Nanoscale polymer movement is induced by a tightly focused laser beam in an azo-polymer film just at the diffraction limit of light. The deformation pattern that is produced by photoisomerization of the azo dye is strongly dependent on the incident laser polarization and the longitudinal focus position of the laser beam along the optical axis. The anisotropic photo-fluidity of the polymer film and the optical gradient force played important roles in the light induced polymer movement. We also explored the limits of the size of the photo-induced deformation, and we found that the deformation depends on the laser intensity and the exposure time. The smallest deformation size achieved was 200 nm in full width of half maximum; a value which is nearly equal to the size of the diffraction limited laser spot.
©2007 Optical Society of America
Light-induced patterns of surface deformations in azobenzene containing polymer films have attracted much attention because of possible applications in optical data storage and in micro/nano fabrication, and it is well known that such patterns reflect the state of the incident light polarization and the light intensity distribution [1–5]. Trans ↔ cis photoselective isomerization and molecular reorientation play important roles in the deformation process. Since photoisomerization was shown to enhance molecular mobility far below the glass transition temperature (Tg) of azo-polymers in the beginning of the past decade [6–9], considerable exploration of sub-Tg photo-induced molecular movement was performed especially targeting polymer structural effects, including Tg, the free volume and free volume distribution, the mode of the attachment of the chromophore to a rigid or flexible chain, the molecular weight, and so on . Light induced mass movement in azo polymers, i.e., surface relief gratings (SRGs) [8,9] triggered much studies to understand the mechanism of polymer migration, and most of the studies have focused on the SRGs that are fabricated by the interference pattern of two coherent laser beams [11–14]. Yet, there are few reports on surface deformations that are induced by a single focused laser beam [15–17]. To fabricate deformation structures with high spatial resolution, a small irradiation spot is required; a feature which can be achieved by focusing the laser beam by using a high numerical aperture (N.A.) objective lens. In this paper, we report on surface deformations of azo polymers by irradiation with a single tightly focused laser beam with a high N.A. objective lens (N.A. = 1.4). We discuss the effect of the incident light polarization and the position of the laser focus on the deformation pattern. In particular, we found that the deformation pattern is strongly dependent on the z- position of the focused laser spot. In addition to the well known trans ↔ cis surface deformation, it will be shown that a gradient force pertaining to laser trapping pulls the polymer towards the laser focus. Then we present a systematic study exploring the limits of the size of photo-induced deformation by changing the irradiation intensity and the exposure time.
We prepared 50 nm thin films of poly(Disperse Red 1 methacrylate) (PMA-DR1, product No. 579009, Aldrich; Tg= 82 °C) by spin-casting from a chloroform solution. The remaining solvent was removed by heating the films for an hour at 100 °C. The chemical structure and the absorption spectrum of the film, i.e., trans-DR1 are shown in Fig. 1. Disperse Red 1 (DR1) is a nonlinear optical azo dye which is well known for its trans ↔ cis photoisomerization and for its ability to undergo efficient orientation and trigger important polymer movement when it is excited by polarized light . The orientation effect is due to the highly anisometric nature of its polarizability tensor (rodlike molecule) . The irradiation light source was a linearly polarized 460 nm light from a diode pumped frequency doubled laser (Sapphire 460 LP, Coherent Japan). The wavelength of the irradiation laser corresponds to the maximum absorption band of the film sample. The laser beam was focused by an objective lens (N. A. = 1.4) (Plan Apo 60×, Nikon). The diffraction limited spot diameters in the lateral (X or Y) and longitudinal (Z) axes are ∼ 400 nm and 1.0 μm, respectively. Computer controlled piezo stages (P-517 for X and Y axes and P-721 for Z axis, Physik Instruments (PI)) were used to control the position of the focused laser spot in three dimensions. The induced surface deformation of the films was measured by an atomic force microscope (AFM) (SPA-400, SEIKO Instruments Inc.). The AFM was operated in the tapping mode using a Si cantilever to eliminate the mechanical deformation of the films by the cantilever itself.
3. Results and discussion
Figure 2 shows AFM images of the surface deformation induced by (a, b) linear and (c) circular polarizations. The polarizations were controlled by half and quarter wave plates. The estimated irradiation intensity at the sample film surface and the exposure time were 12.5 mW/cm2 and 30 s, respectively, and the laser beam was focused on the film surface. Irradiation with linearly polarized light induced the deformation pattern shown in Fig. 2 (a) and (b). It is clearly shown in this figure that the polymer moved along the polarization direction from the center to the outside of the focused spot, thus producing two side lobes along the polarization direction and a pit at the center. Indeed this polarization-dependent deformation was confirmed by an experiment in which the polarization direction of the irradiation light was rotated through an angle of 90 degrees and the surface relief followed the polarization of the light (see Fig. 2 (a) versus (b)). Similar polarization dependence was observed by Gilbert et al. , who used an irradiation intensity of 40 kW/cm2. At such high intensities, photo-bleaching occurs and influences the surface deformation pattern . In our experiments, we used an appropriate light intensity range; e.g., 6.25 ∼ 62.5 W/cm2 (vide infra). In contrast to irradiation with linear polarization, irradiation with circularly polarized light induced a deformation pattern in which the polymer moved from the center to the outside of the focused laser spot, thus forming a doughnut shape pattern (Fig. 2 (c)). For both linear and circular polarizations, the polymer migrates in the direction of the polarization of the light from high to low light intensity regions, and the polarization dependence demonstrates that the light-induced polymer movement is anisotropically photo-fluidic .
The observed polarization dependence is consistent with the one obtained after irradiation with a low N.A. lens . When a laser beam is tightly focused by a high N.A. objective lens, a non negligible electric field Ez component is created along the optical axis. The intensity distributions corresponding to Ex, Ey, and Ez at the focal position are different, and shown in Fig. 3, and they should lead to different deformation patterns. However, in our experimental conditions, only Ex contributes appreciably to the deformation. With N.A. = 1.4 and wavelength = 460 nm, the maximum intensity corresponding to Ez and Ey are 7 and 200 times smaller than that of Ex, respectively.
Figure 4 shows AFM images of the photo-induced deformation that have been obtained by changing the Z-position of the focused laser spot. The Z-position was controlled by the z-axis piezo stage that was attached to the objective lens. The irradiation started 500 nm under the film surface (Z = -500 nm), then the Z-position was moved to upper positions with an interval of 100 nm, and the next irradiation was done at a different lateral (X-Y) position. This procedure was repeated until the Z-position reached 500 nm upper the film surface (Z = +500 nm). For each irradiation, the irradiation intensity and the exposure time were 12.5 mW/cm2 and 60 s, respectively. When the Z-position was just on the film surface (Z = 0 nm), the deformation pattern was the same as the one shown in Fig. 2 (a) or (b). It is interesting to note that at distances larger than 200 nm above the film surface in air, the polymer formed a protrusion coming out towards the center of the laser focus and suggesting the existence of a optical gradient force  that pulls the polymer towards the region of maximum intensity (see Fig. 5). This is optical trapping of a viscoelastic polymer showing nanoelasticity over 20 nm; i.e., the maximum height of the protrusion obtained at z = + 500 nm. For distances between 200 and 0 nm, the overlap of the laser intensity and the film are large enough to produce dips at the center as explained above. When the laser is focused into the glass substrate, there is no protrusion formed, because the polymer movement is blocked by the substrate. The observed deformation patterns in the lateral and longitudinal directions are caused by the anisotropic photo-fluidity due to Ex, and the optical gradient force due to the intensity distribution of the electric field, respectively. The optical gradient force depends only on the intensity and not polarization. In our experimental conditions, Ex is dominant, and the intensity distribution corresponds to that of Ex.
In a systematic set of experiments, we studied the dependence of the size of the photo-induced deformation on the intensity of the irradiation light and the exposure time. In those experiments, the irradiation light is linearly polarized and focused by a 1.4 N.A. objective lens. The deformation pattern was studied just at the laser focus, i.e., Z = 0 nm, for three irradiation intensities (6.25, 12.5, 62.5 mW/cm2), and exposure times according to the series 1 to 500 s for 6.25 and 12.5 mW/cm2, and 1 to 100 s for 62.5 mW/cm2. The deformation patterns obtained at all intensities at all times of irradiation were the same, but the size of the deformation was different. Figure 6 shows the dependence of the height and the full width at half maximum (FWHM) of deformation pattern along the direction parallel to the light polarization on the irradiation intensity and the exposure time. The height is defined as the difference between the top of the side lobes and the bottom of the central pit as shown in Fig. 6. As is can be seen as well from this figure, the rate of the deformation of the height and FWHM decreased with the increasing irradiation dose, and the higher the irradiation intensity, the faster the increase of both the height and FWHM. The height increases more rapidly than FWHM which needs more time to reach saturation. The height increases rapidly at small irradiation doses, and saturates at larger irradiation doses near 90 nm. 90 nm corresponds to near twice the initial film thickness; a thickness modulation which is usually observed in SRGs [13,14]. The minimum FWHM of the fabricated pattern is about 200 nm; a value which corresponds to the size of the diffraction limited laser spot.
We studied anisotropic nanoscale polymer movement induced by a tightly focused laser beam in an azo-polymer film. We found that the deformation pattern strongly depends on the incident laser polarization and the z-position of the focus. Light induced anisotropic photo-fluidity moves the polymer along the polarization direction in the film plane, and the optical gradient force pulls the polymer along the optical axis; a feature which allows for the control of the deformation pattern (protrusion versus pit) by changing the z-position of the focus. From a set of systematic studies, we found that the smallest deformation size that could be achieved is 200 nm in full width of half maximum; a value which is nearly equal to the size of the diffraction limited laser spot. The present work unambiguously demonstrates light induced nanoscale polymer movement, and it will trigger future studies addressing near field fabrication with a resolution beyond the diffraction limit of light as well as fundamental studies and theories of photo-induced nano movements in polymers.
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