Olgun Adem Kaya

Refraction-type sonic crystal junction diode

Unidirectional sound transmission across a junction of two square sonic crystals with different orientations and lattice constants is numerically investigated. Re-scaling and rotating the wave vectors through refractions across the air-first sonic crystal interface and the junction, respectively, facilitate coupling into the spatial modes of the second crystal. Unidirectional transmission, demonstrated through finite element method simulations, is accomplished between 10.4 kHz and 12.8 kHz. Transmission values to the right and left are greater than 60% and less than 1.0%, respectively, between 11.0 kHz and 12.4 kHz, resulting in a contrast ratio greater than 0.9.

Refraction-based photonic crystal diode

A system composed of air holes in a dielectric host to form two square photonic crystals, with the same orientation and lattice constant but different scatterer radii, making an interface along their body diagonals, is numerically demonstrated to facilitate unidirectional light transmission. Band structure computations are carried out via the plane wave expansion method, whereas finite-difference time-domain simulations are carried out to investigate the transient behavior. Unidirectional light transmission is achieved over two adjacent stop bands along the ΓX direction, which are circumvented in the forward direction by scaling down the wave vector and rotating the surface normal. Contrast ratios as high as 0.9 are attained within the lower stop band.

Slow sound propagation in a sonic crystal linear waveguide

A linear waveguide along the [11] direction of a triangular sonic crystal, composed of aluminum cylinders in air is shown both experimentally and numerically to facilitate slow sound propagation. Supercell-based calculations through the finite element method reveal a band centered at approximately 16.0 kHz with 255 Hz span, exhibiting linear variation away from band edges, for the lattice constant and cylinder radii of 21.7 mm and 10.0 mm, respectively. The experimental setup is based on monitoring the propagation of a Gaussian-enveloped sinusoidal pulse at 16.0 kHz inside the waveguide. Numerical behavior of the Gaussian pulse is investigated by time-dependent finite-element computations. The experimental and numerical group velocities are found to be 26.7 m/s and 22.6 m/s, respectively. Being congruous with the experimental findings, numerical transient study of the system reveals significant longitudinal compression commensurate with the calculated group index.

Wide-band all-angle acoustic self-collimation by rectangular sonic crystals with elliptical bases

Self-collimation of acoustic waves in the whole angular range of ±90° in the second and third bands of a two-dimensional rectangular sonic crystal with elliptical basis is demonstrated by examining the band structure and equifrequency contours. 70% and 77% of the second and third bands are available for wide-band all-angle self-collimation spanning a bandwidth of approximately 29% and 25% of the central frequencies of the all-angle self-collimation frequency ranges, respectively. Self-collimation of waves over large distances with a small divergence of beam width in the transverse direction is demonstrated through computations based on the finite element method. The second and third bands available for self-collimation are seen to vary linearly in the vast mid-range where a small group velocity dispersion prevents temporal divergence of waves with different frequencies.

Self-collimated slow sound in sonic crystals

Self-collimated slow-sound propagation in a two-dimensional rectangular sonic crystal composed of elliptical scatterers in air is numerically demonstrated. The group velocity at the centre and the edges of the fourth acoustic band is reduced to 45 m s−1 and 30 m s−1, corresponding to 1/8 and 1/12 of the speed of sound in air, respectively. Elimination of omni-directional reflections encountered in linear waveguides and the reduction of group-velocity dispersion at the mid-band frequencies lead to preservation of pulse shape and amplitude upon traversal of the sonic crystal. Wave transmission is increased from approximately −20 to −2.5 dB, with almost an order of magnitude enhancement, via injector layers optimized through a pattern search algorithm. Self-collimating performance of the system is not degraded under oblique incidence, except for pulse broadening due to increased effective source width.

Guiding airborne sound through surface modes of a two-dimensional phononic crystal

Existence and guiding properties of surface modes bound to the interface between a finite two-dimensional phononic crystal and the host medium are experimentally and numerically demonstrated. Surface modes can be observed on both (1 0) and (1 1) surfaces of a square phononic crystal of steel cylinders in air. Numerical investigations of band properties and simulations of mode excitation are carried out through the finite-element method. Excited by the far field of a speaker, existence of surface modes is investigated by recording the sound field in the vicinity of the respective crystal surfaces. Both surface bands of the square phononic crystal depart from bulk bands and extend into the band gap for sufficiently high filling fractions. While such a surface band can be obtained for considerably smaller scatterer radii for the (1 0) surface, significantly higher radii around 0.49 of the lattice constant are required to obtain propagating surface modes on the (1 1) surface. Persistence of the guided surface mode along the (1 0) surface, where it diminishes in a length scale of the lattice constant in the transverse direction is demonstrated. The modes of the (1 1) surface decay faster into the air in the transverse direction. Guided modes on both surfaces propagate in a beating manner where the beat length can be determined by the wave number of the mode.

Low-concentration liquid sensing by an acoustic Mach–Zehnder interferometer in a two-dimensional phononic crystal

Mach–Zehnder interferometer formed by liquid-filled linear defect waveguides in a two-dimensional phononic crystal is numerically realized for sensing low concentrations of an analyte. The waveguides in the square phononic crystal of void cylinders in steel, as well as their T branches and sharp bends are utilized to construct interferometer arms. Sensing low concentrations of ethanol on the order of 0.1% in a binary mixture with water is achieved by replacing the contents of a number of waveguide core cells on one arm of the interferometer with the analyte. Computations are carried out through the finite-element method in an approach that takes the solid-liquid interaction at the waveguide core cells into account. Band analyses reveal linear variation of the central frequency of the transmission band within a band gap for ethanol concentrations up to 3.0%. Phase difference due to the imbalance of the sample and reference arms of the interferometer also varies linearly with ethanol concentration, leading in turn to a cosine variation of the Fourier component of the temporal interferometer response at the central input-pulse frequency. The induced phase difference in the investigated configuration becomes a and per percent increase of ethanol concentration as calculated from the band-structure and transient data, respectively. This is confirmed by transient finite-element simulations where totally destructive interference occurs for a concentration of approximately 1.5%. The proposed scheme, which can easily be adopted to other binary mixtures, offers a compact implementation requiring small amounts of analyte.

Acoustic waveguiding by pliable conduits with axial cross sections as linear waveguides in two-dimensional sonic crystals.

Pliable conduits composed of periodically arranged concentric aluminum tori in air, with their axial cross sections acting as linear waveguides in two-dimensional sonic crystals, are numerically shown to guide acoustic waves in three dimensions in a flexible manner. Waveguide band structures are obtained by exploiting axial symmetry in a super-cell approach through two-dimensional finite-element simulations under the periodic boundary conditions. One isolated band having a bandwidth of 19.66% or 10.10% is observed for each guide, whose cross section is either in square or triangular geometry, respectively. Corresponding mode profiles indicate efficient guiding, as the acoustic energy is mainly concentrated in the hollow-core region of the guides. Transmittance spectra calculated through finite-element simulations are in agreement with the computed guiding bands. Transmittance along the waveguides with square and triangular axial cross sections around mid-band frequencies of their guiding bands varies slightly from -6.05 and -6.65 dB to -5.98 and -8.86 dB, respectively, as the guide length is increased from 10 to 200 periods. Efficient guiding across the smooth bends over circular arcs up to 90 deg is also demonstrated through three-dimensional finite-element method simulations.

Acoustic beam splitting in a sonic crystal around a directional band gap

Beam splitting upon refraction in a triangular sonic crystal composed of aluminum cylinders in air is experimentally and numerically demonstrated to occur due to finite source size, which facilitates circumvention of a directional band gap. Experiments reveal that two distinct beams emerge at crystal output, in agreement with the numerical results obtained through the finite-element method. Beam splitting occurs at sufficiently-small source sizes comparable to lattice periodicity determined by the spatial gap width in reciprocal space. Split beams propagate in equal amplitude, whereas beam splitting is destructed for oblique incidence above a critical incidence angle.

Superprism effect in a deformed triangular sonic crystal

The superprism effect in a two-dimensional sonic crystal composed of lead cylinders in water aligned on a lattice obtained by varying the angle between the primitive vectors of triangular lattice is numerically investigated. Symmetry breaking influences the equi-frequency contours to reflect the lattice symmetry, so that compression along a direction leads to smaller critical angles of incidence. The whole 0°–90° range is spanned by the refracted waves at the water/sonic crystal interface for frequencies between 165 and 183 kHz, in the third band, and angles of incidence between 0° and 15°. The studied superprism behaviour can be used to achieve both spectral and angular resolution. The refraction angle varies linearly for small angles of incidence below 3° at a constant frequency, while its frequency dependence at a given angle of incidence is quadratic for small frequencies. Finite-element computations reveal that waves are refracted into the angles calculated from the equi-frequency contours with small beam divergence at any frequencies and angles of incidence.