Institut des
NanoSciences de Paris
Accueil > Evénements > Séminaires > Archives 2010 > Imaging of 3D acoustic

Séminaire Nanostructures et systèmes quantiques

Imaging of 3D acoustic wave-fronts by means of picosecond laser ultrasonics : towards a SONAR dedicated to nanosciences

par : Emmanuel Péronne

Picosecond laser ultrasonics is a well-spread technique in solid state physics and semiconductor industry. It is basically a pump-probe technique where the laser pulse (pump) launches acoustic longitudinal pulses into the sample. Subsequent reflections of acoustic pulses are then detected by measuring the time dependence of the sample optical properties with a laser pulse (probe). It is known to be very efficient to measure surface layer thicknesses with nanometric accuracy. Recently, experimental improvements have shed new lights on the technique in such a way that a SONAR dedicated to nano-systems seems achievable. In this presentation an overview of the technique and its capabilities will be given. One of the main issues in nano-imaging is to get an acoustic emitter combining high GHz frequency and broad wave-vector distribution. In the frame of laser ultrasonics, we will discuss how such an emitter is achievable. Experimental performances and limitations will be presented through the study of sound propagation and diffraction in semiconductor substrates. When ultrafast acoustic waves propagate in crystals, acoustic waves can be either transverse or longitudinal. Moreover dispersion and non linear effects occur during the propagation leading to intriguing features such as acoustic solitons. The discussion will be divided in two sections. The first one will be devoted to bulk acoustic wave imaging at room temperature showing how transverse waves give rise to phonon focusing patterns. The second part will focus on how laser ultrasonics open new ways in producing ultra high acoustic frequency generation up to 1 THz by means of non linear effects or nano-emitters such as super-lattices and quantum dot.