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Here we describe liquid crystalline binary mixtures of [60]fullerene with a wedge-shaped tris-(alkyl)gallate group attached via a flexible spacer, where alkyls are n carbons long (C60-n). Phase behaviour was investigated as a function of blend composition. We show that the isotropization temperature is determined only by the volume fraction of the hydrocarbon moiety, irrespective of whether the material is a blend or a pure compound. Moreover, we find that above a certain fraction of the short-chain component, a transition appears from the low-temperature lamellar phase with triple layers of fullerene (LT) to a high-temperature phase containing quadruple fullerene layers (LQ). The LQ phase has an isotropization temperature 50–80 K higher than the LT phase. The behaviour of crystal melting suggests that there is a preference for the longer-chain component to occupy the outermost of the three fullerene layers of the LT phase. The extra-large lamellar thickness observed specifically in a 6 : 4 blend of C60-4 and C60-16 suggests the existence of a new superlattice with a structure yet to be determined. Our studies on the liquid crystalline binary blends provide a facile way to adjust the cross-sectional ratio of C60 and the hydrocarbon moiety, and deliberately tailor the desired level of cross-sectional mismatch. This in turn is likely to lead to further unusual soft matter self-assembly modes when adoption of interfacial curvature is not a solution for such a mismatch. [ABSTRACT FROM AUTHOR]

Adaptive optics systems often work in a closed-loop configuration due to the hysteretic and nonlinearity properties of conventional deformable mirrors. Because of the high-precision wavefront generation and nonhysteretic properties of liquid-crystal devices, the open-loop control becomes possible. Open-loop control is a requirement for advanced adaptive optics concepts. We designed an open-loop adaptive optics system with a liquid-crystal-on-silicon wavefront corrector. This system is simple, fast, and can save much more light compared to conventional liquid-crystal-based closed-loop systems. The detailed principle, construction, and operation are discussed. The 500 m horizontal turbulence correction experiment was done using a 250 mm telescope in the laboratory. The whole system can reach a 60 Hz correction frequency. Evaluation of the correction precision was done at closed-loop configuration, which is 0.22 [lambda] ([lambda] = 0.633 [micro]m) in peak to valley. The dynamic image under open-loop correction got the same resolution compared to closed-loop correction. The whole system reached 0.68 arc sec resolution capability at open-loop correction, which is slightly larger than the system's diffraction-limited resolution of 0.65 arc sec. OCIS codes: 230.3720, 010.1080, 220.4830.

To allow angular separation of the beam reflected off a liquid crystal wavefront corrector from the incident beam, it is convenient to introduce a small incident angle. This avoids using a beam splitter and the associated energy losses. The effect of the tilt incidence on the liquid crystal wavefront corrector was investigated in this paper. For a parallel aligned liquid crystal wavefront corrector, a simplified model was established and used to analyze the change of the phase modulation under the tilt incidence. The simulated results showed that the effect of the tilt incidence on the phase modulation can be ignored when the angle of tilt incidence is less than 6[degrees]. The phase modulation related to the incident angle was measured and the changing trend was similar to the calculated results. The effect of the tilt incidence on the diffraction efficiency of the liquid crystal wavefront corrector was also discussed. The simulated results indicated that the reduction of the diffraction efficiency is less than 1% for incidence angles under 3[degrees]. Last, a closed loop correction experiment was done with an incident angle of 1[degrees]. After correction, the averaged peak to valley (PV) and root mean square (RMS) of the wavefront were down to 0.15 [lambda] and 0.03 [lambda], respectively, and a resolvable image was acquired. OCIS codes: 010.1080, 230.3720, 230.6120.

A collimator with a long focal length and large aperture is a very important apparatus for testing large-aperture optical systems. But it suffers from internal air turbulence, which may limit its performance and reduce the testing accuracy. To overcome this problem, an adaptive optics system is introduced to compensate for the turbulence. This system includes a liquid crystal on silicon device as a wavefront corrector and a Shack-Hartmann wavefront sensor. After correction, we can get a plane wavefront with rms of about 0.017[lambda]([lambda] = 0.6328[micro]m) emitted out of a larger than 500 mm diameter aperture. The whole system reaches diffraction-limited resolution. OCIS codes: 220.4840, 010.1080, 230.3720.

A twisted nematic liquid crystal wavefront corrector (TN-LCWFC) partially modulates the incident polarized light. A blazed grating may be preapplied on the TN-LCWFC to filter the unmodulated light for the purpose of stable adaptive correction. However, for broadband light, the dispersion of the blazed grating affects the image resolution. An achromatic method is presented to eliminate the dispersion of the blazed grating. Based on a prism model, we analyze the achromatic principle. An achromatic system with a conjugated blazed grating and an achromatic lens is given to eliminate the dispersion. An experiment was done with two transmitted blazed gratings so as to validate our method. Finally, a liquid crystal spatial light modulator was used as a conjugated grating to eliminate the dispersion of the blazed grating in an adaptive optics system. The results showed that the dispersion was partially compensated, and a resolvable image was achieved with a 600-700 nm wave band. OCIS codes: 010.1080, 230.3720, 220.1000, 230.6120.

We propose a thermal modulation structure made of two identical SiO2 slabs coated by Dirac semimetal (DSM) films and separated by a nanoscale vacuum gap. The energy transmission probability reveals that the coupled surface plasmon polaritons (SPPs) between the two DSM films, and the surface phonon polaritons (SPhPs) supported by the SiO2 substrate can vary sensitively with the Fermi level, the degenerate factor of 3D Dirac points and the thickness of the DSM film, thus providing the possibilities for modulating the radiative heat transfer by tuning these parameters. Based on Maxwell's equations incorporating fluctuational electrodynamics, the effects of these parameters on the heat transfer coefficient and the thermal modulation contrast are numerically analyzed. Under proper parameters, higher modulation contrasts are obtained by continuously tuning the Fermi level from 0.05 eV to 0.3 eV. The obtained results might be helpful in designing a DSM-based thermal modulator with higher modulation contrasts. [ABSTRACT FROM AUTHOR]

This study proposes a simple method of designing a high-speed liquid crystal variable retarder (LCVR) with reflection mode. First, a series of simple formulas is provided for analysing the effects of tilt incidence and birefringence of the liquid crystal on the phase retardation and response time of the LCVR. Then, a reflective LCVR is fabricated to validate the theoretical analysis. Measured results show that the response speed can reach 2.7 kHz with a phase retardation of 1 λ. Furthermore, the theoretical curve is close to the measured curve while the incident angle is less than 10°. However, the theoretical and measured values show a considerable difference under a large incident angle. This problem is discussed, and a modified method is given. This work is helpful for the design and fabrication of high-speed LCVR. [ABSTRACT FROM AUTHOR]

We propose a voltage-tunable hyperbolic dispersion metamaterial (HDM) consisting of silver nanowires and nematic liquid crystals (NLCs). The metamaterial shows a novel electro-optic effect, in which the dispersion relation switches between elliptic and hyperbolic dispersions at the visible waveband with an external electric field. HDM functionality is analysed in detail based on the effective medium theory. 5CB NLC is chosen for the simulation, and tunability is achieved between 454 and 475 nm. The effects of filling ratio on tunability are also analysed. Tunable bandwidth widens from 21 to 45 nm with decreased filling ratio from 0.27 to 0.09. Finally, the effects of birefringence (Δn) are considered. Tunable bandwidth widens from 21 to 63 nm as Δnchanges from 0.2 to 0.6. These novel tunable performances of the HDM can be applied to optical devices and optical systems. [ABSTRACT FROM PUBLISHER]

A series of 4ʹ-(4-alkyl-phenyl)-2ʹ,6ʹ-difluorotolane isothiocyanate liquid crystal (LC) compounds were synthesised, and their phase transitions and electrooptical properties were investigated. These compounds exhibit high birefringence, about 0.47–0.52. As the number of carbon atoms in the alkyl chains increases, these LC compounds can exhibit smectic phases. When these LCs were mixed into the commercial LCs, the birefringence values of LC mixtures become higher than pure commercial LCs, and the visco-elastic coefficients of five LC mixtures are very close to each other at every test temperature. The results of response properties indicate that the compounds with a tri-fluorine substitution andn-propyl end group possess better response performance than the others. These LC compounds are particularly attractive for achieving fast response times in LC optic devices. [ABSTRACT FROM AUTHOR]

It has been a crucial technique to improve the dynamic response characteristics of a liquid crystal wavefront corrector (LCWFC) with optimal cell gap since the LCWFC needs at least 2π(orπ) phase modulation in adaptive optics systems (AOSs). We have given a complete process for obtaining the optimal cell gap accurately from a single photoelectric measurement, which can be conducted with a liquid crystal (LC) cell of any known thickness. This method has been analysed theoretically and confirmed experimentally by using a wedge-shaped cell; the experimental results match very well with the theoretical analysis. The response time of an optimal gap cell can be a novel evaluation method of response performance of LC materials. [ABSTRACT FROM PUBLISHER]

Self-aligned liquid crystals containing randomly dispersed nanoparticles are proposed to realise hyperbolic metamaterials at visible spectrum. Opposite signs for anisotropic permittivity tensors originated from the scattering of metallic nanoparticles in liquid crystals are achieved. Tunable effective wavelength properties are also demonstrated by choosing dielectric-metal core-shell nanoparticles. [ABSTRACT FROM AUTHOR]

Ultrafast switchable wide angle negative refraction in a novel dual-frequency liquid crystal mixture is demonstrated experimentally. Laterally fluorinated isothiocyanato phenyl-tolane liquid crystal compound is doped into Merck MLC-2048 to acquire the high birefringence dual-frequency liquid crystal. The maximum negative refraction angle achieved is about 12° and the switching time is only 25 ms for a 40-μm cell which is two orders faster than switchable negative refraction achieved before in nematic liquid crystals. Such properties provide a feasible approach to design negative refraction devices based on liquid crystals. [ABSTRACT FROM AUTHOR]

Surface-enhanced Raman scattering (SERS) has been widely reported to improve the sensitivity of Raman spectra. Ordinarily, the laser is focused on the sample to measure the Raman spectrum. The size of the focused light spot is comparable with that of micro-nano structures, and the number of micro-nano structures contained in the light spot area (defined as duty cycle) will severely affect the spectrum intensity. In this study, flower-like silver nanostructures were fabricated with a soft lyotropic liquid crystal template in order to investigate the effect of duty cycle. They were observed under a scanning electron microscope, and their spectrum enhancement factor was computed with the obtained Raman spectrum. Then, their duty cycles were measured using a SERS substrate at different locations. A formula was derived to represent the relation between the duty cycle of the nanoflowers and the Raman spectral intensity. This work could promote the actual applications of SERS in high-sensitivity spectrum testing. [ABSTRACT FROM AUTHOR]

Negative refraction (NR) is a promising technique that provides an opportunity to manipulate beam behaviour. We have demonstrated experimentally the controllable NR with large refraction angle at visible wavelength in the novel high birefringence (Δn) liquid crystals. When the Δnof liquid crystal reaches 0.42, the NR angle and critical incident angle reach –14° and 23.2°, respectively, which are much larger than those achieved before by other liquid crystals. By applying the electric field, we control the switch from NR to positive. [ABSTRACT FROM AUTHOR]

In this study, three kinds of low-viscosity difluorooxymethylene-bridged (CF2O) liquid crystals (LCs) were synthesised. These materials exhibit low melting point and low viscosity at room temperature. The viscosity of the final LC mixture significantly decreased when LC compounds were mixed with high-birefringence (Δn) isothiocyanato LCs. The tested curves with varied temperatures of LC mixtures showed that some CF2O LC compounds could enhance the response performance of high-ΔnLCs in a specific temperature range. Therefore, LC compounds with low viscosity could become a component of LC mixtures with fast response performance. [ABSTRACT FROM AUTHOR]

Abstract: A method to improve the loop frequency of liquid crystal adaptive optics systems (LC AOSs) was investigated. Based on open-loop control, a concurrent control scheme was proposed and the correction frequency of the LC AOS was increased from 85Hz to 212Hz. A simulated correction analysis illustrates that the distortion rejection ability of our LC AOS was improved with −1.1dB by using the concurrent control technique. A 500m horizontal turbulence correction experiment was done to validate our considerations. Firstly, 500m horizontal turbulence was measured and its Greenwood frequency and atmospheric coherence length is 31.5Hz and 4–5cm, respectively. Then, 500m horizontal turbulence was corrected in real time by using consecutive and concurrent control methods. The corrected results indicate that the resolution and contrast of the image was significantly improved by the concurrent control method. These results show that the concurrent control method can be used for LC AOSs to improve the correction frequency. [Copyright &y& Elsevier]

Abstract: Temperature effect on the diffraction efficiency of the liquid crystal spatial light modulator is investigated. The birefringence of the liquid crystal as functions of the temperature is measured with and without the power supply. It is shown that the birefringence reduces while the temperature increases. And the change magnitude of the birefringence has an exponential decay relation with the applied voltage for different temperature intervals. The scalar diffractive theory is used to analyze this effect on the diffraction efficiency. It indicates that the diffraction efficiency decreases from 98.7% to 27.2% while the temperature increases from 10 to 90°C for 16 quantified levels. At last, temperature effect on its applications in optical testing and wavefront correction is discussed. It indicates that it has almost no effect on optical testing, but has an important effect on wavefront correction. And two solutions are given to eliminate this effect. [Copyright &y& Elsevier]

The poor time stability of surface-enhanced Raman scattering (SERS) substrates greatly limits their application potential. Although core–shell structures are commonly used to enhance stability, their complex preparation processes, high costs, and susceptibility under acidic or alkaline conditions result in serious disadvantages for practical applications. Here, we propose a new method of external oxygen barrier to improve spectral stability, in which SERS substrates are stored in an oxygen-free environment. Controlled experiments are carried out under air and vacuum. Raman spectrum intensity is measured 11 times within six months for each group. Using the attenuation formula, the Raman spectrum intensity decay results of each SERS substrate over time are obtained. The effectiveness of the external oxygen barrier method is demonstrated through curve fitting using the corresponding function. The substrate spectral attenuation rates of the vacuum group and the argon group within six months are <20%, proving the effectiveness of the external oxygen barrier method. [ABSTRACT FROM AUTHOR]

Large-scale controllable fabrication of highly roughened flower-like silver nanostructures is demonstrated experimentally via electrodeposition in the liquid crystalline phase. Different sizes of silver flowers are fabricated by adjusting the deposition time and the concentration of the silver nitrate solution. The density of the silver flowers in the sample is also controllable in this work. The flower-like silver nanostructures can serve as effective surface-enhanced Raman scattering and surface-enhanced fluorescence substrates because of their local surface plasmon resonance, and they may have applications in photoluminescence and catalysis. This liquid crystalline phase is used as a soft template for fabricating flower-like silver nanostructures for the first time, and this approach is suitable for large-scale uniform fabrication up to several centimetres. [ABSTRACT FROM AUTHOR]

Electrically tunable distributed feedback (DFB) laser emission from a dye-doped holographic polymer dispersed liquid crystal (HPDLC) transmission grating is demonstrated. The homogenously aligned liquid crystal (LC) in this polymer scaffolding morphologic grating enables a large refractive index (RI) change for the TM wave under applied electric field, which in turn leads to a red-shift of 8 nm in the output laser emission. The tuning behavior of the DFB laser is well explained on the basis of DFB waveguide theory and amount of phase-separated LCs. [ABSTRACT FROM AUTHOR]