Yen-Chen Liu

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.

Photo- and electro-isomerization of azobenzenes based on polymer-dispersed liquid crystals doped with azobenzenes and their applications

We report the photo-isomerization and electro-isomerization effects in azobenzenes-doped polymer-dispersed liquid crystals during the switching of the liquid crystal (LC) device between transparent (cis-isomers dominant) and scattering states (trans-isomers dominant). The isothermal phase transition, which is a result of the illumination of UV light and the application of DC voltage, was the main mechanism to switch the LC device between transparency, scattering, and gray scales. This study discusses in detail the variations in the population of cis-isomers as functions of the period and the amplitude of the applied DC voltage.

Direct learning coverage control based on expectation maximization in wireless sensor and robot network

This paper proposes a direct learning controller for wireless sensor and robot network to coverage an environment by utilizing expectation maximization algorithm. In addition to sensors, including high-level and low-level sensors, mounted on mobile robots, low-level stationary sensors are considered to provide information for enhance the performance of coverage control. The main objective is to maximize the information quantity of high-level sensors from the sensing density generated by a group of low-level sensors. This direct method uses the parameter of basis function to design controller based on Expectation Maximization (EM) algorithm. Moreover, the proposed estimation and learning law are also used in indirect method for other coverage controller. Subsequently, this paper proposes a transformation method based on EM algorithm for coverage problem with complicated sensing function such as Gaussian function. Numerical examples are introduced to demonstrate the performance of the proposed direct coverage control in wireless sensor and robot network.

Slip-ratio-based torque distribution control strategy for electric vehicles over split friction regions

This paper addresses a novel slip-ratio-based torque distribution strategy for electric vehicles (EVs) to avoid slippage when driving over a split friction surface. With the combination to an upper and middle controller, the desired driving force and yaw moment can be distributed to four driving wheels by utilizing the proposed lower controller. Stability analysis by using Lyapunov theorem is presented to show that the distribution method can equalize the slip ratios for the wheels on the same side of EVs. In addition, we also propose two torque distribution parameters for driving force Kf and moment Km so that it is more flexible and easier to design a lower controller for EVs. This distribution method is able to equalize the slip ratios for wheels on the same side of EVs, so that the slipping motion can be avoided. Simulation results by using CarSim and MATLAB/Simulink are presented to show the performance of the proposed methods.

Isothermal electrically and optically induced phase separation of liquid crystal and poly(N-vinylcarbazole) films

This paper reports the electro- and photo-isomerization-induced isothermal phase separation of liquid crystals (LCs) and poly(N-vinylcarbazole) (PVK). The proposed phase separation process determines reformed PVK films on substrates to obtain switchable LC light valves. UV illumination induces simultaneous isothermal phase transition of the mixture and dissolution of PVK into the LCs. Phase separation of PVK and LCs occurs by the reversed phase transition via rapid electro-isomerization and slow dark-relaxation. During rapid phase separation, micron-sized LC domains (branch-like PVK structures) are generated to develop stable light scattering; during slow dark-relaxation, a uniform PVK film is obtained, thereby providing stable transparency.