Vincent Laude

Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter: pulse compression and shaping

We demonstrate experimentally that an arbitrary phase and amplitude profile can be applied to an ultrashort pulse by use of an acousto-optic programmable dispersive filter (AOPDF). Our filter has a large group-delay range that extends over 3 ps and a 30% diffraction efficiency over 150 THz. Experiments were conducted on a kilohertz chirped-pulse amplification laser chain capable of generating 30-fs pulses without additional pulse shaping. Compensating for gain narrowing and residual phase errors with an AOPDF in place of the stretcher results in 17-fs transform-limited pulses. Arbitrary shaping of these 17-fs pulses is also demonstrated in both the temporal and the spectral domains.

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.

A full 3D plane-wave-expansion model for 1-3 piezoelectric composite structures

The plane-wave-expansion (PWE) approach dedicated to the simulation of periodic devices has been extended to 1-3 connectivity piezoelectric composite structures. The case of simple but actual piezoelectric composite structures is addressed, taking piezoelectricity, acoustic losses, and electrical excitation conditions rigorously into account. The material distribution is represented by using a bidimensional Fourier series and the electromechanical response is simulated using a Bloch-Floquet expansion together with the Fahmy-Adler formulation of the Christoffel problem. Application of the model to 1-3 connectivity piezoelectric composites is reported and compared to previously published analyses of this problem.

Arbitrary dispersion control of ultrashort optical pulses with acoustic waves

Acousto-optic programmable dispersive filters (AOPDF) can compensate in real time for large amounts of group-delay dispersion. This feature can be used in chirped-pulse amplification femtosecond laser chains to compensate adaptively for dispersion. An analytical expression relating the group delay at the output of the AOPDF to the input acoustic signal is obtained with coupled-wave theory in the case of collinear and quasi-collinear bulk acousto-optic interactions and also in the case of planar waveguides and optical fibers. With this relation, the acoustic signal that will induce an arbitrary group-delay variation with frequency can be easily obtained. Numerical simulations are shown to support the principle of arbitrary group-delay control with an AOPDF.

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.

Stable scattering-matrix method for surface acoustic waves in piezoelectric multilayers

A scattering matrix approach is proposed to avoid numerical instabilities arising with the classical transfer matrix method when analyzing the propagation of plane surface acoustic waves in piezoelectric multilayers. The method is stable whatever the thickness of the layers, and the frequency or the slowness of the waves. The computation of the Green’s function and of the effective permittivity of the multilayer is outlined. In addition, the method can be easily extended to the case of interface acoustic waves.

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.

Twisted-nematic liquid-crystal pixelated active lens

Liquid-crystal active lenses are classical imaging systems in which a liquid-crystal spatial light modulator is placed in a pupil plane. The modulator controls the phase of the optical wave in the pupil and achieves an arbitrary wavefront shape, that is limited only by the available modulation depth and resolution. The possibility of using a twisted-nematic liquid-crystal television in the pupil plane is investigated both experimentally and theoretically. The precise evaluation of the modulation characteristics is first discussed. Experimental results obtained both with monochromatic and polychromatic light illustrate the different sampling, quantization and chromatism effects discussed theoretically. Useful models for the computation of the point spread function and the modulation transfer function of a generic pixelated liquid-crystal active imaging system are presented for both in-focus and out-of-focus systems, and their numerical implementation is discussed.

Design of coupled resonator filters using admittance and scattering matrices

The longitudinal coupling of thin film bulk acoustic resonator (FBAR) is investigated in order to design coupled resonator filter (CRF). Such devices are dedicated to high-frequency filter applications. A scattering matrix algorithm that was already used to simulate simple FBARs devices is used to study the behavior of these structures. The principle of the algorithm is recalled and is explained how it can be adapted to the simulation of multiport devices. Then an example of filter design is given.

Scattering matrix method for modeling acoustic waves in piezoelectric, fluid, and metallic multilayers

Many ultrasonic devices, among which are surface and bulk acoustic wave devices and ultrasonic transducers, are based on multilayers of heterogeneous materials, i.e., piezoelectrics, dielectrics, metals, and conducting or insulating fluids. We introduce metal and fluid layers and half spaces into a numerically stable scattering matrix model originally proposed for solving the problem of plane wave propagation in piezoelectric and dielectric multilayers. The method is stable for arbitrary thicknesses of the layers. We discuss how the surface Green’s functions can be computed for an arbitrary stack of homogeneous materials with plane interfaces. Aditionnally, we set up a backscattering algorithm to compute the distribution of electromechanical fields at any point in the stack. The model is assessed by considering some well-known examples.