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Schematic of the experimental setup. A strip of metal-coated PVDF film 1 is clamped between two glass slides 2. After being illuminated with laser beam 3 (that can be moved laterally in vertical and horizontal direction with respect to the strip), the strip bends and takes new position 4. A segment of optical fiber 5 is glued to the top free end of the strip. When the strip bends, the fiber takes position 6. The displacement of the free end of the fiber is monitored by a telescopic microscope 7 with an attached CCD camera 8. The microscope is put on a mount with three translational degrees of freedom. Camera 8 is connected to TV monitor 9. The force generated by the bending strip is measured with a torsion balance that consists of a rod 10 suspended on a string 11. The lower end of the string is fixed in point 12. The upper end is connected to a knob 13. By turning the knob, the angular position of a pointer 14 attached to the string can be changed. Reading of the angular position of pointer 14 is done with dial 15. Inset in the upper left corner shows the images of the tip of the fiber when the PVDF strip is not illuminated (top picture) and illuminated with a pulsed beam at resonant frequency (bottom). Images correspond to a strip of cut from 52 μm film with resonant frequency near 340 Hz. Inset on the right shows two sequential frames of a video record of the tip of a fiber (bright vertical line) attached to the strip illuminated with two pulsed laser beams. A time delay between light pulses equal to the quarter of the period of pulsation generates elliptical motion of the fiber tip. On its way from left top extreme position (top picture) to the right bottom extreme position (bottom picture), the fiber tip pushes the oscillating wheel (the side view of its part appears as a horizontal bar in the top of the pictures) of a conventional mechanical alarm clock.
Theoretical model of bending strip of PVDF film. The strip is assumed to consist of two adjacent elastic half-plates 1 and 2 with different thermal coefficients of linear expansion and , respectively , fixed at the bottoms separated by distance ( is the thickness of the film) and sharing a pivot on the top. As the strip, initially in position 3, is heated by the laser beam, it “bends” into position 4. Next, the torsion balance applies a horizontal force necessary to return the strip into symmetric position 5. Forces 6 and 7 are the components of the returning force along the half-plates. Inset: data points of the static force versus the power of the beam measured by torsion balance and the linear fit (solid line).
Force (1) and deflection (2) versus the position of the beam in a PVDF strip. The diameter of the round-shaped laser beam was 2 mm. The strip was coated from one side by an 80 nm-thick chromium layer. Dimensions of the strip were: . The strip was illuminated from the side of the chromium coating. Horizontal solid lines in the middle of plots 1 and 2 mark the region where the force and deflection remain approximately constant at various positions of the beam.
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