Localized irreversible thermal switching in ferroelectric TGS by an argon laser

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 A similar law had been found by G. W. Taylor for expressing a 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 focussed on a very small area to give an heterogeneous electric field leading to the reversal of one very thin domain. Thus, we have been able to find local variations of t+ on the TGS surface. An improved pyroelectric probe technique using a variable phase detection has allowed us 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 illuminated surface is observed and finally a lateral growth.Application to memory devices are considered. One megabit/cm2 capacity has been demonstrated. Writing, Reading and Erasing are made by a flying spot laser. At the present time the energy, needed to write one bit is 3.10-6 J, and the reading speed is 1 Megabit/sec.