Graham Bell's Photo Phone
Our modern era of scientific and technological progress
has brought us a high-speed internet, nuclear medicine for non-invasive
diagnostics, computers that make our lives easier, and genetic
engineering that has yielded better crops and designer therapies for
all sorts of diseases.
Many technologies that we associate with modern times
had taken root more than a century ago. Using the crude tools available
to him in the late 1800s, Alexander Graham Bell is believed to have
been the first to observe and use photomechanical effects.
The Figure above shows a photophone. Light
is reflected from a flexible membrane that is set in motion by sound.
The reflected light is deflected by the vibrating membrane and detected
by a photocell. The photocell converts the light into an electrical
signal that drives a speaker. In this demonstration, Bell intelligibly
transmitted his voice over a couple of city blocks on a beam of light -
an impressive feat given the crude instruments available at the time.
The above figure shows a collimated beam of
light whose intensity is modulated into pulses by a spinning plate with
holes, called a chopper. The blinking light then hits the
photomechanical material. The pitch of the sound emitted from the
sample increases as the speed of the chopper is increased.
Bell found that sound can be perceived from
light pulses that directly hit the human eardrum. While Bell's work
with the telephone lead to the highly successful Bell Telephone
Company, photomechanical effects remained a laboratory curiosity with
no technological implications aside from the technology's use for
eavesdropping by reflecting light from a window pane.
Electrostriction and the Uchino Walker
A photostrictive material changes length in response to
light. Photostriction is a two step process. First, the light releases
charges in the material. The electric field due to these charges leads
to a change in the shape of the material through the piezoelectric
 For a more detailed history, see: M. G. Kuzyk,
Polymer Fiber Optics: Materials, Physics, and Applications, CRC
Press, Taylor & Francis Group, Boca Raton (2007).
A bimetalic strip, made from two sheets of dissimilar
metal will bend as a function of temperature due to the differing
amount of thermal expansion. In analogy, Uchino used two sheets of
ceramic materials, one that is photostrictive and the other, which is
not. When exposed to light, the photostrictive sheet expands and causes
the layered structure to bend.
Using such layered structures as legs, Uchino made a
little machine that walks when its legs are illuminated with light
pulses. The figure above illustrates the approximate design of such a
The Importance of Photomechanical Effects
All of modern technology is based on 5 electronic
devices: Wires, which transmit information from one place to another;
sensors, which convert some physical property such as temperature or
stress into an electrical signal; transistors; which can be used to
perform logic and control the flow of electricity; actuators, which
convert electrical energy into mechanical energy - such as motors; and
powers supplies, which provide electrical power.
The table above summarizes the five
electrical device classes and the analogous optical technologies. The
blue shaded areas represent mature technologies such as lasers, which
are found in many commercial products like checkout scanners and DVD
players; optical fibers that carry huge amounts of information across
the oceans and between high-speed computers; and optical sensors that
are capable of measuring small variations in pressure and temperature.
Much of the field of nonlinear optics has
focused on developing the optical equivalent of the transistor (green
shaded region above). An optical transistor would control the flow of
light as a transistor controls the flow of electricity. Intense
research over the last two decades is finally coming to fruition with
the introduction of commercial electrooptic devices.
Photomechanical effects are the missing
ingredient for making all-optical technologies possible. Since light
allows for the design of novel new architectures, and does not suffer
some of the drawbacks associated with electronics, the development of
photomechanical effects would provide the basis of amazing new