282 research outputs found
Spontaneous radial liquid crystals alignment on curved polymeric surfaces
No full textWe report on the fabrication and characterization of curved periodic microstructures formed through the controlled phase separation of a liquid crystal and a polymerizing matrix comprising self-aligned liquid crystal. Imaging through a "Fresnel like" structure imparts an intensity profile onto a photosensitive mixture which subsequently forms periodic alternating curved polymeric and liquid crystal slices. The phase separated concentric rings of nematic liquid crystal self-align in a radial alignment in between the polymer walls as indicated by polarizing optical microscopy analysis (Maltese cross). Electro-optical experiments confirm the possibility to control this alignment and the optical properties of the macroscopic structure by means of a quite low external voltage. The system exhibits high-quality and self-alignment of an ordered (liquid crystal) fluid without the need of surface chemistry or functionalization. © 2014 AIP Publishing LLC
Chirped POLICRYPS gratings containing self-aligning liquid crystals
No full textA novel chirped polymer/liquid crystal diffraction grating (POLICRYPS) structure with periods
ranging from 10 μm to 100 μm is reported. The chirped gratings are realized by using a single beam
curing process through a diffractive waveplate mask with varying periodicity. The quality of the
nematic liquid crystal (NLC) alignment inside the chirped structures as a function of periodicity
was investigated using a polarized optical microscope technique. The extent of phase separation
between the polymer and the NLC improves as the average spacing between the polymeric walls
gets smaller. In the regions where phase separation is most complete, the duty cycle of the grating
is proportional to the initial concentration of the two main components (polymer and NLC). The
degree of order of the NLC within the phase separated channels was measured with a Mueller Matrix
Spectroscopic Polarimeter. There is strong experimental evidence that the orientation of the NLC
molecular director inside the structure is perpendicular to the surface of the polymer phase separated
walls. Electro-optic measurements reveal a strong correlation between the on–off response times and
the average distance between the polymeric walls. Our findings opens new avenues to realize a new
generation of LCs based devices without using surface treatment or functionalization
Nanosecond switching of photo-responsive liquid crystal diffraction gratings
No full textThe diffraction efficiency of periodic microstructures composed of photoresponsive liquid crystals and polymer gratings is modulated using visible CW or pulsed radiation. A polymeric template (static grating) enabled by holographic photopatterning was infiltrated by capillary flow with a high performance photosensitive (green) liquid crystal after the initial formation step. The infiltration realizes high quality diffractive structures containing well-aligned photosensitive LC molecules and layers separated by polymer walls in a transmission grating geometry. The optical anisotropy present due to the oriented liquid crystals induces a strong polarization dependence of the grating diffraction efficiency. Exposure of the periodic structure to green CW or pulsed excitation modulates both the spectral and amplitude transmission behavior of the grating structure. In particular, under the influence of single pulse excitation, the diffraction efficiency can be modulated from 87% to 17% in 4 ns with a spontaneous back relaxation time three orders of magnitude (microsecond) faster than the one observed for the same material conventionally aligned in standard glass cells. The ability to remotely and quickly change the diffraction properties of soft-composite periodic microstructures marks a breakthrough towards the realization of ultra-fast all-optical devices. This journal is © the Partner Organisations 2014
POLICRYPS-based electrically switchable Bragg reflector
No full textThe formation and characterization of a switchable volume reflective element fabricated from a polymer liquid crystal (LC) polymer slice (POLICRYPS) structure by holographic photopolymerization at high temperature (65 °C) using a photosensitive/nematic liquid crystal prepolymer mixture is reported. The submicron Bragg structure formed consists of periodic continuous polymeric walls separated by periodic LC channels. The phase separated NLC self-aligns in a homeotropic alignment between the polymer walls as indicated by polarizing optical microscopy analysis (Maltese cross). The resulting periodic grating structure results in a Bragg reflection notch upon illumination with white light due to the periodic variation in refractive index. Electro-optical experiments realized through in-plane electrodes and temperature experiments confirm that the multilayer structure acts as a Bragg mirror whose reflection efficiency can be controlled by either a small (∼3V/μm) electric field or temperature
Ultra-fast solid state electro-optical modulator based on liquid crystal polymer and liquid crystal composites
No full textA different generation of polymer-dispersed liquid crystals (PDLCs) based on a liquid crystalline polymer host is reported wherein the fluid behavior of the reactive mesogenic monomer is an enabler to concentration windows (liquid crystal polymer/liquid crystal) (and subsequent morphologies) not previously explored. These liquid crystal (LC) polymer/LC composites, LCPDLCs, exhibit excellent optical and electro-optical properties with negligible scattering losses in both the ON and OFF states. These systems thus have application in systems where fast phase modulation of optical signal instead of amplitude control is needed. Polarized optical microscopy and high resolution scanning electron microscopy confirm a bicontinuous morphology composed of aligned LC polymer coexisting with a phase separated LC fluid. Operating voltages, switching times, and spectra of LCPDLCs compare favourably to conventional PDLC films. The LCPDLCs exhibit a low switching voltage (4-5 V/μm), symmetric and submillisecond (200 μs) on/off response times, and high transmission in both the as formed and switched state in a phase modulation geometry
Hidden Gratings in Holographic Liquid Crystal Polymer-Dispersed Liquid Crystal Films
No full textDynamic
diffraction gratings that are hidden in the field-off state
are fabricated utilizing a room-temperature photocurable liquid crystal
(LC) monomer and nematic LC (NLC) using holographic photopolymerization
techniques. These holographic LC polymer-dispersed LCs (HLCPDLCs)
are hidden because of the refractive index matching between the LC
polymer and the NLC regions in the as-formed state (no E-field applied).
Application of a moderate E-field (5 V/μm) generates a refractive
index mismatch because of the NLC reorientation (along the E-field)
generating high-diffraction efficiency transmission gratings. These
dynamic gratings are characterized by morphological, optical, and
electrooptical techniques. They exhibit a morphology made of oriented
LC polymer regions (containing residual NLC) alternating with a two-phase
region of an NLC and LC polymer. Unlike classic holographic polymer-dispersed
LC gratings formed with a nonmesogenic monomer, there is index matching
between the as-formed alternating regions of the grating. These HLCPDLCs
exhibit broad band and high diffraction efficiency (≈90%) at
the Bragg angle, are transparent to white light across the visible
range because of the refractive index matching, and exhibit fast response
times (1 ms). The ability of HLCPDLCs not to consume electrical power
in the off state opens new possibilities for the realization of energy-efficient
switchable photonic devices
Dynamic photonic materials based on liquid crystals
No full textLiquid crystals, combining optical non-linearity and self-organizing properties with fluidity, and being responsive to a wide variety of stimuli, have reached a key point in their development for photonic applications, for the realization of devices that could be dynamically reconfigurable, widely tunable and ultra-fast controlled. In this framework, ranging from Holography to Plasmonics, we review our recent efforts on developing a new generation of dynamic, tunable, electro- and all-optical photonic systems. © 2013 Elsevier B.V
Control of the plasmonic resonance of a graphene coated plasmonic nanoparticle array combined with a nematic liquid crystal
No full textWe report on the fabrication and characterization of a switchable plasmonic device based on a conductive graphene oxide (cGO) coated plasmonic nanoparticle (NP) array, layered with nematic liquid crystal (NLC) as an active medium. A monolayer of NPs has been immobilized on a glass substrate through electrostatic interaction, and then grown in place using nanochemistry. This monolayer is then coated with a thin (less then 100nm) cGO film which acts simultaneously as both an electro-conductive and active medium. The combination of the conductive NP array with a separate top cover substrate having both cGO and a standard LC alignment layer is used for aligning a NLC film in a hybrid configuration. The system is analysed in terms of morphological and electro-optical properties. The spectral response of the sample characterized after each element is added (air, cGO, NLC) reveals a red-shift of the localized plasmonic resonance (LPR) frequency of approximately 62nm with respect to the NP array surrounded by air. The application of an external voltage (8Vpp) is suitable to modulate (blue shift) the LPR frequency by approximately 22nm
Plasmonic photoheating of gold nanorods in thermo-responsive chiral liquid crystals
No full textWe report on the thermo-optical properties of gold nanorods (GNRs) dispersed in a thermotropic cholesteric liquid crystal (CLC). We have characterized the CLC reflection band behavior for two different cell thicknesses under the influence of a suitable (resonant) pump beam. It turns out that for the 1.6 μm thick cell there is a suppression of the CLC reflection band for both pure CLC and CLC/GNRs. For the 10 μm thick cell, the presence of GNRs desensitizes the shift of the CLC reflection band to temperature. Suitable cell design enables one to 'turn off' the wavelength shift of the peak reflection, thereby turning the system into a pure amplitude measurement tool. This has implications where the probe wavelength is fixed at a common, single wavelength
Polymer dispersed liquid crystals
No full textLow molar mass liquid crystals (LCs) are typically not soluble in polymer systems to any great degree. When the two different materials are mixed, this leads to two-phase systems whose morphology depends on a variety of factors including, primarily, the concentration. The resulting two-phase structures can have inclusions with nanometer through macroscopic dimensions. Although there are a large number of variants, these structures are generically called 'polymer dispersed liquid crystals' (PDLCs) when the resulting morphologies lead to systems that scatter light. This is often achieved in the intermediate concentration region (30-70% LC), in which morphologies with large mesoscale inclusions are typically formed. If the refractive index matching is done correctly, upon application of an electric field, the scattering can be turned off by an electric field, leading to dynamic transparency. This is a review of past literature with a focus on the type of morphologies that can be exhibited. Basic electro-optic properties are discussed as is the large variety of morphologies that can be induced. Also included is the related research area of 'periodic' PDLC systems, wherein the phase separation process is induced spatially. This leads to anisotropic systems where an electric field can control diffraction, instead of scattering
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