A biaxial nematic phase had been predicted with D(2h) symmetry, wherein the mesogen's long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, n and m, respectively. The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality. The results of x-ray diffraction can be readily reproduced by ab initio calculations that explicitly include the bent-core shape in the form factor and assume short-range positional correlations.

This letter describes the use of nanotransfer printing (nTP) for forming three-dimensional (3D) structures with feature sizes between tens of nanometers and tens of microns over areas of several square millimeters. We demonstrate three different approaches−deep etching through printed hard masks, direct transfer of three-dimensional structures, and purely additive fabrication of multilayer stacks−for using nTP to fabricate a range of complex 3D nanostructures, including closed channels, suspended beams, and nanochannel stacks, that would be difficult or impossible to build with other methods.

Two dicationic salts with bis(triflimide) as counterions exhibited crystal-to-smectic liquid crystalline phase transitions (T m~4 1 and 37 ‡C) and smectic-to-isotropic liquid phase transitions (T i~1 12, 136 ‡C). They had a broad liquid crystalline phase range (71-99 ‡C) and an excellent range of thermal stability (360-364 ‡C). Their mixtures of various compositions also displayed liquid crystalline properties from r.t. to an extended range of temperatures. They exhibited fluorescence in 1, 2-dimethoxyethane and methanol.

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Surfaces functionalized with a self-assembled monolayer (SAM) formed from a mixture of two alkylsilanes with different chain lengths have been designed to simultaneously improve the liquid crystal (LC) wettability and promote homeotropic anchoring of the LC. Most chemically functionalized surfaces (e.g., long alkyl chain SAMs) that promote homeotropic alignment of LC possess low surface energy and result in poor LC wettability, inhibiting LC infiltration into microstructured surfaces and sometimes resulting in LC dewetting from the surface. However, a surface modified with a mixed SAM of octadecyltriethoxysilane (C18) and ethyltriethoxysilane (C2) exhibited very low LC contact angle while providing homeotropic anchoring. Ellipsometry was used to correlate the bulk concentration of C18 in the deposition solution to the surface coverage of C18 in the mixed monolayer; these bulk and surface concentrations were found to be equal within experimental uncertainty. The LC contact angle was found to depend nonmonotically with the surface coverage density, with a minimum (14.4 ± 0.1°) at a C18 surface coverage of 0.26 ± 0.08. Homeotropic LC anchoring was achieved at a C18 surface coverage of ≥0.11 ± 0.04, in the regime where a minimum in the LC contact angle was observed. The practical application of this approach to surface modification was demonstrated using a micropillar array sensor substrate. When the array was functionalized with a conventional C18 SAM, the LC did not infiltrate the array and exhibited a contact angle of 47.4 ± 0.5°. However, the LC material successfully infiltrated and wetted the same microstructured substrate when functionalized with a C18/C2 mixed SAM, while still exhibiting the desired homeotropic anchoring.

A highly sensitive nitrogen dioxide (NO2) sensor based on orientational transition of a thin film of liquid crystal (LC) supported on a gold surface is reported. Transport of NO2 molecules through the LC film to the LC-gold interface induces an orientation transition in the LC film. The dynamic behavior of the sensor response exhibits a concentration-dependent response rate that is employed to generate an algorithm for quantitative determination of unknown concentrations. Sensitive, selective and reversible detection with minimal effects of environmental fluctuations suggest that these sensors can be used for quantitative NO2 detection for a number of applications.

This letter describes classes of tunable microfluidic fiber ͑FF͒ devices that use specially designed long-period gratings in which the phase matching condition is satisfied over a wide spectral range. Dynamic tuning is achieved by electrowetting-based pumping of microfluidic plugs back and forth over the gratings. As specific examples, we demonstrate dynamically tunable broadband attenuators and filters with adjustable profiles by using fluids with different refractive indices. These devices have attractive features that include in-fiber design and polarization-independent behavior together with low-power, nonmechanical, fully reversible, and latchable tuning.Tunable optical fiber devices are emerging as valuable components of high-speed optical communications systems. Long period gratings ͑LPGs͒, which couple the fundamental core mode to higher order copropagating cladding modes, have been studied extensively because of their potential applications in band rejection, 1 and gain control. 2 We recently demonstrated that microfluidics actuated by electrowetting effects provides a convenient means to tune the optical properties of these and other fiber structures. 3,4 These microfluidic fiber ͑FF͒ devices offer low power operation and a rich range of tuning mechanisms. A drawback of FF devices that use conventional LPGs, however, is that the tuning range is typically only a few nanometers, primarily limited by the bandwidth of the LPGs. Although LPGs based on large index variations, induced by microbending of fiber, yield relatively large bandwidth ͑ϳ30 nm for 20 dB mode conversion͒ 5 they suffer from polarization dependent characteristics, and high insertion losses ͑ϳ0.4 dB͒. Recently, we showed that with specially engineered few-mode fiber it is possible to achieve a turn around point ͑TAP͒ in the LPG phase matching condition, which yields a wide bandwidth ͑20 dB coupling over 63 nm͒, low loss, and polarization independent performance with only modest index variations. 6 In this letter, we combine FF designs and TAP LPGs to build a class of fiber device that provides continuously tunable attenuation and spectral reshaping over a large bandwidth. The transmission characteristics of these FF devices are controlled by pumping fluids with different refractive indices around the TAP LPG by use of electrowetting pumps and planar microfluidic networks. These devices have low power, latchable operation with polarization independent and low loss behavior over a wide wavelength range.In LPGs, the spectral dependence of the resonant coupling is primarily determined by the difference in the effective indices between the two coupling modes (⌬n) and the difference in their corresponding group indices (⌬n g ) and is given by:where res is the resonant wavelength and ⌳ is the grating period. Conventional LPGs exhibit strong mode conversion at a specific resonant wavelength, with near-monotonic decrease in coupling over bandwidths of 3-5 nm. If a fiber is designed such that one of the higher order cladding modes transitions, as a f...

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