Naima Khelfaoui

Subwavelength focusing of surface acoustic waves generated by an annular interdigital transducer

We propose and demonstrate experimentally the concept of the annular interdigital transducer that focuses acoustic waves on the surface of a piezoelectric material to a single, diffraction-limited, spot. The shape of the transducing fingers follows the wave surface. Experiments conducted on lithium niobate substrates evidence that the generated surface waves converge to the center of the transducer, producing a spot that shows a large concentration of acoustic energy. This concept is of practical significance to design new intense microacoustic sources, for instance for enhanced acouto-optical interactions.We propose and demonstrate experimentally the concept of the annular interdigital transducer that focuses acoustic waves on the surface of a piezoelectric material to a single, diffraction-limited spot. The shape of the transducing fingers follows the wave surface. Experiments conducted on lithium niobate substrates evidence that the generated surface waves converge to the center of the transducer, producing a spot that shows a large concentration of acoustic energy. This concept is of practical significance to design new intense microacoustic sources, for instance for enhanced acousto-optical interactions.

Temporal behavior of two-wave-mixing in photorefractive InP:Fe versus temperature

The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc electric field at different temperatures has been studied. In particular, the temperature dependence of the characteristic time constant has been studied both theoretically and experimentally, showing a strongly decreasing time constant with increasing temperature.

Fast self-induced waveguides in photorefractive semiconductor InP:Fe for reconfigurable optical switching

This paper presents a theoretical and experimental investigation of the self-focusing of a single infrared laser beam in the photorefractive semi-conductor InP : Fe for applications in the telecommunications wavelengths as reconfigurable optical switching. The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc electric field at different temperatures has been studied showing that the temperature as well as the intensity can be used to tune the photorefractive response time. In that way, we analyze both experimentally and theoretically the space-charge field build-up with respect to time and space and provide strong hints on the short time self-focusing of an infrared laser beam in InP:Fe with time response in the range of the microseconds.

Fast photorefractive self focusing in InP:Fe semiconductor at near infrared wavelengths

Self-trapping of optical beams in photorefractive (PR) materials at telecommunications wavelengths has been studied at steady state in insulators such as SBN [1] and in semiconductor InP:Fe [2], CdTe [3]. PR self-focusing and soliton interactions in semiconductors find interesting applications in optical communications such as optical routing and interconnections because of several advantages over insulators: their sensitivity to near-infrared wavelengths and shorter response time. Photorefractive self focusing in InP:Fe is characterized as a function of beam intensity and temperature. Transient self focusing is found to occur on two time scales for input intensities of tens of W/cm2 (one on the order of tens of μs, one on the order of milliseconds). A theory developed describes the photorefractive self focusing in InP:Fe and confirmed by steady state and transient regime measurements. PR associated phenomena (bending and self focusing) are taking place in InP:Fe as fast as a μs for intensities on the order of 10W/cm2 at 1.06 μm. Currently we are conducting more experiments in order to estimate the self focusing response time at 1.55μm, to clarify the temporal dynamic of the self focusing and to build up a demonstrator of fast optical routing by photorefractive spatial solitons interactions.

P4L-3 Anisotropic Wave-Surface Shaped Annular Interdigital Transducer

Interdigital transducers (IDT) are widely used to generate surface acoustic waves directly on piezoelectric materials. However, in most applications, the generating fingers are straight, giving rise to the emission of plane waves. One notable exception is the circular IDT proposed by Day and Koerber for isotropic substrates [IEEE Trans. Sonics and Ultrason. SU-18, 461 (1972)]. More recently, the focused interdigital transducer (FIDT) has been used to obtain high intensity generation at the focal spot. The FIDT uses surface wave emission inside a circular arc for concentrating acoustic energy at its focus. However, the anisotropy of the substrate can lead to aberrations at the focal point. We investigate the problem of constructing an extended source that will focus elastic energy to a single point on the surface of a piezoelectric crystal. On the surface of a piezoelectric solid that is mechanically excited at a single point, concentric waves originate and form in the far field a ripple pattern that follows the shape of the wave surface, obtained by plotting the group velocity as a function of the emission angle. We conversely propose the concept of an annular interdigital transducer (AIDT), in which the shape of the fingers follows the wave surface. The surface acoustic waves generated by an AIDT are expected to converge to the center of the transducer, producing a spot that is limited in resolution by diffraction only. Experiments have been conducted on Y and Z cut lithium niobate (LiNbO3). AIDTs operating at a resonance frequency of 75 MHz have been constructed. Electrical measurements show that despite anisotropy in-phase emission at all angles is obtained for Rayleigh waves. In addition, spatial maps of the displacements at the surface have been obtained using a heterodyne optical probe, showing an important focusing of surface acoustic waves in the center of the device. The measured displacement fields at resonance show surface ripples converging to a spot at the center of the transducer. This result is promising for several applications including intense microacoustic sources.

Fast photorefractive self-focusing in InP:Fe semi-conductor at infrared wavelengths

The fast self-trapping behaviour of an infrared beam in photorefractive InP:Fe is studied experimentally and theoretically versus the intensity. The laser is shown to be self-focused in less than a millisecond at telecommunications intensities.

Fast photorefractive self-focusing in semiconductors

Photorefractive (PR) spatial soliton propagation hints that all optical routing can be achieved through soliton interactions. This requires, however, fast build up and sensitivity to telecommunication wavelengths. We have investigated the build up of infrared (1,06m) photorefractive solitons in iron doped indium phosphide (InP:Fe) and shown that PR self focusing occurs at input powers of hundreds of W and intensities in the range of W/cm2, showing a build up time down to the microsecond.

Temporal Behavior of Photorefractive Self-Focusing in InP:Fe Crystals at Infrared Wavelengths

Temporal and spatial dependency of photorefractive self focusing in InP:Fe from intensity and temperature is compared to a theoretical model; self focusing and bending occur on a microseconds timescale for low beam intensities.

Theoretical and Experimental Temporal Self-Focusing Studies in Photorefractive InP:Fe at Telecommunication Wavelengths

We propose a theoretical and experimental analysis of the temporal self focusing phenomena in InP:Fe semiconductor for low irradiations of continuous laser beams at infrared wavelengths for optical telecommunication applications.