Chun-Hong Lee

Random lasing in blue phase liquid crystals

Random lasing actions have been observed in optically isotropic pure blue-phase and polymer-stabilized blue-phase liquid crystals containing laser dyes. Scattering, interferences and recurrent multiple scatterings arising from disordered platelet texture as well as index mismatch between polymer and mesogen in these materials provide the optical feedbacks for lasing action. In polymer stabilized blue-phase liquid crystals, coherent random lasing could occur in the ordered blue phase with an extended temperature interval as well as in the isotropic liquid state. The dependence of lasing wavelength range, mode characteristics, excitation threshold and other pertinent properties on temperature and detailed make-up of the crystals platelets were obtained. Specifically, lasing wavelengths and mode-stability were found to be determined by platelet size, which can be set by controlling the cooling rate; lasing thresholds and emission spectrum are highly dependent on, and therefore can be tuned by temperature.

Temperature dependence of refractive index in blue phase liquid crystals

The refractive indices of a class of Blue-Phase liquid crystals (BPLCs) and their temperature dependence have been measured and analyzed. In general, the thermal index gradients in blue phases, BPI and BPII, are both larger than in isotropic liquid state; the index gradient of BPII phase is steeper than that of BPI, and is attributed to the difference between the expansion coefficients of simple and body-centered cubic lattices. Besides their obvious importance in photonics and nonlinear optical processes and applications, the investigation of the phase dependence of the index gradient also provides a useful way for phase identification of BPLCs, namely the second-order and weakly first order phase transitions corresponding to the ISO/BPII transition and the BPII/BPI transition.

Meta-q-plate for complex beam shaping

Optical beam shaping plays a key role in optics and photonics. In this work, meta-q-plate featured by arbitrarily space-variant optical axes is proposed and demonstrated via liquid crystal photoalignment based on a polarization-sensitive alignment agent and a dynamic micro-lithography system. Meta-q-plates with multiple-, azimuthally/radially variant topological charges and initial azimuthal angles are fabricated. Accordingly, complex beams with elliptical, asymmetrical, multi-ringed and hurricane transverse profiles are generated, making the manipulation of optical vortex up to an unprecedented flexibility. The evolution, handedness and Michelson interferogram of the hurricane one are theoretically analysed and experimentally verified. The design facilitates the manipulation of polarization and spatial degrees of freedom of light in a point-to-point manner. The realization of meta-q-plate drastically enhances the capability of beam shaping and may pave a bright way towards optical manipulations, OAM based informatics, quantum optics and other fields.

Photo and electrical tunable effects in photonic liquid crystal fiber

This work demonstrates photo alignment and electrical tuning effects in photonic liquid crystal fiber (PLCF). Applying voltages of 0 approximately 130V and 250 approximately 400V shifts the short and long wavelength edges of the transmission bands by about 45 nm and 74 nm toward longer wavelengths, respectively. An electro-tunable notch filter is formed in the PLCF without the use of gratings. The range of tunability of the notch filter is around 180 nm with an applied voltage of 140 approximately 240 V. This photo-induced alignment yields a permanently tilted LC structure in PCF, which reduces the threshold voltage, and can be further modulated by electric fields. The polarization dependent loss and fast response time of photo-aligned PLCF is also demonstrated. The finite-difference frequency-domain method is adopted to analyze the shift of the transmission bandgap, and the simulation results are found to correlate well with experimental data.

Loss-reduced photonic liquid-crystal fiber by using photoalignment method

We present a loss-reduced photonic liquid-crystal fiber (PLCF) using the noncontact photoalignment method. The photoexcited and adsorbed azo dye on the capillary surface of a PLCF induces uniform and highly ordered orientation of the liquid crystal (LC). The anchoring force of the photoalignment effect is combined with that generated by surface boundary conditions of the photonic crystal fiber (PCF). Transmission loss resulting from LC scattering can be reduced from -2.8 to -1.3 db/cm within 10 min. This photoinduced alignment yields a permanent boundary for the LC in the PCF that reduces scattering loss and can be further modulated by electrical fields. The electrical tunable effect and fast dynamic response of the photoaligned PLCF are also presented. This low-loss PLCF can be applied conveniently in various PLCF devices.

Polarization-independent bistable light valve in blue phase liquid crystal filled photonic crystal fiber

This article demonstrates a bistable optical valve in a photonic liquid crystal fiber using the thermal hysteresis effect of the phase transition between the cholesteric phase and the blue phase (BP). The attenuation is due to various scattering losses in different phases. Both cholesteric and BPs can exist stably at room temperature (RT) and can also be switched to each other using temperature-control processes. The transmission spectrum and the intensity of the guided light can be controlled with various extents of scattering loss. For optical communications, this device can be manipulated over a loss difference of 10 dB at RT and insensitive to the polarization of light.

Bistable light-driven π phase switching using a twisted nematic liquid crystal film.

A light-activated optical phase switch was developed, exploiting the conversion between left-handed and right-handed twisted nematic liquid crystals. Theoretical and experimental analyses revealed that the handedness inversion of the twisted nematic film altered the optical phase of the output waves by π. Herein, the competition between the helical twisting powers of the two reverse-handed chiral dopants determines the handedness of the twisted nematic film. The photo-responsibility and the bistability are attributed to the azobenzene chromophores in one of the chiral additives.

Optical bistability in a silicon nitride microring resonator with azo dye-doped liquid crystal as cladding material.

This investigation reports observations of optical bistability in a silicon nitride (SiN) micro-ring resonator with azo dye-doped liquid crystal cladding. The refractive index of the cladding can be changed by switching the liquid crystal between nematic (NLC) and photo-induced isotropic (PHI) states by. Both the NLC and the PHI states can be maintained for many hours, and can be rapidly switched from one state to the other by photo-induced isomerization using 532 nm and 408 nm addressing light, respectively. The proposed device exhibits optical bistable switching of the resonance wavelength without sustained use of a power source. It has a 1.9 nm maximum spectral shift with a Q-factor of over 10000. The hybrid SiN- LC micro-ring resonator possesses easy switching, long memory, and low power consumption. It therefore has the potential to be used in signal processing elements and switching elements in optically integrated circuits.

All-optical control of polarization splitting with a dielectric-clad azobenzene liquid crystal

We report the design, fabrication, and characterization of an optically switchable polarizing beam splitter with a prism/azobenzene liquid crystal/prism hybrid structure. The beam splitter can operate in the polarization-splitting mode and the non-splitting mode. The switching between the modes is realized by the photoisomerization-induced phase transitions in the azobenzene liquid crystal, featuring all-optical control, bistability, and fast response. Such an active polarization-handling element is highly desirable as it not only simplifies and compacts sophisticated optical systems but also increases the degree of freedom in optical circuit design.

Multi-stable variable optical attenuator based on a liquid crystal gel-filled photonic crystal fiber.

This work demonstrates a multi-stable variable optical attenuator (VOA) that is fabricated by infiltrating a photonic crystal fiber (PCF) with a liquid crystal (LC) gel. Varying the cooling rate or biasing the electric field during gelation yields various degrees of scattering. Therefore, LC gel-filled PCFs with various transmittances can be realized. At a wavelength of 1550 nm, an attenuation rate of -33.4  dB/cm is obtained at a cooling rate of 30°C/min and a biasing voltage of 400 V during gelation. The proposed all-in-fiber VOA exhibits tunable attenuation and multiple stable states at room temperature.