Dong-Xiang Qi

Multiple-band transmission of acoustic wave through metallic gratings

In this work, we demonstrate that acoustic waves can achieve extremely flat transmission through a metallic grating under oblique incidence within multiple frequency bands separated by Wood’s anomalies. At the low-frequency band, the transmission of acoustic wave is independent of the frequency and presents a flat curve with the transmission efficiency reaching about 100%; while at high-frequency bands, the transmission decreases to be lower flat curves due to the diffraction effect. The transmission efficiency is insensitive to the thickness of the grating. This phenomenon is verified by experiments, numerical simulations, and an analytical model. The broadband high transmission is attributed to the acoustic impedance matching between the air and the grating. This research may open up a field for various potential applications of acoustic gratings, including broadband sonic imaging and screening, grating interferometry, and antireflection cloaking.

Oblique metal gratings transparent for broadband terahertz waves

In this work, we experimentally and theoretically demonstrate that oblique metal gratings with optimal tilt angles can become transparent for broadband terahertz waves under normal incidence. Direct imaging is applied to intuitively prove this broadband transparency phenomenon of structured metals. The transparency is insensitive to the grating thickness due to the non-resonance mechanism, and the optimal tilt angle is determined only by the strip width and the grating period. The oblique metal gratings with broadband transparence may have many potential applications, such as transparent conducting panels, white-beam polarizers, and stealth objects.

Phononic frequency combs through nonlinear resonances.

We explore an analogue of optical frequency combs in driven nonlinear phononic systems, and present a mechanism for generating phononic frequency combs through nonlinear resonances. In the underlying process, a set of phonon modes is simultaneously excited by the external driving which yields frequency combs with an array of discrete and equidistant spectral lines of each nonlinearly excited phonon mode. Frequency combs through nonlinear resonance of different orders are investigated, and in particular the possibility of correlation tailoring in higher-order cases is revealed. We suggest that our results can be applied in various nonlinear acoustic processes, such as phonon harvesting, and can also be generalized to other nonlinear systems.

Broadband enhanced transmission of acoustic waves through serrated metal gratings

In this letter, we have demonstrated that serrated metal gratings, which introduce gradient coatings, can give rise to broadband transmission enhancement of acoustic waves. Here, we have experimentally and theoretically studied the acoustic transmission properties of metal gratings with or without serrated boundaries. The average transmission is obviously enhanced for serrated metal gratings within a wide frequency range, while the Fabry-Perot resonance is significantly suppressed. An effective medium hypothesis with varying acoustic impedance is proposed to analyze the mechanism, which was verified through comparison with finite-element simulation. The serrated boundary supplies gradient mass distribution and gradient normal acoustic impedance, which could efficiently reduce the boundary reflection. Further, by increasing the region of the serrated boundary, we present a broadband high-transmission grating for wide range of incident angle. Our results may have potential applications to broadband acoustic...

Tunable electric and magnetic resonances in multilayered metal/dielectric nanoplates at optical frequencies

In this work, we investigate electromagnetic responses in multilayered Ag/SiO2 nanoplates at optical frequencies. Electric and magnetic resonances, which originate from localized surface plasmons, are demonstrated by the effective permeability and permittivity, electric and magnetic field distributions, and measured transmission at oblique incidence. Furthermore, electric and magnetic resonances can be tailored by the geometrical parameters of the nanoplates. In a rectangular nanoplate, magnetic resonance only shifts with the width along the incident polarization, and electric resonance shifts obviously as the aspect ratio of the nanoplate changes. The investigation may provide a tunable building block for optical metamaterials.

Resonant transmission and mode modulation of acoustic waves in H-shaped metallic gratings

In this work, we demonstrate that resonant full transmission of acoustic waves exists in subwavelength H-shaped metallic gratings, and transmission peaks can be efficiently tuned by adjusting the grating geometry. We investigate this phenomenon through both numerical simulations and theoretical calculations based on rigorous-coupled wave analysis. The transmission peaks are originated from Fabry-Perot resonances together with the couplings between the diffractive wave on the surface and the multiple guided modes in the slits. Moreover, the transmission modes can be efficiently tuned by adjusting the cavity geometry, without changing the grating thickness. The mechanism is analyzed based on an equivalent circuit model and verified by both the theoretical calculations and the numerical simulations. This research has potential application in acoustic-device miniaturization over a wide range of wavelengths.

Multiple Dirac points and perfect transmission in graphene with a dimerlike potential

In this work, we investigate electronic band structures and transport properties in dimerlike graphene superlattices (DGSLs), where the modulated potentials of square barrier A and well B on graphene are arranged as S(m) = (AB)m(BA)m. Here m is the repeated number of units. It is found that the mirror symmetry of the potential distribution on graphene can induce extra Dirac points (DPs), which originates from the dimerlike positional correlations in the system. The induced DPs, which are exactly located at the energy corresponding to zero averaged wave number, do not exist in the periodic graphene superlattices of (AB)m. The number and the position of DPs in the zero averaged wave number gap of DGSL can be manipulated. Correspondingly, multiple perfect transmissions are observed at the resonant modes. Moreover, the conductance for DGSL presents extra resonant peaks accompanying with the emergence of the induced DPs. The investigations may have potential applications in graphene-based electronic devices.

Tunable energy bands and spin filtering in two-dimensional superlattices with spin-orbit interaction

We theoretically investigate the electronic energy bands and spin filtering tuned by Rashba spin-orbit coupling (SOC) and magnetic field in two-dimensional superlattices (2DSLs), where the square rods of quantum barriers, matrix, and wells are imposed periodically. It is shown that electronic energy spectra form a band structure and the energy levels are split up by the Rashba SOC. Correspondingly, the electrical conductance presents a “band-gap” structure against the electron energy. With manipulating the strength of SOC, the conductance in the “band” is enhanced for the spin-up electrons, while it is reduced for the spin-down electrons. Interestingly, by introducing a magnetic modulation, conductance curves for spin-up and spin-down electrons are translated in the different directions. As a result, high spin polarization is observed, and fully spin-polarized conductance is achieved in this 2DSL. Furthermore, the electronic wavefunctions have been obtained, which presents a clear picture of spin filterin...

Magnetic-flux-induced persistent currents in nonlinear mesoscopic rings

We investigate magnetic-flux-induced persistent currents (PCs) in a one-dimensional nonlinear mesoscopic ring based on the Frenkel–Kontorova (FK) model. By applying a transfer-matrix technique, the energy spectra, the PCs, and the Thouless exponent are theoretically obtained. It is shown that the energy spectrum splits into sub-bands when the on-site energy is gradually increased, and in the flux-dependent energy spectra, the energy levels show different behaviors over the transition by breaking of analyticity. Meanwhile, the PC is determined by the magnetic flux, the on-site energy, and the Fermi level. The increment of the on-site energy leads to a dramatic suppression of the PC. When the Fermi level is in the vicinity of “band” gaps, the PC is limited considerably; otherwise, the PC increases by several orders of magnitude. The suppressed PC is related to the electronic localization of the FK ring, which is described by the Thouless exponents. Our investigations provide detailed information about the i...