Lalgudi V. Natarajan

Phenomenological model of anisotropic volume hologram formation in liquid-crystal-photopolymer mixtures

The real time formation of anisotropic volume holographic reflection gratings in a liquid-crystal/photopolymer mixture is studied. We develop a phenomenological model of grating formation that incorporates the photophysics and photochemistry of the initiator dye, reaction-diffusion kinetics of the monomer-polymer system, phase separation of the liquid crystal, nematic order evolution of liquid-crystal droplets, and volume shrinkage of the polymer. We then test this model by experimentally monitoring the diffraction efficiency for s and p polarization, Bragg wavelength, and laser scattering in real time as the grating is formed. The model yields good agreement with experimental data for different recording intensities and exposure times. We discuss the physics of the system as it evolves in time and explain the major features of anisotropic grating formation in acrylate-based holographic polymer-dispersed liquid crystals.

Tunable two-photon pumped lasing using a holographic polymer-dispersed liquid-crystal grating as a distributed feedback element

A holographic polymer-dispersed liquid-crystal (H-PDLC) grating film was employed as an angle-dependent and narrow spectral-band feedback control element for two-photon pumped lasing in a dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulfonyl) stilbene (APSS) in dimethyl sulphoxide. The grating film contained about 80 layers of liquid-crystal domains periodically dispersed in an ∼15 μm thick polymer film, featuring a maximum reflectance of 75% at 561 nm position with an ∼9 nm spectral bandwidth. The output lasing wavelength could be tuned from 561.5 to 548.5 nm and the lasing bandwidth changed from 5 to 3 nm when the incidence angle on the grating film varied from 0° to 22°. The overall lasing efficiency was measured to be 10%.

Switchable-focus lenses in holographic polymer-dispersed liquid crystal

Fine-grained polymer dispersed liquid crystals have recently become available for electrically switchable holographic elements. We explore applications of this novel material to switchable focus diffractive lenses. Several fabrication approaches, holographic and non-holographic, are demonstrated and compared with respect to design flexibility, diffraction efficiency, switching dynamic range, and optical quality. It appears possible to shift optical power between widely separated focal points with a modulation ratio 100:1 on a 10 - 50 microsecond(s) time scale.

Electrically switchable lasing from pyrromethene 597 embedded holographic-polymer dispersed liquid crystals

One-dimensional photonic band gap (PBG) materials created from holographic polymer dispersed liquid crystals (H-PDLCs) provide enhanced light localization in an organic electro-optic device. Distributed feedback within the reflection notch of a H-PDLC grating narrowed the bandwidth of pyrromethene 597 fluorescence from 56 to 8.4nm at a lasing threshold of 0.12mJcm−2, compared to 2.6mJcm−2 required to observe amplified spontaneous emission in a nonstructured, but comparable floodlit (PDLC) sample. Application of an electric field (10–40V∕μm) continuously decreased the diffraction efficiency of the grating and the commensurate dynamic lasing intensity thus demonstrating electrically modulated gain from an optically pumped, all-organic PBG.

Electrically switchable, photoaddressable cholesteric liquid crystal reflectors

We report on the development of photoaddressable cholesteric liquid crystal (CLC) mixtures capable of large range color tuning as well as direct on-off electrical switching. The continuously photoaddressable CLC mixtures are based on a high HTP azo-containing chiral material mixed with an off-the-shelf nematic liquid crystal (QL9/E44). By polymer stabilizing the QL9/E44 mixture, it is demonstrated that the photoaddressable reflection of the notch can be switched on and off with an AC voltage. The novel combination of these effects has potential utility in lasing, dynamic notch filters, and spatial light modulators.

Switchable holograms in new photopolymer-liquid-crystal composite materials

Switchable holograms open up the possibility of real-time electro-optical control of diffractive optic components. We have developed a novel photopolymer-liquid crystal material system which allows fast, single-step recording of holograms with diffraction efficiency controllable by conveniently applied electric fields. With the addition of a surfactant to our standard material recipe, we have achieved complete switching of a first-order Bragg diffracted beam into the zero-order with an applied field of approximately 5 V/micrometers and microsecond response time. We have also demonstrated image storage and electro-optical readout with these materials. Low voltage, high resolution scanning electron microscopy studies have confirmed that gratings formed in this material system consist of periodic polymer-dispersed liquid crystal planes. The critical fields for switching and the response times agree very well with a simple liquid crystal shaped-droplet model which we have applied to these gratings.

Optical limiting in solutions of diphenyl polyenes

The optical-limiting behavior of a series of trans-alpha, omega-diphenyl polyene compounds was observed in solutions of chloroform. The influence of planarity, the substitution of donor and acceptor groups, and the extent of pi-electron delocalization on the nonlinear thresholds of the diphenyl polyenes in an optical-limiting geometry were examined. A saturation effect of optical-limiting nonlinear thresholds, consistent with the theoretical work, was observed at 10-11 pi-electron bonds. The temporal profiles of the transmitted laser pulses and the power dependence of the nonlinear thresholds as a function of spot size and wavelength were examined. These examinations led to the conclusion that the predominant nonlinear mechanism was quasi-steady-state self-focusing. Nonlinear thresholds an order of magnitude lower, and thus effective n(2) and X(3) values an order of magnitude higher, than the well-known self-focusing medium CS(2) were observed. Our studies demonstrate that this series of polyenes consists of efficient broadband nonresonant optical-limiting materials.

Electro-thermal tuning in a negative dielectric cholesteric liquid crystal material

The thermal and electrical tunability of a cholesteric liquid crystal containing a negative dielectric anisotropy liquid crystal in a planar alignment was studied. The physical, optical, and electro-optical characteristics of mixtures containing different ratios of chiral dopant S811 and the negative dielectric anisotropy liquid crystal ZLI-2806 were examined. A smectic A phase was seen at room temperature for S811 loadings >20wt%. Below 20%, a room temperature cholesteric phase was observed. Upon heating mixtures with composition S811 >20%, the selective reflection notch of the cholesteric phase appeared and blueshifted with temperature. Thermal tuning from 2300to500nm was observed over the temperature range of 23–55°C. Polarized optical microscopy, differential scanning calorimetry, and x-ray studies were utilized to confirm the temperature-dependent phase behavior. Tuning of ∼50nm by the application of a direct current electric field was also observed with no onset of electrohydrodynamic instabilities ...

Thermally Induced, Multicolored Hyper-Reflective Cholesteric Liquid Crystals

where n s and n o are the extraordinary and ordinary refractive indices, respectively. The maximum refl ection of unpolarized light is 50% because a planar aligned cell only refl ects circularly polarized light of the same handedness as the helical pitch. Methodologies to overcome this limitation include the stacking of two opposite-handed CLC fi lms or stacking two same-handed CLC fi lms separated by a half waveplate. [ 1–8 ] Here a methodology is reported to obtain near 100% refl ectivity in a single CLC fi lm (so called hyper-refl ectivity) in which surface-bound polymer stabilization is used to spatially segregate disparate regions of opposite handedness in a single cell. Through the use of a thermally tunable CLC mixture, the high contrast condition can be induced with temperature. The versatility of the approach is further demonstrated by creating a cell with two static refl ection notches at different wavelengths and thermally inducing high contrast at each notch by varying the temperature. High-contrast, selectively refl ecting materials have a variety of prospective applications in displays and photonics, allowing for on/off control of a portion of the spectrum while passing all other wavelengths. A simple method to attain high contrast refl ectors has utilized chiral polymeric materials mixed with opposite-handed CLC mixtures to yield hyper-refl ective CLCs (defi ned here as single-cell CLCs with a refl ection greater

Holographically directed assembly of polymer nanocomposites

Layered polymer/nanoparticle composites have been created through the one-step two-beam interference lithographic exposure of a dispersion of 25 and 50 nm silica particles within a photopolymerizable mixture at a wavelength of 532 nm. The polymerizable mixture is composed of pentaerythritol triacrylate (monomer), 1-vinyl-2-pyrrolidinone (monomer), and photoinitiator. In the areas of constructive interference, the monomer begins to polymerize via a free-radical process and concurrently the nanoparticles move into the regions of destructive interference. The effects of exposure time, power density, nanoparticle size, and periodicity on the final nanocomposite structure were measured with transmission electron microscopy to determine the mechanism for particle segregation. Diffraction from the sample was monitored as well, though its magnitude was not a good predictor of nanostructure in this relatively low index contrast system. Exposure time did not have a strong effect on the final structure. The best nanoparticle sequestration was observed at reduced laser power density, smaller interferogram periodicity, and decreased nanoparticle size, indicating that particle segregation is dominated by diffusion-limited nanoparticle transport directed by a matrix containing a gradient of polymerization kinetics.