We present the results and critical analyses of recent studies of ultrafast optical nonlinearities of liquid crystals in the isotropic and ordered phases for time scales spanning femtoseconds – microseconds. Pure undoped liquid crystals as well as liquid crystals containing plasmonic nano-particles have been investigated. Individual molecular electronic optical nonlinearities are found to be useful for femtoseconds – nanoseconds nonlinear transmission clamping applications. On the other hand, laser induced order parameter and birefringence modification in aligned nematic cells allow very rapid transmission switching of visible as well as near infrared lasers with response times in the sub-microseconds - few nanoseconds regime.

This paper demonstrates light-induced tuning of optical spectrum from optical microfiber knot resonator overlaid with an azobenzene-doped nematic liquid crystal (azo-doped NLC). The high-quality fiber resonator is made by drawing the single mode fiber to the micro-size diameter and self-twisting the microfiber as a knot shape. During the UV light irradiation the azobenzene molecules perform trans-to-cis photoisomerization which disrupts the NLC orientation. The disrupted NLC changes the effective refractive index within the LC overlaid fiber area and shifts the optical spectrum of microfiber knot resonator. The 0.25 nm spectral shifting of resonance wavelength was observed under the irradiation of 50 mW UV light.

Multiwall Carbon Nanotubes (MWCNTs) form a nematic liquid crystalline (LC) phase in their lyotropic form, enabling their mixing and coupling of their director to that of nematic LCs. An important aspect of this LC/MWCNT interaction, for applications other than display technology, is looking at the ways the MWCNTs affect the physical properties of the LCs. We study the effect of MWCNTs on the nematic to crystal (N-C) phase transition of 4-cyano-4-npentylbiphenyl (5CB). Our Differential Scanning Calorimetry (DSC) results show a dramatic increase in N-C phase transition temperature of 14°C for only 0.1% and of 20°C for 1% MWCNT, due to the crystal nucleation activity of the nanotubes. Using Polarized Microscopy we observe a change in the crystalline order of 5CB from spherulitic at 0% MWCNTs to a multidomain in presence of MWCNTs. The new crystals resemble those formed by a smectic LC 4- Decyloxybenzoic acid. This is in line with predictions from simulations, that the MWCNTs form smectic order in nematic 5CB at their interface. MWCNTs induced modifications of the crystal phase of 5CB promise to create controlled novel crystal forms for the purposes of optical transmission and other applications.

By utilizing a thin layer of supertwisted liquid crystal and potassium sodium strontium barium noibate crystal material with excellent pyroelectric effect, a broadband coverage optical sensing concept was proposed. Coating the pyroelectric substrate with a carbon layer of excellent absorption in the frequency range of interest, the intensity of an incident mid- or far-IR radiation can be converted to a corresponding intensity variation of a reflected near-IR beam via optical modulation of the liquid crystal film. As the result, the spatial intensity distribution of an incident mid- or far-IR radiation can thus be perceived directly by a low-cost semiconductor sensor/ sensor array. With flexible design of wave collecting arrangement, the broadband coverage sensor is suitable for viewing IR-giving objects with a large field-of-view.

We report on novel liquid crystals with extremely large flexoelectric coefficients in a range of ultra-fast photonic modes, namely 1) the uniform lying helix, that leads to in-plain switching, birefringence phase devices with 100 μs switching times at low fields, i.e.2-5 V/μm, and analogue or grey scale capability, 2) the uniform standing helix, using planar surface alignment and in-plane fields, with sub ms response times and optical contrasts in excess of 5000:1 with a perfect optically isotropic or black “off state”, 3) the wide temperature range blue phase that leads to field controlled reflective color, 4) chiral nematic optical reflectors electric field tunable over a wide wavelength range and 5) high slope efficiency, wide wavelength range tunable narrow linewidth microscopic liquid crystal lasers.

A new method for photopatterning of a bisanthracene-functionalized mesogenic compound 1 was developed. The monomer 1 had two anthracene moieties on each molecular end, and showed crystalline and liquid-crystalline phases at room temperature and at an elevated temperature, respectively. Upon UV irradiation of 1 in the molten state, intermolecular photodimerization of the anthracene moieties was induced, and consequently resulted in the formation of a linear polymer. In contrast to the monomer 1, the obtained polymer exhibited amorphous phase at room temperature. When 1 was irradiated with UV light through a photomask in the molten state, the irradiated areas changed to amorphous phase due to photopolymerization, whereas the non-irradiated areas remained the ordered phase. This phenomenon provided visual images with a clear contrast under polarized light. In addition, the images could be erased by heating the whole sample at a temperature above ca. 200 °C, because the amorphous phase changed to the ordered phase due to a reproduction of the monomer 1 from the polymer associated with thermal back-reaction of the anthracene photodimer. Photopatterning could be performed for the erased sample again and the process was found to be fairly reversible.

Multiwall Carbon Nanotubes (MWCNT) align by coupling to the liquid crystals’ (LC) nematic director in LC/MWCNT dispersions. This coupling is so strong that the LC molecules act as molecular motors to reorient the MWCNTs when an electric field is applied across oriented electro optic cells. On the other hand, MWCNTs also improve the LC order and modify the crystal phase of LCs. We investigate the physical reasons for those strong effects by studying the molecular interactions between a host LC and MWCNTs. It has been predicted theoretically that the aromatic rings could stack with their π orbitals in 4-Cyano-4’-pentylbiphenyl (5CB) and MWCNT nanocomposites. Experimentally 5CB modifies the MWCNTs Raman breathing modes in the same nanocomposites. In turn, we look for evidence of this interaction between MWCNTs and LCs at the 5CB molecules. Using FTIR spectroscopy we found that the modes corresponding to 5CB aromatic rings vibrations are affected in the presence of MWCNTs which confirms that π-π stacking of 5CB’s biphenyl rigid core to the carbon rings on the MWCNTs’ surface may indeed be major mechanism for MWCNT/LC nematic coupling. It shows also that the Raman breathing mode effects on MWCNTs can be due to this π-π stacking interaction with 5CB. Further investigations of the MWCNTs interactions with 5CB can lead to developing of a complete model of this phenomenon and help applications for electro optic cells, nanoswitches, new crystal forms for optics, communication technology and others.