Our research highlight

Field-induced second-harmonic generation induced by distorted soft chiral crystal

Journal of Physics D: Applied physics,  56, 125303 (2023)

Hui-Yu Chen, Fumito Araoka, Fang-Yu Lee, Chi Lee and Jui Jiun Chen

The nonlinear optical response of soft chiral crystal system-blue-phase (BP) liquid crystals was studied experimentally using a second-harmonic-generation (SHG) microscope. With the aid of the SHG microscope (SHM), the internal coupling between the polarization and structural deformation was visualized in a short time. In this study, a fringing field, formed at the electrode edges, causes lattice deformation of the cubic BPs, which contributes to the flexoelectric-optic response and field-induced SHG at low frequencies. Using the SHM, we can observe the spatial distribution of the induced polarization in the BPs, and the mean SHG intensity of the cubic BP depends quadratically on the strength of the electric field at a lower value. As the applied electric field increases, the structure of the BPs transfers to the chiral nematic phase (N*), and then the SHG intensity remains constant. Compared to the mean intensities of the SHG signal in N* and the different BPs in the low electric field, the SHG signal caused by the lattice deformation in BPs is weaker in N* and depends on the cubic structure of the BPs. The experimental results demonstrate that through the SHM, the influence of the inhomogeneous electric field on the BPs can be exhibited clearly because the response of the SHG signal in BPs is sensitive to field-induced lattice deformation and phase transitions between the BPs and chiral nematic. This will help us elucidate the mechanism of the secondary electro-optical response in BPs and for further improvement and development of high-performance photonic devices using BPs.


Formation of monodomain polymer-stabilized blue phase liquid crystals using surface acoustic waves

Optics Letters, 48(1), 77-80 (2023)

Risti Suryantari, Yi-Hong Shih, Yu-Han Shih, Hui-Yu Chen, Cen-Shawn Wu, and Chia-Yi Huang

This work uses surface acoustic waves (SAWs) that are generated by a piezoelectric substrate containing an interdigital transducer (IDT) to which a low voltage of 2 mV was applied at a frequency of 1 kHz to fabricate a polymer-stabilized blue phase liquid crystal (PS-BPLC) layer. The PS-BPLC layer has a more uniform optical microscope (OM) image at a voltage of 2 mV than at zero voltage, and its reflective spectrum exhibits a smaller full width at half maximum (FWHM) at the former than at the latter. The uniform OM image and small FWHM reveal that the lattices in the PS-BPLC layer have monodomain structure. The monodomain PS-BPLC layer is formed because the SAWs cause longitudinal and transverse vibrations of the PS-BPLC lattices in the vertical plane along their traveling direction. The proposed method for fabricating the monodomain PS-BPLC layer using the SAWs has potential for the development of reflective optical devices that consume low power during their fabrication.


A stable reflective state induced by a disturbed planar texture in surface- treatment-free chiral nematic liquid crystals

Optics Express, 29(19), 30644-30654 (2021).

Hui-Yu Chen,Yu-Chuan Tsao, Chin-Hao Chang, and Jui Jiun Chen  

Chiral nematic liquid crystals possess a one-dimensional periodic helical structure and are one of the oldest known materials with the ability of selective reflection of light. Their helix orientation, determining their optical properties, can be changed by a variety of stimuli, and it is also dominated by the surface treatment, ratio of the elastic constants and cell thickness. Here, we present a simple method to realize an angular independence reflective state, induced by a stable disturbed planar texture, in a surface-treatment-free chiral nematic liquid crystal cell. The scattering state caused by the defect-rich focal-conic texture can be electrically tuned to the reflective state from the disturbed planar texture in a very short time, and vice versa. These two optical conditions are both stable states in the null field until the next trigger. We find that the disturbed planar texture in the chiral nematic can provide a 100viewing angle without reflected wavelength shift. The gray level of the reflected intensity can be tuned via application of the voltage pulses. Moreover, in this work, we discuss the effect of the chiral concentration on stabilizing the disturbed planar texture. When the chiral concentration is higher to induce the blue phases, the change in the texture of the ChNLCs after removing the strong electric field can stop at the disturbed planar texture with high reflectivity. In this work, the optical performance and the bistability based on the disturbed planar texture exhibits great potential for many applications, such as tunable filters, see-through/reflective displays and large-area smart windows.


Electrically tunable and reversible selective reflection due to the transition between simple cubic and tetragonal blue-phase liquid crystals

Liquid Crystals, 47(9), 1330-1337 (2020).

Hui-Yu Chen and Kuo-Cheng Hsu

In this paper, we present experimental results on electrically tunable reflected wavelength in the full visible spectral range from 460 nm to 620 nm in one simple cubic blue-phase cell due to the simple-cubic–tetragonal lattice transition, confirmed by Kossel lines. The reflected wavelength is switched less than few hundreds milli-second and it is reversible. From the experimental observations in the reflection spectra and Kossel lines, we can see the following phenomena as increasing the applied field: the lattice-plane realignment in the simple-cubic blue phase (BPII), the transition between BPII and tetragonal blue-phase liquid crystal (BPX), and the lattice deformation of the BPX. Among them, the lattice deformation of the BPX shows larger electrostriction coefficient to shift the wavelength in a wider range, but it takes the longest response time. Because the reflected wavelength in each applied field exhibited extraordinarily sharp photonic bandgaps (less than 20 nm) with high reflectivity, the BP cell here shows a great potential to be a tunable photonic device.

Free copy: https://www.tandfonline.com/eprint/9AFNE9JWTYY9J9JS2PVF/full?target=10.1080/02678292.2019.1710868 

Symmetry breaking induced Pockels effect in a tilted-field switching BPIII cell

Crystals 2019, 9(11), 598

Hui-Yu Chen and Yen-Wen Wang

n this study, we propose driving the amorphous BPIII with a tilted electric field to enhance or magnify its inherent linear electro-optical properties. The electro-optical properties of an in-plane-switching (IPS) BPIII and a tilted-field-switching (TFS) BPIII cells are compared here. According to the change in the induced birefringence with varying the strength of the electric field in the TFS-BPIII cell, the Kerr effect occurs in the low electric field and the Pockels effect dominates in the high electric field. In addition, the transmittance of the TFS-BPIII cell depends on the polarity of the applied field from the 1 Hz to 10 kHz. It also results in the rise time of the TFS-BPIII cell being almost a half of that of the IPS-BPIII cell. These experimental results and discussion let us unravel the mystery of the amorphous BPIII step by step and provide the potential application of BPIII in photonic devices.


Optical polarization states of a liquid-crystal blue phase II 

OSA Continuum 2(2), 478-485 (2019).

Hui-Yu Chen and Hao-Yu Tu

The advantages and uniqueness of blue-phase-based electro-optical devices are predicted. In this paper, we present relevant electro-optics behaviors of the transmitted and reflected lights of BPII and try to explain those phenomena through studying the polarization states of the lights. There are two stages of electro-optical behaviors seen in an in-plan-switching BPII cell. Because of the Kerr effect, the birefringence of the linear polarized light is induced and saturated 0.021 at 150 V, and the Kerr constant is ≈3×10^−10mV^−2. As the applied voltage is stronger (>200 V), the influence of the deformation of the lattice structure dominates the transmitted/reflected intensity at different wavelengths, which causes a discontinuous change in the transmitted light intensity and a huge variation in the azimuth angle (80∘) of the polarization state of the transmitted light. As a result the deformation of the lattice structure of the BPII not only induces a linear birefringence but also induces a change in optical rotatory power and then affects the polarization state of the light. These experimental results show that the electro-optical nature of the BPII cell is more complicated than the well-known BPI phase. They also show that BPII can be used not only in transflective devices, but also in field-tunable optical devices.


Transflective BPIII mode with no internal reflector

Liquid Crystals 44(3), 473-478 (2017).

A transflective device without reflector using room temperature blue phase III (BPIII) material is demonstrated in this study. In this device, the coupling of an induced birefringence and field-induced BP, relating the ordered orientation of the double-helix cylinders, causes the reflection and transmission. Compared with other reported transflective liquid crystal devices, the BPIII device shown here does not need any type of internal reflector. Well-matched voltage-dependent transmittance and reflectance curves can be obtained easily without considering the cell gap and incident wavelength. The total response time is less than 2 ms, which is also independent of the cell gap. The experimental results exhibit a simple way to get a transflective device with good ability based on the electro-optical properties of BPIII.

Liquid Crystals 44(3), 473-478 (2017).

Investigation on morphology of polymer structure in polymer-stabilized blue phase

Liquid Crystals 43(10), 1351 (2016).

Polymer stabilization effect to increase the thermal stability of blue phases (BPs) for potential application in flat-panel display has been studied in detail though directly observing the morphology of the polymer network on the substrate formed under different experimental conditions during the polymerization process. The tiny and dense polymer network, in which the diameter of the polymer chain is not greater than 250 nm, is useful for stabilizing the BPI structure. Moreover, these experimental results can be explained by the free-energy density of BPI, where its temperature range depends inversely on the square of the polymer-chain diameter. In terms of the experimental results shown in this study, we can conclude an optimal photo-polymerization process for a wide-temperature-range polymer-stabilized BP.

Free access for this work: http://www.tandfonline.com/eprint/HByv7ziVpEm8FeGIQNaA/full

Lattice structure in liquid-crystal blue phase with various chiral concentrations

Liquid Crystals, 42 (10), 1472-1477 (2015)

Hui-Yu Chen* and Yi-Chun Hsieh

Lattice structures, including reflection lattice planes and lattice constant, of liquid-crystal blue phase I (BPI) is studied via the measurements on reflection spectrum and Kossel diagram as concentration of a chiral dopant is changed. Peaks of the reflection wavelength in BPI are mainly dominated by the lattice plane and the lattice constant, which are affected by the chiral concentration. In the chiral nematic state, as decreasing the chiral concentration the reflection peak will shift to a longer wavelength because the helical pitch linearly depends on the chiral concentration and becomes longer. However, this dependence of the chiral concentration and reflection wavelength is broken in the BPI. The reflection peak of BPI moves to a short wavelength when the chiral concentration is less due to the contraction of the lattice constant as well as helical pitch. Moreover, when the concentration of the chiral dopant is over a certain value, a discontinuous shift in reflection peak occurs due to the production of the different lattice planes. It means that the relationship between the chiral concentration and the helical pitch in BPI is not the same as it in the chiral  nematic phase and should be reconsidered.

Liquid Crystals, 42 (10), 1472-1477 (2015/06).

Unusual electro-optical behavior in a wide temperature BPIII cell

Optics Express 21, 9774 (2013)

Hui-Yu Chen* Sheng-Feng Lu, and Yi-Chun Hsieh

A low driving voltage and fast response blue phase III (BPIII) liquid-crystal device with very low dielectric anisotropy is demonstrated. To stabilize BPIII in a wide temperature range (> 15°C), a chiral molecule with good solubility was chosen. By studying field-dependent polarization state of the transmitting light, it was found that the field-induced birefringence becomes saturated in the high field. However, the transmitting intensity exhibits a tendency to increase as the electric field increases. This indicates that the electro-optical behavior in BPIII device may be from the flexoelectric effect, which induces tilted optical axis and then induces birefringence. Because the phase transition from BPIII to chiral nematic phase does not happen, the device shows no hysteresis effect and no residual birefringence, exhibits fast response, and can be a candidate for fast photonic application.


Fast tunable reflection in amorphous blue phase III liquid crystal

JAP 113, 123103 (2013)

Hui-Yu Chen, Jia-Liang Lai, Chun-Cheng Chan, and Chih-Han Tseng

The dynamic reflection spectra of amorphous blue phase III were investigated. When an electric field is applied to a blue phase III cell, the reflected wavelength does not shift obviously, but the intensity of reflection increases or decreases in a few ms. This fast intensity-tunable phenomenon in blue phase III relates to the dielectric anisotropies and chiralities of the liquid crystal and can be explained by the double twist model consisting of randomly orientated double-twisted cylinders. This study shows that blue phase III can act as a fast intensity-tunable reflector for a specific wavelength.

JAP 113, 123103 (2013)