We present details of a recently-discovered novel phenomenon of reversible
and controllable contraction and dilatation in a solid (a chalcogenide
glass) driven by polarized light. This effect could lead towards an optimum
means of actuation of a molecular assembler.
We have found that polarized light causes a bilayer cantilever (consisting of an amorphous As50Se50 film, 250nm thick, deposited on a silicon nitride, V-shaped atomic-force microscope (AFM) cantilever 200 µm long and 0.6 µm thick) to bend reversibly either up or down by up to about +/-1mm, depending on whether the E-vector of the incident polarized light and the long axis of the cantilever are parallel or orthogonal, respectively [Fig. 1.].
Fig. 1. The relative orientation of the light electric
vector Ex,y with respect to the cantilever. Orientation of the electric
vector E of the inducing light parallel to the main axis of the cantilever
causes contraction (>
c < )
of the chalcogenide film (A); orientation of E orthogonal to the main axis
of the cantilever results in expansion (<
) of the chalcogenide film (B). The chalcogenide film is evaporated on
the top of the cantilever.
This is the first observation that an anisotropic mechanical response
can result from a polarized light stimulus. Thus, this smart material (a
chalcogenide glass) can be incorporated into an optical actuator, and is
seen thereby as the basis of the future construction of a general-purpose
Five distinct features make the phenomenon of polarized-light induced
optical actuation much more advantageous than other kinds of actuation
for use in an assembler. The first is that the opto-mechanical effect is
reversible and non-hysteretic ,
unlike the piezoelectric effect. The second is that since light is used
as the stimulus, and not electricity as in piezoelectric devices, noise
(e.g. due to electrical pick-up) is greatly reduced; moreover, since the
stimulus -polarized light- can be carried over a long distance, the design
of an assembler would be greatly simplified (with respect to wiring, vibrations,
safety). Thirdly, it is cheap and readily fabricable using current technology.
Fourthly, the effect is wavelength-selective (the maximum response is for
light having an energy equal to the chalcogenide bandgap). Finally, and
most importantly, the optical actuators lend themselves to miniaturization
unmatched in resolution by present sources of actuation. Large arrays of
independent cantilevers can be envisaged for applications requiring massively
parallel processing. Present knowledge of the phenomenon even suggests
speculation of polarized-light driven motions on atomic range length scale,
thus providing a way to drive molecular machines consisting of a few hundred
atoms. It is believed, that proper materials and mechanical design and
control of the polarization state of the inducing light can enhance this
cantilever deflection with sub-nanometer resolution.
Results regarding our current state of understanding of the phenomenon
on microscopic and macroscopic levels, future studies and possible applications
will be given.