During the last decade, active metasurface has attracted significant attention from academia and industry because of its unparalleled advantages over conventional technologies. Active metasurface using electrically controlled liquid crystal (LC) is one of the most promising types of active metasurface towards industrial applications. In this work, we report a silicon metasurface immersed in a nematic liquid crystal for transmission amplitude modulation. Tunable resonance was realized by applying an AC voltage and the resonance was tuned in the spectral range of 1524 nm ~ 1573 nm, which covers the telecommunication C band. The corresponding phase shift at 1555 nm with a maximum value approaching 2π was measured using a Michelson interferometer, which supports the tuning of metasurface resonance by LC. As a result, the maximal modulation depth in transmission amplitude of 94% as experimentally recorded at 1530 nm. In addition, the high quality of LC photoalignment on metasurface was evaluated by examining the transmitted light intensity between crossed polarizers. The response time of sub-milliseconds can be obtained, thanks to the thin cell thickness of only 1 μm. The high alignment quality and fast response time demonstrated in this work shows a promising future for metasurface-integrated liquid crystal on silicon (meta-LCoS) spatial light modulators (SLMs), especially for telecommunication applications.
Mobile video and gaming are now widely used, and delivery of a glass-free 3D experience is of both research and development interest. The key drawbacks of a conventional 3D display based on a static lenticular lenslet array and parallax barriers are low resolution, limited viewing angle and reduced brightness, mainly because of the need of multiple-pixels for each object point. This study describes the concept and performance of pixel-level cylindrical liquid crystal (LC) lenses, which are designed to steer light to the left and right eye sequentially to form stereo parallax. The width of the LC lenses can be as small as 20-30 μm, so that the associated auto-stereoscopic display will have the same resolution as the 2D display panel in use. Such a thin sheet of tunable LC lens array can be applied directly on existing mobile displays, and can deliver 3D viewing experience while maintaining 2D viewing capability. Transparent electrodes were laser patterned to achieve the single pixel lens resolution, and a high birefringent LC material was used to realise a large diffraction angle for a wide field of view. Simulation was carried out to model the intensity profile at the viewing plane and optimise the lens array based on the measured LC phase profile. The measured viewing angle and intensity profile were compared with the simulation results.
LCOS for phase-only holography is ideally made to better optical tolerance than that for conventional
amplitude modulating applications. Die-level assembly is suited to custom devices and pre-production
prototypes because of its flexibility and efficiency in conserving the silicon backplane. Combined with
automated assembly, it will allow high reproducibility and fast turnaround time, making a way for
pre-production testing and customer sampling before mass production. Optical testing is the key
element in the process. By taking into account the flatness of both the backplane and the front glass
plate, we have assembled LCOS devices. We have reached our aim of less than one quarter wavelength
phase distortion across the active area.
Nonresonant random lasing from a dye-doped smectic A* scattering device is demonstrated. The field-induced scattering state of a low molar mass liquid crystal in the smectic A* phase is found to provide sufficient feedback to generate random lasing when a gain material, such as a fluorescent dye, is doped into the liquid crystal host. Furthermore, we found that the emission intensity of the random laser at a given excitation energy can be adjusted by altering the strength of the applied electric field so as to modify the scattering texture and consequently the transport mean free path. This change in the transport mean free path results in a change in the random lasing threshold. Large values for the transport mean free path, which indicate a weak scattering strength, result in large threshold values and vice-versa. Finally, we discuss the benefits of controlling the scattering strength with an applied electric field in terms of potential device applications.
In this paper, we review our recent experimental work on coherent and incoherent liquid crystal lasers. For the coherent lasers, results are presented on thin-film photonic band edge lasing using dye-doped low molar mass liquid crystals in the self-organised chiral nematic phase. We show that potentially high Q-factor lasers can be fabricated from these materials by demonstrating that a single mode output with a very narrow linewidth is readily achievable in well-aligned monodomain samples. Moreover, from our investigations we have found that the performance of the laser, i.e. the slope efficiency and the excitation threshold, are dependent upon the physical parameters of the low molar mass liquid crystal. Specifically, the slope efficiency was found to vary from 1% to 12% depending upon the liquid crystalline material employed. Using this information, the important parameters of the host liquid crystal are highlighted. As regards to the functionality, we demonstrate how the wavelength of the laser can be tuned using an in-plane electric field in a direction perpendicular to the helix axis. Finally, for the incoherent lasers, we summarise our findings on random lasers that are fabricated from liquid crystals which exhibit a smectic A* phase.
We demonstrate an electro-optic switch and a variable attenuator for
telecommunication applications at λ=1550nm by employing the
ferroelectric and electroclinic properties of an organosiloxane liquid crystal. In the ferroelectric SmC* phase an optical switch has been realised with an extinction ratio of 36dB between crossed polarisers. The switching time was ~200microseconds. In the SmA* phase the analogue nature of the electroclinic effect was employed to obtain a variable attenuator. The maximum attenuation range between crossed polarisers was 35dB for an applied electric field of +-9V/micron. The response time of the device was about 100microseconds, independent of the applied electric field. Both devices where demonstrated in the same 21.5micron thick cell which provided a retardance of λ/2 at λ=1550nm.
In this paper we investigate Photonic Band Edge (PBE) lasing from a homologous series of non-symmetric bimesogen liquid crystals (the mesogenic units are not identical) with varying physical parameters. A homologous series was synthesised, where the number of methylene units in the linking flexible spacer chain ranged from 6 to 12. Our results show a clear odd-even effect within the threshold values and slope efficiencies, of the PBE lasers, when plotted as a function of the number of methylene units in the spacer chain. The even spacer bimesogen PBE lasers performed with an overall higher efficiency (< 2μJ/pulse threshold values and 8% slope efficiency) than the odd spacer bimesogens PBE lasers (3.5μJ/pulse threshold values and 1% slope efficiency). We believe that this odd-even effect is due to the odd-even effect observed within the host physical parameters; a consequence of the molecular shape anisotropy
In this paper, we measure the emission energies of three different photonic band edge lasers containing nematic hosts which have slightly different physical properties. The low molar mass monomers used as nematic hosts are identical in terms of molecular shape except for the length of the terminal alkyl chain. The photonic band edge laser which contains an even number of methylene units in its alkyl chain is found to have the lowest lasing threshold at both the same absolute temperature and the same shifted temperature. At the same excitation energy the laser output for each sample is found to be proportional to the orientational order parameter of the nematic host. However, when plotted simultaneously the absolute value of the order parameter does not correlate with the same emission energies in each sample. As a result we consider other factors that maybe of equal importance to the operating efficiency of the PBE laser in order to obtain viable candidates that are able to explain the discrepancy in our results. Finally, we introduce a figure of merit parameter which contains the candidates most likely to affect the operating efficiency and obtain much better agreement with our results than with the orientational order parameter alone.
The lasing output characteristics of two different types of photonic band edge liquid crystal lasers have been investigated. The required helical structure or periodic change in the refractive index was realised by using either a chiral nematic or a chiral smectic C as the liquid crystal host. The fluorescent dye doped photonic band edge lasers exhibited very different emission characteristics. A Q-switched Nd:YAG pulsed laser, frequency doubled to 532nm with pulse lengths of 5ns was used to excite the samples. Typical laser parameters such as slope efficiency and input energy threshold values were examined for each laser and results indicated that the chiral smectic C laser was more efficient. We believe that the higher performance of the chiral smectic C laser is attributed to the increase in the degree of order of the host.
The birefringence of some different monomesogen and bimesogen liquid crystals has been determined. Birefringence is an important liquid crystal property in the context of display and optical communication applications. Members of the well known homologous series of liquid crystals nOCB were measured for comparison with a series of cyanobiphenyl bimesogens containing a novel fluorinated mesogenic group and novel conjugated ring structures. Comparative measurements were made by a rotating analyser technique, using a Helium-Neon laser as an illumination source. We present the birefringence and phase characterisation for several different liquid crystal materials, inferring structure-property relations and the effect of forming a bimesogen.
The paper describes an enhancement of the electroclinic characteristics of low molar mass dimeric organosiloxane liquid crystals. The degree of polymerisation of the siloxane core unit was varied in order to study the effect on phase transitions and electro-optic properties. It was found that the SmA*-SmC* phase transition temperature could be moved to any position in the range from 50°C to 10°C if we varied the number of SiMe2 groups in the flexible linkage of the dimeric molecule. More importantly, because the organosiloxane liquid crystal material had some distribution of the number of SiMe2 groups, it showed a rather broad SmA*-SmC* phase transition in contrast to the sharp phase transition of conventional electroclinic materials. The electroclinic coefficient reached the maximum value of 8 degrees/V/μm and was at least 1 degree/V/μm over a temperature range as broad as 10°C. The induced electroclinic tilt angle was as high as 22-23 degrees with good linearity and moderate applied electric fields.
New chiral compounds 3R-methylcyclohexanone derivatives were synthesized. These compounds were revealed to exhibit the mesomorphic behavior within rather wide temperature ranges. Types of formed mesophases and phase transition temperatures were determined by polarizing microscopy, differential scanning calorimetry and small angle scattering of X-ray. Mesomorphic properties of the new chiral compounds were compared with those for the chiral 2-arylidene derivatives of 3R,6R-3-methyl-6-isopropylcyclohexanone (d-isomenthone) studied earlier. Distinctions between these two types of compounds in an ability to form mesophases and also in twisting properties as chiral dopants in induced cholesteric mesophases are considered.
Some1R,4R-2-(4-phenylbenzylidene)-p-methane-3-one derivatives containing the ether or ester linking group between benzene rings of the arylidene fragment have been studied as chiral dopants in ferroelectric liquid crystal systems based on the eutectic mixture (1:1) of two phenylbenzoate derivatives (see paper for formula) The ferroelectric properties of these compositions (spontaneous polarization, rotation, viscosity, smectic tilt angle as well as quantitative characteristics of their concentration dependences) were compared with those for systems including chiral dopants containing no linking group. Ferroelectric parameters of the induced ferroelectric compositions studied have been shown to depend essentially on the presence of the linking group between benzene rings and its nature as well as on the number of the benzene rings in the rigid molecular core of the chiral dopants used. For all ferroelectric liquid crystal systems studied, the influence of the chiral dopants on the thermal stability of N*, SmA and SmC* mesophases has been quantified. The influence of the linking group nature in the dopant molecules on the characteristics of the systems studied is discussed taking into account results of the conformational analysis carried out by the semi-empirical AM1 and PM3 methods.
A novel pair of the E- and Z-isomeric 1R,4R-2-)4- heptyloxyphenyl)-benzylidene-p-menthan-3-ones has been prepared and the influence of distinctions in their molecular geometry on macroscopic properties of liquid crystal systems with the induced supra-molecular helical structure has been studied. The significantly lower helical twisting power of the chiral Z-isomer in comparison with that of E- one has been confirmed in the case of induced cholesteric systems based on 4-pentyl-4-cyanobiphenyl. The phase behavior and ferroelectric characteristics have been investigated for smectic-C* compositions based on the eutectic mixture of the homological 4-hexyloxyphenyl-4'- hexyloxy- and 4-hexyloxyphenyl-4'-octyloxybenzoates containing the novel isomeric chiral dopants. The spontaneous polarization of the opposite signs induced by the isomeric chiral components has been revealed for the compositions studied. Distinctions in phase states, absolute values of the spontaneous polarization, smectic tilt angle and rotation viscosity of the systems obtained are discussed.
In a surface stabilized ferroelectric liquid crystal cell, optical transmission oscillations have been revealed accompanying mechanical vibrations caused by fast field reversal. Special bookshelf textures, so-called 'rainbow', were used in the experiments. Temperature dependences of the oscillation parameters have been studied. The temperatures dependence of the oscillation frequency suggests that the some oscillation resonance correspond to modes of the liquid crystals.
Oscillation processes have been revealed in the course of reversible polarization study in ferroelectric liquid crystals. The oscillation frequency of polarization vector has been found to be from 1 to 30 kHz. The oscillation parameters were studied as functions of temperature. Temperature dependences of the oscillation amplitude and damping decrement have been measured.
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