Here we will show the methodology to prepare multiple topological defects in a microfluidic device. The topological defects were stabilized by optimizing the size and shape and satisfying the boundary conditions inside each micro-well. Furthermore, we introduced the liquid on liquid crystals by modifying the microfluidic device. The introduction of various liquidscould show a variety of alignments for the liquid crystals, which could provide topological defects in a well. When the photo-responsive molecules at the interfaces could control the on/off behavior of the topological defects. This is a new platform for studying the properties of topological defects and their functions.
A colossal optical nonlinearity has been observed for the dye-doped liquid crystals under the condition that the nematic phase is very close to the isotropic condition, and it has been usually explained by the effect of the dye-induced torque on the liquid crystal. However, the direct observation of the photo-response by the time-resolved transient grating phase imaging revealed that the optical nonlinear polarization, causing the extraordinary refractive index change, was observed in the liquid crystal region, where the light was not irradiated. Furthermore, a shock-like wave was observed after this nonlinear response ended. We propose that a compression wave generated at the disordered/ordered interface induced the reorientation of liquid crystal molecules to generate a larger polarization, causing the optical nonlinearity.
It is crucial for the efficiency of photocatalytic reactions how to separate the photo-excited electrons and holes and how to utilize them at interfaces. There are two main difficulties to make these possible; variations of defects and co-catalysts. Most of the metal oxide photocatalysts have shallow and deep trap states, whose structure is always controversial. It is hard to tell which state helps reaction or not. Various co-catalysts have been applied, but also it is difficult to tell the real effect; charge separation or the surface passivation. Here, we will show the method to distinguish the defect type from the electron dynamics by using the transient grating (TG) method, which has a high sensitivity at the interface. We prepared a film sample by heating a TiO2 paste on a glass substrate. The film was contacted with a reactant solution sandwiched by another glass and a spacer. The TG method is one of the time-resolved techniques, which measures the refractive index change at the interface after shining a pulse excitation light. We could distinguish three different routes for photo-excited electrons; bulk trap (<100 ns), surface trap (1-5 us), Ti trap (0.5 – 10 s). Only the surface trap showed reactivity with reactants on the solution side. Ti trap had a longer lifetime, which was only observed when the photoexcited holes were scavenged. This trap seems to affect the cycle of the photocatalyst. This method offers simultaneous measurements of different trap states, and gives an insight of which defects have an actual reactivity.
The molecular interaction sometimes propagates in a collective manner, reaching for a long distance on the order of millimeters. Such interactions have been well known in the field of strongly-correlated electron systems in a beautiful crystal interleaved by donor and acceptor layers, induced by photo-stimulus. The other examples can be found in liquid crystals (LCs), which could be found in many places in nature such as bio-membrane. Different from crystals, LCs features “softness”, which enables it to be a curved structure such as a cell. In LCs, even a small molecular change would trigger the overall structural change by the propagation of the molecular interaction. Here we will show, for the first time, how long and how fast the molecular interaction propagates in LCs. The patterned phase transition was induced in a LC, causing the phase transition propagation in a controlled way and the propagation was measured with an time-resolved optical technique, called the transient grating. A LC sample doped with azobenzene was put into a thermally controlled LC cell. A grating pattern of a pulse light with 355 nm was impinged to the LC cell, and the light was absorbed by the dyes, releasing heat or photomechanical motion. We could observe the fringe spacing dependence on the phase transition response, which indicates that phase transition was delayed as the fringe spacing due to the delay by the phase transition propagation. This is the first direct evidence of the molecular interaction propagation of the LC molecules.
We studied the photo-induced dynamics in a LC polymer film including azobenzene by using a time-resolved and a
microscopic technique. The film was confined in a liquid crystal cell, while it is a photomobile film under a free standing
condition, which is triggered by the photoisomerization of azobenezene. From the result of the time-resolved
measurements, the change inside the film induced by UV irradiation was highly anisotropic polarization change.
Microscopic observation revealed that the film was consisted of ordered and disordered region with a patched structure,
and the UV induced change was travelled in the ordered region on the order of seconds.
We studied the effect of the ratio between the monomer and cross-linker molecules in the azobenene included liquid crystal polymer films by using the heterodyne transient grating (HD-TG) technique, which is one of the time-resolved measurement techniques. Depending on the ratio, the magnitude of the refractive index change, its anisotropy, and the lifetime of the cis isomer of azobenzene, generated by a UV pulse irradiation. By increasing the cross-linker ratio, the refractive index change and its anisotropy was reduced, indicating less ability for the motion, while slower lifetime was observed by increasing the monomer ratio, indicating that the film is difficult to return the original shape by a visiblelight irradiation. The obtained dynamics was consistent with the functionality of the films.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.