Yuan-Di Chen

Modulation Instability and Phase-Shifted Fermi-Pasta-Ulam Recurrence

Instabilities are common phenomena frequently observed in nature, sometimes leading to unexpected catastrophes and disasters in seemingly normal conditions. One prominent form of instability in a distributed system is its response to a harmonic modulation. Such instability has special names in various branches of physics and is generally known as modulation instability (MI). The MI leads to a growth-decay cycle of unstable waves and is therefore related to Fermi-Pasta-Ulam (FPU) recurrence since breather solutions of the nonlinear Schrödinger equation (NLSE) are known to accurately describe growth and decay of modulationally unstable waves in conservative systems. Here, we report theoretical, numerical and experimental evidence of the effect of dissipation on FPU cycles in a super wave tank, namely their shift in a determined order. In showing that ideal NLSE breather solutions can describe such dissipative nonlinear dynamics, our results may impact the interpretation of a wide range of new physics scenarios.

Particular thermally induced phase separation of liquid crystal and poly(N-vinyl carbazole) films and its application

In the current study, a method of particular thermally induced phase separation (TIPS) of liquid crystals (LCs) and polymers is presented. The method involves a combination of dissolution process and TIPS. The LCs and poly(N-vinyl carbazole) (PVK) play the roles of solvent and solute, respectively, during the processes of particular TIPS. The nematic LC sample fabricated by two substrates coated with uniform PVK films is heated and then cooled, generating the rough PVK layers onto the surfaces of the substrates. The LC sample having rough PVK layers produces micron-sized, multiple domains of disordered LCs that can scatter incident light. Additionally, an application of a scattering mode light shutter, having the advantages of low driving voltage, polarization-independent scattering, fast response, high contrast ratio, and being polarizer free, is reported.

Direct optical switching of bistable cholesteric textures in chiral azobenzene-doped liquid crystals

Bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis-isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.

Radial liquid crystal alignment based on circular rubbing of a substrate coated with poly(N-vinyl carbazole) film

This paper presents a simple method to produce radial liquid crystal (LC) alignment layers using circular rubbing of poly(N-vinyl carbazole) (PVK) films. The produced layer can be used for fabricating axially symmetric homogeneous?radial, homeotropic?radial and radial?radial LC alignment devices by combining a rubbed PVK-coated substrate with another one with a desired LC alignment layer. The transmittance?voltage curves of the fabricated LC devices at various positions are measured to examine the uniformity of the alignment effect. Additionally, the PVK film does not absorb visible light, and can be operated at high temperatures.

Formation of holographic gratings in polymer-dispersed liquid crystals using off-resonant light

This paper presents the electro-optical characteristics of polarization-independent holographic gratings (HGs) recorded in polymer-dispersed liquid crystals (PDLCs) using off-resonant laser beam. The key mechanism is based on the low light absorbance by the materials used, which enables a slow polymerization-induced phase separation. The off-resonant light can penetrate through the cell without much energy loss (absorbance) and can be uniformly absorbed across the LC cell to produce uniform PDLC structures. The intensity-modulated interference field, which is generated by two linearly polarized off-resonant laser beams, is adopted to record the HGs. The fabricated HGs are electrically switchable and polarization independent. Moreover, the diffractions of the HGs fabricated using off-resonant light is better than those produced by resonant light.

All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films

The present study reports that isothermal phase transition induced by photoisomerization of azobenzene liquid crystals (azo-LCs) from trans- to cis-isomers results in the dissolution of poly(N-vinylcarbazole) (PVK) into azo-LCs. Transparent (scattering) states can be demonstrated using uniform (rough) morphologies of PVK generated by slow (rapid) phase separation of PVK and azo-LCs from cis- to trans-isomers. The PVK films were examined in detail using scanning electron microscopy. Scattering performance resulting from the rough PVK surface induced micron-sized LC domains, and transparent performance resulting from the reformed uniform PVK surface can be optically and reversibly switched. Finally, all-optically controllable and highly efficient (contrast ratio of 370:1) scattering mode light modulators based on azo-LCs and PVK films were demonstrated.

Optically and thermally controllable light scattering based on dye-doped liquid crystals in poly(N-vinylcarbazole) films-coated liquid crystal cell.

This paper presents the optically controllable light scattering based on dye-doped liquid crystals (DDLCs) in a cell, whose substrates are coated with poly(N-vinylcarbazole) (PVK) films. The optical control mechanism is the light-induced dissolution of PVK in DDLCs, which reforms the disordered LC distribution into multiple and micron-sized LC domains. The induced thermal effect on the process is investigated in detail. Scanning electron microscopy images are obtained to show the surface structures of the produced PVK films. The generated scattering can be switched back to the original one by particular thermally induced phase separation. Results indicate that the light-induced thermal effect and photoisomerization lead to the dissolution of PVK in DDLCs. Finally, scattering mode light shutter with different transmission is successfully achieved by illuminating the cell under various light intensities.

Polarization rotators fabricated by thermally-switched liquid crystal alignments based on rubbed poly(N-vinyl carbazole) films.

This paper demonstrates the thermally-switched liquid crystal (LC) alignments based on a rubbed poly(N-vinyl carbazole) (PVK) film, and their application for polarization rotators. The mechanically rubbed PVK layer can induce a planar alignment of LCs with their director axis perpendicular to the direction of rubbing. This direction can be switched toward the rubbing direction by thermal treatment. Experimentally, the angle of re-orientation of the director axis increases with the temperature in a specific range of temperatures. In this study, the optical properties of linear and concentric polarization rotators, fabricated using a rubbed PVK film with thermal treatment, are examined.

Dual liquid crystal alignment configuration based on nanoparticle-doped polymer films

This paper proposes an approach for producing dual liquid crystal (LC) alignment configuration based on nanoparticle-doped polymer films. Experimental results indicate that illuminating a nanoparticle-doped pre-polymer film, coated onto a substrate with a homogeneous alignment layer, with unpolarized UV light through a photomask causes the polymerization of pre-polymer, ultimately generating homogeneous and vertical alignment layers in unpolymerized and polymerized regions, respectively. The dual LC alignment configuration of the homogeneous (vertical) and hybrid alignments can be achieved by combining the treated substrate with another substrate that has a homogeneous (vertical) alignment layer. Additionally, the applications of the dual LC alignment layer in phase gratings and transflective LC displays are demonstrated.

Independent Manipulation of Topological Charges and Polarization Patterns of Optical Vortices.

We present a simple and flexible method to generate various vectorial vortex beams (VVBs) with a Pancharatnam phase based on the scheme of double reflections from a single liquid crystal spatial light modulator (SLM). In this configuration, VVBs are constructed by the superposition of two orthogonally polarized orbital angular momentum (OAM) eigenstates. To verify the optical properties of the generated beams, Stokes polarimetry is developed to measure the states of polarization (SOP) over the transverse plane, while a Shack–Hartmann wavefront sensor is used to measure the OAM charge of beams. It is shown that both the simulated and the experimental results are in good qualitative agreement. In addition, polarization patterns and OAM charges of generated beams can be controlled independently using the proposed method.