Zhaohong Wang

Flexible optical waveguides based on the omnidirectional reflection of one-dimensional photonic crystals

An optical wave guiding structure based on the omnidirectional reflection of one-dimensional photonic crystals is presented. The dispersion and transmission characteristics of flexible waveguides using this structure are investigated by numerical simulations using plane wave expansion method and finite-difference time-domain method, respectively. Calculated results indicate that for waveguide bends of arbitrary angles with a curved radius less than several wavelengths, a very high transmission (>97%) over wide frequency range is observed. Owing to this unique advantage of flexibility, it is expected to be applied to highly dense photonic integrated circuits after further research.

Pentamode metamaterials with asymmetric double-cone elements

Pentamode metamaterials are a very interesting set of artificial solids. They are difficult to compress but flow easily, hence imitating somehow the behaviours of liquids. In this paper, three-dimensional (3D) pentamode metamaterials based on the asymmetric double-cone element (ADCE) are proposed. The ADCE is composed by two connected truncated cones with different thin diameters. The phonon band structures of the ADCEs pentamode metamaterials are numerically analysed by using the finite element method (FEM). Besides the single phonon mode, the complete 3D band gaps are also found in the phonon band structures. Here, the influence of the degree of asymmetry of ADCE to the figure of merit (FOM), which is the ratio of bulk modulus and shear modulus, is discussed. The FOM can be increased by 21–30% (at FOM ≥ 103) when the degree of asymmetry m ranges from 0.4 to 0.6.

Field analysis and calculation of interdigital transducers with arbitrary finger shapes

The interdigital transducer (IDT) is a key component in surface-acoustic wave (SAW) and acousto-optical devices and has extensive applications in signal processing and optical communication at present. Properties of the acoustic field are mainly dominated by the geometric shapes of the electrodes (i.e. fingers) of the IDT and the piezoelectric characteristics of the substrate. However, the studies on excitation and acoustic wave field characteristics of IDTs are still not matured or perfect. In this paper, the 2D interface Greens function method (GFM) for simulating numerically the SAW field of IDTs with arbitrary finger shapes is presented. The electric charge densities originated from the electrostatic field and the generated SAW on the IDT electrodes are calculated first using the 2D interface GFM. Then the charge density distribution as the distributed source of SAW used to calculate the SAW field. IDT with arbitrary finger shapes can be treated. As an example, the properties of a SAW field generated by focused IDTs with the shape of concentric circular arc fingers on Y–Z LiNbO3 and c-oriented PZT substrates are also analysed and discussed. The method can be applied to numerical analysis of IDTs with arbitrary finger shapes, such as broadband chirp transducers, curved-finger transducers and finger-length weighted transducers.

Omnidirectional Reflection Extension in a One-Dimensional Superconducting-Dielectric Binary Graded Photonic Crystal with Graded Geometric Layers Thicknesses

The omnidirectional re∞ection characteristics of one- dimensional (1D) superconducting-dielectric binary graded photonic crystals (PhCs) are studied by using transfer matrix method. The in∞uences of thickness changing rate, numbers of periods, incident angles are analyzed. And the omnidirectional photonic band gaps are extended markedly in the 1D thickness-graded superconducting- dielectric PhC.

Shear-horizontal acoustic wave propagation in piezoelectric bounded plates with metal gratings

In this paper, shear-horizontal (SH) acoustic wave propagation in metal gratings deposited on piezoelectric bounded plates is investigated. The spectral characteristics of the electromechanical coupling coefficient are studied first, which are very important for acoustic wave device designs. And, an effective mathematic method based on even- and odd base functions is also presented for overcoming the large frequency thickness product problem. Then, the characteristics of the grating modes are studied, and the nature and characteristics of the stop bands are investigated fully. The results show that the width and attenuation of the stop bands are dominated by the electromechanical coupling coefficient at the frequency centers of the stop bands.

Electro-optic thin films of organic nonlinear optic molecules aligned through vacuum deposition

Nonlinear optical molecules can be vacuum deposited into uniform thin films using thermal evaporation. Alignment order can be achieved during thin film deposition by an in-plane electrical field poling using electrodes patterned on the substrate. Electro-optic (EO) coefficients, r33 and r13 are independently measured using Youngs interferometry technique. Thin-films of N-benzyl-2-methyl-4-nitroaniline (BNA) can exhibit an EO coefficient, r33, comparable to that of BNA single crystals. EO coefficients of BNA at different poling fields, wavelengths, and frequencies are investigated.

The Bloch Theorem Generalized for Semi-Infinitely Periodic Systems with Free Surface

In this paper, we present the Bloch theorem generalized for semi-infinitely periodic systems with free surface. For these structures the conventional Bloch theorem is effective only for some special modes, there exist more modes determined by the generalized Bloch theorem. As an example, the propagation characteristics of shear horizontal acoustic waves in these structures are investigated by this generalized theorem. In the semi-infinitely periodic systems, we also find that in a same dispersion curve there are gaps, and surface and bulk waves can be transformed into each other.

Surface electrical charge and field analysis of single-electrode type interdigital transducers with bus-bar

A single-electrode type interdigital transducer (SIDT) is extensively used to excite surface acoustic waves (SAWs) on piezoelectric substrates because of its simple structure and fabrication process. In a SIDT a bus-bar is used for feeding in electrical voltage. However, systematic analyses about bus-bar effects of SIDT are still sparse among the existing studies concerning the propagation of acoustic wave of SIDT. In this paper, a two-dimensional interface Greens function method is adopted for calculating the electrical charge density on the electrode surfaces and acoustic wave fields in a Y-cut Z-propagation LiNbO3 substrate excited by SIDTs with a bus-bar. The computation results show that the charge density associated with SAW cannot be neglected; the bus-bar has essential effects on the excited SAW fields and the SAW field generally diverges strongly due to the influence of the bus-bar. Approaches for reducing this effect are discussed.

Composite pentamode metamaterials with low frequency locally resonant characteristics

Locally resonant phononic crystals have excellent low-frequency characteristics and can result in a small size controlling a long wavelength. Based on the locally resonant phononic crystal theory, locally resonant pentamode metamaterials with composite materials are proposed and studied. The acoustic band frequencies of the locally resonant pentamode metamaterials were found to be two orders of magnitude smaller than those of conventional Bragg-Scattering pentamode metamaterials with the same lattice constant. Therefore, the single-mode frequency regions and locally resonant acoustic bandgaps which are under 110 Hz can be obtained by locally resonant pentamode metamaterials with a centimeter-scale lattice constant. The figure of merit can be increased by 60.78%–138.08%. This kind of locally resonant pentamode metamaterial opens a new path for applications in low-frequency acoustic cloaking and control of long wavelengths using small structure dimensions.

Pentamode metamaterials with tunable acoustics band gaps and large figures of merit

In this paper, we propose a class of pentamode metamaterials for which the frequency range of the acoustics band gaps can be tuned and large figures of merit can be obtained. The band structures of the pentamode metamaterials are calculated systematically by using the finite element method. The numerical results show that the lower edge frequency of the first acoustics band gaps of pentamode metamaterials can be tuned between 3.72 kHz and 10.6 kHz by changing the diameters of the bottom and top touch cones slightly, and the relative bandwidth of the first acoustics band gaps can also be expanded. In addition, compared with the results seen in the previous research in this area, the volume filling fraction of pentamode metamaterials can be decreased by 15.7%–24.4% and the maximum figure of merit can be increased by 39.2%.