Sarah Benchabane

Guiding and bending of acoustic waves in highly confined phononic crystal waveguides

We demonstrate experimentally the guiding and the bending of acoustic waves in highly confined waveguides formed by removing rods from a periodic two-dimensional lattice of steel cylinders immersed in water. Full transmission is observed for a one-period-wide straight waveguide within the full band gap of the perfect phononic crystal. However, when the waveguide width is doubled, destructive interference causes the transmission to vanish in the center of the passband. Waveguiding over a wide frequency range is obtained for a one-period-wide waveguide with two sharp 90° bends. Finite-difference time-domain computations are found to be in good agreement with the measurements.

Simultaneous existence of phononic and photonic band gaps in periodic crystal slabs

We discuss the simultaneous existence of phononic and photonic band gaps in a periodic array of holes drilled in a Si membrane. We investigate in detail both the centered square lattice and the boron nitride (BN) lattice with two atoms per unit cell which include the simple square, triangular and honeycomb lattices as particular cases. We show that complete phononic and photonic band gaps can be obtained from the honeycomb lattice as well as BN lattices close to honeycomb. Otherwise, all investigated structures present the possibility of a complete phononic gap together with a photonic band gap of a given symmetry, odd or even, depending on the geometrical parameters.

Tailoring simultaneous photonic and phononic band gaps

The simultaneous existence of photonic and phononic band gaps opens up many possibilities for enhancing acousto-optical interactions at a common wavelength scale. We term such structures phoxonic crystals. By computing the existence and dependence of phoxonic band gaps on the choice of lattice and unit cell, we obtain a hierarchy of two-dimensional phoxonic crystal structures. The single-atom hexagonal and square lattices, and some multiple-atom hexagonal lattices, including honeycomb and heterometric lattices, are investigated. For definiteness, arrays of air holes in lithium niobate are considered in the computations. It is observed that decreasing the symmetry of the lattice by adding atoms of different sizes inside the unit cell leads to larger phoxonic band gaps. Examples of designs for operation at an optical wavelength of 1550 nm are given. The corresponding phononic frequencies are in the gigahertz range.

Simultaneous guidance of slow photons and slow acoustic phonons in silicon phoxonic crystal slabs

We demonstrate theoretically that photons and acoustic phonons can be simultaneously guided and slowed down in specially designed nanostructures. Phoxonic crystal waveguides presenting simultaneous phononic and photonic band gaps were designed in perforated silicon membranes that can be conveniently obtained using silicon-on-insulator technology. Geometrical parameters for simultaneous photonic and phononic band gaps were first chosen for optical wavelengths around 1550 nm, based on the finite element analysis of a perfect phoxonic crystal of circular holes. A plain core waveguide was then defined, and simultaneous slow light and elastic guided modes were identified for some waveguide width. Joint guidance of light and elastic waves is predicted with group velocities as low as c/25 and 180 m/s, respectively.

Acoustic channel drop tunneling in a phononic crystal

We study both theoretically and experimentally the possibility of resonant tunneling of acoustic waves between two parallel guides created in a phononic crystal composed of steel cylinders in water. In the absolute band gap of the phononic crystal, ranging from 250to325kHz, a full transmission band exists for propagation inside a straight waveguide. We show that the transfer of a particular wavelength can occur between two parallel waveguides coupled together through an appropriate coupling structure. The latter is composed of isolated cavities interacting with stubs located at the sides of the waveguides.

Optical in-well pumping of a vertical-external-cavity surface-emitting laser

A scheme is demonstrated for optical pumping of a vertical-external-cavity surface-emitting laser. The scheme is based on absorption of the pump light within the wells of the multiple-quantum-well gain structure rather than the conventional approach of absorption of a shorter wavelength in the barrier regions. The operation of a laser around 850 nm pumped by an 808 nm source demonstrates the potential of this technique for allowing operation at a significantly shorter range of wavelengths for these devices in general and specific application of high-brightness pump lasers for devices in this spectral region. A further advantage is the smaller quantum defect which results in reduced heating of the gain medium. These advantages are achieved while maintaining a slope efficiency of up to 18%, which is comparable to results obtained with a traditional pumping scheme with a similar gain medium.

Scattering of surface acoustic waves by a phononic crystal revealed by heterodyne interferometry

Surface acoustic wave propagation within a two-dimensional phononic band gap structure has been studied using a heterodyne laser interferometer. Acoustic waves are launched by interdigital transducers towards a square lattice of holes etched in a piezoelectric medium. Interferometer measurements performed at frequencies lying below, within, and above the expected band gap frequency range provide direct information of the wave interaction with the phononic crystal, revealing anisotropic scattering into higher diffraction orders depending on the apparent grating pitch at the boundary between the phononic crystal and free surface. Furthermore, the measurements also confirm the existence of an elastic band gap, in accordance with previous electrical measurements and theoretical predictions.

Acousto-optically tunable lithium niobate photonic crystal

We report on an active two-dimensional lithium niobate photonic crystal (PhC) driven by stationary Rayleigh surface acoustic waves. The configuration relies on two interdigital transducers that modulate the refractive index through the acousto-optical effect. Highly efficient, compact acousto-optical PhCs with an active length of only 13 μm and a driving electrical power of 20 mW have been fabricated and characterized. Experiments show that an enhancement factor of the elasto-optical interaction of the order of 61 is obtained thanks to slow light effects in the PhC.

Observation of surface-guided waves in holey hypersonic phononic crystal

We observe experimentally the propagation of surface-guided waves in a hypersonic phononic crystal, both in the radiative and nonradiative regions of the spectrum. Combining electrical measurements in reflection and transmission as well as optical maps of the surface displacement, a band gap extending from 0.6 to 0.95 GHz is identified in a square lattice array of 1 μm radius air holes milled in lithium niobate. The optical measurements reveal the transmission of surface-guided waves above the band gap, well inside the sound cone.

Band gaps and cavity modes in dual phononic and photonic strip waveguides

We discuss theoretically the simultaneous existence of phoxonic, i.e., dual phononic and photonic, band gaps in a periodic silicon strip waveguide. The unit-cell of this one-dimensional waveguide contains a hole in the middle and two symmetric stubs on the sides. Indeed, stubs and holes are respectively favorable for creating a phononic and a photonic band gap. Appropriate geometrical parameters allow us to obtain a complete phononic gap together with a photonic gap of a given polarization and symmetry. The insertion of a cavity inside the perfect structure provides simultaneous confinement of acoustic and optical waves suitable to enhance the phonon-photon interaction.