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(a) An isotropic island, generated by the laser with a frequency of 70 kHz and a laser power of 800 mW, as seen between crossed polarisers and the l -plate inserted. The laser was focused on one of the two ITO layers, thereby inducing NLC melting from that interface. The black double-headed arrow represents the rubbing direction of a $ 19 m m-thick planar cell. Yellow and blue bars show the direction of local molecular alignment at the isotropic – nematic interface, which is planar. (b) The isotropic island, generated by the laser, is now continuously moving along a 50 m m line, and the scanning direction is indicated by the dashed arrow. (c) Mark left on the surface after the laser was turned o ff . Black lines indicate the orientation of polariser and analyser, and the yellow line the direction of the retardation plate.
Source publication
We present a new method for the in-plane realignment of nematic liquid crystals in already fully assembled cells with uni-directionally rubbed polyimide as an aligning layer. We use nematic liquid crystals (NLCs) with a relatively high nematic-isotropic transition temperature and we focus the IR laser beam of the laser tweezers selectively onto one...
Contexts in source publication
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... a better understanding of the method we should rst describe the effect of the laser on the LC and on the polymer. The power of the laser was adjusted in a way that the heat generated by absorption of IR light on only one of the two ITO layers was sufficient to locally melt the liquid crystal, giving rise to a thin isotropic island, as shown in Fig. 2a, which does not extend from one plate to the other. This can be achieved in a cell with a thickness down to 5 mm. At the nematic-isotropic (N-I) interface LC molecules are oriented planarly; this can be assessed by using a full-wave retardation plate. 9,27 In Fig. 2a, bluish and yellowish regions (for an explanation on the origin of ...
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... melt the liquid crystal, giving rise to a thin isotropic island, as shown in Fig. 2a, which does not extend from one plate to the other. This can be achieved in a cell with a thickness down to 5 mm. At the nematic-isotropic (N-I) interface LC molecules are oriented planarly; this can be assessed by using a full-wave retardation plate. 9,27 In Fig. 2a, bluish and yellowish regions (for an explanation on the origin of these colours please refer to the ESI †) of the NLC close to the circumference of the isotropic island clearly indicate planar alignment of the LC molecules at the nematic-isotropic inter- face. 28 When the laser is moved fast along a line the alignment of the LC ...
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... of these colours please refer to the ESI †) of the NLC close to the circumference of the isotropic island clearly indicate planar alignment of the LC molecules at the nematic-isotropic inter- face. 28 When the laser is moved fast along a line the alignment of the LC molecules follows the tailing part of the nematic droplet as clearly visible in Fig. 2b, where the laser is moving from bottom to top; therefore the alignment on the two sides of the line corresponds to the alignment of the bottom part of the isotropic island in Fig. 2a. When the laser is turned off we notice that the alignment is imprinted on the polymer layer as shown in Fig. 2c; this happens when the laser is kept ...
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... inter- face. 28 When the laser is moved fast along a line the alignment of the LC molecules follows the tailing part of the nematic droplet as clearly visible in Fig. 2b, where the laser is moving from bottom to top; therefore the alignment on the two sides of the line corresponds to the alignment of the bottom part of the isotropic island in Fig. 2a. When the laser is turned off we notice that the alignment is imprinted on the polymer layer as shown in Fig. 2c; this happens when the laser is kept moving along the line for a few seconds. The "tailing" side of the moving island therefore imprints the molecular orientation, which is present at that interface. Now, having demonstrated ...
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... part of the nematic droplet as clearly visible in Fig. 2b, where the laser is moving from bottom to top; therefore the alignment on the two sides of the line corresponds to the alignment of the bottom part of the isotropic island in Fig. 2a. When the laser is turned off we notice that the alignment is imprinted on the polymer layer as shown in Fig. 2c; this happens when the laser is kept moving along the line for a few seconds. The "tailing" side of the moving island therefore imprints the molecular orientation, which is present at that interface. Now, having demonstrated the effect of the laser on the liquid crystal medium and on the polymer layer we want to assess if the ...
Citations
... [93,94] The optical gradient force along the laser scanning direction was calculated to be about 3.02 × 10 −5 fN (details in Section S1, Supporting Information). Besides, the hydrodynamic force caused by shear flow was applied to the LC molecules during the laser irradiation process, [95][96][97] which could be described by the following Equation (1) [98,99] F hydro = 4 lv ln (2l∕r) − 0.72 (1) where v represents the fluid velocity, is the fluid viscosity, and l and r denote the length and radius of the LC molecules, respectively (details in Section S2, Supporting Information). The hydrodynamic force was calculated to be about 293 fN, obviously greater than the optical gradient force. ...
The precise construction of hierarchically long‐range ordered structures using molecules as fundamental building blocks can fully harness their anisotropy and potential. However, the 3D, high‐precision, and single‐step directional assembly of molecules is a long‐pending challenge. Here, a 3D directional molecular assembly strategy via femtosecond laser direct writing (FsLDW) is proposed and the feasibility of this approach using liquid crystal (LC) molecules as an illustrative example is demonstrated. The physical mechanism for femtosecond (fs) laser‐induced assembly of LC molecules is investigated, and precise 3D arbitrary assembly of LC molecules is achieved by defining the discretized laser scanning pathway. Additionally, an LC‐based Fresnel zone plate array with polarization selection and colorization imaging functions is fabricated to further illustrate the potential of this method. This study not only introduces a 3D high‐resolution alignment method for LC‐based functional devices but also establishes a universal protocol for the precise 3D directional assembly of anisotropic molecules.
... Imaging a bubble in between crossed polarizers [22] indicates a uniform radial orientation of the molecules WGMs circulating in any vertical plane (black lines) pass through the two gain regions and are therefore excited. (b) Another bubble but illuminated at its rim, which generates a bright ring of circulating light. ...
Soap bubbles are simple, yet very unique and marvelous objects. They exhibit a number of interesting properties such as beautiful interference colors and the formation of minimal surfaces. Various optical phenomena have been studied in soap films and bubbles, but so far they have not been employed as optical cavities. Here we demonstrate that dye doped soap or smectic liquid crystal bubbles can support whispering gallery mode lasing, which is observed in the spectrum as hundreds of regularly spaced peaks, resembling a frequency comb. The lasing enabled the measurement of size changes as small as 10 nm in a millimeter-sized, ∼100−nm-thick bubble. Bubble lasers were used as extremely sensitive electric field sensors with a smallest measurable electric field of 110 Vm−1 Hz−1/2. They also enable the measurement of pressures up to a 100 bar with a resolution of 1.5 Pa, resulting in a dynamic range of almost 107. By connecting the bubble to a reservoir of air, almost arbitrarily low pressure changes can be measured while maintaining an outstanding dynamic range. The demonstrated soap bubble lasers are a very unique type of microcavities which are one of the best electric field and pressure microsensors to date and could in the future also be employed to study thin films and cavity optomechanics.
... An in-situ realignment technique was developed by Mirri et al. by laser-imprinting onto one or the other of the LC cell inner interfaces [38]. Laser imprinting melts the LC locally, increases the surface roughness of the scanned area, and changed the topography slightly. ...
... Copyright 2015, Royal Society of Chemistry. Reprinted with permission from[38]. ...
... The thickness was estimated by the amount of transmitted light [19]. Imaging a bubble in between crossed polarizers [20] indicates an uniform radial orientation of the molecules ( Fig. 3c and d). ...
Soap bubbles are simple, yet very unique and marvelous objects. They exhibit a number of interesting properties such as beautiful interference colors and the formation of minimal surfaces. Various optical phenomena have been studied in soap films and bubbles, but so far they were not employed as optical cavities. Here we demonstrate, that dye doped soap or smectic liquid crystal bubbles can support whispering gallery mode lasing, which is observed in the spectrum as hundreds of regularly spaced peaks, resembling a frequency comb. The lasing enabled the measurement of size changes as small as 10 nm in a millimeter-sized, ~100 nm thick bubble. Bubble lasers were used as extremely sensitive electric field sensors with a sensitivity of 11 Vm$^{-1}$Hz$^{-1/2}$. They also enable the measurement of pressures up to a 100 bar with a resolution of 1.5 Pa, resulting in a dynamic range of almost 10$^7$. By connecting the bubble to a reservoir of air, almost arbitrarily low pressure changes can be measured while maintaining an outstanding dynamic range. The demonstrated soap bubble lasers are a very unique type of microcavities which are one of the best electric field and pressure microsensors to date and could in future also be employed to study thin films and cavity optomechanics.
... ordinary, axis) and a high nematic-to-isotropic phase transition temperature (T c = 387 K). However, although the mixture is referenced in some publications, little information is available and it originates from the manufacturer [18,19]. To determine the advantage of MLC2132 for the above-mentioned applications, an in-depth characterization of its anisotropic optical properties is needed. ...
In this paper, broadband Spectral Domain Interferometry provides single-shot optical characterization of dispersive thermotropic nematic liquid crystals. The proposed experimental setup enables measuring the chromatic dispersion, the extended Cauchy equation parameters knowing the optical index for one wavelength, and the thermo-optical coefficients to ascribe the dependence of the optical index with the inner temperature. The analysis is applied to the commonly known E 7 mixture and to M L C 2132 , whose chromatic properties are not referenced although the mixture is commercial, demonstrating the latter’s interest for electro-optical or thermo-optical applications of thick nematic liquid crystal cells.
... It should be noted that both these potentials V i and V ij describe the interactions of the ith layer with confining surfaces and the pair interactions between ith and jth molecules, respectively. Here (...) i is the statistical-mechanical average with respect to the one-particle distribution function of the ith layer [7,38] ...
... where z i is the dimensionless distance through the smectic film, V 0 is the force potentials which is responsible for the molecule-molecule interaction, W 0 is the parameter corresponding to "enhanced" pair interactions in the bounding layers, and the constant α implicitly characterizes molecular packing across smectic layers [38]. Physically, these approaches indicate that we replace V 0 by W 0 within the first and last layers, whereas for all interior layers 1 < i < N the interaction coefficient V 0 has not been changed. ...
... Taking into account this fact, in the theoretical investigations the initial thickness of the film was chosen as being equal to N = 25 [7,11,12,17,[19][20][21]. According to the McMillan's theory [38], the first-order bulk AI transition occurs for α ≥ 0.98, so, the choosing of α = 1.05 is acceptable. When choosing the value of W 0 /V 0 , one is usually guided by the fact that the partially fluorinated free-standing smectic films composed of the H10F5MOPP molecules are stable above the θ AI (bulk). ...
The problem of predicting structural and dynamic behavior associated with thin smectic films, both deposited on a solid surface or stretched over an opening, when the temperature is slowly increased above the bulk transition temperature towards either the nematic or isotropic phases, remains an interesting one in the physics of condensed matter. A useful route in studies of structural and optical properties of thin smectic films is provided by a combination of statistical–mechanical theories, hydrodynamics of liquid crystal phases, and optical and calorimetric techniques. We believe that this review shows some useful routes not only for the further examining of the validity of a theoretical description of thin smectic films, both deposited on a solid surface or stretched over an opening, but also for analyzing their structural, optical, and dynamic properties.
... This way of indirect heating is com- monly used for colloidal trapping and manipulation by the strong light of the optical tweezers and the mechan- ism was described and used in a large number of experiments [7,10,11,13]. It is interesting to note that this indirect heating by light was used for successful in- situ patterning of the nematic liquid crystal-ITO inter- face in an already assembled cell [20]. The mechanism relies on scanning a strong and focused laser spot along the alignment layer of the LC cell. ...
... Due to the softness of the LC and modified elasticity, it was possible to influ- ence the structure of topological defects and induce their rewiring [10,11]. In another study, Mirri et al. [20] have used strong absorption of laser light in ITO to modify the surface alignment and literally "draw" alignment patterns of the LC in situ. While the opto- thermal method was found to be very useful in these topological studies, little was known about thermal and temporal properties of the method. ...
We report on the thermal response of a thin nematic liquid crystal layer to a strong laser pulse, which is partially absorbed by the Indium Tin Oxide electrodes of the measuring LC cell. We measure local increase and temperature profile of the heated region and we measure the time response of the birefringence of this nematic layer. It is found that local increase of the temperature due to absorption of a 90 Mw laser light of 10 MS duration can easily exceed °C and depends on the localisation and focusing of the laser beam. Local thermal response of the heated nematic cell is measured by monitoring the time dependence of birefringence during the cooling and we found that nematic layer can be cooled down at an extremely fast rate of 10000 K/s. Optical retardation of 200 nm can be switched in 100 μs by laser pulse absorption and heating/cooling.
... 4 Alternative methods involve photolithography, enabling to imprint appropriate patterns onto photoactive polymer substrates, 5 the technique of creating self-assembled alignment surfaces or the process of controlled self-assembly a) Electronic mail: izasliwa@ifmpan.poznan.pl of LC molecules on surfaces, 6 the preparation of laser-induced periodic layer structures on polyimide surfaces, 7 as well as the process of realigning LC molecules at surfaces by applying the infrared laser beam. 8 Theoretical approaches to study the effect of the molecular alignment of LC molecules in thin cells have usually been focused on geometrical aspects of the molecular anchoring at surfaces with different topographies. [9][10][11][12][13][14] These methods allow the determination of the local orientation profile not only strictly at the grooved surface but also away from it. ...
The effect of the interplay between attractive nonlocal surface interactions and attractive pair longrange
intermolecular couplings on molecular structures of liquid crystals confined in thin cells with
flat solid surfaces has been studied. Extending the McMillan mean field theory to include finite
systems, it has been shown that confining surfaces can induce complex orientational and translational
ordering of molecules. Typically, local smectic A, nematic, and isotropic phases have been shown
to coexist in certain temperature ranges, provided that confining cells are sufficiently thick, albeit
finite. Due to the nonlocality of surface interactions, the spatial arrangement of these local phases
can display, in general, an unexpected complexity along the surface normal direction. In particular,
molecules located in the vicinity of surfaces can still be organized in smectic layers, even though
nematic and/or isotropic order can simultaneously appear in the interior of cells. The resulting surface
freezing of smectic layers has been confirmed to occur even for rather weak surface interactions. The
surface interactions cannot, however, prevent smectic layers from melting relatively close to system
boundaries, even when molecules are still arranged in layers within the central region of the system.
The internal interfaces, separating individual liquid-crystal phases, are demonstrated here to form
fronts of local finite-size transitions that move across cells under temperature changes. Although the
complex molecular ordering in surface confined liquid-crystal systems can essentially be controlled
by temperature variations, specific thermal properties of these systems, especially the nature of the
local transitions, are argued to be strongly conditioned to the degree of molecular packing. Published
by AIP Publishing. [http://dx.doi.org/10.1063/1.4989543]
... The alignment of liquid crystal at solid substrates is very complicated, yet relatively well understood 45 , as this knowledge was required to develop mature LCD technology. These processes are intrinsically related to the mechanisms behind liquid crystal alignment at any surface, which continue to be studied extensively [46][47][48][49][50][51][52][53][54][55][56][57][58][59] . Rod-like liquid crystals are routinely aligned in the plane parallel to the cell surfaces by coating the surface with a thin polymer film for alignment parallel or perpendicular to the surface structures. ...
... The liquid crystal director aligns normal to the thickness variation (along the rubbing direction), since that will minimize the elastic distortion energy of the system. Although other physical and chemical mechanisms are also important for alignment 55,[60][61][62][63][64][65] , the role of direct molecular interactions in the alignment is much less understood, because these are intermingled with surface roughness on solid substrates. However, the interaction between complex molecules and surfaces is a critical component of many important biological and technological processes. ...
We briefly review studies of liquid crystal films suspended in submillimeter size grids for biosensing applications and beyond. Due to intense recent research, the sensitivity of liquid crystal films to targeted biologically relevant agents can be increased, and the LC surface can be functionalized to be sensitive only to pre-assigned pathogens. Beyond sensor applications, we show that novel liquid crystal defect structures can be used to manipulate separation and deposition of lipids. Finally, we demonstrate that not only the nematic liquid crystal phase, but also chiral nematic (cholesteric and blue phase) and smectic liquid crystals can be used for sensing and may extend the sensitivity and/or the selection of biomaterials, which can be sensed.
We report a necklace structure made of liquid crystal dispersed in polyvinyl alcohol (PVA) aqueous solution, which is fabricated by a microfluidics device. In the necklace structure, liquid crystal droplets of tens of micrometer diameter are connected by micro-tethers, which are birefringent, are not penetrating the droplets, and can be elastically stretched by applying external force. The necklace structure was analyzed by fluorescent confocal microscopy, and the tethers are made of liquid crystal and PVA composite. The elastic constant of the tether was determined by using laser tweezers to stretch the tether. The Whispering Gallery Modes (WGM) circulating inside individual droplets in the necklace structure were also observed.
