Tong-Biao Wang

Enhanced absorption of monolayer MoS2 with resonant back reflector

The optical absorption of monolayer MoS2 on top of one-dimensional photonic crystal (1DPC) or metal films with spacer layers is theoretically investigated by extracting the permittivity of monolayer MoS2 from existing experimental results [K. F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010)]. The absorption of graphene with 1DPC across a broad spectral range is substantially enhanced because of the photonic localization at the optical micro-cavity on top of the 1DPC or metal films. The absorption of monolayer MoS2 can be tuned by varying either the distance between the monolayer MoS2 and the back reflector or the thickness of the cover layers.

Tunable THz absorption in graphene-based heterostructures

We investigate THz absorption properties of graphene-based heterostructures by using characteristics matrix method based on conductivity. We demonstrate that the proposed structure can lead to perfect THz absorption because of strong photon localization in the defect layer of the heterostructure. The THz absorption may be tuned continuously from 0 to 100% by controlling the chemical potential through a gate voltage. By adjusting the incident angle or the period number of the two PCs with respect to the graphene layer, one can tailor the maximum THz absorption value. The position of the THz absorption peaks can be tuned by changing either the center wavelength or the thicknesses ratio of the layers constituting the heterostructure. Our proposal may have potentially important applications in optoelectronic devices.

Simultaneous large band gaps and localization of electromagnetic and elastic waves in defect-free quasicrystals.

We report numerically large and complete photonic and phononic band gaps that simultaneously exist in eight-fold phoxonic quasicrystals (PhXQCs). PhXQCs can possess simultaneous photonic and phononic band gaps over a wide range of geometric parameters. Abundant localized modes can be achieved in defect-free PhXQCs for all photonic and phononic polarizations. These defect-free localized modes exhibit multiform spatial distributions and can confine simultaneously electromagnetic and elastic waves in a large area, thereby providing rich selectivity and enlarging the interaction space of optical and elastic waves. The simulated results based on finite element method show that quasiperiodic structures formed of both solid rods in air and holes in solid materials can simultaneously confine and tailor electromagnetic and elastic waves; these structures showed advantages over the periodic counterparts.

Acoustic multimode interference and self-imaging phenomena realized in multimodal phononic crystal waveguides

We report an acoustic multimode interference effect and self-imaging phenomena in an acoustic multimode waveguide system which consists of M parallel phononic crystal waveguides (M-PnCWs). Results show that the self-imaging principle remains applicable for acoustic waveguides just as it does for optical multimode waveguides. To achieve the dispersions and replicas of the input acoustic waves produced along the propagation direction, we performed the finite element method on M-PnCWs, which support M guided modes within the target frequency range. The simulation results show that single images (including direct and mirrored images) and N-fold images (N is an integer) are identified along the propagation direction with asymmetric and symmetric incidence discussed separately. The simulated positions of the replicas agree well with the calculated values that are theoretically decided by self-imaging conditions based on the guided mode propagation analysis. Moreover, the potential applications based on this self-imaging effect for acoustic wavelength de-multiplexing and beam splitting in the acoustic field are also presented.

Novel 1 × N ultrasonic power splitters based on self-imaging effect of phononic crystal waveguide arrays

We present an appropriate design and simulated results of novel 1 × N (N represents an integer larger than 1) ultrasonic power splitters based on self-imaging effect with symmetric interference of phononic crystal waveguide arrays. Such sonic devices with two and three output channels are discussed in detail as examples. The finite element method is used to calculate the distribution of total displacement field and evaluate the efficiency of these structures. Results show that these devices exhibit new and interesting characteristics, such as compact size, wide bandwidth, and high-transmission. The approach provides a novel method and compact model for exporting freely ultrasonic waves to N channels and can present practical applications in future acoustic wave circuits.

Tunable plasmonic band gap and defect mode in one-dimensional photonic crystal covered with graphene

We study the transmission characteristics of surface plasmons at the interface of a one-dimensional photonic crystal (1DPC) and a monolayer graphene sheet. The composite structure with a plasmonic band gap (PGB) was obtained by covering a graphene sheet on the lateral side of the 1DPC. Defect modes in these structures appeared when defect layers were inserted in the 1DPC. On the condition that the chemical potential of graphene is much larger than , the PBG and defect mode shift to higher frequencies as the chemical potential increases. As the carrier density in graphene can be controlled by the gate voltage, both the PBG and defect mode can be easily tuned by the chemical potential, which is determined by the carrier density.

Simultaneous localization of photons and phonons within the transparency bands of LiNbO 3 phoxonic quasicrystals.

We report the properties of dual phononic-photonic band gaps and localized modes of eightfold lithium niobate (LiNbO3) phoxonic quasicrystals (PhXQCs). Complete and large phoxonic band gaps are easily achieved despite the low refractive index of LiNbO3 substrate. Point defect intentionally introduced can form localized modes within both forbidden and transparency bands over a wide range of geometric parameters. Further analysis indicates that the localized modes within transparency bands originate from the intrinsic high-order rotational symmetry of quasiperiodic structures, which resemble whispering gallery modes. LiNbO3 PhXQCs provide a good candidate to enhance phononic-photonic interaction and show considerable advantage over the periodic counterparts.

Enhancement of near-field radiative heat transfer via multiple coupling of surface waves with graphene plasmon

Coated silicon carbide (SiC) thin films can efficiently enhance near-field radiative heat transfer among metamaterials. In this study, the near-field heat transfer among graphene–SiC–metamaterial (GSM) multilayer structures was theoretically investigated. Graphene plasmons could be coupled both with electric surface plasmons supported by the metamaterial and with symmetric and anti-symmetric surface phonon polaritons (SPhPs) supported by SiC. The heat transfer among GSM structures was considerably improved compared to that among SiC-coated metamaterials when the chemical potential of graphene was not very high. In addition, the near-field heat transfer was enhanced among SiC–graphene–metamaterial multilayer structures, though the heat transfer among these structures was less than that among GSMs owing to the absence of coupling between symmetric SPhPs and graphene plasmons. Hence, heat transfer could be flexibly tuned by modifying the chemical potential of graphene in both configurations. These results pr...

Contribution of terahertz waves to near-field radiative heat transfer between graphene-based hyperbolic metamaterials

Hyperbolic metamaterials alternately stacked by graphene and silicon (Si) are proposed and theoretically studied to investigate the contribution of terahertz (THz) waves to near-field radiative transfer. The results show that the heat transfer coefficient can be enhanced several times in a certain THz frequency range compared with that between graphene-covered Si bulks because of the presence of a continuum of hyperbolic modes. Moreover, the radiative heat transfer can also be enhanced remarkably for the proposed structure even in the whole THz range. The hyperbolic dispersion of the graphene-based hyperbolic metamaterial can be tuned by varying the chemical potential or the thickness of Si, with the tunability of optical conductivity and the chemical potential of graphene fixed. We also demonstrate that the radiative heat transfer can be actively controlled in the THz frequency range.

Dependence of the sliding distance of a one-dimensional atom chain on initial velocity

In our daily lives, a body with a high initial velocity sliding freely on a rough surface moves a longer distance than that with a low initial velocity. However, such a phenomenon may not occur in the microscopic world. The dynamical behavior of a one-dimensional atom chain (1DAC) sliding on a substrate is investigated in this study by using a modified Frenkel–Kontorova model, in which the vibration of atoms on the substrate is considered. The dependence of sliding distance on initial velocity is examined. Result shows that although sliding distance is proportional to the initial value for most velocities, such a linear relation does not exist in some special velocities. This phenomenon is explained by a theoretical analysis of phonon excitation. The physical process is divided into three stages. The first stage is a superlubric sliding process with small amplitude of the vibrication of the atoms. The single-mode phonon is excited in the second stage. In the third stage, the system exhibits instability because of multiple-mode phonon excitations. In addition, the dependence of the coupling strength between 1DAC and the substrate is investigated. The findings are helpful in understanding the energy dissipation mechanism of friction.