Localized irreversible thermal switching of spontaneous polarization in ferroelectrics by laser

Abstract

Abstract Irreversible thermal switching of spontaneous polarization which has been recently described1 for triglycine sulphate (TGS) is studied with a higher power laser (Argon ion laser), and a more efficient device to focus the light on a small area of the crystal (radius ≃ 1 μm). It is seen that an external depolarizing field is not needed to achieve irreversible switching. Laser pulses are sufficient if their duration is longer than a characteristic time t *. It is shown that laser flux φ * and time t* are related by an exponential law t* = BeA/φ. Such an exponential law had been found by G. W. Taylor for expressing some threshold time t * he defined, vs. electric field E.6 The most important difference is probably that Taylor's law is a macroscopic law where unipolar electric pulses E are uniformly applied to the whole crystal for time t * while the laser pulse is focused on a very small area to give an heterogeneous electric field leading to the reversal of a very thin domain. Thus, we have been able to find local variations of t* on the TGS surface, and an improved pyroelectric probe technique using a variable phase detection has allowed to see the effect of successive pulses of time duration t*. First appears an inner irreversible domain switching. Then a forward growth up to the illuminate surface is observed and finally a lateral growth.