Azobenzene-containing materials (or briefly azomaterials) are systems in which the azobenzene molecules are bonded or simply dispersed into another material. Many of the peculiar physical properties of this class of materials are attributed to the particular kinetics that the azobenzene molecules undergo when illuminated with UV and visible light. The absorption of photons with suitable energy produces cyclic structural transitions of the molecules between the trans and cis isomerization states. As the two isomers differ significantly in terms of structural conformation and volume, the photoisomerization of azobenzene is accompanied by a large motion at molecular scale. Such nanoscopic movement influences the surrounding chemical environment and can be highly amplified through the interaction of the photo-responsive units with a properly designed material system. In this respect, the motion of the photoisomerizing azobenzene molecules can be considered as the basic movement occurring at nanoscale in any azomaterial under light irradiation. Depending on the material architecture, the microscopic photo-induced movement can then eventually originate a series of macroscopic effects involving the collective rearrangement of the azobenzene molecules and the structural reconfiguration of the hosting material matrix at increasing spatial scales . The technological implications of such multiscale phenomena are enormous, and azomaterials have been largely used to successfully design molecular compounds and material systems having specific light-tunable chemical, optical and mechanical properties. A huge amount of literature about the synthesis, the characterization and the applications of azobenzene-containing materials already exists. Each of these aspects is an active and rich field of research on its own, so all the perspectives involved cannot be exhaustively summarized together into a single concise review. Here, we shall try to provide a brief overview of the general scenario related to the many facets of azomaterials and their applications, focusing mainly the attention on the effects of macroscopic motions initiated by azobenzene photoisomerization.
The main focus of this review lies on the intriguing macroscopic phenomenon of the light-induced directional mass migration. Such phenomenon involves a mass transport at the free surfaces of thin amorphous azomaterial films (polymers, oligomers, molecular glasses), which translates into topographic film modulation in the form of surface reliefs. Superficial structuration is obtained only in the illuminated areas of the film and is reversible and high directional, with a very peculiar sensitivity to the intensity distribution; to the polarization state; and, in some cases, even to the wavefront of the irradiating light beam. The vectorial dependence of the mass migration on the irradiating optical field components offers many possibilities for its use as a superficial photo-patterning technique. Here, the recent advances in this field are reviewed, together with several applications of the light-structured azomaterial surfaces in photonics, biology, surface engineering and lithography. In addition, the review includes also an overview of the macroscopic photo-actuation driven by structural trans-cis photo-switch of the azomolecules in azobenzene-containing liquid crystalline elastomers (azo-LCEs). The light-fueled motion of the azo-LCEs is the expression of a direct optical-to-mechanical energy conversion and represents a promising framework for emerging research fields aimed at the realization of light-driven artificial muscles, biomimetic soft-robotic locomotors and energy-harvesting devices.
2 The light-induced molecular motions of azobenzene
Due to the large range of possible applications derived from their inclusion into different material systems, azobenzene molecules and their photochemistry have been the subject of intensive studies. The detailed description of such aspects can be found in several reviews already existing in the literature , , , , ,  and is out of the scope of the present work. Here, however, the main features of the azobenzene molecules and their photo-induced dynamics are briefly discussed with the aim to provide an overview on the microscopic mechanisms underlying the macroscopic effects presented in the next sections and to highlight many application scenarios directly related to the use of azobenzene molecules as molecular photo-switches in azomaterials.
2.1 The azobenzene molecule
The chemical structure of the azobenzene molecules is characterized by two phenyl rings linked together through the azo bond (–N=N–). Azobenzene can exist either in the thermodynamically stable trans isomerization state or in the meta-stable cis isomerization state (Figure 1A). The trans isomer has a rigid and planar rod-like geometry, while in cis-azobenzene, the two phenyl rings form an angle of about 90° , , , , , , . The azobenzene isomerization proceeds via rotation or inversion mechanisms around the azo-bond  and results in a reduction of the distance between the carbons in position 4 in the aromatic rings, which passes from 9.9 Å in the trans isomer to 5.5 Å in the cis isomer (Figure 1A). Besides generating volume variation , ,  and large motion at molecular scale, the conformational switch between the trans and cis isomers of azobenzene is also accompanied by an increase of the molecular dipole moment (from ptrans≈0 to pcis≈3D) , . The trans-azobenzene absorption spectrum presents a broad absorption band associated to the pi-pi* transitions with peak at around 320 nm and a weak absorption band centered at about 450 nm originated by the n-pi* transitions. The latter absorption band becomes strengthened for the cis-azobenzene isomer , .
The most interesting feature of azobenzene molecules is the fact that the switch between the two isomeric forms can be initiated efficiently and repeatedly by the absorption of photons. The irradiation of an ensemble of azobenzene molecules lying in the energetically favored trans-state with UV light causes the switch of the molecular population to the cis isomerization state, which then can return back to the trans form either by thermal relaxation or by a new photo-reaction initiated by blue/green light (Figure 1A). The photo-induced reactions happen on time scales of a few picoseconds , , , while the thermal relaxation time from the cis to the trans state can range from fraction of milliseconds up to several days  depending on the molecular environment and on the actual chemical configuration of the phenyl rings, which can be eventually substituted with some functional groups. The substitution in azobenzene rings originates from a series of azobenzene-like compounds, generally referred to as azomolecules , which are characterized by the same structural trans-cis isomerization observed in the parent azobenzene molecules but eventually having different and tunable photophysical properties, like the position of absorption peak wavelength  or extremely long cis-state lifetimes , , .
In typical azomolecular systems, the absorption resonances initiating the trans-to-cis and the cis-to-trans isomerization transitions are broad and significantly overlapping. Then, even monochromatic photons (e.g. photons from a laser) of suitable energy (typically with wavelength in the range 400–550 nm) are able to efficiently activate both the photoisomerization reactions. As a consequence, an ensemble of continuously irradiated azomolecules undergoes repeated trans-cis-trans light-triggered isomerization cycles, which are able to initiate another light-driven molecular scale motion in azobenzene-containing systems. Such microscopic movement involves the stochastic photo-reorientation of the azounits irradiated with linearly polarized light , . The photo-reorientation of the azomolecules is derived from the enhanced absorption probability, and hence from the more efficient isomerization dynamics, for the molecules excited with light polarized along the direction of their molecular transition dipole, this being oriented along the molecular main axis of the trans isomer. The dynamics of the molecular reorientation process is schematized in Figure 1B. As the orientation of the molecular axis of a trans-azomolecule varies randomly in each photoisomerization cycle, after many of them, the excited azounit can become stochastically oriented in the direction orthogonal to the light polarization direction. The photoinduced dynamics of such reoriented azomolecule stops as it does not absorb photons efficiently anymore, remaining oriented at 90° in respect to the light polarization direction even if the light irradiation continues. Such polarization-induced “invisibility” to new polarized photons of the reoriented azocromophores is commonly referred to as orientational hole-burning and causes a lowering of the population of trans isomers oriented in the polarization direction, together with a concomitant enrichment of the population of trans molecules oriented in the direction orthogonal to the polarization plane. Naturally, the molecular reorientation is reversible and reconfigurable. A new alignment can be realized in any desired direction simply by a rotation of the irradiating linear polarization direction. Eventually, even the initial isotropic molecular distribution can be easily restored by the irradiation of circularly polarized or non-polarized light .
2.2 Photo-switch in azomaterials
The geometrical reconfiguration of the azomolecules and their reorientation under light irradiation directly affect the configuration and the functionality of the molecular environment in which they are incorporated, behaving essentially as controllable and reversible photo-switches. For example, the conformational molecular rearrangement associated with the photoisomerization reaction able to transform the mesogenic trans isomer into the non-mesogenic cis isomer has been largely used to efficiently trigger phase transitions in azobenzene-containing liquid crystalline material systems , , , . The trans-cis structural photo-switch has recently been employed in several biological applications , , . For example, the isomerizing azomolecules have been used to photo-activate molecular mechanisms able to control on demand the aperture and the closure of the channels through the cellular membranes for controlled ion transport , ,  or the three-dimensional (3D) structure of proteins . The variation in the molecular dipole moment occurring in the isomerization photo-switch has been also successfully exploited as a light-controllable switch of compound solubility  and for the reversible tailoring of the wetting properties of solid azobenzene-functionalized surfaces , , , , , , , . Even the directional and controllable light-driven displacement of liquid droplets has been demonstrated . Furthermore, recent studies used photo-switchable azobenzene-containing surfactants to control the movement of particles at the liquid/solid interface through light-driven diffusioosmosis , , . The photo-orientation effect of the azomolecules gives origin to strong birefringence, dichroism and non-linear optical effects , , , , , , . Based on these properties, azobenzene-containing materials have been proposed as a platform for reversible data storage and holographic recording , , , , . The photo-orientation is also able to control the direction of ordering in liquid crystalline matrices, doped with cyclically isomerizing and re-orientable azocromophores , , , . Furthermore, the in-plane photo-reorientation of molecular azobenzene monolayers deposited onto a substrate has been used as aligning layer in liquid crystal (LC) cells, allowing the realization of spatially structured alignment geometries not easily accessible with conventional mechanical rubbing , .
3 The directional light-induced mass migration in azomaterials
In 1995, Natansohn and Rochon  and, independently, Kim et al.  reported for the first time about the large-scale periodic modulations of the free surface of a thin amorphous azobenzene-containing polymer film irradiated with the sinusoidal intensity light pattern produced by the interference of two mutually coherent p-polarized laser beams (Figure 2A). The resulting sinusoidal superficial texture, now commonly referred to as surface relief gratings (SRGs), presented an amplitude modulation of several hundreds of nanometers and a groove pitch matching the periodicity of the illuminating interference pattern (Figure 2B–C), hence showing an exact replica of the irradiated light intensity pattern in the form of surface modulations.
The light-induced superficial texturing was observed to occur at very low irradiation intensities, discarding any explanation of the phenomenon in terms of the ablative or destructive processes , . On the contrary, topographic modulation was immediately understood as the result of an actual material movement, induced efficiently by the irradiating light even for temperatures significantly lower than the glass transition temperature TG of the employed amorphous azomaterial , . The non-destructive nature of the phenomenon emerged immediately also from the almost complete volume conservation in the written area  and from the possibility of erasing (and then eventually rewriting) the inscribed superficial textures either by rising the temperature of the film above TG or by irradiating the structured film with circularly polarized or non-polarized uniform light distributions , , .
Since the first fundamental discoveries, a large amount of research has been oriented toward the study of this intriguing light-induced phenomenon, with great multidisciplinary interest in both theoretical and applied research fields. This can be immediately understood by thinking of the phenomenon as the macroscopic manifestation of a unique complex microscopic light-matter interaction, whose actual underlying mechanism is still debated in the scientific community. Moreover, and more importantly, the non-destructive reversible process of the light-driven mass migration offers countless possibilities in several applied and technological areas. The high scientific impact of the subject is also witnessed by the high number of reviews already available in literature, which cover many of the aspects of the light-driven superficial structuration and several its possible applications , , , , , , .
The next sections of the present chapter include an overview on the well-established general rules governing the light-induced mass migration and the resulting superficial structuration of the azomaterial films. The dependence of the surface reliefs on the intensity; on the polarization; and, in some cases, also on the wavefront distributions of the irradiating light is described, providing a solid global picture of the phenomena involved in the large number of applications presented later in the chapter. Finally, the differences in the behavior of the material transport observed in some cases for different material systems and/or illumination configurations are also evidenced, with the aim of triggering further research in both fundamental and technological fields related to this intriguing light-matter interaction.
3.1 The phenomenology
3.1.1 Material design strategies
From the perspective of material design, the prerogatives for an efficient light-induced mass transport are the efficient trans-cis-trans cyclic photoisomerization of the azomolecules and the inclusion of these molecules in amorphous material matrices. A well-defined chemical interaction between the hosting material and the photo-active azounits has been demonstrated to increase the efficiency of the material transport in respect to the simple blended azomaterials , , and several binding strategies (including covalent attachment , , ionic interactions ,  and supramolecular interactions as hydrogen and halogen bonds , , , ) have been exploited to design light-responsive azomaterials showing efficient mass migration. The most commonly used material platform involves the use of amorphous polymers (simply referred to as azopolymers) as inert hosting matrix. Also, other materials lying in the glassy state, like molecular glasses , , oligomers ,  and dendrimers , , , have turned out to give rise to efficient light-driven material transport, while only smaller effects in terms of surface modulation have been found in liquid crystalline azopolymers , , , .
3.1.2 Intensity and polarization dependence
The fingerprint of the light-induced mass migration phenomenon in azomaterials is the peculiar sensitivity of the material transport to the intensity distribution and the polarization state of the irradiating light field. Under the irradiation of polarized light, the material motion turns out to be highly directional, with the polarization direction constituting a preferential direction along which the material transport mainly occurs.
The general aspects of the intensity/polarization-dependent directional material motion occurring in the presence of a spatially structured illumination pattern can be easily understood from the surface reliefs generated in the irradiation of an azopolymer film with focused linearly and circularly polarized Gaussian beams. In the case of linear polarization, the surface structuration reflects a polarization-driven anisotropic material displacement. The surface relief is indeed constituted by a depression appearing in the region of the laser focal spot, with two lobes raised above the film average quote and oriented along the direction of the linear light polarization (Figure 3A). When the Gaussian beam is circularly polarized, no preferential direction exists for the material motion, as the optical electric field in this polarization configuration is, on average, isotropically oriented in all the directions of the film plane. In this situation, the two overhead lobes obtained in the case of linear polarization are substituted by a symmetric annular relief (Figure 3B) , . As a Gaussian beam has an isotropic intensity gradient in the focal plane, the surface reliefs obtained in this illumination condition allow to further characterize the direction of the mass migration in respect to the intensity distribution of illuminating light field. Indeed, the appearance of the hole in correspondence with the intensity maximum of the focal spot reflects another general feature of the directional material displacement, actually proceeding from high- toward low-intensity regions of a structured irradiating light pattern.
The directional mass transport can occur also under uniform-intensity distributions whenever the azomaterial surface presents a structural discontinuity able to break the in-plane isotropy of the (typically spin-coated) homogeneous flat films. A simple example of such anisotropic azopolymer surface is presented in Figure 3C, where a series of scratches are mechanically practiced in orthogonal directions over a flat azomaterial film , . The irradiation of a single collimated and linearly polarized beam produces a selective filling only of the canals oriented perpendicularly to the linear polarization direction (Figure 3D–F), highlighting again a highly directional azomaterial displacement in the light polarization direction. This observation reveals an azomaterial motion behaving similarly to a light-fueled directional liquid flow, which is able to reconfigure the pre-patterned azopolymer surface analogously to the isotropic thermally induced material fluidization used in the lithographic technique named self-perfection by liquefaction (SPEL) , . The phenomenological analogy with SPEL has nevertheless led to the introduction of the concept of directional photo-fluidization (DPF) ,  to describe the anisotropic light-driven mass migration phenomenon in azomaterials. However, the eventual light-induced liquefaction of the azomaterials has not been unambiguously demonstrated in the literature, with the actual viscoelastic and mechanical properties of the azomaterials under irradiation being still subject of intense investigation , , , , , , , , ,  and debates in the scientific azo-community .
From the described intensity/polarization dependence, it is clear that, when the irradiating light field has a preferential direction in the spatially modulated intensity pattern (as in the case of the sinusoidal intensity distribution produced in the interference of two parallel polarized laser beams), the direction of the intensity gradient should match the direction of the linear light polarization in order to achieve a highly efficient surface structuration. Indeed, differently from the case of p-polarized beams, only small surface modulation is observed upon the irradiation of an intensity interference pattern obtained by two s-polarized interfering beams, where the intensity gradient and the polarization direction are mutually orthogonal.
Such behavior has been confirmed several times by polarization-dependent experiments involving the monitoring of the growing dynamics of SRGs inscribed with intensity patterns generated by two interfering beams in different polarization configurations , . More recently, these kinds of polarization-dependent studies have been accurately performed by direct in-situ analysis , , , , , , ,  conducted through microscopic setups having the powerful ability to monitor in real time both the evolution of the topographic surface structuration and the irradiating light fields (Figure 4A–F). Also the relative π-phase shift between the maxima in the intensity patterns and the topography maxima in SRGs resulting from a material migration from illuminated to dark areas of the spatially structured interference intensity pattern has been unambiguously observed with such setups , , , , , , , . Similar polarization-dependent experiments have highlighted a high efficiency for the SRGs formation even under pure polarization interference patterns, in which the intensity distribution is uniform but the orientation of the electric field vector is spatially modulated over the sample (Figure 4G–I) and the material motion is driven by a pure polarization gradient , , , , , , , , .
In some cases, however, the polarization-dependent structuration of flat azomaterial surfaces is obtained even under both uniform intensity and polarization light distributions. Indeed, also the irradiation of a single collimated beam can induce periodic textures over the illuminated azomaterial film , , , , , , , , , , , , , , , , , , . Linearly polarized beams produce unidimensional (1D) periodic grooves, whose orientation depends on the light polarization direction (Figure 5A–F). The irradiation of a circularly polarized or non-polarized light beam induces instead protrusions arranged in hexagonal arrays (Figure 5G–I). The structural features of such spontaneous superficial patterning of the azopolymer films share several aspects of the laser-induced periodic superficial structuring (LIPSS) occurring at the surface of many solids when irradiated with intense pulsed laser beams , , . However, differently from such thermally induced phenomenon involving material ablation, the spontaneous structuring of the azomaterial film occurs efficiently also for continuous low intensity regimes and has been recognized as resulting again from the mass migration phenomenon , , . Because of the phenomenological and structural analogies with the large-scale sinusoidal superficial texture of the SRGs, the periodic large-scale superficial modulations obtained with a single irradiating beam are typically referred to as spontaneous surface relief gratings (SSRGs).
3.1.3 The effective wavefront sensitivity
The appearance of spiral-shaped surface reliefs ,  under the irradiation of an azopolymer film with focused Laguerre-Gauss (L-G) beams (Figure 6A) has allowed the discovery of an effective wavefront sensitivity of the light-induced mass migration, which, in particular illumination configurations, can be added to the already mentioned dependence of the phenomenon on the intensity and polarization distributions of the illuminating optical field.
In these experiments , , the inscribed spiral surface reliefs showed a direct dependence on the beam parameter (the vortex topological charge, q), which characterizes the properties of the helically shaped wavefront of the L-G beams , , , . In particular, larger values of |q| translate into larger spiral diameters, while the inversion in the sign of the topological charge produces an inversion in the handedness of the inscribed spiral reliefs (Figure 6B–C). As information about such helicity is contained only in the beam wavefront and not in the intensity and polarization distributions of the doughnut-shaped irradiating L-G beam, the natural hypothesis for the interpretation of the observed spiral mass transport is that light-induced mass migration is sensitive to the wavefront of the irradiating light beam in this particular illumination condition. Even if the spiral surface reliefs are, until now, the only experimental situation where such wavefront sensitivity has been clearly highlighted, the inclusion of this degree of freedom can enlarge significantly the possibilities offered by the light-driven structuration of the surface of azopolymer films for applications in nano-lithography.
3.1.4 A complex light-matter interaction
The rationalization of the huge phenomenology associated to the azomaterial light-induced mass migration in a complete theoretical framework is still an open question. While the photoisomerization of the azomolecules is thought as absolutely necessary in order to observe any significant superficial structuration, the physical link between this microscopic movement and the observed macroscopic azomaterial displacement has not been unambiguously recognized yet. Several different theoretical models have been proposed over the years with the aim of elucidating this intriguing light-matter interaction. One of the general requirements, common to several hypothesized microscopic mechanisms, is the presence of optical gradients in the intensity and/or in the polarization of the illuminating light able to induce some variation in the physical properties of the material as a consequence of spatially dependent photoisomerization dynamics of the azounits. Effects including a gradient in the pressure forces inside the material , , the electric interactions among the molecular dipoles of the re-aligned azos , the mechanical stress induced by the azo photo-reorientation , , the motion driven by gradients in the electromagnetic forces , , ,  and the anisotropic diffusion or random-walk , , ,  of the azomolecules have been proposed as driving microscopic mechanisms of the phenomenon. Recently, also the phase separation among the trans-rich and the cis-rich microdomains, eventually induced in the azomaterial film by the irradiating light, has been hypothesized as a possible fundamental microscopic driving mechanism . However, none of the proposed models were able to effectively describe all the experimental observations, their validity being typically limited to specific material systems or to specific illumination conditions.
In the literature, also reports about few situations where the mass migration phenomenon gives rise to a different phenomenology can be found. For example, in some conditions, the interfering beams in s-s polarization configuration can drive the efficient formation of SRGs, whose amplitude can depend on material characteristics (e.g. material molecular mass) ,  or on the presence of an assisting beam recycling the orientational population of the isomerizing azomolecules during the inscription process , , . Even an inversion in the direction of the material displacement has been observed in the SRGs inscribed in certain amorphous ,  and in liquid crystalline azopolymers , , , , , where the topographic maxima arise in correspondence with the maxima in the light intensity pattern. A similar inversion in the mass migration direction has also been found in experiments where a focused laser beam is scanned across a glassy azopolymer surface , , with a polarization direction chosen to be parallel to the scanning direction (Figure 7A–B). Also, the direction of the light-induced deformation observed in isolated micropillars under uniform linear light polarization (Figure 7E–G) has been found to depend on the nature of the employed azomaterial . Furthermore, “conch-shaped” surface reliefs, instead of spirals, have been obtained on the surface of an azopolymer illuminated by a focused L-G beam in intense irradiation conditions, as reported in Ref. .
The observed huge phenomenology is certainly attributed to the simultaneous coexistence of different microscopic physical processes driving the mass migration under light irradiation, whose relative efficiency has been hypothesized as dependent on material chemistry ,  and on illumination parameters as the light intensity level or the complex spatial distribution of the optical electromagnetic fields inside the azomaterial film , , , , , , , , , , . However, while unveiling the nature of these microscopic effects is of great interest from the perspective of the rationalization of complex light-matter interactions at a fundamental level, in all the practical applications aimed to use light-induced surface structuration of azomaterials, comprehension of the exact and general microscopic mechanism can be effectively overcome by phenomenological descriptions of the phenomenon. With a such approach, the surface modulation can be qualitatively, but precisely, predicted for each particular azomaterial system of interest on the basis of the knowledge of the characteristic empirical response to simple illumination configurations (e.g. focused polarized beams) and the actual vectorial distribution of the optical field inside the irradiated azomaterial film.
A promising powerful phenomenological model able to reproduce and qualitatively predict almost all the experimental observations is given in Refs. , . In these works, the azopolymer surface structuration is described as related to the optical field components of the irradiating light by the following relation:
Here, Δh(x, y) is the variation of the surface amplitude describing the surface relief across the polymer film; E=(Ex, Ey, Ez) is the optical electric field of the irradiating light; and c1, c2, c3 and cB are phenomenological constants characteristic of the actual employed azomaterial. The phenomenological relation includes terms (proportional to c1, c2, c3) depending explicitly on the intensity distribution and its gradient, whose sign and relative strength can be empirically adjusted in order to reproduce most of the phenomenology observed in typical illumination schemes (SRGs and focused beams) . Moreover, the last term in the phenomenological expression, proportional to the constant cB, describes a surface-mediated interference between longitudinal and lateral field components of the irradiating beam. Such a term is identically vanishing in almost all the simple illumination schemes but is the one responsible for the spiral mass transport observed in the irradiation of focused L-G beams  (Figure 8) and hence for the effective wavefront sensitivity observed experimentally in this illumination configuration. Such surface-mediated interference term has been found to play a main role also in the explanation of the light-induced directional mass migration phenomenology observed in the reconfiguration of sharp edges (scratches and isolated micro-volumes) of prestructured azomaterial surfaces under uniform illumination distributions. In this illumination configuration, both intensity and polarization gradients in the light field are identically vanishing and the only non-vanishing contributions to the light-induced mass current flow causing the surface reconfiguration comes from the coupling of the homogenous optical field with the discontinuities of the pristine prestructured surface. As described in Ref. , this surface-initiated material motion is again well-described by the term proportional to the constant cB in the phenomenological model.
3.2 The photo-patterned azomaterial surfaces and their applications
The general rules governing the phenomenology of the macroscopic light-induced surface structuration of amorphous azomaterials were soon understood, and the literature contains a large number of examples where the light-patterned azomaterial films are concretely used for fundamental and technologically oriented studies. Applications range from the high-resolution photochemical imaging of optical fields, in which the intensity and polarization sensitivity of the light-induced surface reliefs are directly used to extract the information about complex optical fields, to large-scale surface photo-patterning. In relation to this last aspect, the next sections of the present chapter cover several situations in which the large-scale photo-structuration of azopolymer surfaces is employed to modify different structural properties and functionalities of the azomaterial films themselves or to structure the surface of other materials. In particular, the possibilities in terms of complex and controllable photo-patterning offered by the SRGs and the light-induced reconfiguration of azopolymer isolated microstructures and microvolumes are discussed together with their possible applications in wettability, biology, photonics and photolithography.
3.2.1 Photochemical imaging of optical fields
As the light-induced mass migration behavior depends on the illumination parameters in a vectorial manner, the surface reliefs in azopolymers can be used to map the vectorial distributions of unknown optical fields . A first example of such kind of application is represented already by the spiral-shaped surface reliefs described above. In particular, it is legitimate to suppose that, whenever a spiral surface relief appears on an azopolymer surface, the irradiating beam is probably endowed with an optical vortex. In this scheme, the peculiar spiral-shaped surface reliefs become a fingerprint impressed into the azopolymer surface of a beam endowed of an optical vortex.
One of the key features making the light-induced surface reliefs attractive for applications in the photochemical imaging of optical field is the possibility to reach spatial resolutions far beyond the ones achievable with diffraction-limited far-field imaging techniques. Such high spatial resolution is, moreover, achieved by using a simple AFM analysis, in which the reconstruction of the high-resolution information about the optical fields comes directly from the topographic characterization of the irradiated azomaterial film, without the necessity of using powerful but sophisticated near-field imaging techniques (e.g. near-field optical microscopy, NSOM , , , , ).
This optical-to-topographical information conversion was revealed to be a powerful tool for the imaging of polarization-dependent near-field optical distributions in the proximity of metal nano-structures. An example of such sub-diffraction resolution imaging approach relies on the photochemical imaging of the local field enhancement obtained in proximity of an illuminated metallic-coated nanometric tip. The light field confined at subwavelength scales near the tip produces a local nanometric protrusion onto an exposed azopolymer film, whose structure is directly dependent on the tip geometry and polarization state of the irradiated light , . Such nanometric surface reliefs have also been suggested as usable for lithographic patterning of the azopolymer films at subwavelength spatial resolution . In other experiments, the surface reliefs have been used to image the plasmonic field produced by metallic nanostructures. For such applications, an azopolymer film is typically spin-coated onto an array of metallic nanostructures, which were fabricated onto a substrate by means of high-resolution standard lithographic techniques. The optical near-field in proximity to the nanostructure generates surface reliefs whose configuration is strongly correlated with the intensity pattern of the plasmonic field predicted from theoretical models. For example, Bachelot et al. found a qualitative agreement of the surface reliefs experimentally observed in proximity of silver nanodisks (Figure 9A–F) and bowtie nanoantennas (Figure 9G–I) with the negative images of the intensity distribution predicted by theoretical calculation on the plasmonic fields in different polarization configurations , , . These results are simply interpreted assuming the typical material motion in the presence of intensity gradients along the direction of light polarization. In similar experimental conditions, also the interference of surface plasmons has been successfully recorded in an azopolymer film , , .
3.2.2 Surface relief gratings as large-scale periodic superficial textures
One of the key features making the SRGs attractive as a large-scale, easy and flexible technique for the micro-nanofabrication of textured surfaces is the possibility of tuning few irradiation parameters affecting the behavior of the material transport to precisely control the final surface topography. Indeed, at fixed irradiation wavelength, the periodicity Λ of the SRGs in a selected direction over the surface is determined only by the angle θ between the two interfering beams according to Bragg’s relation Λ=2/[sin(θ/2)], while the amplitude of the grooves, inscribed in any chosen polarization configuration, depends on the light fluence. Periodic surface reliefs, extended over areas of a few square centimeters, with amplitudes up to 1 μm  and periods ranging from hundreds of nanometers to several micrometers, can be easily obtained in very simple and cost-effective experimental conditions, typically constituted by a single irradiation step of azomaterial film with two interfering light beams.
Besides the 1D SRGs, more complex bidimensional surface reliefs arranged in tetragonal, hexagonal or even more complex configurations can be easily obtained by two or multiple successive inscription steps of 1D SRGs in different directions over the film , , . An example of the hexagonal grating induced in a three-step illumination procedure in which the azomaterial film is rotated by 120° at each step is shown in Figure 10A–B, while a quasi-crystal pattern ,  with 10-fold symmetry resulting from five subsequent irradiation steps (each for a rotation angle π/5) is shown in Figure 10C–F. Other complex and periodic superficial textures can be achieved also in experiments where irradiation is accomplished with three or more interfering beams , , , .
Also the surface structuration obtained in the case of the SSRGs can be of particular interest for cost-effective, all optically driven surface structuration. The periodicity of the SSRGs is controlled by the wavelength of the irradiating light ,  and by the incidence angle of the collimated beam , offering a possible deterministic tuning of the inscribed surface modulation. Even if the number of accessible superficial textures of SSRGs is lower when compared to the SRGs, these being limited to parallel groves or hexagonal arrays, the large-scale tunable superficial textures of the SSRGs in the simplest conceivable illumination configuration constituted by a simple single irradiation step with a collimated light beam.
Regardless of the degree of geometrical complexity achievable onto the surface, the texture of the azomaterial film can be erased, and eventually rewritten, at will, taking advantage of the reversible nature of the photo-driven phenomenon of the mass transport , . Particularly relevant for the realization of reversible all-optical patterned surfaces is the possibility to remove the inscribed superficial structuration by irradiating the film with circularly polarized or unpolarized light (Figure 11A–B). In this situation, even an eventual spatially selective erasure of desired areas of the sample can be easily realized by shining an erasing beam with a spatially structured intensity profile. An example highlighting this spatial-selective optical erasure/rewrite process is given in Ref. , where previously inscribed SRGs are deleted and then rewritten with a new grating vector only in specified areas of the film through the use of a transmission mask (Figure 11C–D).
3.2.3 Applications of the SRGs
Many of the technological applications of the periodic modulations induced by the light onto the surfaces of the azomaterial films exploit the physical proprieties gained by the textured surfaces in respect to the flat azomaterial films. In such applications, the structured azomaterial films can be used directly as obtained after the light structuring exposure or can be used as molding templates and lithographic masks for the transfer of the superficial texture on other materials.
The surfaces with 1D sinusoidal SRGs are characterized by significant structural anisotropy, with a preferential direction represented by the direction of the grooves. A similar structural anisotropy translates also in anisotropy in the superficial properties like wettability and adhesion, both strongly dependent on the actual spatial configuration of the superficial roughness. A report about the use of a patterned 1D SRGs as a substrate with controlled anisotropic wettability is found in Ref. . The direction parallel to the grooves in a 1D textured surface, like the 1D SRG, represents a preferential direction along which liquid volume can anisotropically propagate , . The deposition of a liquid drop on such anisotropic surface results hence in asymmetric elongation of the liquid contact line (Figure 12A) (triple-phase contact line, TLC), which is quantified by an anisotropy in the observed liquid contact angle along different directions of the substrate (Figure 12B–C). The degree of wetting anisotropy of the texture depends explicitly on the modulation depth and on the periodicity of the rough surface , . Both these structural parameters are easily tunable through the control of the irradiation parameters in the light-induced SRG inscription process, making the SRGs an easy and cost-effective fabrication framework of large-scale periodic patterned substrates for fundamental studies about anisotropic surface wettability  and their applications .
Another situation where the modulated anisotropic texture of the 1D SRG presents advantages with respect to other micro- and nanofabrication techniques concerns the study of the substrate-influenced behavior of living cells , , , , , , , . A large number of works emphasize, indeed, the role of the substrate topographic pattern in influencing cell properties like attachment, spreading and differentiation , , opening the way to crucial advances in clinical and tissue bioengineering. Besides the ability of producing controlled large-scale micro-textures whose geometry directly affects cell behavior, the optically induced surface reliefs in the azomaterials add also to the unique possibility of dynamic light-induced reconfiguration of the inscribed superficial textures. This aspect makes the surface reliefs particularly attractive for the investigation of the dynamical response of the cell properties to time-varying controlled topographical stimuli. An example of cells elongated and reoriented from the interaction with a biocompatible SRG support is presented in Figure 12D–E, while Figure 12F shows the image of a cell grown on the optically erased SRG, which recovers the initial structural properties of the cells directly grown on the flat pristine azopolymer substrate (Figure 12G–H).
The research field where SRGs find many other applications is photonics , . The applications related to the direct use of azopolymer SRGs as photonic elements are typically based on their tunable and efficient properties as diffraction gratings , , , , , , . SRGs acting as Bragg reflectors have been largely used as light couplers and spectral filters , , , , , , . In these optical devices, the modulated surface of the azopolymer enhances the coupling of the propagating light in waveguided modes at specific wavelengths, resulting in resonant peaks in the angular reflection and transmission spectra, which depend also on the periodicity of surface texture. The waveguided coupling can be obtained in the textured azopolymer film itself (Figure 13A) or in a slab of another material onto which the textured azopolymer film is deposited (Figure 13B). Other fabrication designs can involve also the deposition of materials with enhanced optical properties onto the light-induced SRGs , .
The diffractive properties of the periodic superficial texture of the SRGs can be used also to realize the optical feedback mechanism necessary for the operation of the distributed feedback lasers (DFB lasers). In such mirrorless lasers, the optical feedback is provided by Bragg scattering, which originated from periodic perturbations in the gain and/or refractive index of the active medium, while the emission wavelength depends on the period of such modulations , . Organic DFB lasers based on the SRG induced onto azomaterial surfaces can be easily realized by covering the inscribed SRG with a layer of active material, giving rise to a stacked structure presenting a spatially modulated effective refraction index , , . Other fabrication possibilities concern the effective modulation of gain of an active layer obtained from the direct inscription of the SRG onto a fluorescent-doped azomaterial film. An example of such a device is found in Ref. , while its operating principle, able to transform the amplified spontaneous emission (ASE) in laser emission, is shown in Figure 13C–F. Despite the fact that fluorescence quenching can be a limit for the performance of DFB lasers realized through this SRG-based design, it has the remarkable advantage of direct tunability of the emission wavelength by the proper choice of the fluorophores and grating periods (Figure 13D). Furthermore, it should be mentioned that some of the problems associated to fluorescence quenching and fluorophore bleaching occurring during the SRG structuring have been overcome in recent works that reported about the use of azomaterials for the holographic inscription of refractive gratings (polarization gratings) instead of SRGs , , , , , . Even if the process does not involve mass migration phenomenon but only the photo-reorientation of the azochromophores, the successful application of such polarization grating for DFB lasing widens considerably the range of possible azobenzene-based materials suitable for this application.
Many other applications in photonics, as well as in other technological fields, could in principle take advantage from the periodic textured surfaces of both 1D and two-dimensional (2D) SRGs inscribed in azomaterial films. For example, controlled textured surfaces acting as light-harvesting layers can be used to improve the performances of solar cells ,  and other optoelectronic devices , , , to record holograms ,  or to mimic the structure of some functional surface found in nature . Recently, SRGs have been even used as patterned electrodes for the fabrication of flexible light-healable triboeletric generators , which can find applications in the emerging technological field of wearable electronic devices.
In several practical situations, especially related to the design of optical devices with enhanced performances, the direct use of a layer of photosensitive azomaterial can deteriorate structural stability as a consequence of undesired mass migration effects. Moreover, the azomaterial layers are not optically transparent in the UV/visible portion of the light spectrum , and their direct use in optical devices can limit the wavelength operation range outside the azobenzene absorption bands. However, such limitations can be easily overcome by transferring the light-induced texture of the structured azomaterial surface onto other organic or inorganic materials, having eventually different chemical, optical, electronic and thermal properties in respect to the original azomaterial films. The pattern transferring onto organic materials generally involves a soft lithographic approach , , ,  in which the light-induced SRGs inscribed onto the azomaterial film are used as master templates for the fabrication of elastomeric stamps by a replica molding process . Typically, stamps are made of poly(dimetylsiloxane) (PDMS), a material platform that allows the reproduction of the complementary superficial SRG texture with high fidelity (Figure 14).
The fabricated stamps then can be either used themselves as functional substrates or can serve as molding templates for the new soft-lithographic fabrication steps on other materials (e.g. photoresist or optical active layers). This approach has been successfully employed for the fabrication of transparent patterned substrates in UV-curable photopolymers, suitable for the operation of a DFB laser  and for the realization of microlens arrays . Furthermore, the PDMS-based replica molding of 1D and 2D SRG has been also used to improve the performances of different optoelectronic devices. Taking advantage from the enhanced absorption efficiency for photons propagating in a textured active layer, organic solar cells with improved efficiency have been fabricated from SRGs templates , , ,  (Figure 15), while the replica of hexagonal SRGs onto the substrate used for the fabrication of organic light-emitting diodes (OLEDs) has been demonstrated to increase the light extraction efficiency of the LED , .
Several reports about the use of SRG as etch masks for texture transfer onto inorganic materials can also be found in the literature. In these micro-fabrication applications, the azopolymer film is typically deposited onto the flat inorganic substrate intended to be structured. The SRGs are then simply inscribed onto the azopolymer, and once the light-induced azomaterial structuration is accomplished, a chemical etching (typically realized using a reactive ion etching) of the azomaterial layer is used to partially expose some regions of the substrate, whose spatial configuration depends on the SRGs geometry. In the last fabrication step, the substrate is finally etched though the azopolymer mask by using standard wet or dry etching processes. With this approach, periodic structures ranging from groves, posts and more complex bidimensional arrays have been successfully fabricated onto metal, silicon and indium tin oxide (ITO) substrates , , , , , finding potential applications also in the fabrication of controlled scattering arrays for resolution tests of optical microscopes , .
3.2.4 Light-induced reconfiguration of pre-patterned azomaterial microtextures and microvolumes
The possible applications of the light-induced micro- and nano-structuration of azomaterial surfaces in the fields of material science and technology can be significantly enlarged by the light-induced deterministic reconfiguration of properly prestructured azomaterial films and microvolumes. There are several aspects that make the coupling of prestructuration and mass migration particularly relevant for the fabrication of complex 3D superficial micro-textures, which can extend eventually even over large areas of several square centimeters . One of the most crucial points of this approach is certainly the possibility of using a very simple illumination configuration for the 3D geometrical reconfiguration of the surface. The illumination scheme, indeed, is typically constituted by just a single polarized and collimated light beam. However, despite the simple optical configuration, the range of achievable 3D geometries is very large and, in some cases, able to produce superficial textures that are difficult or even impossible to achieve with standard, and more expensive, lithographic techniques. The reconfiguration of azopolymer microstructures offers also the attractive ability to re-tune (even reversibly) several times the actual geometry of the superficial texture by means of simple all-optical steps. This aspect can represent a crucial advantage when compared to other lithographic techniques, which only rarely allow further tuning of the structures once fabricated.
The preparation of the pristine superficial microtexture onto the azomaterial film is typically accomplished by a PDMS-based replica molding process, which makes use of a lithographically fabricated master for a solvent-assisted structuration of the azomaterial surface , ,  (Figure 16A). Such soft-lithographic azomaterial texturing has been successfully used to fabricate large-scale pristine arrays with various geometries, including parallel groves, holes, pillars (Figure 16B) and 2D microtextures in several shapes and geometrical arrangements. Similar pristine textured surfaces can be fabricated also through the all-optical patterning approach of the proximity field nanopatterning . In this all-optical method, the surface structuration is achieved by irradiating a flat azomaterial film through a prestructured PDMS mask, which serves as a template for the spatially selective accumulation of the light-fueled moving azomaterial (Figure 17A). Recently, Choi and coworkers  showed that the quality of the optically fabricated 3D azopolymer textures can be highly improved by using two sequential light irradiation steps at slanted and normal incidence (Figure 17A–B), demonstrating the proximity field nanopatterning to be a concrete alternative to the solvent-assisted preparation of high-quality prestructured azomaterial surfaces. This technique can be of particular relevance for the patterning of particular azomaterial systems, like the supramolecular azopolymers , in which the solvent-assisted soft-imprinting could produce the diffusion of the not strongly bonded azomolecules in the PDMS mask during the solvent evaporation, causing then a complete loss of the material photoresponse. The all-optical proximity field nanopatterning allows also a facile fabrication of complex hierarchical textures  (Figure 17C–F), which can be of particular interest for applications in several technological fields.
Once fabricated, the prestructured azomaterial surfaces can be reconfigured by a light irradiation step, whose phenomenology follows the general polarization-dependent directionality of the mass migration phenomenon. In particular, few trivial illumination parameters can be tuned to deterministically tailor the final reconfigured geometry of the irradiated structures. The degree of structural deformation is controlled by the light irradiation fluence , while the actual 3D geometry of the structures is determined by the polarization state of the irradiating light , with the direction of the irradiated optical electric field being the direction along which the structures become mainly deformed. Eventually, also the inclusion of some spatial constraint for the material movement at the top surface of the structures can be used to further tailor the 3D architecture of the reconfigured microstructures. This can be easily realized by capping the pristine textured surface with a transparent PDMS thin layer during the light irradiation , , . Also, the incident angle of the irradiating light can be used to further tune the 3D symmetry of the reconfigured structures. As reported in Refs. , , a slanted irradiating beam induces non-uniform material motion along the polarization direction, resulting in asymmetrically reconfigured structures whose degree of directional anisotropy increases by increasing the incidence angle. Figure 18 shows the SEM images of a series of reconfigured azopolymer microstructures exemplifying the effects of different illumination conditions on the final superficial microtexture achievable starting from the same pre-patterned array of cylindrical micropillars. Figure 18A–D report the reconfigured posts obtained from the irradiation at normal incidence of the pristine array with collimated linearly and circularly polarized beams, respectively. Figure 18E–H show instead the structures obtained in the same previous irradiating polarization configurations but with the inclusion of a PDMS thin capping layer attached to the pristine structures during the reconfiguration process. This illumination configuration results in flat-top 3D reconfigured structures, having symmetric or asymmetric architecture depending on the laser polarization state. The slanted flat-top structures resulting from the irradiation of linearly polarized light at the incidence angle of 40° are shown in Figure 18I. Many other structures obtained from different pristine textures and reconfigured in similar illumination conditions can be found in the literature. These include arrays of micropillars of different sizes and shapes , , , , parallel groves , ,  and array of microporous , , , , ,  and hemispherical caps , . Another set of pristine azopolymeric structures that can be deformed with a phenomenology similar to the lithographic pre-texture surfaces pertains to the agglomerates of azopolymer colloidal particles , , , .
3.2.5 Applications of reconfigured azopolymer microstructures
The light-reconfigurable prestructured azomaterial surfaces represent a versatile framework for the micro/nano-fabrication of complex textured surfaces, whose functionality in several applications is determined by the engineered 3D superficial roughness. For example, the 3D architecture of the light-reconfigured microstructures can be designed to sustain a strong repellence to the wetting of almost any liquid, a property commonly referred to as omniphobicity, which finds applications in several technological areas involving liquid-repellent and self-cleaning surfaces , , . The realization of such strong hydrophobic surfaces requires a specific superficial texture, constituted by an array of 3D microstructures having a re-entrant geometry , , . Examples coming from the natural world show that the strongest omniphobic surfaces have a double re-entrant structure, with a 3D geometry similar to a mushroom . While the fabrication of such omniphobic microstructures through conventional lithographic techniques (like silicon photolithography and silicon micromachining) is highly delicate, time-consuming and expensive , , their realization can be much easier and cost-effective by using the single-step light-induced reconfiguration of array of cylindrical azopolymer microposts, irradiated with circularly polarized light  (Figure 19A–B). In Ref. , the omniphobic behavior of a reconfigured mushroom-shaped azopolymer surface has been clearly demonstrated (Figure 19C–E), showing that the light reconfiguration of azopolymeric microstructures can provide an unprecedented powerful approach for the facile fabrication of robust, flexible and cost-effective omniphobic surfaces.
If the irradiation of an azopolymer prestructured cylindrical micropillar array with circular polarized light preserves the circular symmetry of the pristine soft-imprinted structures and affects isotopically the wetting behavior in all the directions over the surface, the asymmetric structures resulting from the irradiation of linearly polarized light  can induce wetting anisotropies along different directions of the film. In this case, the light polarization direction (or its projection onto the sample plane) determines, indeed, a preferential direction for the structural asymmetry and for the consequent wettability anisotropy. In Ref. , we reported about the accurate tailoring of the anisotropic wetting behavior of such reconfigured azopolymer surface (Figure 20A). Besides the controlled and deterministic wettability anisotropy, the same pristine array of cylindrical azopolymer micropillars has been demonstrated to achieve even tunable uni-directional anisotropic wetting by simply varying the incidence angle of the linearly polarized light (Figure 20B). Furthermore, structured polarization/intensity light distributions have been successfully used to design 2D wetting pathways onto the surface of azopolymer substrate having a fixed pre-patterned geometry. Examples of arrays of radially and azimuthally oriented azopolymer micropillars, obtained over a 1-cm2 area in a single irradiation step through radially and azimuthally polarized light beams, are shown in Figure 20C and E, respectively. The corresponding radially and azimuthally varying wetting anisotropy is highlighted by the overall elongation directions of the liquid droplets placed in different positions of the irradiated film, as shown in Figure 20D and in Figure 20F. Even if the two mentioned experiments already demonstrate the huge potential of the light-reconfigurable array of azomaterial microstructures in the wettability-oriented applications, we believe that many new applications in this field will appear in the next few years, probably further exploiting also the reversible behavior of the light-driven mass migration able to drive reversible structural deformation at command , , .
The light-reconfigurable pre-patterned azopolymer superficial textures can find several applications also as photonic elements able to manipulate and nano-focus the light. Mushroom-shaped dielectric microstructures, similar to the one easily achievable onto the reconfigured azomaterial cylindrical micro-posts, are known to behave as optical micro-resonators with extremely high q-factors , . Even if studies about such azopolymer micro-resonators have not been reported yet, we believe that there are no particular conceptual limitations for this application. Imprinted pre-patterned azomaterial surfaces can also be used in the fabrication of hierarchical structures, in which the 1D or 2D nanometric texture of the SRGs is inscribed directly onto the surface of azopolymer micro-post arrays or onto large-scale azopolymer wavy surfaces. Such light-reconfigurable hierarchical textures have been demonstrated to be able to precisely control light reflection and diffraction properties, providing light-tunable color structuration ,  and enhanced OLED outcoupling efficiency . In Ref. , the reconfigured 3D structures resulting from the irradiation of an array of sub-micron-sized holes with circularly polarized light are used as templates for the fabrication of 3D tapered Au nanoholes (Figure 21A). Such metallic nanostructures have been proven, both theoretically and experimentally, to provide light nano-focusing (Figure 21B–E), opening to the use of such light-reconfigured structures in surface-enhanced Raman spectroscopy (SERS)  and in integrated nano-optical circuitry .
4 Light-induced deformation in films of azobenzene-containing LC elastomers
Another class of materials in which the nanoscopic light-induced isomerization of the azobenzene molecules is transferred into a macroscopic movement involves azo-LCEs. LCEs are loosely cross-linked polymer networks that share the properties of both liquid crystalline polymers and elastomers . Similar to normal LCs, LCEs are characterized by an ordered organization, in which the molecular constituents of the material are preferentially aligned in a liquid crystalline phase. In addition, the design of an LCE architecture involves the inclusion of slightly dense crosslinks, which confer specific structural and elastic properties to the polymeric network. Such materials are then solid polymeric systems, cuttable in free-standing films with well-defined shapes, possessing furthermore a defined molecular order. The peculiarity of LCE films is the large structural deformation they undergo in response to an external stimulus capable to disrupt the molecular liquid crystalline order. The competitive mechanism is obviously the heat transfer to material matrix. A temperature increase, indeed, can cause liquid crystalline into isotropic phase transition, turning the ordered material system into a disordered one. The increase of structural disorder at molecular level translates into a macroscopic contraction of the LCE along the LC director, which defines the direction of the molecular alignment, and the concomitant material expansion in the direction orthogonal to the director. As this structural deformation is reversible, LCEs have immediately been regarded as soft actuators and artificial muscles able to convert at command external energy into mechanical movement. A particularly relevant aspect associated with the technological implications of LCEs is that the reversible structural deformation can be driven even using light as energy source once the azobenzene molecules have been included into the material matrix. In this case, indeed, the non-mesogenic cis-azobenzene isomer produced into the photoisomerization destabilizes the initial LC phase, efficiently sustained by the rod-like trans-azobenzene, and causes the lowering of the LCE molecular order, which eventually leads to the macroscopic material deformation. Furthermore, as the photoisomerization reaction is fully reversible, the initial geometrical configuration of the LCE film is restored by the thermal relaxation of the cis-azomolecules back to the mesogenic trans isomers. The relaxation induces a recovery of the original LC phase and makes the material available for a new photo-actuated deformation. Eventually, the LC order recovery can be also accelerated by the irradiation of the LCE film with visible light, which further stimulates the depletion of the cis-azomolecule population. Reversible light-induced uniaxial contraction of about 20% along the director direction of monodomain azobenzene-containing LCEs has been reported since the first studies about the light-induced mechanical actuation of these materials , . However, the exact geometry of the film deformation depends on the actual distribution of the LC directors inside the film, and in azobenzene-containing LCEs, it can be different for monodomain and polydomain liquid crystalline networks. Details about LCE chemistry, design, molecular alignment strategies and macroscopic deformation engineering can be found in several already existing reviews , , , , , , , . Here, we want to provide a general and brief overview on the main aspects of the photo-actuation occurring in azo-LCEs as a manifestation of a nanoscopic-to-macroscopic propagation of light-induced movements in azomaterials, highlighting also several applications offered by such material systems as light-driven artificial muscles, self-propelling locomotors and energy-harvesting devices.
4.1 The photo-actuation in azo-LCEs
The light-induced deformation of a free-standing monodomain azo-LCE films (typical thickness in the order of 10–100 μm) involves typically the bending of the film toward the direction of the incoming irradiating light , , , . Such bending behavior is determined by the large absorption extinction coefficient of the trans-azomolecule in the UV optical range, which generates a gradient in the photoisomerization efficiency for the azomolecules located at different positions along the direction of the film thickness. Indeed, most of the incident light is absorbed in the proximity of the irradiated surface within a thickness of 1–2 μm, while the trans-azomolecules in the bulk of the film and at the bottom surface remain almost unaffected . This isomerization gradient causes the contraction only of the exposed surface of the film, which finally bends toward the incoming light direction (Figure 22).
A large range of strong and complex photo-actuation modes can be achieved by controlling such bending behavior by means of an accurate spatial patterning of the molecular orientation directions across the film. For example, using a splay alignment of the mesogens (Figure 23A), the degree of structural bending is increased in respect to the monodomain ordered matrices , , while the bending direction of the film with this molecular alignment depends on which surface of the film is exposed to the light (Figure 23B–F) . Even very complex 3D programmable deformations of the LCE films can be obtained by accurately patterning the molecular alignment into two dimensions. Examples from literature show geometrical deformations similar to waves, springs, cones and saddles , , , , , opening the way to the arbitrary tailoring of the actuation geometry by means of a careful control over the molecular alignment , , , .
Differently from LCE monodomains, the bending direction of azobenzene-containing polydomain LCE films is determined by the direction of the irradiating linearly polarized light . In a polydomain LCE, the alignment direction is uniform only at the microscale, while macroscopically, the direction of the molecular alignment is essentially random. Each micro-domain of an azo-LCE contains trans-azomolecules oriented along the local molecular director. Due to the orientation-dependent efficiency of the light absorption for the trans-azomolecule, the irradiation of an azo-LCE with linearly polarized light drives an efficient order-disorder transition only in microdomains where the mesogenic trans-azomolecules are aligned with the light polarization direction. As a result, a reversible and directional photo-selective film bending is achieved only in the direction of the linearly polarized light (Figure 24).
4.2 Light-fueled robots and actuators
Light-fueled azo-LCE actuators can present several advantages with respect to electrically driven ones because they convert light energy directly into mechanical work without the aid of batteries, electric contacts or electrical engines. In several reported applications, the use of azo-LCEs as light-driven actuators requires a controlled irradiation of light at two wavelengths, able to fuel at command the disruption of the molecular alignment and the acceleration of its recovery via the photoisomerization reactions of the azomolecules. For more complex deformation modes and functionalities, also localized and spatio-temporally modulated irradiating light can be used.
Simple examples of light-to-mechanical energy conversion accelerated by the wavelength switch of the irradiating light have been already given in the previous section describing the light-induced bending and unbending behavior of different LCE films , , , , , . However, using the same approach, the photo-controlled deformation of azo-LCEs can be transferrable into the generation of actual mechanical work in clever experimental configurations. A light-fueled motor has been realized through a ring of an azo-LCE obtained connecting together the ends of a flat single-layer stripe of the elastomer . The simultaneous irradiation of UV and visible light at different positions across the ring has been demonstrated to cause the local expansion and contraction of the material, which drives the rotation of the ring and even the actuation of pulleys, realizing in fact a motor fueled only by light .
The design of the macroscopic deformation by the molecular alignment patterning and the selective actuation with spatio-temporal modulated light can be used to drive complex structuration and motion of LCE films, which can act, for example, as light-driven switchable adhesives , , microvalve and micropumps for microfluidic applications , , robotic arms , ,  and biomimetic locomotors. A light-driven micro-robot (Figure 25A), able to pick, lift, move and place an object heavier than its weight, has been reported in Ref. . The robot constitutes three stripes of a bilayer LCE film, joined together to form a photo-actuable arm endowed with a wrist and a hand (Figure 25B). Each section of the robot can be selectively activated by switching on and off a spatially localized illuminating spot of visible light. This is a clear example in which a light-fueled soft robot can be artificially manipulated to perform some action on its environment, acting similarly to an artificial muscle. Recently, even the possibility of automatic smart-responsive robots has been highlighted by Zeng and coworkers. They demonstrated the realization of a self-regulating LCE iris, whose structure deforms similarly to the iris of the human eye, which regulates the amount of transmitted light on the basis of the incident light intensity level . The same group proposed also a smart azo-LCE robot able to selectively distinguish, and eventually grab, different objects on the basis of their absorption properties, as a natural flytrap does with insects and other random particles .
Another application scenario for azo-LCEs is the biomimetic light-induced locomotion of soft robots. This technological field has been recently reviewed elsewhere , . In Figure 26, two examples of azo-LCE films actuated by a scanning laser beam able to drive a directional macroscopic displacement are reported. The robots mimic the natural locomotion modes of millimeter-sized walking caterpillars (Figure 26A–D) and swimming flat fishes (Figure 26E–F). The motion of such robots is provided by the activation of structural deformations of the LCE film by means of spatially or temporally modulated light patterns, able to induce the non-reciprocal deformations necessary for the directional locomotion at different positions of the robot body. Several other examples of similar walking and swimming biomimetic azo-LCE robots can be found in the literature , , , , , , accessing even to the micrometer spatial scale . However, even stationary illumination light fields have been demonstrated to generate continuous movement and locomotion of an LCE film incorporating azomolecules with fast cis-to-trans thermal relaxation dynamics. An example of continuous wave-like deformations obtained under stationary illumination conditions in an LCE film with splayed alignment has been recently reported by Gelebart et al. , . The light-induced continuous motion has been explained in terms of a self-shadowing mechanism initiated by the fast photo-driven deformations of the films at different positions under constant irradiation. Such effect is very promising for the realization of remotely controlled light-driven devices exhibiting interesting functionalities, like self-cleaning and self-propelling directional locomotion , . The self-shadowing feedback mechanism from which a continuous movement originates under stationary illumination conditions is potentially of great interest also in energy-harvesting applications, where the mechanical energy of the continuously oscillating LCE films , ,  can be converted in other forms of energy, e.g. electricity . In this regard, the possibility of directly using the light coming from the sun to activate photo-driven motions like bending  and high-frequency and chaotic oscillations ,  in azobenzene-containing LCE films can represent a new frontier for the applications of these materials for the realization of solar energy-harvesting devices.
5 Summary and outlook
Due to the efficient and reversible light-induced isomerization involving the significant variation of molecular electrochemical and structural properties, azobenzene molecules can act as versatile molecular photo-switches for materials in which they are included. Several azomaterial systems able to harness the microscopic motion of the isomerizing azomolecules in light-controllable optical, chemical and mechanical properties have been synthesized over the years. In the present review, we have described two classes of properly designed azomaterial systems in which the light-induced azobenzene molecular dynamics at the nanoscale is propagated at macroscopic spatial scales. In particular, we have mainly focused attention on the light-induced mass migration phenomenon occurring at the surface of films of amorphous azobenzene-containing materials. In such materials, the irradiation of Uv/visible light directly translates into the appearance of topographic surface reliefs, whose structural features depend on the intensity and polarization distributions of the irradiating light and on the pristine geometry of the irradiated azomaterials surface. These classes of azomaterials represent a versatile platform for light-induced micro- and nanopatterning of surfaces. Here, we have reviewed several scenarios in which the light-textured azomaterial surfaces have been used. Applications range from the photochemical imaging of optical fields at nanometric spatial resolution, to the light-induced reconfiguration of flat and pre-patterned surfaces for wettability, adhesion, photonics and lithography. New applications in all these fields will surely come in the future by exploiting new material design strategies for efficient and large surface modulation, complex illumination/prestructuration configurations and even the reversible nature of the phenomena involved. However, a further widening of their applicability would also come from the unveiling of the exact mechanism, bridging the initiator light-fueled motion of the isomerizing azomolecules and the macroscopic material transport, which is still an open question related to fundamental light-matter interaction at the basis of such nanoscopic-to-macroscopic motion propagation.
In addition, here, we described the general aspects of the photo-actuation observed in azobenzene-containing liquid-crystalline elastomers in which light is converted in controlled and reversible isomerization-initiated mechanical motion. The technological potentialities of such direct optical-to-mechanical energy conversion are enormous, and applications of azo-LCE films as artificial muscles, photo-actuators, soft robots, self-propelling locomotors, light-fueled machines and energy-harvesting devices have been highlighted. The complex photo-actuation geometries used in many of these applications are typically the result of both the accurate engineering of the patterned LC matrix and the modulation in time and/or space of the irradiating light. Future experiments will certainly explore further these design strategies to realize advanced deformation geometries and functionalities, taking advantage also from the eventual automated photo-response to external light stimuli in smart photo-driven actuators and devices. Furthermore, the continuous reversible actuation demonstrated recently in azo-LCEs with fast thermal isomerization dynamics have traced the path toward self-sustained moving light-driven actuators, which could be used in the future to continuously convert the energy of light, coming even from the sun, in mechanical or electric energy.
This study was supported by “Centro Servizi Metrologici Avanzati (CeSMA)” of the University of Naples “Federico II”, Italy.
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