Ultrafast Photostriction in Ferroelectric Domains, Walls and Nanostructures – UP-DOWN

UP-DOWN project aims at laying the basis for the design of next generation nano-transducers and mechanical actuators based on ferroelectric materials controlled by light. For this, the mechanism of ultra-fast photostriction (control of the deformation by light) will be studied from the fundamental point of view in typical ferroelectrics, then used to develop first micro-nano-devices. In particular, we propose to control the mechanical deformation over an extended frequency range up to 100GHz-1THz. This frequency goes far beyond the current limit frequency of mechanical transducers/actuators which is in the MHz range (because of the use of an electrical stimulus that is intrinsically limited in frequency by electronic technology). In this project, we will show that short pulses of light can excite ferroelectric materials in the GHz-THz regime without any electrical contact. However, to ensure the efficiency of the process a fine understanding of the mechanisms of photostriction is necessary, both in the ferroelectric domains and domain walls, features that are currently absent in the literature. In addition, the use of small gap ferroelectric materials would optimize the performance of photostrictive processes. Thus, our project is naturally structured in 3 axes (3 WorkPackages). Because of its fundamental aspect and the aim to demonstrate a proof-of-concept through first prototypes, our project is at level 3 in the TRL-Technologies Readiness Level scale.

The first WP concerns the understanding of the role of domains and domain walls in the inverse piezoelectric process that drives photostriction. This inverse piezoelectric effect is obtained by rapid transient screening of the depolarization field (screening induced by electron-hole pairs excited optically by laser pulse). For this we will conduct a systematic study of the photostrictive response in different ferroelectrics known and mastered by the partners (BiFeO3, Pb(Zr,Ti)O3, BaTiO3) whose domains (size, density, orientation) will be controlled by PiezoForce Microscopy (PFM) or by epitaxial strain. We will study the GHz-THz acoustic wave emission of polarization LA (longitudinal), TA (transverse) or surface (SAW), and quantify photo-induced strains at the atomic scale by combining optical and X-ray diffraction time-resolved experiments (pump-probe).

The second WP aims to propose new photostrictive ferroelectric materials more efficient than those currently used. For this, we will favor a high piezoelectric coefficient and a small gap (in the visible) for an injection (at lower cost) of electron-hole carriers in the ferroelectric domains and domain walls. Different solid solutions of newly discovered ferroelectric materials will be developed and characterized.

Finally, in the route towardst future devices, UP-DOWN project proposes in its last WP devices based on capacitive geometry and micro-nanostructured cantilevers obtained by micro-nanofabrication. The first devices will be made of ferroelectric thin films (PZT, BTO) with integrated electrodes grown during the fabrication process, allowing to control in-situ the ferroelectric polarization and thus to adapt the photostrictive response. This approach will then be transferred to self-supported devices such as micro-nano-cantilevers.