Photothermal Heating
The
most common photomechanical mechanism is photothermal heating. When
energy is absorbed from a beam of light, it is converted into heat,
which results in a temperature increase. For a given temperature, the
change in the size of the material is proportional to the coefficient
of thermal expansion.
Molecular Reorientation
A
dye-doped polymer is made of a transparent host polymer that is
impregnated with dye molecules that give the material color. These dye
molecules can absorb light to generate heat, or, the molecules can
change shape. The change in molecular shape stresses the polymer, which
can cause the bulk material to change its shape.
The animation below shows how molecular reorientation works.
- A molecule sits inside a void in its trans form (a polymer is like swiss cheese with lots of holes).
- The
electric field due to a beam of light excites the molecule and causes
its shape to change into the cis form. The smaller molecule can rotate
more freely in the cavity.
- The molecule de-excites back to its
trans form and pushes against the polymer. The internal force due to
many molecules acting together leads to a shape change of the material.
- When
the light is turned off, the molecule can remain stuck, for a while, in
its new orientation. Due to thermal jiggling, the molecule can relax
back to its original orientation, thus allowing the polymer to relax
into its original shape.
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The Photomechanical Cantilever
When
a beam of light is launched down the center of a dye-doped polymer
optical fiber, its length will change due to photothermal heating,
molecular reorientation, or other mechanisms. If the light is launched
into a fiber off-center, the differential stress that is induced causes
the fiber to bend. The photomechanical cantilever was demonstrated in a
dye doped fiber by Bian and coworkers.[1]
The
video below shows the change in length and angle of a polymer fiber
cantilever made of PMMA polymer that is doped with DR1 dye. The fiber
diameter is 0.3mm. The grid is a guide to the eye, and the two arrows
show the edge of the fiber when the light beam is off.
Bian
and coworkers found that the motion of this polymer fiber cantilever
was dominated by photothermal heating and molecular reorientation
mechanisms.[1]
[1] S. Bian, D. Robinson, and M. G. kuzyk, J. Opt. Soc. Am. B. 23, 697-708 (2006).
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