Optical Trapping of Photosoftened Solid Polymers

Abstract

We present evidence of optical trapping of solid polymer films by a tightly focused near-infrared (NIR) laser beam (785 nm) when the polymer is softened by green laser (532 nm) irradiation, which is uniformly distributed over the film surface. While the whole film is softened by the green laser, only the part which is subjected to the NIR laser is trapped. The film, which is transparent in the NIR range and strongly absorbs the visible light, is deposited on a glass substrate. The NIR laser is focused at the sample plane and attracts the polymer to the laser focus, by the optical gradient force, during the green light irradiation, indicating that molecular mobility is strongly reduced and the optical gradient force of the NIR laser is inefficient in moving the polymer below the glass-transition temperature of the polymer without photosoftening. The observations are consistent with the theory of photochemical tweezing, which rests on the assumption that photosensitive polymers, containing azo dyes, are softened, i.e., molecular mobility is enhanced, by photoisomerization of the azo dyes during green light irradiation, and are moved in gradients of light fields. We also found that this two-color laser tweezing scheme is sensitive to light polarization, owing to the polarization sensitivity of the azo dye chromophores.

Molecular-Level Photo-Orientation Insights into Macroscopic Photo-Induced Motion in Azobenzene-Containing Polymer Complexes

“…Several studies have shown experimentally that photo-induced free volume plays a key role in light-induced motion in azomaterials and that the "photo-softened" material can be manipulated, for instance, through optical tweezers. 12,51,73,74 A detailed model recently published by Sekkat provides a quantitative connection between the absorbed energy dose and the photo-induced decrease in relaxation time, analogous to the effect of temperature on nonirradiated systems. 8 Our group, in an IR spectroscopy investigation, showed that the photo-isomerization of the azobenzene under unpolarized light strongly perturbs the chemical environment around the azo, as expected, whereas it does not affect the chemical environment of molecular moieties located far from the azo such as the polymer chain in the case of a covalent sidechain azopolymer.…”

Efficient Surface Structuring of Photomechanical Solid Polymers by Near-Zero Light Absorption

“…Photoisomerization of azobenzene derivatives, induced by resonant laser light irradiation, strongly enhances molecular mobility in azo-polymers by tens to hundreds of °C below T g , a feature that can be rationalized by a photoinduced increase of the nanocogent free volume of the dye, which in turn enhances molecular mobility, and mass motion occurs in gradients of light intensity. − Photoisomerization softens the solid polymer, and external potentials due to light intensity gradients move it . Intensity gradients, produced by interference patterns of coherent light beams, can be used as one of such potentials to create motion and spatially arrange the polymer in a periodic manner, i.e., create surface relief gratings (SRGs).…”

Optical Characterization of Ultra-Thin Films of Azo-Dye-Doped Polymers Using Ellipsometry and Surface Plasmon Resonance Spectroscopy

“…Azo-dye-containing materials have been studied extensively in past decades for applications in holography and optical data storage [28,29], nonlinear optics (NLO), electro-optic modulation (EO), second-harmonic generation (SHG) [30][31][32][33], photomechanical actuation and matter motion, and so on [34][35][36][37][38][39][40]. Azo dyes also have potential applications in dye chemistry and bio-photonics [41,42], driving a natural interest in their optical characterization, namely the determination of the real and imaginary parts of n c as a function of the wavelength of light.…”