Laser beam homogenization based on a multifocal liquid crystal microlens array.

A novel, to the best of our knowledge, tunable multifocal liquid crystal microlens array (TMLCMA) was fabricated with a triple-electrode structure consisting of a large-hole, a small-hole array, and planar electrodes. The electro-optical performances of the TMLCMA are characterized, demonstrating the monofocal convex, multifocal convex, and multifocal concave functions when the TMLCMA is manipulated with various driving schemes. Furthermore, the homogenization of a laser beam is realized using the fabricated TMLCMA. The multifocal convex and multifocal concave functions of the TMLCMA successfully suppress the lattice phenomenon caused by the monofocal microlens array, homogenize the Gaussian beam to a flattop intensity distribution, and broaden the beam size.

Polarization-insensitive tunable multifocal liquid crystal microlens array with dual lens modes.

Microlens has significant applications in integrated micro-optical systems. Recently, multifocal microlens arrays are expected to extend the depth of field for imaging systems and realize a highly efficient laser beam homogenizer. This work presents what we believe to be a novel approach for developing a tunable multifocal liquid crystal microlens array (TMLCMA), which can be operated in convex and concave modes through voltage control schemes. The TMLCMA is manufactured using nematic liquid crystals (LCs) with negative dielectric anisotropy, in conjunction with a triple-electrode structure consisting of top large-hole, middle small-hole array, and bottom planar electrodes. When a voltage is applied, the axially symmetric fringing electric field induced by the large-hole electrode causes the focal length of the microlens to gradually and radially change from the TMLCMA border toward the center. The gradient in the change of focal length is electrically tunable. The calculated spatial potential distributions qualitatively explain the multifocal characteristic and dual lens modes of the TMLCMA. The LC molecules in each microlens are reoriented in an axially symmetrical form, resulting in a polarization-insensitive TMLCMA. The imaging functions of the TMLCMA operated with dual lens modes are shown through practical demonstrations. The simple fabrication and versatile function make the developed TMLCMA highly promising for various optical system applications.

Polarization-insensitive liquid crystal Fresnel lens based on self-assembly polymer gravels and chiral dopant.

A polarization-insensitive liquid crystal (LC) Fresnel lens is developed with binary LC configurations of 90°-twisted nematic (TN) and vertically-aligned (VA) domains in the adjacent zones. A LC mixture comprised of nematic host, photopolymer and chiral material is initially filled into the VA cell with orthogonal rubbing treatment. After the ultraviolet irradiation on the filled LC cell through a photomask with Fresnel zone plate pattern, the interactions among orthogonal rubbing treatment, self-assembly polymer gravels, and chiral material induce the 90°-TN structure in the odd zones, whereas the initial VA structures are maintained in the even zones. The fabricated LC Fresnel lens with binary configuration emerges a maximum diffraction efficiency of around 35% at a voltage of 2.3 V, close to the theoretical diffraction limit of around 41%. The diffractive focus of the LC Fresnel lens is polarization-insensitive at the voltage above 2 V. When the voltage reaches 10 V, the diffractive focus vanishes. The numerical calculation confirms that the polarization-insensitive property appears in the primary focus of the LC Fresnel lens. This work reports a simple method to develop a highly efficient, polarization-insensitive, and electrically tunable LC Fresnel lens which is favorable for imaging system.

Rubbing-free liquid crystal electro-optic device based on organic single-crystal rubrene.

Liquid crystals (LCs) have been a vital component of modern communication and photonic technologies. However, traditional LC alignment on polyimide (PI) requires mechanically rubbing treatment to control LC orientation, suffering from dust particles, surface damage, and electrostatic charges. In this paper, LC alignment on organic single-crystal rubrene (SCR) has been studied and used to fabricate rubbing-free LC devices. A rubrene/toluene solution is spin-coated on the indium-tin-oxide (ITO) substrate and transformed thereafter to the orthorhombic SCR after annealing. Experimental result reveals that SCR-based LC cell has a homogeneous alignment geometry, the pretilt angle of LCs is low and the orientation of LCs is determined with capillary filling action of LCs. LC alignment on SCR performs a wider thermal tolerance than that on PI by virtue of the strong anchoring nature of LCs on SCR due to van der Waals and π-π electron stacking interactions between the rubrene and LCs. SCR-based LC cell performs a lower operation voltage, faster response time, and higher voltage holding ratio than the traditional PI-based LC cell. Organic SCR enables to play a role as weakly conductive alignment layer without rubbing treatment and offers versatile function to develop novel LC devices.

Tunable focal waveguide-based see-through display with negative liquid crystal lens.

A see-through display based on a planar holographic waveguide with a tunable focal plane is presented. A negative liquid crystal lens is attached on the outcoupling location of the waveguide to manipulate the image distance. The continuous tunable range for the focal length is from negative infinity to -65 cm. The demonstrated prototype system provides 10.5° field-of-view (FOV) for the images not locating at infinity. The FOV for the images not locating at infinity is limited by the diameter of the liquid crystal lens. The lens function of the liquid crystal lens is polarization dependent. By controlling the polarization states of the real scene and the input information image, the liquid crystal lens keeps the see-through function for a real scene and simultaneously plays the role of a negative lens for the input information image. Compared to the see-through display system with a single focal plane, the presented system offers a more comfortable augmented reality (AR) experience.

Low-voltage tunable liquid crystal lens fabricated with self-assembled polymer gravel arrays.

A novel approach for fabricating liquid crystal (LC) lenses is presented. The approach involves the use of a photocurable prepolymer dispersed in a cell fabricated with vertically aligned substrates. A radial gradient UV irradiation intensity distribution is produced using a radial variable neutral density filter. Under UV irradiation, the prepolymer diffuses and is then polymerized on the substrate surfaces owing to vertical phase separation. After polymerization, the diameter of the self-assembled polymer gravel on the substrates has a radial gradient distribution, causing a radial gradient pretilt angle (RGPA) distribution on the substrates and producing LC lenses. By numerical simulation, RGPA LC lens has significantly lower supplied voltage than conventionally hole-patterned electrode (HPE) LC lens, and higher lens power. In the experiment, the fabricated RGPA LC lens with aperture size of 5 mm possesses a simple planar electrode structure, low operation voltage (< 4 V), small root mean square wavefront error (< 0.08 λ), and acceptable focusing quality. By the overdriving scheme, the switched-off time of the fabricated RGPA LC lens reaches 0.27 s. With the novel approach, low-voltage LC lenses with different optical aperture sizes can be easily fabricated.

Liquid crystal lens with doping of rutile titanium dioxide nanoparticles.

A 4 mm-aperture hole-patterned liquid crystal (LC) lens has been fabricated using a LC mixture, which consisted of rutile titanium dioxide (TiO2) nanoparticles (NPs) and nematic LC E7, for the first time. The TiO2 NP dopant improves the addressing and operation voltages of the LC lens significantly because it strengthens the electric field surrounding the TiO2 NP and increases the capacitance of lens cell. Unlike the doping of common colloidal NPs, that of rutile TiO2 NPs increases the phase transition temperature and birefringence of the LC mixture, thereby helping enhance the lens power of LC lens. In comparison with a pure LC lens, the TiO2 NP-doped one has approximately 50% lower operation voltage because of the strengthened electric field around the NPs and has roughly 2.8 times faster response time because of the decreased rotational viscosity of the LC mixture and the increased interaction between the LC molecules by the NP dopants. Notably, the doping of rutile TiO2 NPs improves the operation voltage, tunable focusing capability, and response time of LC lens simultaneously. Meanwhile, this method does not degrade the focusing and lens qualities. The imaging performances of TiO2 NP-doped LC lens at various voltages are demonstrated practically by tunable focusing on three objectives at different positions. These results introduce NP in the application of LC lenses.

Fast-response liquid crystal lens with doping of organic N-benzyl-2-methyl-4-nitroaniline.

In this study, a large-aperture hole-patterned liquid crystal (LHLC) lens was prepared from a mixture of nematic liquid crystal (NLC, E7) and organic material (N-benzyl-2-methyl-4-nitroaniline, BNA). The electro-optic properties of doped and undoped samples were measured, compared, and analyzed. The doped sample exhibited a response time that was ∼6 times faster than that of the undoped sample because BNA doping decreased the rotational viscosity of the NLC. BNA dopant effectively suppressed the RMS error of LHLC lens addressed at the high voltage. Furthermore, the BNA dopant revealed a considerable absorbance for short wavelengths (< 450 nm), automatically providing the LHLC lens with a blue light filtering function for ophthalmic applications.

Large aperture liquid crystal lens with an imbedded floating ring electrode.

We propose a hole-patterned large aperture (LA) liquid crystal (LC) lens with a diameter of 6 mm. In our design, a floating ring electrode is embedded into the interface between the dielectric layer and the LC layer. This structure increases the electric field strength around the floating ring electrode located near the aperture center and assists in distributing the fringing electric field throughout the LC layer. Therefore, the thick dielectric layer used in the conventional hole-patterned LA LC lens can be effectively decreased. Consequently, the proposed LA LC lens has low operation voltage, large lens power, and introduces a low wavefront error of approximately 0.07 λ.

Controlling liquid crystal pretilt angle with photocurable prepolymer and vertically aligned substrate.

We demonstrate a new approach to control the pretilt angle of liquid crystals (LCs) with photocurable prepolymer in a cell fabricated with vertically aligned substrates. During UV exposure, prepolymer approaches and is polymerized on the substrate surfaces because of the vertical phase separation induced by differences in the surface tensions of the employed materials. After polymerization, the polymer structure formed on the substrate alters its surface polarity and changes the pretilt angle of the LC cell. The LC pretilt angle can be controlled from 87.3° to 2.5° when the prepolymer concentration ranges from 0 wt% to 2.5 wt%.

Holographic polymer networks formed in liquid crystal phase modulators via a He-Ne laser to achieve ultra-fast optical response.

The holographic polymer network formed in liquid crystal (LC) phase modulators via a He-Ne laser in this study demonstrates ultra-fast optically response and low light scattering. These advantages are mainly caused by the small LC domains and uniform polymer network when processing LC cells via holographic exposure to a He-Ne laser. The use of this method to fabricate LC cells as phase modulators results in a decay time of 49 µs under 2π phase modulation at room temperature. The predicted fast optical response can be achieved when operating devices at high temperatures.

Effects of silica nanoparticles on electro-optical properties of polymer-stabilized liquid crystals.

We control the pretilt angle of liquid crystals (LCs) by simultaneously doping silica nanoparticles (SNs) and reactive monomers into the LC cell. Application of AC high voltage (ACHV) to the cell compels the lifting force and the facilitation of polar groups to move the SNs and monomers toward the substrate surface. Polymer networks and SNs are stabilized at the substrate surface after UV exposure, sustaining the LCs at high pretilt angles. The deposited SNs on the substrate surface increases the anchoring energy of the substrate; the dispersed SNs in the cell decrease the relaxation constant of LCs. Therefore, the response time of the high-pretilted-polymer-stabilized LC cell is decreased. The method enables the control of the LC pretilt angle over a broad range. The slow response time of the polymer-stabilized LC cell from high monomer dose can also be prevented following the addition of SNs.

Polarization-insensitive liquid crystal microlens array with dual focal modes.

We demonstrate a liquid crystal (LC) microlens array (MLA) fabricated by LCs possessing negative dielectric anisotropy, in conjunction with a cell with a three-electrode structure. The presented LC MLA is polarization-insensitive and can be operated in both concave and convex modes. The shortest focal length of the LC MLA is -2.54 and 2.22 mm in concave and convex mode, respectively. Disclination lines that are usually observed in conventional hole-patterned LC lens can also be avoided because of the vertical alignment treatment of LCs.

Using photopolymerization to achieve tunable liquid crystal lenses with coaxial bifocals.

Liquid crystal (LC) lenses with circular hole-patterned electrodes possess the excellent capabilities of tunable focal lengths. In this paper, we demonstrate the performance of a specific LC lens with tunable coaxial bifocals (CB) synthesized via photopolymerization of LC cells. The characteristics of tunable CB are clearly exhibited when the voltage applied is continuously increased, eventually disappearing until only one focus is left when significantly higher voltages are applied. We simultaneously demonstrate two types of tunable CB LC lenses fabricated via different photocurable processes and determine their optical functions.

Preventing occurrence of disclination lines in liquid crystal lenses with a large aperture by means of polymer stabilization.

Liquid crystal (LC) lenses with a circularly hole-patterned electrode possess excellent characteristics in optical performance, especially for the capability of tunable focal lengths. But, non-uniformly symmetrical electric fields in LC lenses usually induce disclination lines when operating. In general, the occurrence of disclination lines not only degrades their optical capability such as imaging performance, but also spends more time for tuning focal lengths. In this paper, we use a way of polymer stabilization to successfully prevent the disclination lines in LC lenses. Even arbitrarily adjusting the applied voltages in LC lenses, it seems no occurrence of disclination lines again. In addition, we compare the basic optical performance for LC lenses with or without polymer stabilization. From experimental results, it shows that they almost have the same optical performance.

Superior improvement in dynamic response of liquid crystal lens using organic and inorganic nanocomposite.

In this study, the response time of a 4 mm-aperture hole-patterned liquid crystal (HLC) lens has been significantly improved with doping of N-benzyl-2-methyl-4-nitroaniline (BNA) and rutile titanium dioxide nanoparticle (TiO2 NP) nanocomposite. The proposed HLC lens provides the focus and defocus times that are 8.5× and 14× faster than the pristine HLC lens, respectively. Meanwhile, the focus and defocus times of the proposed HLC lens reach the order of millisecond. Result shows that the synergistic effect of BNA and TiO2 NP induces a 78% decrement in the viscosity of pristine LC mixture that significantly shortens the focus and defocus times of HLC lens. The remarkable decrement in viscosity is mainly attributed to spontaneous polarization electric fields from the permanent dipole moments of the additives. Besides, the strengthened electric field surrounding TiO2 NP assists in decreasing the focus time of HLC lens. The focus and defocus times of HLC lens are related to the wavefront (or phase profile) bending speed. The time-dependent phase profiles of the HLC lenses with various viscosities are calculated. This result shows the decrease in wavefront bending time is not simply proportional to viscosity decrement. Furthermore, the proposed HLC lens emerges a larger tunable focus capability within smaller voltage interval than the pristine HLC lens.

A Comparative Study on Electro-Optic Effects of Organic N-Benzyl-2-Methyl-4-Nitroaniline and Morpholinium 2-Chloro-4-Nitrobenzoate Doped in Nematic Liquid Crystals E7.

Improvements in electro-optical responses of LC devices by doping organic N-benzyl-2-methyl-4-nitroaniline (BNA) and Morpholinium 2-chloro-4-nitrobenzoate (M2C4N) in nematic liquid crystals (LCs) have been reported in this study. BNA and M2C4N-doped LC cells have the fall time that is fivefold and threefold faster than the pristine LC cell, respectively. The superior performance in fall time of BNA-doped LC cell is attributed to the significant decrements in the rotational viscosity and threshold voltage by 44% and 25%, respectively, and a strong additional restoring force resulted from the spontaneous polarization electric field of BNA. On the other hand, the dielectric anisotropy (Δε) of LC mixture is increased by 16% and 6%, respectively, with M2C4N and BNA dopants. M2C4N dopant induces a large dielectric anisotropy, because the phenyl-amine/hydroxyl in M2C4N induces a strong intermolecular interaction with LCs. Furthermore, BNA dopant causes a strong absorbance near the wavelength of 400 nm that filters the blue light. The results indicate that M2C4N doping can be used to develop a high Δε of LC mixture, and BNA doping is appropriate to fabricate a fast response and blue-light filtering LC device. Density Functional Theory calculation also confirms that BNA and M2C4N increase the dipole moment, polarization anisotropy, and hence Δε of LC mixture.

Electro-optical effects of organic N-benzyl-2-methyl-4-nitroaniline dispersion in nematic liquid crystals.

The dispersion of organic N-benzyl-2-methyl-4-nitroaniline (BNA) in nematic liquid crystals (LCs) is studied. BNA doping decreases the threshold voltage of cell because of the reduced splay elastic constant and increased dielectric anisotropy of the LC mixture. When operated in the high voltage difference condition, the BNA-doped LC cell has a fall time that is five times faster than that of the pure one because of the decrements in the threshold voltage of the cell and rotational viscosity of the LC mixture. The additional restoring force induced by the BNA's spontaneous polarization electric field (SPEF) also assists to decrease the fall time of the LC cell. The decreased viscosity can be deduced from the decrements in phase transition temperature and associated order parameter of the LC mixture. Density functional theory calculation demonstrates that the BNA dopant strengthens the absorbance for blue light, enhances the molecular interaction energy and dipole moment, decreases the molecular energy gap, and thus increases the permittivity of the LC mixture. The calculation also shows that the increased dipole moment, polarizability, and polarizability anisotropy increase the dielectric anisotropy of the LC mixture, which agrees with the experimental results well. BNA doping has a promising application to the fields of LC devices and displays.