Precise control of the surface topographies of polymer coatings is crucial to developing high-performance materials and devices for a wide variety of applications, such as optical displays, micro/nanofabrication, photonic devices, and microscale actuators. Mainly, photocontrolled polymer surfaces, such as photoinduced surface relief, have been studied through photochemical mass transport. In this study, we propose a novel method triggering the mass transport by photopolymerization of liquid crystals with structured light and demonstrate the single-step formation of microscale well and canal structures on the surface of polymer films. Such well and canal structures can be arranged in two dimensions by designing light patterns.
With the rapid development of flexible electronics and soft robotics, the topic of preventing fracture in materials and devices integrated on largely bending film substrates is emerging. The high demand for strategically reducing strain in bending materials requires a facile method to accurately and precisely analyze the surface bending strain in various materials. This study reports the fundamental and efficient techniques for measuring the bending behavior of various polymer films. The fracture limit of a hard coating overlying flexible substrates is successfully determined by the accurate and precise quantification of surface bending strains. Furthermore, a multilayer film substrate with surface bending strain prevents fractures of hard coatings and organic thin film transistors.
With the rapid development of flexible electronics and soft robotics, there is an emerging topic of preventing fracture in materials and devices integrated on largely bending film substrates of >100 µm thickness. The high demand for strategically reducing strain in bending materials requires a facile method that enables one to accurately and precisely analyze the surface bending strain in a wide variety of materials. This study proposes the surface-labeled grating method that is the fundamental and efficient technique for measuring surface bending strains merely by labeling a thin, soft grating onto various film substrates composed of flexible polymeric and rigid inorganic materials. The surface strain with a single-nanoscale (<1.0 nm) can be quantified in real time with no need of material information such as Poisson's ratio, Young's modulus, and film thickness. The fracture limit of a hard coating overlying flexible substrates is successfully determined by the accurate and precise quantification of surface bending strains.
Precise control of the surface topographies of polymer materials is key to developing high-performance materials and devices for a wide variety of applications, such as optical displays, micro/nanofabrication, photonic devices, and microscale actuators. In particular, photocontrolled polymer surfaces, such as photoinduced surface relief, have been extensively studied mainly through photochemical mass transport. In this study, we propose a novel method triggering the mass transport by photopolymerization of liquid crystals with structured light and demonstrate the direct formation of microscale well and canal structures on the surface of polymer films. The wells and canals with depths of several micrometers and high aspect ratios, which are 10 times larger than those of previously reported structures, were found to be aligned in the center of non-irradiated areas. Furthermore, such well and canal structures can be arranged in two dimensions by designing light patterns. Real-time observations of canal structure formation reveal that anisotropic molecular diffusion during photopolymerization leads to a directed molecular alignment and subsequent surface structure formation.
Nonlinear optical (NLO) materials whose optical properties change in accordance with incident light intensity are
attracting much attention in various fields. Liquid crystals (LCs) exhibit the largest nonlinearity among functional
materials due to their photoinduced molecular reorientation. In particular, doping dichroic oligothiophene dye into LCs
increases the light sensitivity of materials based on the interaction between dyes and an optical-electric field.
Furthermore, the absorbance of this LC system drastically increases through the dye molecules' reorientation, promising.
for application to the optical limiter; however, practical applications require better light sensitivity. In this study, we
investigated the effect of the host LC structure such as fluorinated LCs on optical limiting behavior derived from
nonlinear molecular reorientation. Irradiation of dye-doped LCs with a laser beam brought about molecular reorientation,
and the transmittance decreased with an incident light intensity. Furthermore, the threshold light intensity for optical
limiting behavior depended on the host LCs structure. Trifluorinated LCs effectively increased the light sensitivity of the
dye-doped LCs compared to LCs without fluorine substituents. This result contributes to the material design for the low threshold optical devices utilizing the NLO of dye-doped LCs.
Functional soft materials with controlled molecular alignment are attracting much attention in various fields due to their excellent flexibility and functional properties. Among conventional alignment methods, mechanical methods such as rubbing the polymer surface are well-known as a facile route to align various molecules. Besides, photoalignment methods, using photoresponsive molecules and polarized light, enable precise alignment control towards advanced functions. As a novel alignment method combining the advantages of both mechanical and photoalignment methods, we have developed scanning wave photopolymerization (SWaP) where phototriggered molecular diffusion is applied to align molecules. Since it uses the molecular diffusion as a driving force for alignment control, SWaP has the potential to align a variety of molecules. For further exploration of the mechanism, it is necessary to understand the polymer properties; thus, the synthesis of polymers applicable to solution-based analyses is highly desired. In this study, we conducted SWaP to synthesize soluble liquid-crystalline polymer films with one-dimensional alignment. Furthermore, we compared the molecular alignment behavior between SWaP and the conventional rubbing alignment technique using a soluble polymer, and revealed that only SWaP can induce a unidirectional molecular alignment in film.
Flexible electronic devices composed of polymers and elastomers require high mechanical durability to maintain their performance during cyclic bending. To design the appropriate structure for such devices, it is important to identify the position of a neutral mechanical plane (NMP) where there is no strain inside a bending material. In this study, the NMP position of bending polydimethylsiloxane (PDMS) film, which is a common soft material used in flexible electronic devices, is experimentally identified through internal strain measurement using a cholesteric liquid crystal sensor. Notably, the NMP of the bending PDMS film reversibly shifts toward the inner bending surface. Further, considering the NMP shifting enables us to fabricate a flexible electronic device with high mechanical durability. Quantifying the NMP position facilitates the development of device designs for flexible electronics.
Optical limiting is the phenomenon widely recognized as the potential application for a protector of human eyes and optical sensors from irradiation with lasers. However, much high optical limiting threshold and less flexibility have restricted such applications. Here, we report that oligothiophene-doped liquid crystals (LCs) function as a low-threshold optical limiter with deformability. Irradiation of dye-doped LCs with a continuous wave (CW) laser beam brings about the formation of diffraction rings, and the number of rings changes depending on the incident light intensity due to their photoinduced molecular reorientation. Utilizing such reorientation enables reversible optical limiting without additional multilayered optical components. Furthermore, softness of LCs allows for the fabrication of the deformable optical limiter; optical limiting due to the molecular reorientation occurs even in largely bent states. The low-threshold and deformable optical limiter based on oligothiophene-doped LCs thus will enable one to develop the protector of eyes and optical sensors from glaring-light-induced damage.
Photoalignment technology has attracted much attention from the viewpoint of noncontact patterning of large-area coatings. We report on direct alignment patterning in coatings by scanning wave photopolymerization of liquid crystals and nonlinear optical reorientation of dye-doped liquid crystals.
The control of molecular alignment patterns in liquid crystals is key to developing high-performance optical devices. In particular, two-dimensionally designed patterns have attracted much attention due to their potential application to novel optical devices such as a high efficiency polarization grating and a vortex converter. However, there remain challenges in obtaining molecular alignment patterns by a simple method. We have recently proposed a novel method for controlling the alignment of liquid crystals termed scanning wave photopolymerization (SWaP). In this method, a mass flow triggered by spatiotemporal photopolymerization causes shear stresses to anisotropic molecules, resulting in the generation of alignment patterns finely guided by the scanned light. In this study, we present the direct fabrication of polymer films with cycloidal molecular alignment patterns by SWaP.
Liquid crystals (LCs) have been utilized for the innovative optical devices because the molecular reorientation triggers a change in their optical properties. Among them, oligothiophene-doped LCs can induce the molecular reorientation by irradiation with a laser beam, which is useful for all-optical devices. However, the photoinduced reorientational behavior was observed only by irradiation with a high-intensity laser beam. In this study, we investigated the effect of incident light properties on photoresponsive behavior of oligothiophene-doped LCs by irradiation with a collimated laser beam from the perspective of improvement of the photoresponsive sensitivity of the molecular reorientation.
As a result, the irradiation of oligothiophene-doped LCs with a collimated laser beam enhanced the sensitivity of the molecular reorientation.
Surface bending strains in various flexible films is quantitatively analyzed by a surface-labeled grating method with a single-nanometer resolution. The real-time strain analysis we achieved has multiple benefits: high resolution, precision, and a wide range of measurable materials. The reliability of the measurements was confirmed using the modified Elastica theory. This method revealed that the cracking of hard coatings on the surface of PET films occurred at surface strains exceeding 1.45%, regardless of the film thickness and curvature. The multilayering of two PET layers with a soft PDMS layer between them reduced the surface bending strain by 50% compared with that of a single-layer film with the same thickness. This triple-layer film successfully suppressed the cracking of the hard coating and the breakdown of the OTFT. The surface-labeled grating method, therefore, is a practical tool that allows the analysis of surface bending strain in the elaborately designed materials.
Inorganic materials such as nanotubes and nanorods have attracted much attention due to their anisotropic properties. Although controlling the alignment of inorganic materials is able to enhance their functionality, macroscopic alignment over a large area remains a challenge. We have recently proposed a simple method for inducing unidirectional alignment of ZnO nanorods on a rubbed polyimide layer. In this method, ZnO nanorods grafted with liquid-crystalline (LC) polymers are aligned by cooperative interaction between the LC moieties in the grafted polymers and surrounding LC host molecules. In this study, we investigated the unidirectional alignment of surface-modified ZnO nanorods in nematic LCs in a micrometer-thick cells. Alignment of LC polymer-grafted ZnO nanorods along nematic LC host molecules has been revealed by polarized optical micrography and ultraviolet-visible absorption spectroscopy.
Over the past decades, flexible electronics such as flexible liquid crystal devices composed of polymer film substrates have been dramatically growing. Understanding bending behavior of polymer films is the key to designing flexible electronic devices with high mechanical durability. Although various bending analysis methods have been proposed, they are still limited to macroscopic and qualitative analyses. Recently, we have newly proposed a method for analyzing the surface bending strain in flexible materials, termed surface labeled grating method. This method enables us to quantitatively evaluate the surface bending strain by monitoring the diffraction angle of a He-Ne laser beam that passes through a grating label attached on a sample. In this study, we measure the surface bending strain in polyethylene terephthalate films and reveal that tension and compression occurs in their outer and inner surfaces, respectively.
Macroscopic and precise alignment control of functional molecules represented by liquid crystals (LCs) and polymers is the key to generating a new function and enhancing their performances. Among various alignment techniques, a photoalignment method offers the greatest potential to finely control molecular alignment because of the capability of micro- to nano-patterning with remote processes. Recently, we have proposed a novel photoalignment method based on a new concept of scanning wave photopolymerization (SWaP). This method utilizes molecular diffusion triggered by the localized photopolymerization, enabling to generate an arbitrary alignment patterns merely by single-step photo-irradiation. In this study, we fabricated liquid-crystalline polymer networks directed by SWaP.
We present a dye-free alignment patterning technique, based on a scanning wave photopolymerization (SWaP) concept, that achieves a spatial light–triggered mass flow to direct molecular order using scanning light to propagate the wavefront. This enables one to generate macroscopic, arbitrary 2D alignment patterns in a wide variety of optically transparent polymer films from various polymerizable mesogens with sufficiently high birefringence (>0.1) merely by single-step photopolymerization.
To develop flexible devices that have mechanical durability, understanding the bending behavior of soft material components is quite important. However, measuring bending strain in soft materials has been limited to their surface due to experimental constraints. In addition to the surface strain analysis, internal strain analysis will further clarify the bending behavior of soft materials. In this study, we quantified internal strain in bending polydimethylsiloxane (PDMS) films, which are a common soft material, through the selective reflection of a cholesteric liquid crystal elastomer (CLCE). The strain analysis with the CLCE revealed that internal strains depend on the position of the bending PDMS films. This internal strain quantification of soft materials leads to the development of flexible devices with high mechanical durability.
The control of the surface topology is important to utilize their high functionality for a wide variety of applications such as optics and mechanics. Several methods of controlling the surface topology have been proposed; in particular, the light irradiation can precisely control the surface topology in a non-contacting manner. However, conventional methods using the light irradiation need complicated processes with specific photoresponsive molecules. Recently, we revealed that patterned photopolymerization, based on a novel method of molecular alignment, induced surface relief structures on a film; yet, the relationship between the surface structures and molecular alignment has not been clarified. In this study, we made the detailed investigation of surface structures and molecular alignment of the film induced by patterned photopolymerization.
Molecular alignment control in polymer films is key to the development of high-performance materials with optical, electronic and thermal functions. Among molecular alignment techniques, a photoalignment method offers the fine and remote control of two-dimensional molecular alignment by the irradiation with linearly polarized light. We have recently proposed a novel photoalignment method based on the molecular diffusion caused by the polymer concentration gradient, termed scanning wave photopolymerization. In this study, we report specific polymerization behavior occurring during the process of the scanning wave photopolymerization. We investigate the effect of molecular diffusion on the photopolymerization behavior by measuring molecular weight of the polymers obtained under various photopolymerization conditions.
Flexible electronics and soft robotics have been drawing much attention. However, prevention of fracture of materials and devices in three-dimensional deformation remains a challenge. Design of strain in the materials and devices enables durable high-performance materials and devices. In this study, surface strain analysis of bending polymer films by a novel method called a surface labeled grating method is introduced.
Scanning wave photopolymerization of liquid crystal monomers enables arbitrary molecular alignment patterning due to molecular diffusion induced shear stress by spatially selective polymer production. However, the molecular alignment process has not been explored because of the difficulty of measuring such non-equilibrium state. In this study, we investigated molecular alignment behavior by directly observing scanning wave photopolymerization process with a polarized micrograph equipped with a dynamic light processor. Optical anisotropy was generated along with light scanning to trigger photopolymerization, indicating that molecules are aligned parallel to the light scanning direction.
Liquid crystal (LC) is the promising material for the fabrication of high-performance soft, flexible devices. The fascinating and useful properties arise from their cooperative effect that inherently allows the macroscopic integration and control of molecular alignment through various external stimuli. To date, light-matter interaction is the most attractive stimuli and researchers developed photoalignment through photochemical or photophysical reactions triggered by linearly polarized light. Here we show the new choice based on molecular diffusion by photopolymerization. We found that photopolymerization of a LC monomer and a crosslinker through a photomask enables to direct molecular alignment in the resultant LC polymer network film. The key generating the molecular alignment is molecular diffusion due to the difference of chemical potentials between irradiated and unirradiated regions. This concept is applicable to various shapes of photomask and two-dimensional molecular alignments can be fabricated depending on the spatial design of photomask. By virtue of the inherent versatility of molecular diffusion in materials, the process would shed light on the fabrication of various high-performance flexible materials with molecular alignment having controlled patterns.
Nonlinear optics has drawn much attention for its great potential in applications, such as frequency conversion, multiple-photon absorption, self-focusing, and so on. However, such optical nonlinearities are generally observed at very high light intensities. In this study, we designed hybrid-aligned dye-doped polymer-stabilized liquid crystals (PSLC), in which the molecular director orientation gradually changes from homeotropic at one surface to homogeneous at the other. In such film, the threshold intensity required to form self-focusing effect was markedly reduced by a factor of 8.5 compared to that in a conventional homeotropic cell, which enabled the generation of the self-focusing effect using a handheld 1-mW laser pointer. In addition, we investigated the structural effect of dye molecules: azo-dye methyl red (MR, photoisomerizable)-doped PSLC was prepared and its NLO response was evaluated. It turned out that such MR-based LC system was not effective for self-focusing effect compared to oligothiophene-doped systems.
Dye-doped liquid-crystalline (LC) system is known to enhance its photoinduced reorientation behavior significantly compared to undoped LC system. However, aligned LC system is disturbed by thermal fluctuation at high irradiation intensity, due to excess thermal deactivation excited dye molecules with a low emission quantum yield. In this study, photoresponsive behavior of LC systems doped with photostable fluorescent π-conjugated molecules was investigated by irradiation with a single Ar+ laser beam. When the laser beam was incident on the sample cells, diffraction patterns were observed on a screen for the LC cell doped with coumarin derivatives. Appearance of the diffraction rings indicates photoinduced reorientation of LCs. Coumarin derivatives with long molecular length were found to act as efficient triggers for photoinduced reorientation of LCs.
We report linear and crosslinked azobenzene containing liquid-crystalline polymers which can be applied to high-density optical storage and photomechanical analysis. We introduced a molecular design concept of multicomponent systems composed of photoresponse, refactive-index change amplification, and transparency units. Taking advantage of characteristics of liquid crystals (optical anisotropy and cooperative motion), polarization holograms were recorded,
which enabled us higher-density holographic storage. On the other hand, crosslinked liquid-crystalline azobenzene
polymer films were fabricated to investigate the photomechanical behavior. We have found that a large change in
Young’s modulus is induced by several mol%-cis form production. Furthermore, a unique bending behavior, which
cannot be explained by the conventional bending mechanism, was observed in the crosslinked liquid-crystalline polymer films with azobenzene in the side chain.
Amorphous copolymers were synthesized with azobenzene methacrylate, cyanobiphenyl methacrylate, and alkyl
methacrylate, in which the azobenzene acts as a photoresponsive part to undergo photoisomerization, the cyanobiphenyl
is involved in a change in refractive index triggered by the photoisomerization of the azobenzene, and the alkyl
methacrylate is to prevent light scattering, respectively. The obtained polymers showed no LC phase in spite of the
incorporation of the common mesogenic group. When two writing beams were interfered in the thick polymer films, the
diffraction of the probe beam was observed. The first-order diffraction efficiency reached more than 70 % within 500 ms.
On the other hand, the intensity of the zeroth-order beam decreased in proportion to the increase in that of the first-order
diffracted beam. It is clear that the intensity of the zeroth-order beam is transferred to that of the first-order diffraction
with no optical loss due to the formation of the Bragg gratings. When the writing beams were turned off, the diffraction
efficiency remained unchanged. When the recorded grating was kept in the dark at room temperature, sufficient
diffraction efficiency was generally retained after a year. However, the grating was easily erased when heated up above
Tg for several seconds, and rewritability was confirmed more than 100 cycles. No shrinkage was observed through those
processes.
Azobenzene-containing polymer, in which a change in birefringence can be induced by photoisomerization, is one of
promising materials for holographic applications. To develop high-performance holographic materials using azobenzene
polymers, it is necessary to prepare optically transparent thick films with capability of a large photoinduced change in
refractive index to obtain high diffraction efficiency and fast response in the Bragg regime. A number of materials have
been studied so far, but no attention has been paid to the combination of thick amorphous copolymers films containing
photoresponsive and mesogenic groups in the side chain and polarization gratings formed in the Bragg regime. In this
paper, we prepared optically transparent thick polymer films containing an azobenzene moiety with photosensitivity and
a tolane moiety with high birefringence, and formed intensity and polarization gratings with linearly polarized beams and
orthogonal circularly polarized beams, respectively. It was found that the nearly theoretically maximum diffraction
efficiency was successfully achieved, and the recorded hologram could be erased thermally or photochemically. In
addition, a large change in refractive index was induced in the polarization grating, and high diffraction efficiency
(~90%) and quick response (920 ms) was realized simultaneously.
Holography is one of the most promising techniques that enables ultra-high density optical data storage with a simple and small optical setup. The key component in holographic devices is a holographic material that meets various requirements such as high diffraction efficiency, fast response, and stability. Furthermore, additional problems, rewritability and reversibility, must be solved in rewritable holographic materials which are more convenient. In this paper, we successfully achieved rewritable holograms with 55 multiplicities by a simple formulation of optically transparent copolymer films containing azobenzene and mesogenic moieties. The recorded hologram was stable for more than six months and was rewritable over 300 times. In addition, holograms with angle and polarization multiplicity were recorded by controlling the incident angle and the polarization direction of writing beams independently.
We have studied the optical nonlinearities of aligned nematic liquid crystalline films in the near IR communication spectral region (1.55 micrometers ). The measured refractive index coefficients are on the order of 10-3 cm2/W. The origins of the refractive index changes are thermal indexing effect and director axis reorientation. Phase modulation of several (pi) s can be generated with mW-power near IR lasers in micron thick films.
We report optical and photoresponsive behavior of nonlinear liquid crystals in two-dimensional (2D) periodic structure. 2D structure made of photoresist and titania is constructed by interference photolithography using grating mask. Then azobenzene-doped nematic liquid crystal is infiltrated into these arrays, and photoresponsive behavior of the azobenzene-doped liquid crystal in the periodic structure is investigated. In particular, we show that the diffraction from these liquid crystal infiltrated grating structures can be optically modulated by an Ar+ laser at 488 nm.
A quantitative model for nonlinear photonic absorption processes in a liquid and their effects in the nonlinear transmission and limiting of a laser pulse are studied. The theoretical simulation results accounting for two-photon- and excited-state absorption processes, and the dynamic evolution of the molecular level populations and propagation of picosecond and nanosecond laser pulses through a fiber core formed by such liquid are in good agreement with experimental observations. In pump-prove studies using picosecond and nanosecond laser pump pulses and cw probe, we have also observed oscillatory transmission associated with laser induced thermal/density effects. We also discuss applications of the supra optical nonlinearity of nematic liquid crystal film for anti-laser jamming and optical limiting operation against long-pulse or cw lasers.
We found that in nematic liquid crystals doped with methyl-red dye or an azobenzene liquid crystal, the orientational optical nonlinearity can be dramatically enhanced. A concise review of the fundamental mechanisms involved and some application in all-optical image processing, optical limiting and switchable storage lens holograms is presented.
All-optical holography of azobenzene-liquid-crystal (ALC) doped nematic liquid crystals (NLC) is investigated by means of polarization and intensity grating methods. The holographic grating is formed upon a total optical power as low as 200 (mu) W/cm2 without any external biases, and can be switched within hundreds of milliseconds. The polarization dependence of the probe beam on the diffraction intensity reveals that the grating formation is attributed to reorientation and order-parameter change of the NLC molecules induced by the photoisomerization of the ALC molecules.
Manipulation of light by light as a stimulus by the use of liquid-crystalline materials was explored, which was aimed at optical switching, optical image storage and optical display. Two types of the liquid-crystalline materials are used: nematic liquid crystals and ferroelectric liquid (FLCs). Several kinds of optical switching and image storage systems were demonstrated by means of photochromic molecules and photosensitive LC alignment layer to induce the change in alignment of the LCs: 1) Fast nematic to isotropic phase transition of azobenzene LCs was induced by the laser pulse in two modes of analysis, transmission mode and reflection mode. 2) Optical switching in FLCs doped with the azobenzene was investigated with regard to photochemical flip of polarization of the FLCs. 3) Optical control of polymer- dispersed LC was explored by means of the azobenzene LC. 4) Optical switching and fabrication of nonrubbed alignment layer based on the novel photophysical principle was achieved in terms of photosensitive polyimide as an LC alignment layer. 5) Optical control of conductivity was performed by the use of LC thiophene derivatives.
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