Structural color originates from the interaction between light and periodic submicron structures and has led to many unique functions in nature, inspiring the development of material science. In sensing changes in the external environment, animals use color changes to transmit signals and accomplish behaviors such as communication, camouflage, vigilance, and courtship. Responsive photonic crystals are considered to be one of the best artificial materials for preparing color-changing functions, i.e., when induced by external stimuli, photonic crystals change their own periodic structure to modulate the properties of light waves, producing color changes in visual effects associated with wavelength changes. Among them, the cholesteric liquid crystal (CLC) exhibits unique selective reflective properties due to its self-assembled helical structure forming a periodic arrangement of dielectric constant and refractive index, is endowed with advantages such as polarization-dependent generation of structural color and dynamic tunability, and is rapidly developing in research fields such as dynamic display, information storage, and optical security. People have been changing the working wavelength and reflectivity by using optical, electrical and thermal responsive chiral molecules, multilayer composite structures, phase transitions and many other methods. However, there are still problems such as easy destabilization and limited regulation mechanisms, so it is still a challenge to design and prepare reflective liquid crystal photonic devices with multiple responses, real-time reconfigurability, and dynamic broadband tunability.
Microfluidics is a technology for the precise control and manipulation of microscale fluids, and its greatest advantage is that it allows flexible combination and scale integration of multiple unitary technologies on an overall controllable tiny platform. The selectivity and manipulability of microfluidic components can confer novel response properties to CLCs, which is one of the effective strategies to address the above challenges.
Recently, the research group led by Prof. Lu-Jian Chen at Xiamen University (XMU) achieved real-time continuous modulation of the working wavelength in anisotropic polymerized CLC (PCLC) films with a dynamic range of up to 210 nm by using a microfluidics-based "wash-out/refill" strategy. They also demonstrated structural color patterns with reversible changes and multipitch gradients. This work was published in Chinese Optics Letters 2022, Vol. 20, No. 9 (Y. Cao, et al., Dynamic coloration of polymerized cholesteric liquid crystal networks by infiltrating organic compounds) and was selected as the cover of the issue.
Principle: The acrylate-based liquid crystal monomers are cross-linked under UV initiation to form a polymer backbone, which can avoid the collapse of the polymer network during the "wash-out/refill" process and stabilize the chiral microstructure of CLCs. After integrating such chiral PCLC films into microchannels, the reversible filling of miscible organic solutions results in a continuous change in the average refractive index nav, and a dynamically tunable photonic band gap is obtained, corresponding to rich and vivid structural colors.
Figure 1 (a) Schematic diagram of PCLC film integrated in a microfluidic device, (b) Shift of reflection band by alternative filling of nematic liquid crystal E7 and benzyl alcohol, (c) Reflection spectrum obtained by diffusion of ethanol vapor, (d) Reversible change of structural color pattern by fluid filling (100th-anniversary logo of Xiamen University), (e) Multicolor gradient pattern by gas diffusion.
The microfluidic device integrated with PCLC film is shown in Figure 1(a). By observing the macroscopic changes in the reflectance color and measuring the reflectance spectrum, the dynamic diffusion process of the flow field within the microchannel is characterized in real time. As shown in Figure 1(b), alternate pumping of the nematic phase liquid crystal E7 (n = 1.64) and benzyl alcohol (BA, n = 1.54) caused a real-time change in the refractive index of the system, making the reflection center wavelength adjustable between 450 nm and 600 nm by fluid filling. As shown in Figure 1(c), the shift of the reflection center was obtained in the range of 410 to 620 nm by the diffusion of ethanol vapor. The diffusion forms a concentration gradient leading to the coexistence of multiple pitches in this composite system. As illustrated in Figure 1 (d) and (e), the structural color patterns with reversible changes and multipitch gradients are obtained using the above two approaches, respectively.
The XMU researchers said that, unlike common scenarios using external stimuli such as light, electricity and heat, the work is a preliminary exploration of modulating broadband reflective photonic devices based on CLCs with the help of microfluidic technology. The current focus of the group is to introduce optical concepts such as geometric phase, construct microfluidic-based liquid crystal planar optical components, and expand the scope of "Liquid Crystal on Chip" for applications such as anti-counterfeit/identification and sensing.