October 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Every time I see
something about "transparent anything" that formerly was know only in an opaque
form, I think about the Star Trek "The Voyage
Home" movie (click on the link if you don't know what I mean). This
transparent ceramic material was a real breakthrough in optics technology in its
day due to the ability to control its degree of transparency or opacity with an
electric field. It would even retain its state with the electric field removed
so use as a data or even image storage device was possible. An ability to be quickly
switched (at up to a 10 MHz rate) held promise for it as a laser or other light source modulator
or even as a high speed facsimile (fax) system. It seems sort of like a solid
version of a liquid crystal display (LCD).
Lots of unique optics terms appear in this article, so I provided hyperlinks to
them for you.
Transparent Ceramics for Electro-Optics
New Ceramic Material Can Be Used to Control
By Nigel S. Hey
Electro-Optical devices have been in use for quite a number of years, from Edison's
first incandescent light bulb to modern light-emitting diodes and research dye lasers.
Simply by closing a switch, these devices operated exactly as they were designed
to. However, in the attempt to modify the light after these devices are turned on,
problems begin to appear.
To solve the problems of light processing, Sandia Laboratories in Albuquerque,
New Mexico, has developed a new ceramic device which offers an alternative to traditional
Kerr and Pockels cells and other mechanical contrivances. The new ceramic is similar
to that used as the piezoelectric transducer in phonograph pickups. The big difference
is that the new ceramic - hot-pressed and doped with lanthanum, a metallic rare-earth
element - is transparent.
When sandwiched between crossed polarizers, the lanthanum-modified
titanate (PLZT) ceramic acts as a variable color filter or transmits light intensity
in response to an applied voltage. Used in a "scattering" mode, it displays black
and white images without the need for polarizers. Most important, PLZT has a built-in
Positive image stored in a ceramic electro-optic device. Plate
is 1 in. in diameter and is 0.0123 in. thick.
At least a minor revolution in electro-optic technology is promised by PLZT.
The ceramic can be used to modulate lasers for communication purposes; extract colors
from white light; store information in binary or decimal form; turn on and off light
beams; and store and display images. Variations of PLZT are suited to special applications
like high-contrast shutters or memories which retain stored information until erased.
How It Works
Let us take a microscopic look at PLZT. The material consists of randomly spaced
PLZT "crystallites," in which each unit cell is arranged like a cube with lead and
lanthanum ions at the corners, oxygen ions in the center of each face, and a titanium
or zirconium ion in the center of the cube.
In this configuration, both the unit cell and the crystallite are electrically
neutral. But in "slim loop" PLZT, the central titanium or zirconium ion is displaced
toward one of the oxygen ions when an electric field is applied. The unit cell distorts
into a tetragon and becomes an electric dipole. Clusters of unit cells align in
the same direction to form a domain dipole.
Increasing the electric field increases both the magnitude and alignment of the
domain dipoles. When the electric field is removed, the domain dipoles disappear
and the ceramic returns to its electrically and optically isotropic state.
In ferroelectric or "wide loop" PLZT, the unit cell is spontaneously distorted,
causing domain dipoles to exist with no applied electric field. When an electric
field is applied, it causes domains favorably aligned with respect to the field
to grow at the expense of others and produces increased domain alignment.
Controllable dipole alignment is the basis of PLZT's remarkable versatility.
This alignment determines whether or not the material is
birefringent (high or low
double refraction) and the magnitude of the birefringence. In another Mode of operation,
dipole alignment controls the ceramic's light-scattering properties.
Birefringence has the effect of changing the polarization of plane-polarized
light. By controlling birefringence, one can also control the amount or color of
light penetrating a polarized filter. Hence, PLZT can be said to be a solid-state
analog of the Kerr cell which exhibits a quadratic electro-optical effect. Alternatively,
using a different chemical mixture, PLZT exhibits a linear electro-optical effect
similar to a Pockels cell.
Unlike the Kerr cell. PLZT is solid-state and compact. It is less expensive to
make and has a much larger electro-optical effect than does potassium dihydrogen
phosphate (KDP) and
lithium niobate crystals commonly used in Pockets cells. For
the same applied voltage, 12 times the effect of KDP is produced by one composition
These variously shaped electro -optic ceramic pieces, made by
new process, are very transparent and homogeneous.
Prototype of ceramic memory element can store 5120 bits/sq.in.
Straight pin at left.
The one feature of PLZT that is unique is its electro-optical memory. When the
polarizing electric field is removed from "wide loop" PLZT, the domain dipoles do
not return to a random orientation. The ceramic remains birefringent and stays this
way until changed by electrically reorienting the dipoles. The dipoles can be returned
to a random orientation by raising the temperature of the PLZT to its Curie point.
In PLZT materials; the applied field-versus-birefringence plot follows a hysteresis
curve. Typically, memory-type PLZT exhibits a certain maximum retardation (saturation)
while the field is applied. When the field is removed, tire material relaxes into
a remanent (residual) state with a lesser birefringence. Subsequent electric field
pulses can be used to switch the ceramic into other
Many device applications present themselves when one considers the effect from
a relatively simple device consisting of an electroded ceramic slice sandwiched
between crossed polarizers. These include:
Memory material - light gates and shutters; optical memories which can he read
without photocell arrays; controlled-persistence displays and electrically controlled
spectral filters. For color effects, back lighting with plane-polarized light is
used. Each frequency component of white light is affected by birefringence. By applying
a specific voltage to the material, one color can he made to dominate as light penetrates
the analyzer. Monochromatic light is used to achieve "amplitude modulation."
Pockets - effect material-linear light modulation and momentary light shutters.
This material could be useful in modulating laser beams at frequencies up to approximately
Kerr-effect material - quadratic light modulators and light shutters. This is
an extremely promising material since it is para-electric at room temperature and,
thus, optically isotropic. Virtually no light passes through the crossed
of the device. When a field is applied, however, the crystallites distort and the
ceramic becomes birefringent. When the field is removed, the material immediately
relaxes to a para-electric state.
So far, devices in which an electric field is applied perpendicular to the direction
of light propagation (transverse mode) have been discussed. But one major development
has been pioneered at Sandia in which the field and light are applied in the same
direction (longitudinal mode). The new device is called
In Cerampic, no birefringence is involved. The device works entirely by scattering
light. Dark areas of the image scatter more light away from the viewing plane than
do light areas. Non-image forming scattered light is then removed from the projected
light beam with a contrast-enhancing pinhole aperture.
To make Cerampic, one side of a PLZT ceramic slice is coated with photoconductive
polyvinyl carbazole (PVK). The device is then coated on both sides with transparent
High-resolution images (up to 1000 line pairs/inch) are stored in this device
by illuminating it with light through a photographic negative or with a scanning
light beam while applying voltage to the electrodes. Light passing through the negative
penetrates the transparent electrode and enters the PVK, decreasing electrical resistivity
anti allowing current to pass through the ceramic to the second transparent electrode.
In this manner, the relative opacity (or scattering capability) of innumerable points
on the ceramic plate is altered so that the device transmits various shades of gray
in the form of an image. The image has memory, is erasable, and can be viewed directly
or projected like a 35-mm slide.
A variety of potential uses is possible with Cerampic. Most promising is the
generation of images from signals received by telephone or radio. These images could
be generated in a few seconds or, conceivably, at rates of up to 15,000 lines/second.
Cerampic is being considered for use in optical information storage and processing
systems. The new scattering effect may also prove useful in devices such as shutters,
optical memories, and page composers for holographic memories.
Many companies are now working on devices that use PLZT as the basic working
medium. Since the material was developed by Sandia Laboratories under a prime Atomic
Energy Commission contract, the basic work is covered by patents held by the U.S.
Government. This means that private companies may be licensed to develop and use
the material on a no-royalty, revocable. nonexclusive basis.
Posted October 1, 2019