Thermocapillary Marangoni Flows in Azopolymers

Structured polarized laser beams for controlled spiral-shaped mass transfer in azopolymer thin films

We present an approach for the realization of controlled spiral-shaped mass transfer in azopolymer thin films and the fabrication of spiral microreliefs. For such laser processing, we propose to use light fields with structured polarization distributions generated by a transmissive spatial light modulator. The projection lithography approach is utilized, transferring the pattern directly to the surface of azopolymer thin films. The shaped polarization distributions with different dependencies of the polarization vector orientation on the azimuthal angle allow us to drive surface waves on the sample along a spiral trajectory. Additionally, the ability to control the concavity of the formed microreliefs is demonstrated. This approach can be effectively modified for the direct laser fabrication of more complex nano-/micro-elements as well as their arrays.

Multi-Spiral Laser Patterning of Azopolymer Thin Films for Generation of Orbital Angular Momentum Light

Recently, the realization of the spiral mass transfer of matter has attracted the attention of many researchers. Nano- and microstructures fabricated with such mass transfer can be used for the generation of light with non-zero orbital angular momentum (OAM) or the sensing of chiral molecules. In the case of metals and semiconductors, the chirality of formed spiral-shaped microstructures depends on the topological charge (TC) of the illuminating optical vortex (OV) beam. The situation is quite different with polarization-sensitive materials such as azopolymers, azobenzene-containing polymers. Azopolymers show polarization-sensitive mass transfer both at the meso and macro levels and have huge potential in diffractive optics and photonics. Previously, only one-spiral patterns formed in thin azopolymer films using circularly polarized OV beams and double-spiral patterns formed using linearly polarized OV beams have been demonstrated. In these cases, the TC of the used OV beams did not affect the number of formed spirals. In this study, we propose to use two-beam (an OV and a Gaussian beam with a spherical wavefront) interference lithography for realization spiral mass transfer with the desired number of formed spirals. The TC of the OV beam allows for controlling the number of formed spirals. We show the microstructures fabricated by the laser processing of thin azopolymer films can be used for the generation of OAM light at the microscale with the desired TC. The experimentally obtained results are in good agreement with the numerically obtained results and demonstrate the potential of the use of such techniques for the laser material processing of polarization-sensitive materials.

Photoinduced movement: how photoirradiation induced the movements of matter

Pioneered by the success on active transport of ions across membranes in 1980 using the regulation of the binding properties of crown ethers with covalently linked photoisomerizable units, extensive studies on the movements by using varied interactions between moving objects and environments have been reported. Photoinduced movements of various objects ranging from molecules, polymers to microscopic particles were discussed from the aspects of the driving for the movements, materials design to achieve the movements and systems design to see and to utilize the movements are summarized in this review.

Writing and reading with the longitudinal component of light using carbazole-containing azopolymer thin films

It is well known that azobenzene-containing polymers (azopolymers) are sensitive to the polarization orientation of the illuminating radiation, with the resulting photoisomerization inducing material transfer at both the meso- and macroscale. As a result, azopolymers are efficient and versatile photonic materials, for example, they are used for the fabrication of linear diffraction gratings, including subwavelength gratings, microlens arrays, and spectral filters. Here we propose to use carbazole-containing azopolymer thin films to directly visualize the longitudinal component of the incident laser beam, a crucial task for the realization of 3D structured light yet remaining experimentally challenging. We demonstrate the approach on both scalar and vectorial states of structured light, including higher-order and hybrid cylindrical vector beams. In addition to detection, our results confirm that carbazole-containing azopolymers are a powerful tool material engineering with the longitudinal component of the electric field, particularly to fabricate microstructures with unusual morphologies that differentiate from the total intensity distribution of the writing laser beam.

Molecular-Level Photo-Orientation Insights into Macroscopic Photo-Induced Motion in Azobenzene-Containing Polymer Complexes

As part of continuing efforts to deepen the understanding of photo-induced mass transport in azo-containing polymers, we compared the diffraction efficiency (DE) during surface-relief grating (SRG) inscription, photo-induced molecular orientation (<P₂>), and thermal stability in two sets of supramolecular azopolymer complexes, namely, hydrogen-bonded (H-bonded) and ionically bonded (i-bonded) complexes, both as a function of the polymer degree of polymerization (DP). To that end, poly(4-vinylpyridine) (P4VP) polymers with DPs of 41, 480, and 1900 were H-bonded at an equimolar ratio with 4-hydroxy-4'-dimethylaminoazobenzene (azoOH), and the fully quaternized derivatives of the three P4VPs (P4VPMe) were i-bonded via ion exchange to sodium 4-[(4-dimethylamino)-phenylazo]benzene sulfonate (azoSO₃), also known as methyl orange, where the OH functionality of azoOH is replaced by a sulfonate group. The i-bonded complexes show much better DE performances and <P₂> levels than those of H-bonded complexes, which we relate to the liquid crystal structure of the former complexes. Fitting the <P₂> curves by a biexponential equation leads to two parameters associated with a fast trans-cis or angular hole burning (AHB) process and a slow angular redistribution (AR) process of the azo, respectively. It is found that AHB is predominant in the H-bonded complexes, whereas the AR contribution is much greater in the i-bonded complexes, assuring their superior SRG efficiency that is enabled by the anisotropic free volume created mainly by the AR process. In each set of complexes, the SRG efficiency is much better for the lowest DP complex, while the AR contribution is constant (and low) for the H-bonded complexes and increases roughly linearly with the decrease in DP for the i-bonded complexes. The latter difference might be related to the presence of entanglements in the complexes with DPs 480 and 1900, which slow down the macroscopic movement during SRG inscription but not the molecular-scale movement in photo-orientation.

Photo-triggered large mass transport driven only by a photoresponsive surface skin layer

Since the discovery 25 years ago, many investigations have reported light-induced macroscopic mass migration of azobenzene-containing polymer films. Various mechanisms have been proposed to account for these motions. This study explores light-inert side chain liquid crystalline polymer (SCLCP) films with a photoresponsive polymer only at the free surface and reports the key effects of the topmost surface to generate surface relief gratings (SRGs) for SCLCP films. The top-coating with an azobenzene-containing SCLCP is achieved by the Langmuir-Schaefer (LS) method or surface segregation. A negligible amount of the photoresponsive skin layer can induce large SRGs upon patterned UV light irradiation. Conversely, the motion of the SRG-forming azobenzene SCLCP is impeded by the existence of a LS monolayer of the octadecyl side chain polymer on the top. These results are well understood by considering the Marangoni flow driven by the surface tension instability. This approach should pave the way toward in-situ inscription of the surface topography for light-inert materials and eliminate the strong light absorption of azobenzene, which is a drawback in optical device applications.