Recently, Professor Huai Yang and his research team in the Department of Materials Science and Engineering made great progresses on responsive soft photonic crystals. The research results are published in international authoritative journals: Advanced Functional Materials, entitled “Asymmetric Tunable Photonic bandgaps in Self-Organized 3D nanostructure of Polymer-stabilized Blue Phase I Modulated by Voltage Polarity” (https://doi.org/10.1002/adfm.201702261).
Responsive photonic crystals (RPCs) have been regarded as fantastic intelligent materials for versatile applications in next-generation nanophotonic technology as they can not only manipulate the flow of light or photons but also respond to external stimuli. Usually, the RPCs have a periodic structure with its dimension close to the wavelenth of light and the periodicity should be tunable. Among various organic and inorganic materials, liquid crystals, as a kind of soft matter that combines crystalline-like solid ordering with liquid-like fluidity from molecular to macroscopic material levels, have become one kind of ideal candidates for RPCs owing to the high optical sensitivity to various stimuli and self-assemble property into periodic nanostructure.
Yang’s research team successfully fabricates a 3D blue phase (BP) liquid crystalline photonic crystal film with electrically switchable photonic bandgap (PBG) recently. The film has the self-organized cubic nanostructures of liquid crystalline blue phase that periodically align in three-dimensional space, and the supernanostructures are stabilized by the selective occupancy of cross-linked polymer network in disclination lines, which exhibits excellent thermodynamic stability in the temperature range between -30 ? to 60?. Interestingly, the PBG of the film is tuanble under direct current and the shift of the reflection band is asymmetrically modulated by the polarity and magnitude of bias voltage. Full colored reflective coloration is achieved when an originally green reflective photonic crystal is biased by either positive or negative polarity voltage. The researchers domenstrate the applications of the film in BP lasers, which has a tunable wavelength range of about 22 nm.
Figure 1. Voltage-polarity-controlled photonic bandgap in the blue phase liquid crystalline photonic crystal film. (a) without electric field, (b) under negative bias and (c) under positive bias.
The researchers propose that the underlying mechanism is attributed to the electromechanical deformation of self-organized cubic lattice nanostructures in BPs induced by the displacement of charged polymer network under a directional electrostatic force. They consider that the nonhomogeneous density distribution of the polymer network across the whole cell gap is crucial to the bias-polarity dependent dynamic optical response in the system. Through the incorporation with polymer compositions, not only the thermal stability but also the electrical tunibility is improved in the area of BP materials. And the alterable morphology of polymer network by controlling the preparation condition in the system may provide new insight to the tunability of RPCs. They think this self-assembled soft material is prospective to produce large-scale electrically responsive photonic crystals due to the facile preparation process, and has enormous potential applications in intelligent optoelectronic devices, such as 3D tunable lasers, reflective full-color displays or photonic integrated circuits.
The first author of this paper is Miss Meng Wang, a Ph.D. candidate in Yang’s Group. This work is supported by grants from the NSFC.