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We present the first report of two-photon induced plastic surface deformation in solid polymer films. Exposure of azo polymer films, which absorb in the visible range (λmax=480 nm), to intense 920 nm irradiation leads to polarization dependent photofluidic polymer nanomovement caused by photoselective two-photon trans ↔ cis isomerization. The deformations were induced by a gradient of light intensity; and strongly depend on the wavelength and the polarization direction of the incident laser light and the position of the focused spot with respect to the plane of the polymer film.
©2008 Optical Society of America
1. Introduction
Polymers containing azobenzene derivatives have been the subject of intensive research for two decades owing to their unique “smartness”, i.e., the ability to tailor and/or control materials properties by photoisomerization [1]. In particular, it was shown that photoisomerization creates optical anisotropy by nonpolar orientation, and poling by polar optical excitation (all optical poling), and it triggers molecular movement far below the glass transition temperature (Tg) of the polymer (photo-assisted poling), and polymer mass movement proceeds in spatial gradients of the excitation light (surface relief gratings) [2–5].
Applications may include optical data storage, birefringent devices, nonlinear optical devices, gratings based devices, and photo-mechanical actuators [6,7]. In solid polymers, photoisomerization of azobenzene derivatives creates free volume and drives efficient chromophore and polymer segmental and chain motion far below the polymer’s Tg [8,9]; an effect which is at the origin of photo-assisted and all optical poling and surface relief gratings. Most of the studies reported to date on azo-polymers used single photon isomerization, and it is of critical importance to investigate two- or multiphoton isomerization of azobenzene derivatives in polymers since it would trigger additional studies and applications of azobenzenes containing polymers at the interface of nonlinear optics and photochemistry, in that all of the effects that have been demonstrated in azo-polymers by one-photon isomerization may be reproduced by two- or multi-photon isomerization with potential applications in nanophotonics. The first study towards this end was reported by us few years ago, and we demonstrated the creation of photo-induced anisotropy in photosensitive polymers by two-photon absorption (the nonlinear extension of the Weigert effect), thus showing that photochromic dyes may be isomerized and reoriented in films of polymer by two-photon absorption [10,11]. In this paper, we report on photoinduced plasticity in azo-polymers, i.e., induced surface relief structures, by two-photon isomerization of the azo dye disperse red one (DR1); a chromophore which is well known for its trans ↔ cis photoisomerization and for its ability to undergo efficient photo-orientation; a feature which is due to the highly anisometric nature of its polarizability tensor (rodlike molecule) [12].
Recently, two-photon isomerization and reorientation of azo dyes in polymers have attracted much attention because of the scientific and technological extension into nanophotonics [10,11,13–27]. Basically, the photoreaction can be induced by tightly focused lasers into confined volumes - a resolution of 120 nm has been achieved for three dimensional nanofabrication in photopolymerizable resins [28] -, and novel scientific information may be obtained, especially with regard to one versus two- or multi-photon reaction pathways — the one and two-photon transition dipole moments of a diarylethene derivative have been found to be perpendicular to each other in two-photon isomerization experiments [10].
Photoinduced surface relief gratings (SRGs) in azo-polymers are of interest because of possible applications in surface micro/nano fabrication, and because the mechanism of the formation of the surface structures, owing to a polarization sensitive isomeric movement that drives a polymer mass movement, is still not understood [4–5, 29–41]. SRGs are usually fabricated by irradiating azo polymer films with interference patterns, while a single focused laser allows for a simple experimental configuration, i.e., simpler than that produced by interference patterns, owing to its gradient of light intensity and polarization thereby allowing for additional information on the mechanism of the formation of such surface relief structures to be obtained [42–45]. Indeed, we found in a recent study, that the optical gradient force produced by a single laser beam, which is tightly focused in the air above the film surface, attracts the film towards the beam focus thus making a protrusion just beneath the focus [45]. This phenomenon adds an additional mechanism to the well known observation in which the polymer moves from higher to lower intensity regions along the polarization of the light when the light spot is at the film surface. In all the observations of the induced surface deformation process, trans ↔ cis photoselective isomerization cycling and successive molecular reorientation play a central role [1].
In this report, we give the first evidence that surface deformation by polymer mass movement may be induced by two-photon isomerization in azo-polymers, and we discuss the effect of the incident light polarization and the position of the laser focus (Z-position of the focus) on the deformation patterns induced by two-photon absorption. We discuss the mechanism of the induced surface deformation for two-photon versus one-photon absorption. The effect of photobleaching is also discussed by studying the effect of the irradiation wavelength, i.e., 780 nm versus 920 nm irradiation, on the induced surface structures.
2. Experiment
We prepared 150 nm thin films of poly(Disperse Red 1 methacrylate) referred to in the text as DR1-PMA (Aldrich, Product No. 579009) (Tg=82 °C) by spin coating from a chloroform solution on a cover glass (~170 µm). In DR1-PMA, the chromophore is flexibly tethered to the main chain. The UV-visible absorption spectrum of the polymer and its structure formula can be found elsewhere [45]. The remaining solvent was removed by heating the films for an hour at 110 °C. The films were irradiated by a linearly polarized near infrared light from a Ti:Sapphire laser (Spectra Physics, Mai Tai, pulse width=130 fs; repetition rate=80 MHz) to induce surface deformations via a two-photon absorption process. The laser light was focused by an objective lens (N.A.=0.55) from the bottom of the cover glass (see a schematic on Fig. 1). The diffraction limited spot diameter in the lateral and longitudinal axes are 2 µm and 12 µm, respectively. A computer controlled piezo stage (Melles Griot, Nanoblock), on which the sample was placed, was used to control the position of the irradiation spot with respect to the sample surface. The surface topology of the deformed films was measured by an atomic force microscope (AFM) (SEIKO Instruments Inc., SPA-400). The AFM was operated in the tapping mode to avoid mechanical deformations of the films by the cantilever itself.
3. Results and discussion
Figure 2 shows an AFM image of the surface deformation induced by a focused, linearly polarized, 920 nm irradiation (irradiation intensity=61 kW/cm2; and exposure time=60 s). This figure corresponds to a spot center 2 µm above the film surface in air. A similar behavior is observed when the center of the spot is 1 µm above and just at the film surface. It can be clearly seen from this figure that the polymer moves 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. This can also be seen at the AFM topographic line scans along and perpendicular to the direction of light polarization. This observed behavior, which is due to a photonic effect, i.e., polarization dependent, and not to heat deposition, is much like that observed with single photon isomerization [45]. At half the 920 nm fundamental wavelength, i.e., 460 nm, the sample presents a strong absorption suggesting that two-photon isomerization is at the origin of the surface deformation.
Fig. 2. AFM images of the deformation induced at 920 nm light when Z-position of the focused laser spot is +2 µm. The polarization direction is indicated by the arrow. The line plots (a) parallel and (b) perpendicular to light polarization are shown. The positions of the each plot correspond to the directions that are between the arrows indicated.
The effect of dye photobleaching on the induced surface reliefs was investigated by changing the irradiation wavelength of the laser light to 780 nm to confirm that the surface deformations fabricated by 920 nm irradiation were in fact due to two-photon isomerization and not to bleaching of the dye. Indeed, in a recent report, we showed that DR1 undergoes bleaching under irradiation at 780 nm, at the same range of light intensity, i.e., GW/cm2, via a multi-photon absorption process [14]. Furthermore, Fig. 3 shows that the surface deformation induced by 780 nm irradiation at the same light intensity and exposure time as those of 920 nm irradiation formed dips at the center of focus, and the patterns are independent from the incident light polarization. The same behavior was observed for all Z-positions of the focus. Only those at +2 µm, and at the film surface, i.e., 0 µm, and -1 µm are shown, and no surface deformation was observed when the Z-positions of the spot were smaller than -2 µm. This finding confirms that the surface deformations shown on Fig. 3 were induced by multi-photon bleaching of DR1. Note that while an ablated polymer surface by a high intensity pulsed laser looks rough under AFM imaging, due to the deposition of fragments of the ablated polymer at the film surface [46], the AFM images of Fig. 3 are smooth outside the irradiated area and reinforce photobleaching versus ablation as a mechanism of the dip formation.
Fig. 3. (Upper row) AFM images of the surface deformation induced at 780 nm light by changing the Z-position of the focused laser spot. The Z-position was varied from -5 µm to +5 µm with an interval of 1 µm. Only images at +2 µm, 0 µm, and -1 µm are shown. The values inside each figure represent the Z-position of the focus (unit is µm). The polarization direction is indicated by the arrow. (Lower row) Line plot of the surface deformation at Z =+2 µm. The position of the plot corresponds to the directions that are between the arrows indicated.
Figure 4 shows AFM images of the photo-induced surface deformations observed by changing the Z-position of the focused laser spot with irradiation at 920 nm. This irradiation, as well as that done at 780 nm which is discussed in the previous paragraph, started 5 µm above the film surface (Z=+5 µm) in air, then the Z-position was moved to lower positions, i.e., towards the film, with an interval of 1 µm, and the next irradiation was done at a different lateral (X-Y) position. This procedure was repeated until the Z-position reached -5 µm (Z=-5 µm), i.e., in the other side of the film, inside the glass slide. For each image, the irradiation intensity and the exposure time were 61 kW/cm2 (peak intensity=5.9 GW/cm2) and 60 s, respectively. Figure 4 shows that when the Z-position of the focus is above the film surface in air (Z>0 µm), 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 (vide supra). A somewhat contrasted behavior is observed when the Z-position of the focus is below the film surface (Z<0 µm), i.e., the polymer moves from the outside to the center of the focused laser spot always along the direction of the polarization of the excitation light and formed a protrusion at the center and two dips, one at each side of the protrusion along the polarization direction (see also Fig. 5). When the distance between the focus and sample surface is larger than 5 µm (Z>+5 µm, Z<-5 µm), no deformation is induced because the intensity of the light spot at the film is not large enough to induce two-photon isomerization and polymer movement.
Fig. 4. AFM images of the surface deformation induced at 920 nm light by changing the Z-position of the focused laser spot. The Z-position was varied from -5 µm to +5 µm with an interval of 1 µm. The values inside each figure represent the Z-position of the focus (unit is µm). The polarization direction is indicated by the arrow.
Fig. 5. (Left column) Line plots of the surface deformations at (a) Z=+2 µm and (b) Z=-2 µm. The positions of the each plot correspond to the directions that are between the arrows indicated in Fig. 4. (Right column) Schematics describing the relationship between the Z-position of the focus and the film surface.
At a low N.A. objective lens, i.e., N.A.=0.55 in the present experiments, the dominant component of electric field at the focus is the one along the polarization direction (Ex) when the incident light is linearly polarized along X. Indeed, electromagnetic calculations that we performed for the intensity distribution at 5 nm below the air (n=1.0)/polymer (n=1.5) interface when the beam focus is at +2 µm and -2 µm (not shown), show that the Y and Z components of the fourth power of the optical field are 7 and 4 orders of magnitude smaller that the fourth power of Ex, respectively (see Fig. 6). So, during two-photon surface relief formation, polymer movement proceeds in direction of the light polarization (X). In this calculation, nonlinear refractive index change induced by two-photon isomerization is not taken into account, because even with one-photon isomerization the maximum refractive index change we measured using the same dye molecules was the order of 10-2. Such a small refractive index change does not affect the field distribution. We also found that the field distribution is nearly unchanged when the Z-position of the focus is at +2 µm (in air) and -2 µm (in the cover glass). Since the field distribution in X-Y plane is quasi-identical from both sides of the polymer film, i.e., the laser is independently focused at the same distance from the film in air and glass, the presence of an intensity gradient along the Z axis leads to the formation of the observed deformation. Note that when the same experiments were performed with one-photon absorption at 460 nm on the same polymer, an optical gradient force [47], which is proportional to the Z-intensity gradient, was observed to be attractive, i.e., a protrusion was formed when the focus was in the air and not in the glass [45]. It is interesting to note that this behavior is opposite to that observed with two-photon isomerization. We do not have yet a clear explanation for this phenomenon, and more experiments are necessary in order to assess how the intensity gradient contributes to the surface relief formation by one and two-photon isomerization. Systematic studies on irradiation wavelength and doses and so on are also needed in order to clarify the mechanism of the formation of the surface relief structures, as well as for assessing the limit of the spatial resolution of two-photon surface nanofabrication on azo-polymers by a single tightly focused laser, especially at a high N.A. objective lens.
Fig. 6. Calculated field distributions of 4th power of electric field components created by a focused linearly polarized laser beam in the presence of the interface between air (n=1.0) and polymer (n=1.5). The distributions were calculated in a XY in-plane which is located at 5 nm below the interface. The figure shows the field distributions when the Z-position of the focus is +2 µm. Each component of the electric fields of (a) Ex, (b) Ey, and (c) Ez are shown. The polarization direction is X, and Z is perpendicular to the film surface, and Y is perpendicular to both X and Z.
4. Conclusion
Both one- and two-photon absorption induce isomerization and surface relief structures in azo-polymers. In this paper, however, we present the first report on two-photon absorption induced surface reliefs. In both one and two-photon cases, the polymer moves in the direction of the irradiation light polarization. We also found that the induced surface pattern strongly depends on the Z-position of the laser focus suggesting the contribution of a light intensity gradient along the Z-axis, i.e., the direction of the propagation of the laser, to the observed surface reliefs. The present findings suggest additional experimental and theoretical studies especially with respect to mechanism of photo-fluidity, in which the assessment of the contribution of the light intensity gradient and radiation forces and torques will be important [48–54].
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