Fermi energy of graphene, at the same time the optical hysteresis width is narrowed. Moreover, we have confirmed that the optical bistability can be tuned by adjusting the incident illumination angle, or by varying the thickness and permittivity of the dielectric slabs. Our contribution might provide a new avenue of fabricating graphene based optical switching device that could even operate at terahertz regime.

Sensitivity Improved SPR Biosensor Based on the MoS2/Graphene–Aluminum Hybrid Structure

MoS₂-graphene-based hybrid structures are biocompatible and useful in the field of biosensors. Herein, we propose a heterostructured MoS₂/aluminum (Al) film/MoS₂/graphene as a highly sensitive surface plasmon resonance (SPR) biosensor based on the Otto configuration. The sensitivity of the proposed biosensor is enhanced by using three methods. First, prisms of different refractive index have been discussed and it is found that sensitivity can be enhanced by using a low refractive index prism. Second, the influence of the thickness of the air layer on the sensitivity is analyzed and the optimal thickness of air is obtained. Finally, the sensitivity improvement and mechanism by using molybdenum disulfide (MoS₂)–graphene hybrid structure is revealed. The maximum sensitivity ∼ 190.83°/RIU is obtained with six layers of MoS₂ coating on both surfaces of Al thin film.

Modulation instability in the oppositely directed coupler with a quadratic nonlinearity

We investigate modulation instability (MI) in a nonlinear oppositely directed coupler with a quadratic nonlinearity, where one channel is made from a positive-index material (PIM) and another channel is fabricated from a negative-index material (NIM), trying to identify the different MI properties from those in a conventional parametric gap system with grating. Both the analytic continuous wave (CW) solutions and dispersion relation are obtained. By using standard linear instability we in detail discuss how the ratio of the backward to forward propagating wave’s power, the phase mismatch, and the coupled coefficient influence the dynamical behavior of MI. Large stable regions are found if the fundamental harmonics (FH) falls in the normal dispersion when compared to the case in the conventional Bragg grating with a quadratic nonlinearity where the CW solutions are unstable in most cases. In addition, we also observe the large stable regimes of the CW solutions even when the coupled strength for the second harmonics (SHs) is weaker than that of the FH. These findings suggest that the oppositely directed coupler with a NIM channel provides more ways to manipulate the MI and soliton.

Low-threshold optical bistability with multilayer graphene-covering Otto configuration

In this paper, we propose a modified Otto configuration to realize tunable and low-threshold optical bistability at terahertz frequencies by attaching multilayer graphene sheets to a nonlinear substrate interface. Our work demonstrates that the threshold of optical bistability can be markedly reduced (three orders of magnitude) by covering the nonlinear substrate with multilayer graphene sheets, due to strong local field enhancement with the excitation of surface plasmons. We present the influences of the Fermi energy of graphene, the incident angle, the thickness of air gap and the relaxation time of graphene on the hysteresis phenomenon and give a way to optimize the surface plasmon resonance, which will enable us to further lower the minimal power requirements for realizing optical bistability due to the strong interaction of light with graphene sheets. These results are promising for realization of terahertz optical switches, optical modulators and logical devices.

Manipulating the optical bistability at terahertz frequency in the Fabry-Perot cavity with graphene

We investigate theoretically the optical bistability from a Fabry-Perot cavity with graphene in the terahertz (THz) frequency. It is demonstrated that the optical bistablility in this cavity can be realized due to the electric field enhancement and the giant third-order nonlinear conductivity of graphene. The optical bistable behavior is strongly dependent on the transmission amplitude of the mirror and the position of the graphene in the cavity. It is especially important that the hysterical behaviors of the transmitted light rely on the optical conductivity of graphene, making the Fabry-Perot cavity to be a good candidate for dynamic tunable optical bistable device in the THz frequencies, owing to the possibility of high tunability of graphene conductivity by means of external electrostatic or magnetostatic field.

Tunable THz Angular/Frequency Filters in the Modified Kretschmann–Raether Configuration With the Insertion of Single Layer Graphene

Tunable terahertz (THz) angular/frequency filters in the modified Kretschmann-Raether configuration with the insertion of single-layer graphene have been numerically demonstrated. Due to the excitation of the transverse magnetic (TM) polarized surface plasmons at the interface of two dielectrics with the insertion of the graphene sheet, the transmission resonance occurs at some range incident angles and frequencies in the THz frequency range, which can be adopted for designing the angular/frequency filters. It is shown that the resonant angle and the resonant wavelength can be tuned by varying the Fermi energy of the graphene sheets via electrostatic biasing. Moreover, we show that the resonant behaviors can be engineered by changing the gap thickness or the incident angle.

Tunable Fano resonances of a graphene/waveguide hybrid structure at mid-infrared wavelength

A planar graphene/dielectric multilayer structure is investigated, where the graphene surface plasmon polariton and the planar waveguide mode are coupled to realize Fano resonances. Few-layer graphene with high doping levels is used to excite surface plasmons at mid-infrared wavelength. Reflectance of the structure is calculated numerically by transfer-matrix method, and tunable Fano resonances with different line shapes are demonstrated by varying doping levels of graphene. Properties of the Fano resonances are discussed qualitatively by calculating electric field distribution in the structure and quantitatively by utilizing an analytical fitting equation. We also calculate Goos-Hänchen shift of the Fano resonances as an example for potential applications, and find that large Goos-Hänchen shift appears for optimized doping levels of graphene.