Fast talk

A new class of materials could make for quicker telecommunication

Published: Saturday 15 April 1995

-- Fibre-optic cables can safely lay claim to being the "concrete and asphalt" of the information highway that is set to usher a telecommunications revolution. But inspite of the high reliability and capacity of these cables, there has always been a natural speed-breaker on the information highway: the sound coded in the optical pulses must be converted through electrical means into mechanical energy which produces the sound that we hear. This considerably slows down the process of communicating human speech. But now a small beginning seems to have been made in bypassing the intermediate electrical process and converting light flashes directly into sound.

Using a new class of materials called photostriction materials, Kenji Uchino at the Pennsylvania State University has made a resonator like the ones we use in telephones. The material, called PLZT (a crystalline compound of lead, lanthanum, zirconium, titanium and oxygen) exhibits a combination of 2 phenomena, the photovoltaic effect and the piezoelectric effect. The photovoltaic effect is the direct conversion of light energy into electrical energy and is used in solar cells. The piezoelectric effect is the transformation of electrical energy into mechanical motion and a common application of this process is in quartz watches (Science, Vol 266, No 5192).

Uchino exploited the properties of the PLZT crystals to make a device that would resonate when light fell on it. He bonded 2 strips of PLZT back to back and connected them electrically. These 2 strips were such that when light illuminated the 2 sides alternately, the device vibrated with the desired frequency.

Though the frequency of sound waves produced in the device is still only barely within the human audible range, the technique holds a lot of promise. The materials are interesting not only because of their properties but also because these properties can be manipulated by changing the composition of the crystals.

Though it is still premature to talk about all the areas where these designer materials can be used, there is certainly a lot of excitement in the communication and microelectronics industry. The speeding up of sound transmission is obviously an application that could enhance the efficiency of telecommunications systems. But there are other applications which are not all that obvious. For instance, there is a proposal to use PLZT crystals in optically controlled robots which are too small to be controlled mechanically. In any case, there is a lot of physics which needs to be explored at the basic level which makes the study of these materials interesting in its own right.

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