Responsive Photonic Crystals Research Articles

Steric‐Repulsion‐Based Magnetically Responsive Photonic Crystals

The first steric-repulsion-based magnetically responsive photonic crystals (MRPCs) is constructed by synthesizing uniform superparamagnetic polyvinylpyrrolidone-coated Fe3 O4 colloidal nanocrystal clusters. The color tunable range of the MRPCs can not only cover almost the entire visible specztrum in solvents of diverse polarities, but also is insusceptible to ionic strength or pH values, facilitating the practical applications of MRPCs.

Kinetically Controlled Crystallization for Synthesis of Monodispersed Coordination Polymer Nanocubes and Their Self‐Assembly to Periodic Arrangements

Under control: A new concept for synthesizing monodispersed coordination polymer (CP) nanoparticles is established. By utilizing a chelating agent that is coordinated to the metal ions, the subsequent coordination process between metal ions and ligands is controllable (see figure). The obtained nanocubes with uniform sizes can be self-assembled into periodically ordered arrangements. This phenomenon is useful for sensing applications, such as those with responsive photonic crystals.

Tunable fiber Bragg grating based on responsive photonic crystals

A tunable fiber Bragg grating (FBG) was developed by using stress-responsive colloidal crystals. In addition, the FBGs with the function of dynamically filtering multiple wavelengths were also demonstrated by incorporating multiple colloidal crystal segments into the fibers.

Non-invasive in vivo imaging of the ionic regimes along the gastrointestinal tract of a freshwater vertebrate model organism (Japanese medaka) using responsive photonic crystal beads.

Microspherical photonic colloidal crystalline beads that are responsive to media ionic strength of cationic electrolytes have been developed for in vivo imaging of the morphology and concentration gradient of cationic electrolytes along the gastrointestinal (GI) tract of live Japanese medaka (Oryzias latipes). These responsive photonic beads were assembled from core-shell nano-sized particles with polystyrene-co-polyacrylic acid (PS-co-PAA) cores and poly(hydroxyethyl methacrylate-co-p-styrene sulfonate) (PHEMA-co-PSS) hydrogel shells. The three-dimensional orderly packing of these nano-sized core-shell particles gave rise to the photonic properties of the resultant colloidal crystalline array of microspheres. The cationic electrolyte-induced volume phase transition of the sulfonate-laden hydrogel shells of the nano-sized particles altered the lattice spacing among those particles and brought about the photonic responses of the colloidal crystalline beads. Unambiguous changes in the diffraction colour of the colloidal crystalline beads were observable under ordinary ambient light in solution media of increasing concentration of sodium chloride up to 500 mM. These photonic colloidal crystalline beads were found to possess enough structural integrity for in vivo imaging of the GI tract of live Japanese medaka. With the use of a conventional optical microscope, the gradient in the ionic strength of cationic electrolytes along the GI tract of live Japanese medaka larvae was readily revealed, with a lower electrolyte concentration in the mid-intestine (<50 mM) compared to that of the posterior-intestine (≥50 mM). Our results demonstrated the potential of stimuli-responsive photonic materials in bio-imaging applications.

Superparamagnetic nickel ferrite colloidal spheres for constructing magnetically responsive photonic crystals

Well-dispersed nickel ferrite (NixFe1−xFe2O4) colloidal spheres with uniform size and tunable composition were synthesized by a simple hydrothermal method. The changes of diameter, grain size, and magnetic properties of nickel ferrite colloidal spheres with the content of nickel element were investigated. The saturation magnetization of nickel ferrite can be tuned via changing the composition, which affects molecular magnetic moment and diameter of colloidal spheres. All nickel ferrite colloidal spheres prepared in this work show superparamagnetic behavior, and thus can be used to construct magnetically responsive photonic crystals. The color of aqueous solution containing Ni0.40Fe0.60Fe2O4 colloidal spheres changed from blue to brown with decreasing external magnetic field.

Size- and Solvent-Dependent Magnetically Responsive Optical Diffraction of Carbon-Encapsulated Superparamagnetic Colloidal Photonic Crystals

The size and solvent dependence of magnetically responsive photonic crystals was investigated using carbon-encapsulated superparamagnetic magnetite nanoparticles as building blocks. The wavelength dependence of the optical diffraction on particle size was demonstrated using colloidal nanoparticles spherical in shape and having an average diameter of 100, 150, 200, and 250 nm. A colloidal suspension with a particle size of 250 nm can only diffract red light, whereas the suspension with a particle size of 100 nm can only diffract blue light, no matter how the magnetic induction changes in strength. The solvent-dependent diffraction of photonic crystals under an external magnetic field was carried out by dispersing the colloidal nanoparticles in solvents, including acetone, ethanol, DMSO, and water. It is shown that the reflection rate was reduced as the hydrogen-bonding ability of the solvent increased.

Responsive Photonic Crystals

This Review summarizes recent developments in the field of responsive photonic crystal structures, including principles for design and fabrication and many strategies for applications, for example as optical switches or chemical and biological sensors. A number of fabrication methods are now available to realize responsive photonic structures, the majority of which rely on self-assembly processes to achieve ordering. Compared with microfabrication techniques, self-assembly approaches have lower processing costs and higher production efficiency, however, major efforts are still needed to further develop such approaches. In fact, some emerging techniques such as spin coating, magnetic assembly, and flow-induced self-assembly have already shown great promise in overcoming current challenges. When designing new systems with improved performance, it is always helpful to bear in mind the lessons learnt from natural photonic structures.

Broad-wavelength-range chemically tunable block-copolymer photonic gels

Responsive photonic crystals have been developed for chemical sensing using the variation of optical properties due to interaction with their environment. Photonic crystals with tunability in the visible or near-infrared region are of interest for controlling and processing light for active components of display, sensory or telecommunication devices. Here, we report a hydrophobic block-hydrophilic polyelectrolyte block polymer that forms a simple one-dimensional periodic lamellar structure. This results in a responsive photonic crystal that can be tuned via swelling of the hydrophilic layers by contact with a fluid reservoir. The glassy hydrophobic layer forces expansion of the hydrophilic layer along the layer normal, yielding extremely large optical tunability through changes in both layer thickness and index of refraction. Polyelectrolyte polymers are known to be highly responsive to a range of stimuli. We show very large reversible optical changes due to variation of the salt concentration of a water reservoir. These one-dimensional Bragg stacks reflect incident light from the ultraviolet-visible region to the near-infrared region (lambda(peak)=350-1,600 nm) with over a 575% change in the position of the stop band. Our work demonstrates the extremely high responsivity possible for polyelectrolyte-based photonic materials.

Mesoscopic monodisperse ferromagnetic colloids enable magnetically controlled photonic crystals.

We report here the first synthesis of mesoscopic, monodisperse particles which contain nanoscopic inclusions of ferromagnetic cobalt ferrites. These monodisperse ferromagnetic composite particles readily self-assemble into magnetically responsive photonic crystals that efficiently Bragg diffract incident light. Magnetic fields can be used to control the photonic crystal orientation and, thus, the diffracted wavelength. We demonstrate the use of these ferromagnetic particles to fabricate magneto-optical diffracting fluids and magnetically switchable diffracting mirrors.