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.
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.
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.
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.
 S. Bian, D. Robinson, and M. G. kuzyk, J. Opt. Soc. Am. B. 23, 697-708 (2006).