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Abstract
A versatile system for the fabrication of surface microstructures is demonstrated by combining the photomechanical response of supramolecular azopolymers with structured polarized illumination from a high resolution spatial light modulator. Surface relief structures with periods 900 nm - 16.5 µm and amplitudes up to 1.0 µm can be fabricated with a single 5 sec exposure at 488 nm. Sinusoidal, circular, and chirped surface profiles can be fabricated via direct programming of the spatial light modulator, with no optomechanical realignment required. Surface microstructures can be combined into macroscopic areas by mechanical translation followed by exposure. The surface structures grow immediately in response to illumination, can be visually observed in real time, and require no post-exposure processing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical microstructures are diffractive optical elements widely used in information display, spectral analysis, and optical communications systems [1–3]. A common fabrication technique involves patterning a photosensitive film using a mask or laser interference pattern, followed by post-exposure chemical processing. The emergence of high resolution spatial light modulators (SLM) has for some applications transformed microstructure fabrication by replacing masks and laser interference fields with the programmable projection of spatially structured intensity patterns [4–7]. Equally versatile but less utilized is the capability of the SLM to project structured polarization fields. These can spatially define the optical axis of polarization-sensitive materials, leading to recent advances in diffractive optical waveplates and phase-encoded imaging [8]. Here we use structured polarization fields from an SLM to drive a photomechanical response in a supramolecular azopolymer film, resulting in the one-step fabrication of surface relief microstructures. Such digital polarization optics combined with the polarization sensitivity and facile fabrication of supramolecular azopolymers enables the programmable generation of a broad range of structures, including linear, circular, and chirped diffraction gratings, among others.
Azopolymers combine an azobenzene chromophore with a polymeric host. They respond to polarized illumination with cyclic cis-trans isomerization, causing a rotational motion of the azobenzene until the linear trans conformation is trapped perpendicular to the optical polarization. The chromophore-polymer coupling interaction acts as a photomechanical transducer, transforming the molecular rotation to a macroscopic mechanical response. This is particularly efficient when driven by a spatially periodic linear optical polarization, which can be generated by overlapping two counter-rotating circularly polarized laser beams on the surface of an azopolymer film, as illustrated in Fig. 1(a). The resulting spatially modulated chromophore alignment ultimately deforms a flat film into a surface relief pattern with the same period as the optical polarization, as shown in the scanning electron microscope images on the right [9–11]. As a general observation, surface relief structures on supramolecular azopolymer films can be generated with lasers of wavelength 400 nm - 550 nm and exposures of less than 1 min. The surface structures grow in immediate response to light, require no further processing, and are stable in ambient conditions [12–14].
Fig. 1. (a) Two-beam laser interferometric setup for surface relief generation on azopolymer film surface. Unpolarized illumination (left) does not alter bulk isotropic chromophore orientation. Counter-rotating beams (middle) interfere, producing linear polarization grating that drives chromophore orientation perpendicular to optical polarization. Modulated molecular reorientation drives surface deformation, forming a permanent surface relief pattern. Surface relief, bulk birefringence, and optical polarization grating are shown in phase for clarity only. SEM images (right) show typical azopolymer film response to unpolarized illumination (background) and illumination with polarization grating (foreground). (b) Supramolecular polymer of poly(4-vinylpyridine) (P4VP) and 4-hydroxy-4’-dimethylaminoazobenzene (OH-DMA) sustained by hydrogen bonding interactions. (c) Experimental setup for single-step fabrication of optical microstructures. Laser is 488 nm diode-pumped solid state laser. Polarization gratings are generated by spatial light modulator (SLM) and quarter-wave plates (λ/4), and are focused onto film with microscope objective. White light source, camera, dichroic mirror, and filter facilitate focal alignment and recording of surface grating evolution. (d). Sawtooth voltage (top) and corresponding gray-scale (middle) applied to SLM and resulting spatial polarization distribution (bottom).
2. Materials
Azopolymers have attracted considerable interest, in large part for the volume birefringence grating that accompanies the surface relief grating [15–17]. Although still not fully understood, the interaction between the azobenzene and polymer plays an important role in the volume vs. surface grating tradeoff. Supramolecular azopolymer materials in general are not favorable for the long-term retention of optically-enscribed volume gratings, but they are however an efficient materials system for surface relief generation. Supramolecular systems exploit weak electrostatic coupling between the azobenzene and polymer, usually in the form of hydrogen bonding [18–20], resulting in a self-assembled, inexpensive, and highly modular material for surface microstructures. The results presented here are based on the supramolecular azopolymer 4-hydroxy-4’-dimethylaminoazobenzene and poly(4-vinylpyridine), denoted by OH-DMA- P4VP [19] and shown in Fig. 1(b). Thin films of OH-DMA-P4VP exhibit excellent optical clarity and homogeneity, and complexations up to 1:1 (1 chromophore per 1 polymer repeat unit) are possible without aggregation. All films used in this work were of order 1 µm thickness and were spincast from solution.
3. Experimental setup
The experimental setup for microstructure fabrication on OH-DMA-P4VP films is shown in Fig. 1(c). The central feature is the optical rotator arrangement, consisting of the λ/4- SLM-λ/4 combination, where λ/4 is a quarter-wave optical retarder. This combination rotates the plane of polarization of a linearly polarized laser beam by dθ, where dθ is set by the programmable optical retardation as determined by the gray-scale value addressed to the SLM. Addressing the SLM with a sawtooth pattern as shown in Fig. 1(d) will therefore generate a spatially periodic orientation of linearly polarized light, identical to that formed by the two-beam interferometric arrangement of Fig. 1(a). Jones vector analyses of this configuration for the generation of 2-dimensional linear polarization distributions can be found in [21–26].
The SLM (Meadowlark Optics, Inc.) is a liquid crystal on silicon device, with 1920 × 1152 pixels on an active area of 17.7 mm x 10.6 mm, and is imaged onto the film surface using a microscope objective. The laser source is a 488 nm diode-pumped solid-state laser, which efficiently pumps the cis-trans isomerization of the azobenzene chromophore. Focusing the SLM pattern on the film surface is facilitated using back-reflected 488 nm light from the film surface, a dichroic mirror and camera. After focusing, a blue filter is placed in front of the camera and a white light source illuminates the film through the substrate, allowing the dynamic surface structure evolution to be visualized and recorded.
This experiment maps a gray-scale function to a spatial orientational arrangement of linearly polarized light, ultimately generating a surface relief structure on an azopolymer film. As a basic demonstration, consider the fabrication of a surface relief grating with a sinusoidal profile as shown in Fig. 2. The SLM is addressed with a sawtooth pattern of 84 cycles, which when projected onto the film surface with a 40X objective results in a linear polarization period of 1.5 µm over an exposure area of 126 µm x 80 µm. A 5 sec exposure with 9 mW on the film surface (corresponding to an intensity of 89 × 103 mW/cm2) results in the surface grating shown in the optical micrograph of Fig. 2(a). The peak-to-peak surface modulation over the 20 µm x 20 µm region (outlined in red) of the grating is 300 nm +/- 35 nm, as measured by atomic force microscopy (AFM). Figures 2(b-d) show the SLM pattern and corresponding AFM scans for this region, respectively, while Fig. 2(e) shows an SEM image of the film cross-section. Figure 2(f) shows an example of surface modulation dependence on exposure time, indicating that peak-to-peak modulations of 1 µm are possible. As with all surface structures photopatterned on azopolymers, the grating in Fig. 2 required no post-exposure processing, was available immediately for AFM and SEM characterization, and is stable in ambient conditions.
Fig. 2. (a) Sinusoidal surface relief grating on OH-DMA-P4VP film. Image is from camera in Fig. 1(c) and was taken after 5 sec exposure. Grating area on film is 126 µm x 80 µm (b) Grayscale pattern addressed to SLM and (c,d) resulting AFM scans taken from 20 µm x 20 µm region of film surface outlined in red. Period is 1.5 µm and peak-to-peak surface modulation dpp is 330 +/- 30 nm. (e) SEM image of grating cross-sectional profile. (f) Experiment repeated as a function of exposure time.
4. Experimental results
4.1 Surface relief microgratings—spatially fixed exposure
One advantage of the SLM over the conventional interferometric approach is that the surface microstructure period can be varied by addressing the SLM with the appropriate pattern, with no optomechanical realignment required. For example, consider Fig. 3, which shows the surface modulation dependence on the period for the basic sinusoidal grating. The period was varied from 900 nm to 16.5 µm by simply addressing the SLM with the appropriate pattern and exposing the film in a previously unexposed location. The results show a strong dependence on period, which is not unexpected in systems such as these in which surface deformation occurs via mass redistribution. While the data in Fig. 3 is based on 10 gratings each with a unique period and each requiring a separate exposure, it is straightforward to address the SLM with a pattern that combines multiple periods into a single smoothly varying period profile.
Fig. 3. Surface grating modulation vs period for 10 individual gratings, all created with 5 sec exposure.
This would enable the single-step fabrication of variable-pitch, or chirped gratings, which have applications as waveguide couplers and spectral filters [27–29]. Figure 4 shows the SLM pattern and the resulting surface relief chirped grating with a pitch that varies from 900 nm to 6 µm over a distance 70 µm, obtained from a single 5 sec exposure.
Fig. 4. Chirped grating fabricated in single exposure. (a) grayscale pattern addressed to SLM. (b,c) SEM and optical image of resulting surface relief grating for single 5 sec exposure. (d) AFM image of chirped grating over region outlined in red. Pitch varies from 900 nm to 6 µm over a distance 70 µm.
Independent of its flexibility in defining feature size, the SLM is particularly versatile in generating on-demand surface structures with a rich diversity of geometrical configurations. As one example, consider circular microstructures, which find use as distributed feedback structures in polymer lasers and as diffractive lenses or kinoforms [30–32]. Figure 5 shows the SLM pattern and the resulting surface relief circular gratings, respectively. The circular grating has a pitch of 1.1 µm and was obtained with a single 5 sec exposure.
Fig. 5. (a) Optical image of circular micrograting structure of period 1.1 µm, fabricated in single 5 sec exposure. Yellow outline denotes AFM scan region. (b) AFM scan of circular grating.
More generally, the SLM and the photomechanical azopolymer comprise an efficient and reconfigurable system for fabricating diffractive optical elements. For example, consider Fig. 6(a) in which the SLM was addressed with six sub-patterns, with periods on the film ranging from 2.5 µm to 1.0 µm and orientations ranging from 0 deg to 50 deg in 10 deg steps. A single 5 sec exposure with this pattern resulted in the surface structure in Fig. 6(b-c), with a corresponding diffraction pattern in Fig. 6(d).
Fig. 6. (a) SLM divided into 6 subpatterns of increasing period and orientation. (b-c) Camera and AFM image after 5 sec exposure showing resulting surface grating. (d) +/- 1 diffracted modes of 633 nm laser at normal incidence on surface grating. Color-coded dashed lines identify individual grating periods and orientations.
Additional structures and their respective diffraction patterns are shown in Fig. 7. Multiple exposures can also be implemented, further enhancing the versatility of this system.
Fig. 7. Additional examples of surface gratings and their respective diffraction patterns. Each grating is 126 µm x 80 µm and was fabricated with a single 5 sec exposure.
Figure 8 shows a 2-dimensional surface structure resulting from 2 separate exposures of 5 sec. The first 5 sec exposure was the basic sawtooth SLM pattern (similar to that shown in Fig. 2), resulting in a sinusoidal surface relief grating of period 1.5 µm. The pattern was then rotated 90 deg on the SLM and the process repeated. This capability to Fourier-synthesize arbitrary surface structures via programmable digital polarization optics has considerable applications potential and is the subject of ongoing study.
Fig. 8. (a) Camera and (b) AFM image for two-step exposure with SLM pattern rotated 90 deg between exposures. Each exposure was for 5 sec with sawtooth gray-scale pattern applied to SLM.
4.2 Surface relief grating matrices
To achieve microstructure periodicities of order 1 µm requires focusing the SLM pattern on the film surface, resulting in microstructures of approximate area 126 µm x 80 µm. A technique to obtain larger area structures is to employ multiple exposures, translating the film 126 µm horizontally (or 80 µm vertically) after each exposure. For linear surface relief gratings, this effectively tiles the film surface with a two-dimensional grid of rectangular microgratings. Figure 9(a) shows a grating of period 1.5 µm fabricated by a series of 12,155 exposures (each of 5 sec) obtained by mechanically raster scanning the film between exposures, producing a 1.0 cm x 1.0 cm composite surface grating. The total fabrication time was 20 hours. This process was repeated for different exposure times and the resulting surface modulation of each composite grating was measured with AFM. The diffraction efficiency was measured using a 633 nm HeNe laser at normal incidence, with the results shown in Fig. 9(b). The diffraction efficiency increases with increasing surface modulation, approaching a maximum of approximately 30%. This is the expected maximum efficiency as predicted from the well-known paraxial analysis of gratings with a sinusoidal profile [33,34]. Note also that the boundary of the individual SLM exposures is visible in Fig. 9(a). This is primarily due to inhomogeneous illumination of the SLM, and will impose higher-order structure on the diffracted modes. This can be minimized by over-filling the SLM aperture combined with more precisely overlapping the fringe structures at the intersection of adjacent exposures.
Fig. 9. (a) 1 cm x 1 cm grating obtained from tiling 12,155 individual microgratings. Constituent gratings have 1.5 µm period and approximately 300 nm dpp obtained with 5 sec exposure. Film was mechanically translated between exposures. (b) Experiment repeated as a function of exposure time. Diffraction efficiency scales with peak-to-peak surface modulation, approaching 30% maximum value.
Combining mechanical raster-scanning with the SLM’s capability to define the configuration, period, and orientation of surface microstructures enables the fabrication of a wide range of macroscopic diffractive structures, with representative examples shown in Fig. 10. The left image in Fig. 10 shows an array of 9 macroscopic gratings, each of area 2 mm x 2 mm. Each of these is the raster-scanned version of its respective micrograting shown in Fig. 7. As a specific example, consider the lower right element (white), which is the raster-scanned version of the lower right grating in Fig. 7. Successive magnification reveals the 336 constituent gratings, each of which is a 126 µm x 80 µm micrograting containing 4 subgratings of different periods. The viewer is therefore presented with a spectral mixture, resulting in a white appearance.
Fig. 10. Tiled version of microgratings in Fig. 7, obtained via raster-scanning. Each is 2.0 mm x 2.0 mm. Lower right grating (white) is raster-scanned version of corresponding lower right grating in Fig. 7. Magnification reveals 336 constituent gratings, each consisting of single 126 µm x 80 µm micrograting.
4.3 Aperiodic microstructures
Using the SLM to project spatially structured polarization fields offers a distinct advantage over two-beam interferometric methods in that aperiodic structures can be easily fabricated. Consider that a grayscale sequence of 128 to 64 to 0 (i.e. the sawtooth function as shown in Fig. 1(d).) generates an optical polarization rotation of 0 deg to 90 deg to 180 deg. This subsequently generates a sinusoidal surface pattern of the same period, although it is impossible to determine the relative phase between the optical polarization and the surface pattern.
Now consider Fig. 11(a), which shows a series of experiments in which the grayscale sawtooth pattern addressed to the SLM is sequentially cropped. The rightmost figure shows a single, aperiodic, surface feature in response to a single sawtooth of 128 to 64 to 0. The material response to this polarization rotation of 180 deg results in a redistribution of mass away (i.e. a trough) from the center of the feature. By phase shifting the polarization pattern by 90 deg so that the grayscale sawtooth goes from 64 to 192 to 64, the same experiment results in a redistribution of mass towards the center of the feature (i.e. a crest) as shown in Fig. 11(d). This can be leveraged to photofabricate aperiodic surface structures, with some examples shown in Fig. 11(b,c). For example, the vertical features of the cube are composed of troughs (i.e. the rightmost figure in 11) of approximately 400 nm depth, while the horizontal features are composed of crests of similar size. These experiments demonstrate that the photogeneration of aperiodic surface distortion in these materials is driven not only by the optical polarization gradient (and resulting alignment of the azobenzene), but also by the initial phase of the polarization pattern. The capability of the SLM to precisely define optical polarization distributions in supramolecular azobenzene films will therefore contribute to a deeper understanding of the role played by molecular architecture and azobenzene-polymer coupling in the photomechanical writing of microstructures in these materials.
Fig. 11. Photofabrication of aperiodic surface microstructures. (a) Three experiments showing sequential cropping of grayscale sawtooth addressed to SLM (top row) and AFM image of resulting surface feature (bottom row). Rightmost image shows single optical polarization cycle resulting in single, aperiodic surface feature of negative displacement. (b,c) Examples of various aperiodic structures fabricated using single polarization cycles. (d) 90 deg phase-shifted optical polarization results in surface feature of positive displacement.
5. Conclusions
In conclusion, we have demonstrated a versatile platform for the programmable fabrication of periodic and aperiodic surface microstructures by combining the photomechanical response of supramolecular azopolymers with structured polarized illumination from a high resolution spatial light modulator. Topographical relief gratings with surface displacements up to 1.0 µm and with periods 900 nm - 16.5 µm can be fabricated with a single 5 sec exposure at 488 nm. Sinusoidal, circular, and chirped surface profiles can be written via direct programming of the spatial light modulator.
Several unique advantages of this system emerge via the azopolymer-SLM combination. The photomechanical response that drives the surface deformation is quite different from the photochemical response of typical materials used for photofabrication. The surface structures demonstrated here grow immediately in response to illumination, can be visually observed in real time, and require no post-exposure processing. Likewise, the entire process from film fabrication through exposure can be accomplished in normal room lighting. Perhaps most interesting is the inherent reversibility of structures generated through a photomechanical process. In a previous work we demonstrated that sinusoidal surface gratings of period Λ on a OH-DMA-P4VP film can be erased and re-written by mechanically translating the film Λ/2 and re-exposing [35]. This same reversibility is even easier to access in the present work, by merely shifting the pattern addressed to the SLM. Finally, the surface microstructures fabricated here can be replicated using nanoimprint lithography, enhancing their potential for applications. Results of these all-optical pattern reversibility and nanoimprint lithography studies will be presented in future publications.
Funding
National Science Foundation Division of Electrical, Communications and Cyber Systems (2024118); National Science Foundation Directorate for Mathematical and Physical Sciences (1919557).
Acknowledgments
J. Stobelt, D. Stolz, and M. Leven acknowledge travel support from the German Academic Exchange Service (DAAD). D. McGee thanks Georg Sommerer for collaborator recommendations and technical assistance.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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