Azopolymer-Based Micro- and Nanopatterning for Photonic Applications

Transcription

1 POLYMER SCIENCE REVIEW Azopolymer-Based Micro- and Nanopatterning for Photonic Applications Arri Priimagi, Andriy Shevchenko Department of Applied Physics, Aalto University, Aalto FI-00076, Finland Correspondence to: A. Priimagi (E- mail: or A. Shevchenko (E- mail: Received 6 September 2013; accepted 17 September 2013; published online 12 October 2013 DOI: /polb ABSTRACT: Azopolymers comprise a unique materials platform, in which the photoisomerization reaction of azobenzene molecules can induce substantial material motions at molecular, mesoscopic, and even macroscopic length scales. In particular, amorphous azopolymer films can form stable surface relief patterns upon exposure to interfering light. This allows obtaining large-area periodic micro- and nanostructures in a remarkably simple way. Herein, recent progress in the development of azopolymer-based surface-patterning techniques for photonic applications is reviewed. Starting with a thin azopolymer layer, one can create a variety of photonic elements, such as diffraction gratings, microlens arrays, plasmonic sensors, antireflection coatings, and nanostructured light-polarization converters, either by using the azopolymer surface patterns themselves as optical elements or by utilizing them to microstructure or nanostructure other materials. Both of these domains are covered, with the aim of triggering further research in this fascinating field of science and technology that is far from being harnessed. VC 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, KEYWORDS: azo polymers; isomer/isomerization; lithography; nanotechnology; periodic arrays; photonics; surface-relief grating INTRODUCTION Azobenzene is a molecule with long history and many faces. Its photochemical isomerization between trans- andcis-forms [Fig. 1(a)] was first observed already in But ironically, as azobenzene derivatives have been extensively used as dyes and colorants, much of the research that followed focused on preventing the photoisomerization reaction (and the subsequent color change) rather than exploiting it. 2 Nowadays, however, photoisomerization and its use in the design of functional materials comprise the very key of azobenzene research. Rather than being used as simple light absorbers, azobenzene derivatives allow for controlling light with light through photomodification of the material systems they are incorporated into. They are attractive in view of various applications that make use of photon absorption, 3 6 controllable light emission or frequency conversion, 7 10 and light propagation The photoisomerization reaction can be exploited in context with functional monolayers, 16,17 gels, 18,19 polymers, 20,21 and liquid crystals, 22,23 and it allows for photocontrol over the structure and function of biomaterials 24,25 and molecular machines. 26,27 The multifaceted nature of azobenzene research is well illustrated in two recent books on azobenzene-based functional materials, with coverage ranging from pure photochemistry and nonlinear optics to light-controllable nanocarriers and optomechanical energy conversion. 28,29 Herein, we concentrate on azobenzene-containing polymers (azopolymers), representing a materials platform that combines the ease of processing and high optical quality of polymers with the photocontrol provided by the azobenzene units. Following the division originally made by Natansohn and Rochon, 20 the motions triggered by the photoisomerization process can be classified into different length scales, each of which comprises a vibrant and active field of research on its own. The basic molecular-level process is the trans cis photoisomerization reaction itself, which for the parent azobenzene involves a decrease in the distance of the para-carbon atoms from about 10 to 6 Å [Fig. 1(a)]. This is a huge geometrical change, generating a significant nanoscale force 30,31 that can be further harnessed in macroscopic photoactuation. 32,33 Another molecular-level process is photoalignment of randomly oriented non-interacting azobenzene molecules within the polymer matrix Upon irradiation with linearly polarized light of appropriate wavelength, the azobenzene molecules statistically reorient and accumulate to the direction perpendicular to the polarization plane [Fig. 1(c)]. The resulting molecular alignment gives rise to optical anisotropy that can be erased by irradiating the sample with circularly polarized or unpolarized light. 20,37 At the mesoscopic scale, the molecular-level processes can be greatly amplified through collaborative motions that arise VC 2013 Wiley Periodicals, Inc. POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

2 REVIEW POLYMER SCIENCE Arri Priimagi earned his Ph.D. Degree in 2009 from the Department of Applied Physics, Helsinki University of Technology (nowadays part of Aalto University), Finland. After a two-year postdoctoral fellowship at Tokyo Institute of Technology, he returned to Aalto University where he presently works as a postdoctoral fellow. His main research interests involve the development of polymeric and supramolecular materials for optics and photonics and the design of light-responsive functional materials. Andriy Shevchenko graduated in optical physics in 1993 from Taras Shevchenko National University of Kyiv, Ukraine. In 2004, he received his D.Sc.(Tech.) degree from Helsinki University of Technology, Finland. In 2007, he was appointed as Docent in optical physics. Since 2012, he holds the position of University Lecturer at Aalto University, Finland. His research interests include theory of light-matter interaction, statistical optics, nanooptics, optical nanomaterials, and methods of micro- and nanofabrication. from intrinsic order within the polymer system. The best examples of this are photoaddressable and liquid-crystalline (LC) azopolymers, which are the materials of choice for azobenzene-based holographic data storage Yet another intriguing feature of azobenzene LCs and LC azopolymers is that isomerization of only a small fraction of azobenzene molecules can significantly (and reversibly) destruct the molecular alignment or even give rise to LC-to-isotropic phase transition The photocontrol over molecular alignment is the key factor behind optical-to-mechanical energy conversion in various light-fuelled motions in free-standing LC azopolymer films [Fig. 1(d)] Due to the lack of cooperative molecular motions, photoinduced actuation is rather weak in amorphous azopolymers. These azopolymers, however, possess another intriguing characteristic, the ability to form surface-relief patterns. 20,47 As an example, when a thin FIGURE 1 (a) Trans-cis isomerization of azobenzene. (b) Chemical structure of poly(disperse Red 1 acrylate). (c) Schematic illustration of the photoalignment of azobenzenes with polarized light. (d) Illustration of photoinduced bending of free-standing azopolymer films upon UV irradiation. The illumination comes from the top and the bending direction is dictated by the molecular director alignment. Reproduced from Ref. [50], with permission from Springer. (e) Schematic representation of the SRG inscription process. An atomic-force micrograph and a surface profile of an inscribed grating are shown on the right. Redrawn from Ref. [51]. 164 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

3 POLYMER SCIENCE REVIEW amorphous azopolymer film is irradiated with an optical interference pattern, the material starts to migrate and move away from high-intensity areas to form a replica of the incident irradiation in the form of a surface-relief grating (SRG) [Fig. 1(e)]. The SRG formation was first observed in 1995 by the Natansohn/Rochon and Tripathy/Kumar research teams. 48,49 They immediately recognized the immense potential of this simple, one-step approach to fabrication of periodic microstructures. The focus of this review lies on the use of azopolymer surface patterns for applications in photonics. The field of photonics covers various techniques to control the creation, propagation, and detection of photons for a wide range of applications 52 analogously to electronics, that accomplishes similar tasks with the help of electrons. High-quality azopolymer surface patterns with easily controllable surface topology can be readily inscribed using a simple procedure and, if necessary, erased, and reconfigured at will. Furthermore, once inscribed, the patterns will preserve their quality at least over several years under ordinary storage conditions. Owing to these features, azopolymers may possess significant application potential in view of fabrication of a large variety of photonic elements, such as diffraction gratings, 53,54 microlens arrays, 54,55 photonic crystals, 56 nanostructured polarizers and wave plates, broad-band antireflection (AR) coatings, 60,61 perfect absorbers, 62 plasmonic nanostructures, and even some of recently proposed optical metasurfaces and metamaterials. 66,67 The azopolymer surface patterns can be used either directly as functional optical materials or indirectly, by using them to microstructure or nanostructure other materials. Recent developments in both domains are reviewed herein, with the aim of launching further research in this highly fascinating area. AZOPOLYMER SURFACE PATTERNING Since its discovery in 1995, 48,49 photoinduced surface-relief formation has triggered a great amount of both fundamental and applied research. The interest toward this topic is twofold. From the perspective of fundamental science, it is important to understand how light can cause macroscopic movements of glassy materials at even more than 100 C below the glasstransition temperature of the polymer. This intriguing question is still unresolved. Although various models have been proposed to account for the macroscopic movements of azopolymers, none of them is able to explain all the experimental observations that may considerably vary in different types of material systems. The fundamental and applied aspects are nonetheless interrelated, and thorough understanding of the light-induced polymer motions is an essential prerequisite for their efficient use in photonic applications. In amorphous azopolymers (as opposed to LC azopolymers), efficient photoinduced surface-pattern formation is thought to be driven by continuous trans-cis-trans cycling of the azobenzene molecules. For this reason, the azobenzene units are typically substituted with electron donor and acceptor groups for which both trans-to-cis and cis-to-trans isomerization can be induced using the same (blue-green) light. 76 A typical azopolymer used for SRG inscription, poly(disperse Red 1 acrylate), is shown in Figure 1(b). The azobenzene units can be attached to polymer chains either covalently or noncovalently, but it is important that there is a well-defined connection; if the interaction is overly weak or if the constituents are simply mixed together, the pattern formation turns out to be very inefficient When an amorphous azopolymer is exposed to a light interference pattern or to a focused laser beam, such that the intensity is not too high, the material migrates from the illuminated to the dark areas, 81,82 referring to the case where the electric-field component directed parallel to the grating vector is not spatially modulated or is simply absent. If the intensity profile is flat, but the amplitude of the electric-field component in question is modulated, the material as a rule piles up in the areas where this amplitude is small. In some cases, however, no intensity or polarization gradient is required; spontaneous surface patterns have been formed even upon irradiating the sample with a collimated laser beam The light-induced azopolymer movements are highly sensitive to the polarization state of the incident light. 87 Typically, s-s geometry (where the pattern is formed by two interfering s-polarized waves) yields only weak gratings whereas p-p and RCP-LCP geometries give rise to much stronger gratings; here RCP and LCP stand for right- and left-handed circular polarizations, respectively. Based on recent experiments, the relative efficiency of the s-s versus p-p geometries may in some materials depend on the molecular weight of the azopolymer. 88 In this context, we would like to bring out an in-situ near-field technique to observe in real time both the light interference pattern and the changes in the azopolymer surface topography, which has revealed that in the s-s geometry, the crests of the SRG actually coincide with the intensity maxima (due to photoisomerization-induced expansion of the material 92 ), whereas in the p-p geometry the grating undergoes a phase shift during the inscription process and the crests gradually move from the intensity maxima to the intensity minima. The polarization-sensitivity of the azopolymer movements is the key feature in view of applications in photochemical imaging and, for example, directional photofluidization lithography. Although the described principles of the surface relief formation hold for many amorphous azopolymers, we once again underline the fact that the mechanisms of the SRG formation can differ in different materials. It would in our opinion be very useful to understand the differences in the behavior of amorphous and LC azopolymers. Also the fact that some photochromic molecules that do not exhibit any trans-cistrans cycling have been successfully used for obtaining SRGs 96,97 can bring additional knowledge on the SRG origins. Finally, we think that the photoinduced mechanical changes observed in various azopolymers must play a role in the macroscopic motions of the material systems. A complete self-consistent theory must be able to account for all possible factors that, for a given azopolymer, can influence the surface pattern formation. POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

4 REVIEW POLYMER SCIENCE FIGURE 3 Microscope and diffraction images of (a) hexagonal, (b) tetragonal, and (c) hierarchical multilayer structures created by using azopolymer SRGs. Reproduced from Ref. [118], with permission from IOP Publishing. FIGURE 2 Atomic-force microscope (AFM) images of (a) oneand (b) two-dimensional azopolymer SRGs. Reproduced from Ref. [116] with permission from Royal Society of Chemistry. AZOPOLYMER SURFACE PATTERNS AS DIFFRACTION GRATINGS Both transmissive and reflective diffraction gratings, which are widely used in optics and photonics, are easy to create using azopolymers. Transmissive phase gratings can be obtained by holographically inscribing an SRG onto an azopolymer film. Reflective gratings can in turn be created by coating an already patterned polymer film with a reflective material, such as metal; note that other coating materials have also been used for improvement of optical properties, quality, and stability of the gratings The overall diffraction efficiency of an azopolymer SRG is always a sum of multiple effects. For instance, bulk reorientation of the azobenzene molecules upon illumination gives rise to a birefringence grating that in principle is always present, but whose contribution to the overall diffraction efficiency of thin films of amorphous azopolymers is small. 78,112,113 As another example, annealing of an azopolymer SRG may give rise to the formation of a density grating below the film surface. 114,115 Even if it is important to account for the contributions arising from these different gratings, in practice it often suffices to consider only the SRGs. For an azopolymer SRG to exhibit high diffraction efficiency, the thickness modulation of the polymer layer must be large enough. Figure 2 illustrates examples of such one- and twodimensional gratings, of which the former can exhibit > 30% first-order diffraction efficiency for visible light. 116,117 It is remarkable that SRGs can be arranged on top of each other by planarizing them using spacer layers of other polymers, which allows one to create three-dimensional diffractive azopolymer structures. 118,119 As an example, Figure 3 shows microscope and diffraction images of (a) hexagonal, (b) tetragonal, and (c) more sophisticated hierarchical multilayer structures fabricated by using such planarized and stacked SRGs. 118 This example demonstrates the possibility to use azopolymers also for creation of three-dimensional photonic crystals and even optical nanomaterials. Azopolymer gratings can also be used to fabricate so-called distributed Bragg reflectors, in which the effective refractive index is modulated along the light propagation direction. If the modulation period is close to k/2, light is fully reflected, penetrating into the grating only to some extent. Such gratings can be applied as spectral filters in optical waveguides and frequency-selective mirrors in laser resonators An example of using an SRG-based Bragg reflector for the creation of a distributed feedback (DFB) laser is introduced in Ref. [127]. The principle of the device is illustrated in Figure 4. The optical inscription of an SRG onto a flat polymeric laser-gain medium [Fig. 4(a)] provides the necessary laser feedback and, by this, switches the device operation from amplified spontaneous emission to lasing [Fig. 4(b)]. Figure 4(c) presents the laser emission spectra, of which the one shown by the green line represents the most stable narrow-band laser operation. An example of a diffraction grating obtained by coating an azopolymer SRG with a layer of titanium dioxide is shown in Figure 5(a). 128 The grating is designed to act as a guidedmode resonant filter that shows sharp dips in the transmission spectrum (along with the corresponding sharp peaks in the reflection spectrum) at normal incidence. The measured and theoretically predicted transmission spectra of the grating are shown in Figures 5(b,c), respectively, for both TE and TM polarizations. The sharp spectral features correspond to the resonant coupling of the incident light to the leaky guided modes of the structure. The observed good agreement between theory and experiments implies that the grating quality is reasonably high. As a rule, diffraction gratings fabricated by applying ordinary photoresist-based lithography are binary, meaning that the grating profile is square-shaped. 54 Holographically recorded smooth sinusoidal gratings could be beneficial compared with the binary gratings, for example, due to reduced unwanted scattering at sharp geometrical features. Usually, however, the assumed limit of 34% for the first-order diffraction efficiency of a sinusoidal phase grating compared 166 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

5 POLYMER SCIENCE REVIEW FIGURE 4 Illustration of an azobenzene-based DFB laser. The device produces (a) an amplified spontaneous emission and (b) lasing before and after the inscription of an SRG, respectively. (c) Emission spectra of the device. Reproduced from Ref. [127], with permission from Wiley. FIGURE 5 (a) SEM image of a TiO 2 -coated SRG. (b) Measured and (c) calculated zero-order transmittance spectra of the grating. Reproduced from Ref. [128], with permission from Optical Society of America. with about 40% calculated for a binary grating 53,129 renders the sinusoidal gratings less attractive. In this respect, we would like to emphasize that these numbers are correct only for diffraction gratings that are treatable within the paraxial approximation of traditional Fourier optics. For gratings beyond this approximation, such as those having a period of few wavelengths or smaller and phase modulation amplitude close to p, the maximum values of the first-order diffraction efficiency can be different. 54,130 In fact, the first-order diffraction efficiency of a reflective sinusoidal phase grating can reach even the level of 100%, for example, when the grating operates near the regime where the diffraction order becomes evanescent. 130 First-order diffraction efficiencies close to or higher than, say, 50% have not been demonstrated with gratings based on azopolymers, but values exceeding the 34% theoretical limit for sinusoidal gratings have been reported in several instances Unlike linear one- and two-dimensional diffraction gratings, gratings with curved fringe patterns have not been studied much. As an exception, circular SRGs have been inscribed on azopolymer films by using a Bessel beam. Figure 6(a,b) present the theoretical and experimental SRG patterns, respectively, inscribed with a radially polarized Bessel-like laser beam. 136 Another example is shown in Figure 6(c). The depicted ring SRG was recorded at the end facet of an optical fiber by using a circularly symmetric interference pattern of the fiber modes. 137 We lastly mention that an interference pattern of a collimated and a focused optical beam was recently used to create chirped holographic lenses on an azopolymer film. 138 In spite of the fact that these lenses were not as good in their performance as typical refractive lenses, we consider the obtained results to be the first successful demonstration of focusing light with curved diffraction SRGs on azopolymer films. Zone plates and diffractive axicons are particular examples of curved grating patterns. They are circularly symmetric diffractive structures that in principle act as lenses. The zone plate focuses light similarly to an ordinary refractive lens, but it can be designed to have a higher numerical aperture and hence allow focusing light into a smaller focal spot. 105,139,140 An axicon, in turn, is an element that transforms an incident light beam into a narrow divergence-free Bessel-like beam. 141 To our knowledge, high-quality azopolymer-based zone plates and axicons have not yet been demonstrated, but we think that there are no substantial obstacles for such demonstration in the future, especially in the reflection geometry. Recall that the optical path-length variation in a reflective SRG is twice as large as its thickness modulation depth. POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

6 REVIEW POLYMER SCIENCE FIGURE 6 (a) A theoretical and (b) the corresponding experimental circular SRG pattern inscribed in an azopolymer film with a radially polarized Bessel beam. Reproduced from Ref. [136], with permission from Optical Society of America. Case (c) illustrates a ring SRG created at the end facet of an optical fiber. Reproduced from Ref. [137]. We believe that a zone plate could be inscribed holographically by using a superposition of two coaxial converging and diverging beams with opposite circular polarizations. An axicon should be possible to create by using an auxiliary microfabricated diffractive axicon as a transmission mask 142,143 and an incident light beam with circular polarization. Another option is to apply so-called proximity field nanopatterning 144,145 to inscribe a diffractive axicon onto an azopolymer film through an appropriately designed microfabricated amplitude mask. The principle of this technique and examples of experimentally obtained surface relief patterns are shown in Figure 7. A fascinating example of microscopic spiral structures inscribed in an azopolymer with a focused Laguerre-Gauss vortex beam is given in ref. [146]. Figure 8 shows the dependence of the shape of such a spiral pattern on the beam vortex topological charge (that is the quantity defining the orbital angular momentum of the beam). The handedness of the pattern is determined by the handedness of the incident wavefront. Although these patterns are not diffraction gratings, they demonstrate a clear possibility to create high-quality curved surface-relief structures on an azopolymer film. Interestingly, the authors explained the spiral mass transport through interference between longitudinal and transverse optical field components, made possible by the symmetry 75,146, see also 147 breaking at the polymer-air interface. PHOTOCHEMICAL IMAGING OF OPTICAL NEAR FIELDS FIGURE 7 (a) The principle of the proximity-field patterning technique. (b) and (c) illustrate examples of surface relief patterns created using this technique. The measured profiles (left) are in perfect agreement with the corresponding theoretical profiles (right). Reproduced from Ref. [144], with permission from IOP Publishing. Another way to make use of azopolymer mass transport is to map, or photochemically image, optical field distributions with a subdiffraction-limited resolution. A natural starting point for this topic is provided by the work of a Japanese team of researchers who observed that irradiation of an azopolymer film through dielectric spheres cast onto the film gives rise to topographical changes of the film [Fig. 9(a)] Dents were always formed directly under the spheres, but for small spheres (that act as electric dipoles) the shape of the dents did not correspond to the shape of the spheres. 150 These results could not be completely explained by the far-field intensity distribution at the azopolymer surface, neither could they be explained purely by photoinduced softening due to trans-cis-trans cycling. A mechanism based on the gradient force of the optical near field was then proposed. 150 Later on, the same authors demonstrated an interesting application of their observations as light-triggered immobilization of various biomolecules (such as DNA, 151 immunoglobulin G, 151,152 actin filaments, 153 and viruses 154 ) on azopolymer surfaces [Fig. 9(b)]. 168 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

7 POLYMER SCIENCE REVIEW FIGURE 9 (a) Atomic-force micrograph ( mm 2 ) of the photoinduced deformation caused by 500 nm polystyrene spheres cast on an azopolymer film. Reproduced from Ref. [148], with permission from Elsevier. (b) Schematic illustration of photoinduced immobilization of biomolecules on azopolymer surfaces. Reproduced from Ref. [151], with permission from American Chemical Society. FIGURE 8 Vortex-beam induced spiral surface relief patterns on an azopolymer film. The vortex topological charge is equal to 1 in (a) and (b), 2 in (c) and (d), 5 in (e) and (f), 10 in (g) and (h). The left and right profiles illustrate the measured and theoretical topologies, respectively. The scale bar corresponds to 900 nm. Reproduced from Ref. [146], with permission from Nature Publishing Group. In context of near-field optics, azopolymers have been used to visualize the optical-field distributions at the vicinity of probe tips of near-field optical microscopes, and to map the optical response of surface-mounted copper nanoislands. 159 As an example, Figure 10(a,b) display the surface deformation of an initially flat film irradiated through a metal-coated tapered optical fiber. 160 In (a), the tip-tosample distance was 130 nm, corresponding to the far-field illumination, and the azopolymer migrated away from the illuminated area along the direction dictated by the light polarization. In (b), the tip sample separation was set to be smaller than 10 nm and, inversely to the far-field illumination, the sample formed a circular protrusion with a 65 nm diameter. Such near-field protrusions were further used to pattern the surface with a k/10 resolution [Fig. 10(c)]. More recently, azopolymers have been employed to map the optical near fields around various types of metal nanostructures Typically, a thin azopolymer layer is spin coated onto a plasmonic array, and the topographic modification upon irradiation is observed to closely follow the near-field intensity distribution around the structures. 161,167 Imaging resolution as high as k/35 has been reported. 166 Figure 11 illustrates the power of the method for silver nanodisk arrays on a glass substrate. 161 The surface deformation depends on the polarization of the incident illumination: linearly polarized light gives rise to formation of two dents along the polarization direction [see (a) and (b)], around the positions where the local field intensity is at maximum [(e)], whereas circularly polarized light leads to more symmetric, rectangular deformations [(c) and (d)] that again match rather well with the computed near-field distributions shown in (f). Another example is an azopolymer-based imprinting of surface-plasmon interference patterns, 168,169 illustrated in Figure 12. Interference of surface plasmons created by using the four planar diffraction gratings of Figure 12(a) gives rise to surface deformation of the azopolymer film [(c)] that perfectly matches with the simulated plasmon interference pattern shown in (e). A good agreement between experiment [(d)] and the simulation [(f)] is obtained also for a circular plasmonic grating shown in (b). This result represents another evidence of the possibility to create curved SRG patterns. For the examples given, similar visualization can be obtained also with conventional photoresists, but azopolymers provide a more facile one-step imaging. AZOPOLYMER-BASED MICROSTRUCTURING AND NANOSTRUCTURING Quite recently, it has been realized that patterned azopolymer films can be used as molds and etching masks for fast POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

8 REVIEW POLYMER SCIENCE FIGURE 10 Shear-force images of the surface deformation of an azopolymer film upon (a) far-field and (b) near-field illumination. The arrows indicate the polarization direction of the incident light. (c) An image of an array of protrusions inscribed on an azopolymer film. The lateral size of each protrusion is 55 nm and their mean height is 5 nm; the central dot has been deliberately omitted. Reproduced from Ref. [160], with permission from American Institute of Physics. and cost-effective microstructuring and nanostructuring of other materials. This has significantly broadened the application potential of azopolymer SRGs, since they can now be used as a starting point for creation of periodic (or selfassembled quasiperiodic ) structures out of essentially any material or combination of materials, and in a variety of shapes. In addition to the already discussed azopolymerbased diffraction gratings that can indeed be used to control light emission, 122, , absorption, and transmission, 123,182 one can also create such photonic elements as microlens arrays for optical signal processing and wavefront sensing, 183 molecular alignment layers for liquid-crystal displays, 22, and various nondiffractive subwavelength gratings that can be used, for example, as ultrathin broadband polarizers and phase retarders, 57 59, moth-eye-type broadband AR coatings, 60,61 and plasmonic microstructures and nanostructures for near-field optical applications including surface-enhanced Raman scattering (SERS) and optics of metamaterials. 65,107,108,190,191 We notice that essentially all FIGURE 11 Photochemical imaging of a silver nanodisk array. Surface deformation upon linearly polarized (a) and (b) and circularly polarized (c) and (d) excitation. (e,f) Present the negatives of the near-field intensity distribution for linear and circular polarizations, respectively. Reproduced from Ref. [161], with permission from American Chemical Society. these periodic structures can be fabricated also with the help of other microfabrication and nanofabrication techniques, such as traditional photoresist-based optical lithography and electron-beam lithography. Compared with them, however, the azopolymer-based approach is remarkably simple and convenient, because (1) the SRGs are inscribed purely optically and on a large surface area, (2) the material is not sensitive to ambient light, owing to which the patterned films stay unchanged under ordinary room lighting, and (3) they can be easily reconfigured or simply erased. Microfabrication and nanofabrication techniques making use of azopolymers can conditionally be divided into three groups. In the first group, SRGs are used as molds for imprint-like fabrication purposes. In the second group, they are used as masks for etching the underlying material, although at subsequent stages of the fabrication the imprint approach can be applied as well. The third group involves a 170 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

9 POLYMER SCIENCE REVIEW FIGURE 12 Photochemical imaging of surface-plasmon interferences. (a,b) Show atomic-force micrographs of the plasmonic structures to be exposed. The arrows indicate the polarization direction of the incident illumination. (c f) AFM (top) and simulated (bottom) images of the printed interference patterns. Reproduced from Ref. [168], with permission from Optical Society of America. technique named as directional photofluidization lithography that is based on a combination of means of microfluidics with directional control of azopolymer motions under illumination with polarized light. SRGs as Molding Templates The principle of essentially all microfabrication and nanofabrication techniques based on the use of SRGs as molding templates is as follows. First, an azopolymer is spin-coated on a flat substrate (usually a glass plate) and an SRG is inscribed on it by interferometric means. Then, the SRG is covered with another material that, after solidification, is detached from the SRG. The obtained stamp can be further used as a mold to create replicas of the original SRG. 192 Figure 13 illustrates the use of this approach in fabricating a DFB laser. 178 In the first step [Fig. 13(a)], an SRG is inscribed onto an azopolymer film, after which a glass cell is formed above the SRG and filled with a UV-curable adhesive. Then the adhesive is polymerized by UV illumination, and the cell FIGURE 13 Fabrication of an optically pumped nanosecondpulsed DFB laser; k nm, FWHM 2 nm. (a) The fabrication steps. (b) The azopolymer SRG (left) and its stamp in a polymerized UV adhesive (right). (c) A photograph of the radiating laser (right) and the laser beam spot (left). Reproduced from Ref. [178], with permission from Japanese Society of Applied Physics. is taken apart to release the obtained photopolymer stamp. Finally, a conducting polymer that plays the role of the laser gain medium is spin coated onto the stamp. Figure 13(b) presents AFM images of the azopolymer SRG (left) and the obtained photopolymer stamp (right). The operating laser and the produced laser-beam spot are illustrated in Figure 13(c). Compared with other approaches of fabricating DFB lasers, the azopolymer-based method is very fast and simple. SRGs have also been used for fabrication of organic solar cells, with the purpose of creating a periodic light-trapping structure in the solar-cell body. The structure acts as a diffraction grating that increases the effective path length of photons in the active layer and thereby the probability of their absorption Figure 14 shows a technique to fabricate such an organic solar cell. 180 An azopolymer SRG is POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

10 REVIEW POLYMER SCIENCE FIGURE 14 Fabrication of organic solar cells using azopolymer SRGs. (a) Creation of a PDMS stamp with the help of an SRG mold. (b) The obtained PDMS stamp. (c) The steps of fabrication of a solar cell using the created stamp. Reproduced from Ref. [180], with permission from American Institute of Physics. used as a master mold for creating a PDMS stamp [Fig. 14(a)]. An AFM image of the stamp is shown in Figure 14(b). Figure 14(c) shows the reported fabrication process flow that includes soft contact imprinting of the SRG onto the active layer and thermal evaporation of calcium and aluminum onto its patterned surface. Such structuring increased the cell efficiency by about 15%. Later on, the technique was used to imprint two-dimensional SRGs onto the solar cells, which increased the photon-to-current conversion efficiency even further. 181 A technique to fabricate close-packed microlens arrays using azopolymer SRGs as templates has been reported in ref. [183]. A two-dimensional SRG on an azopolymer film is used to create a PDMS mold, with the help of which the pattern is imprinted into a UV-curable photopolymer used then as an array of microlenses. Figure 15(a) shows AFM images of the fabricated tetragonal and hexagonal microlens arrays (the thickness modulation depths exceed 500 and 300 nm, respectively), and Figure 15(b) illustrates their optical micrographs taken exactly at the focal plane. The optical images clearly show good focusing performances of the created microlenses. The described microfabrication techniques are representative examples of the use of azopolymer SRGs as molding templates for creation of polymer-based photonic elements. No doubt that in the future many new photonic applications will FIGURE 15 (a) AFM images of a tetragonal (left) and a hexagonal (right) microlens array. (b) The corresponding focal-plane optical micrographs of the fabricated arrays. The scale bar in the optical images is 5 mm. Reproduced from Ref. [183], with permission from Optical Society of America. be realized by using this simple, cost-effective and productive approach. We emphasize, however, that microfabrication techniques in which azopolymer SRGs can be used as templates may significantly differ from the techniques described above (see, e.g., refs. [ ]). In ref. [194], for example, colloidal nanoparticles of titania (with an average diameter of <10 nm) were deposited by using a sequential layer-bylayer electrostatic method and aggregated in the grooves and dimples of one- and two-dimensional SRG templates to yield accurate periodic patterns of TiO 2 (see also Ref. 196). Another way of using azopolymer SRGs as nanostructuring templates is illustrated in Figure 16(a). Herein, a twodimensional SRG is first covered with gold using electronbeam deposition, after which the crests of the gold-coated grating are cut away using ion-beam milling to yield plasmonic array structures with various shapes. The SRG inscription parameters allow controlling the symmetry of the structures (two exposures are required to obtain a twodimensional SRG). The type of the structure is determined by the ion-beam milling time, that is, a short milling time gives rise to dielectric openings in the gold film whereas a long milling time leads to formation of discrete gold nanoislands. A collection of plasmonic nanostructures fabricated with the aforementioned method is given in Figure 16(b). 191 SRGs as Etch Masks Similarly to patterns created in photoresists, azopolymer SRGs can be used as etch masks for periodic structuring of the substrate. The SRGs can be inscribed over a large periodicity range, say from 250 nm to 10 mm. For fabricating periodic structures within this periodicity range, azopolymerbased interference lithography can in our opinion be a better choice than the conventional lithography using photoresists. 172 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

11 POLYMER SCIENCE REVIEW FIGURE 16 (a) Schematic representation of a fabrication method for obtaining subwavelength-sized plasmonic structures from azopolymer SRGs. (b) A collection of scanningelectron micrographs from structures fabricated with the process outlined in (a). Reproduced from Ref. [191], with permission from Royal Society of Chemistry. As we have already mentioned, the photoinduced SRG inscription can be performed under normal room lighting. Furthermore, azopolymers respond to a wider spectral range of light and exhibit better tolerance to overexposure than traditional photoresists. Owing to this, by applying multiple interference exposures one can easily create complex surface patterns on the azopolymer films. For an azopolymer SRG to function as an etch mask, the polymer must be removed from the grating grooves to reveal the substrate. This can be achieved by partially etching the azopolymer using, for example, reactive ion etching (RIE) in oxygen plasma. 123,197,198 Since the SRGs typically have a nearly sinusoidal surface profile, the etching time can be used as a parameter for tuning the open area of the substrate, and hence the dimensions and to some extent the geometry of the mask note the similarity with the last example in the preceding subsection (Fig. 16). For a 2D grating, for example, a shorter etching period results in an array of holes in the mask, while a longer etching time would yield an array of discs. 190 FIGURE 17 AFM images of (a) an azopolymer SRG, (b) a mask obtained by etching the polymer in oxygen plasma, and (c) an ITO layer etched through the created mask. Reproduced from Ref. [197], with permission from Wiley. Depending on the material to be etched, one can use either wet or dry etching. An example of applying wet etching (in a dilute hydrochloric acid with a zinc powder catalyst) to pattern indium tin oxide (ITO) is shown in Figure AFM images of the original SRG, the mask obtained by pre-etching the azopolymer in oxygen plasma, and the resulting patterned ITO layer are shown in Figures 17(a c), respectively. ITO is a very important electrically conductive material used to create optically transparent electrodes for variety of photonic applications. Hence, this technique can be of technological interest. Recently, an interference lithography technique, in which azopolymer SRGs are used as masks for dry etching of silicon, has been introduced. 198 Figure 18(b) shows an SEM image of a silicon wafer etched directly through an azopolymer soft mask [Fig. 18(a)] using RIE. This almost trivial etching technique is seen to yield trapezoidal rather than rectangular silicon patterns because of the inevitable mask trimming; the etch ratio of Si versus the azopolymer was about 4. To increase the etch ratio of the Si wafer to the mask, a 5- nm thick layer of alumina and a 20-nm thick film of amorphous silicon were added beneath the azopolymer film [Fig. 19(a)]. Alumina acts as a hard mask, and the purpose of amorphous silicon is to insure good adhesion of the azopolymer to the substrate. The azopolymer mask pattern is first transferred to the amorphous silicon (using RIE) and then to the alumina layer (using wet etching through the remaining amorphous silicon). Through the obtained alumina mask, the POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

12 REVIEW POLYMER SCIENCE FIGURE 18 (a) An AFM image (top) and surface profile (bottom) of an azopolymer SRG. (b) An SEM image of a silicon wafer dry-etched directly through an SRG mask. Reproduced from Ref. [198], with permission from Wiley. underlying silicon can be etched much deeper. Figure 19(b) shows an example of a silicon pattern obtained in this way. Such mushroom-like structures can find applications, for example, in touch-screen and solar-cell technologies as selfcleaning superhydrophobic and superoleophobic 199,200 AR coatings. We recall that an AR coating, which is based on a subwavelength grating may have excellent characteristics over the whole visible spectral range. 60,61 Azopolymer SRGs can be directly inscribed on highly reflective (e.g., metal) substrates, owing to the sensitivity of the polymer to the light polarization rather than intensity modulation. Hence, azopolymer-based interference lithography can be used to pattern gold or other metals by using Ar ion milling directly through an azopolymer mask. 189,190 This is exemplified in Figure 20. A conceptually different approach to fabricate periodically nanopatterned metal structures has been introduced in ref. [189]. The technique is illustrated in Figure 21(a). A silicon wafer is covered with thin films of alumina, titanium, gold, and azopolymer. Then, an azopolymer SRG is inscribed and used as a soft mask to etch the three underlying layers, through which the substrate is etched afterwards. The resulting silicon pattern has deep grooves (or cylindrical dimples, FIGURE 19 (a) A fabrication process using an auxiliary hard mask: I: deposition of Al 2 O 3 (5 nm), amorphous silicon (20 nm) and azopolymer (100 nm) and inscription of SRG, II: partial etching of the polymer in O 2, III: etching of amorphous silicon and Al 2 O 3, IV: dry etching of Si, and V: stripping the mask. (b) An etched silicon wafer with a two-dimensional azopolymer SRG used as an initial mask. Reproduced from Ref. [198], with permission from Wiley. if the SRG is two-dimensional) with vertical walls. Subsequently, gold is evaporated onto the silicon template and glued onto a glass plate using a UV adhesive. After this, the metal structure is decoupled from the template (here CH 3 Cl 3 Si is used to reduce friction between the metal and the template). Note that a single template can be used to create many nanopatterned metal samples. This technique has been used to create a reflective gold nanogrid wave plate for optical wavelengths [see Fig. 21(b)]. 189 The operation of such a wave retarder is based on the fact that, if the pattern period is smaller than k, the incident light polarized along the grooves is reflected almost completely from the top surface of the array, while orthogonally polarized light is reflected mostly from the bottom of the pattern. 59 By properly selecting the depth of the grooves, one can obtain a quarter- or a half-wave plate with phase differences of p/2 and p, respectively, for the two polarizations. The wave plate shown in Figure 21 is designed to act as a k/4-plate at k nm and 174 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

13 POLYMER SCIENCE REVIEW FIGURE 20 SEM images of Au surfaces etched directly through an azopolymer mask. (a) Stripe- and (b) disc-shaped protrusions with lateral dimensions of about 100 nm. The thickness of the protrusions is several tens on nm. Reproduced from Ref. [190], with permission from Elsevier. as a k/2-plate at k nm. The conducted optical measurements showed a good agreement between the theoretical predictions and the obtained experimental results. 189 To summarize, micro- and nanofabrication techniques making use of azopolymer SRGs as etch masks allow one to create micro- and nanostructures on a large area of a flat substrate that can be optically transparent, absorbing, or highly reflective. The patterned area is limited only by the size of the substrate and the diameter of the laser beam used to inscribe the SRGs. Typically, the areas of the created SRGs are on the order of 1 cm 2. We therefore consider the azopolymer-based interference lithography as a fast and cost-effective alternative to the interference lithography using conventional photoresists. Directional Photofluidization Lithography The third azopolymer-based patterning technique we would like to describe is coined as directional photofluidization lithography. It leans on a combination of the photoinduced motions in azopolymers 100 and a nanofabrication technique termed self-perfection by liquefaction (SPEL). 201,202 When an azopolymer film is irradiated with linearly polarized light of appropriate wavelength (such that the azobenzene units undergo continuous cycling between the trans and cis forms), the material can undergo directional photoinduced motions. This has been convincingly demonstrated in FIGURE 21 (a) The fabrication process used to print periodic metal structures. I: creation of an azopolymer mask on a silicon wafer covered with Al 2 O 3, Ti and Au, II: etching of silicon and removal of other materials, III: ALD of Al 2 O 3 and deposition of CH 3 Cl 3 Si, IV: deposition of Au, V: gluing of a glass plate to gold, and VI: removal of the gold pattern from silicon. (b) A gold nano-grid reflective wave plate created by using this technique. The parameter values of the array are K nm, W nm, and H nm. Reproduced from Ref. [189], with permission from American Institute of Physics. an experiment where an azopolymer film with two orthogonally carved canals was exposed to collimated, linearly polarized laser beam. Upon irradiation, the canal perpendicular to the polarization plane gradually filled up, while the canal parallel to it remained intact (Fig. 22). 100 If instead the irradiation was performed using circularly-polarized light, both canals filled up equally. Such polarization-controlled anisotropic photofluidity was explained through pronounced photoinduced changes in mechanical properties of the azopolymer film, that are caused by photoinduced anisotropic ordering of the chromophores and anisotropic polarization/intensity-dependent stresses during illumination. The entropy changes of the polymer system due to photoinduced reorientation have also been proposed as the primary source POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

14 REVIEW POLYMER SCIENCE for the SRG formation, 73 and according to a recently developed microscopic theory, the light-induced mechanical stresses arising from chromophore reorientation can be larger than the yield stress of the material. 203 SPEL, in turn, uses thermally induced fluidization to improve or modify nanostructures after their preparation. 201,202 The technique is schematically illustrated in Figure 23. It can be applied to various materials, including metals, semiconductors, and polymers. The alreadyprepared nanostructure is melted either isothermally (polymers) or by means of a laser pulse (semiconductors and metals) under proper boundary conditions that guide the material flow into a desired geometry. The absence of a boundary from above yields smoothened structures with rounded sidewalls [Fig. 23(a)], while additional external control is achieved by capping the structure with a quartz plate. Such capped SPEL can improve the structure quality by removing fabrication defects [Fig. 23(b)], but depending on the experimental geometry, it can also (i) reduce the linewidth and increase the height 201 or (ii) reduce the trench width/hole diameter and decrease the height of the structures. 202 The latter is exemplified in Figure 23(c). As the technique is based on thermally induced melting, it is inherently isotropic and incapable of producing more complex structures via postmodification. The anisotropic photofluidization of azopolymers and the nanostructure postmodification through liquefaction were recently put together in a series of research articles that have also been reviewed. 208,209 The procedure consists of preparation of a predetermined azopolymer line or hole arrays using PDMS-based micromolding, and subsequent light-induced reconfiguration of the obtained azopolymer structures via single- or interfering-beam irradiation with desired polarization. Recall that as depicted in Figure 22, the azopolymer movements are dictated by the polarization direction. In contrast to thermally induced material flow, the light-induced reconfiguration can produce a variety of nanostructures with different shapes in an easily controllable and noncontact manner. 206 The azopolymer nanostructures can be further used as templates to obtain, for example, metallic nanostructures. As an example, Figure 24(a) schematically illustrates the fabrication of gold nanowires and ellipsoids, obtained by reconfiguring the pristine azopolymer structure with single-beam and interference-pattern irradiation, respectively. An example of ellipsoidal structures is given in Figure 24(b). The dimensions of the structures, and hence the plasmonic resonances, can be controlled by irradiation time and intensity, as demonstrated in Figure 24(c). The ellipsoids were proposed to be applicable as optical antennas and, due to their sharp ends, as active elements of SERS substrates. 204 Also other types of plasmonic nanostructures, such as funnel-shaped plasmonic tip arrays for applications in SERS, 205 and more recently, plasmonic color filters, 210 have been demonstrated. FIGURE 22 Directional photoinduced movements of azopolymers can be visualized by using a thin film with two perpendicular canals produced with a scanning-force microscope tip. (a) An atomic-force micrograph of a nonirradiated film; (b d) images taken after 30, 150, and 195 min irradiation (532 nm, 60 mw/cm 2 ), respectively. The arrows dictate the polarization direction of the incident light. Reproduced from Ref. 100, with permission from Nature Publishing Group. Directional photofluidization is not only useful as a nanofabrication tool but also as a tool for gaining fundamental understanding of the light-induced motions in azopolymers. This is because the polymer flows are much easier to visualize using discrete azopolymer structures or line arrays rather than homogeneous films. 211,212 An interesting example of using square post arrays of an amorphous azopolymer is shown in the top image of Figure 25. Several different amorphous azopolymers were studied. If the azobenzene units were covalently attached to the polymer backbone, light irradiation caused the posts to elongate along the polarization direction (bottom left). However, an amorphous but supramolecular azopolymer (with the azobenzenes being ionically bound to the polymer chains 213 ) exhibited reverse behavior, and elongation occurred perpendicular to the polarization direction. Such discrepancy between two amorphous azopolymers is not easy to explain, and it highlights that different factors may dominate the photoinduced macroscopic motions in different types of azopolymers (see ref. [214] as another example). SUMMARY AND FUTURE OUTLOOK Azopolymer-based micropatterning and nanopatterning has proven to be a very promising, efficient, and versatile tool for variety of photonic and other scientific and 176 POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

15 POLYMER SCIENCE REVIEW FIGURE 23 (a,b) Schematically illustrate postmodification of semiconductor, metal, or polymer nanostructures through SPEL. Reproduced from Ref. 201, with permission from Nature Publishing Group. (c) SEM images of a polymer grating with 1 mm period, where the spacing between the adjacent grating lines is gradually reduced from 280 to 13 nm upon mechanical pressing at 70 C. Reproduced from Ref. 202, with permission from American Chemical Society. FIGURE 24 (a) Schematic illustration of the main steps of directional photofluidization lithography. (b) SEM images of gold ellipsoids fabricated using the method described in (a). The scale bar corresponds to 20 mm (inset, 1 mm). (c) By controlling the irradiation time and/or intensity, the dimensions and, therefore, the longitudinal and transverse plasmonic resonances of the ellipsoids can be tuned over a wide range. Reproduced from Ref. 204, with permission from American Chemical Society. technological applications. Examples of already demonstrated photonic applications include one- and twodimensional linear diffraction gratings, microlens arrays, distributed Bragg reflectors, spectral filters, near-field sensing and imaging instruments, subwavelength gratings and various periodic microstructures and nanostructures with potential applications in SERS spectroscopy, light polarization control, and broadband AR coatings. We anticipate that in the future, the use of azopolymer-based patterning will be extended towards the creation of periodic patterns with curvilinear symmetries and on curved surfaces (examples are diffractive zone plates and axicons), three-dimensional photonic crystals and optical nanomaterials (including, e.g., negative-index metamaterials), fiber-optical Bragg reflectors and lasers, variety of nanostructured plasmonic elements, Shack-Hartmann-type wavefront sensors (using microlens arrays), and commercially competitive SERS substrates, solar cells and displays for electronic devices. To widen the applicability of the azopolymer-based patterning, several materials issues can be addressed. Naturally, high quality of the azopolymer SRGs is of primary importance in view of photonic applications. However, efficient macroscopic movements, preferably such that all the material would be removed from the troughs of the grating, POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,

16 REVIEW POLYMER SCIENCE Azopolymer-based surface patterning is a promising technique from the perspective of many applications, but the light-induced macroscopic motions in azopolymers and liquid crystals go way beyond the surface patterning. 32,33 An intriguing prospect would be to use the light-controlled macroscopic actuation of azopolymers to study distancedependent physical phenomena, such as Casimir effect 229 and transition between the weak and strong electromagnetic interaction regimes. 230 Finally, the possibility for photonenergy harvesting 6,231 and optical-to-mechanical energy conversion 232 can offer significant advantages in energy storage and, for example, microrobotics, that warrant much further study. Azobenzene has many faces, but many of them are yet to be revealed. REFERENCES AND NOTES FIGURE 25 AFM image of a pristine azopolymer square post array (top), and photoinduced deformation of amorphous covalently functionalized (bottom left) and supramolecular (bottom right) azopolymers. The arrows indicate the polarization direction of the incident light. Reproduced from Ref. 211, with permission from Optical Society of America. would further facilitate the use of the SRGs, for example, as etch masks. Thus, increased quality and modulation depth of SRGs are what is desired. We also emphasize that the use of azopolymer SRGs as etch masks would be considerably simplified if it would be possible to enhance adhesion of the polymer to dielectric substrate materials, such as glass and alumina. Increased resistivity of the azopolymer to various types of wet and dry etching would as well be highly appreciated. We also want to mention here a quite separate materials platform. It is provided by azobenzene-containing block copolymers. 215,216 These materials have great future prospects, since they combine the possibility of optical patterning and controlling the block-copolymer self-assembly with the immense potential of the block copolymers in microfabrication and nanofabrication Another area where we expect to see a boost of research effort in the becoming years is the use of azopolymers as a design tool to control surface wetting characteristics. Photoinduced surface patterns can be readily inscribed directly on fluorinated materials with low surface energy, 80,225,226 and a variety of surface geometries required, for example, for superhydrophobicity or superoleophobicity can be conveniently obtained using the microfabrication and nanofabrication techniques described in this review. Some first steps in this domain have already been taken, 227,228 yet the future is wide open for further advancements. 1 G. S. Hartley, Nature 1937, 140, J. Griffiths, Chem. Soc. Rev. 1971, 1, M. Russew, S. Hecht, Adv. Mater. 2010, 22, R. Klajn, J. F. Stoddart, B. A. Grzybowski, Chem. Soc. Rev. 2010, 39, J. F. Gohy, Y. Zhao, Chem. Soc. Rev. 2013, 42, A. M. Kolpak, J. C. Grossman, Nano Lett. 2011, 11, M. Han, M. Hara, J. Am. Chem. Soc. 2005, 127, L. M. Goldenberg, V. Lisinetskii, S. Schrader, Adv. Opt. Mater. 2013, 1, S. K. Yesodha, C. K. S. Pillai, N. Tsutsumi, Prog. Polym. Sci. 2004, 29, A. Priimagi, K. Ogawa, M. Virkki, J. Mamiya, M. Kauranen, A. Shishido, Adv. Mater. 2012, 24, N. V. Tabiryan, S. R. Nersisyan, D. M. Steeves, B. R. Kimball, Opt. Photonics News 2010, 21, U. Ruiz, P. Pagliusi, C. Provenzano, G. Cipparrone, Appl. Phys. Lett. 2013, 102, J. C. Hong, J. H. Park, C. Chun, D. Y. Kim, Adv. Funct. Mater. 2007, 17, T. J. White, R. L. Bricker, L. V. Natarajan, N. V. Tabiryan, L. Green, Q. Li, T. J. Bunning, Adv. Funct. Mater. 2009, 19, F. Yang, G. Yen, G. Rasigade, J. A. N. T. Soares, B. T. Cunningham, Appl. Phys. Lett. 2008, 92, J. M. Mativetsky, G. Pace, M. Elbing, M. A. Rampi, M. Mayor, P. Samori, J. Am. Chem. Soc. 2008, 130, S. Seo, M. Min, S. M. Lee, H. Lee, Nat. Commun. 2013, 4, Y. L. Zhao, J. F. Stoddart, Langmuir 2009, 25, H. Yamaguchi, Y. Kobayashi, R. Kobayashi, Y. Takashima, A. Hashidzume, A. Harada, Nat. Commun. 2012, 3, A. Natansohn, P. Rochon, Chem. Rev. 2002, 102, V. Shibaev, A. Bobrovsky, N. Boiko, Prog. Polym. Sci. 2003, 28, O. Yaroshchuk, Y. Reznikov, J. Mater. Chem. 2012, 22, T. Ikeda, J. Mater. Chem. 2003, 13, A. A. Beharry, G. A. Woolley, Chem. Soc. Rev. 2011, 40, POLYMER SCIENCE, PART B: POLYMER PHYSICS 2014, 52,