Optical Response Of Photonic Crystal Research Articles

Time-temperature indicator based on the variation of the optical response of photonic crystals upon polymer infiltration

Time-temperature indicators are used for sensing the thermal history of perishable products, like food and pharmaceutical goods. Their working principle is based on a temperature response that mimics the temperature dependence of the deterioration kinetics of a given product. For successful implementation, time-temperature indicators must be of minimum size, cost-effective, change-irreversible, and easy to read. This work presents a time-temperature sensor based on capillary imbibition of thermoplastic polymers into a mesoporous photonic crystal, which was tuned to reflect well-defined wavelengths of visible light. The photonic crystal was made by electrochemical etching of silicon, where a periodic structure was formed with microscale layers of alternating nanoscale porosity. Polymer infiltration induces an irreversible change of the effective refractive index of the crystal, leading to a progressive shift of the reflected light that can be seen by the naked eye. Importantly, the employed thermoplastic polymer (poly(ethylene vinyl-acetate)) presents a temperature-dependent viscosity that is well represented by the Arrhenius law, which is normally used to characterize the temperature-dependence of quality indexes. Therefore, each reflected color is associated to the time-temperature history of the system, representing the deterioration level of a monitored product.

TiO2 Self-Assembled, Thin-Walled Nanotube Arrays for Photonic Applications.

Two-dimensional arrays of hollow nanotubes made of TiO are a promising platform for sensing, spectroscopy and light harvesting applications. Their straightforward fabrication via electrochemical anodization, growing nanotube pillars of finite length from a Ti foil, allows precise tailoring of geometry and, thus, material properties. We theoretically investigate these photonic crystal structures with respect to reduction of front surface reflection, achievable field enhancement, and photonic bands. Employing the Rigorous Coupled Wave Analysis (RCWA), we study the optical response of photonic crystals made of thin-walled nanotubes relative to their bare Ti foil substrate, including under additional charge carrier doping which might occur during the growth process.

Improving analyte selectivity by post-assembly modification of metal-organic framework based photonic crystal sensors.

The porous nature and structural diversity of metal-organic frameworks (MOFs) provide a versatile platform for specific and selective sorption behavior. When integrated as functional layers into photonic crystals (PCs), loading of the porous network with organic solvent vapors translates into an optical response, allowing analyte discrimination according to the specific host-guest interactions and, hence, framework affinity to the analytes. However, the optical response of PCs is critically influenced by the overall PC architecture, leading to batch-to-batch variations, thus rendering unequivocal analyte assignment challenging. To circumvent these problems, we have developed a straightforward and mild "post-assembly" modification strategy to impart differences in chemical selectivity to the MOF layers whilst keeping the overall PC backbone constant. To this end, one-dimensional photonic crystal (1D PC) sensors based on CAU-1 and TiO2 layers were fabricated to obtain a generic platform for post-assembly modification, targeting either the secondary building unit (SBU) or the linker unit of the as-assembled MOF nanoparticle layers. The optical response to solvent vapor exposure was investigated with the pristine CAU-1 based sensor as well as its modifications, showing enhanced analyte selectivity for the post-modified systems.

Magnetic field role on the structure and optical response of photonic crystals based on ferrofluids containing Co0.25Zn0.75Fe2O4 nanoparticles

Ferrofluids based on magnetic Co0.25Zn0.75Fe2O4 ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions of Co (II), ZnSO4, and Fe (III) in an alkaline medium. Ferrofluids placed in an external magnetic field show properties that make them interesting as magneto-controllable soft photonic crystals. Morphological and structural characterizations of the samples were obtained from Scanning Electron Microscopy and Transmission Electron Microscopy studies. Magnetic properties were investigated with the aid of a vibrating sample magnetometer at room temperature. Herein, the Co0.25Zn0.75Fe2O4 samples showed superparamagnetic behavior, according to hysteresis loop results. Taking in mind that the Co-Zn ferrite hysteresis loop is very small, our magnetic nanoparticles can be considered soft magnetic material with interesting technological applications. In addition, by using the plane-wave expansion method, we studied the photonic band structure of 2D photonic crystals made of ferrofluids with the same nanoparticles. Previous experimental results show that a magnetic field applied perpendicular to the ferrofluid plane agglomerates the magnetic nanoparticles in parallel rods to form a hexagonal 2D photonic crystal. We calculated the photonic band structure of photonic crystals by means of the effective refractive index of the magnetic fluid, basing the study on the Maxwell-Garnett theory, finding that the photonic band structure does not present any band gaps under the action of applied magnetic field strengths used in our experimental conditions.

Effect of extinction on the high-energy optical response of photonic crystals

An analysis of the optical response of photonic crystals in the high-order band energy range is herein presented. High and abruptly fluctuating specular reflectance is predicted for perfect lattices at those energies even in the absence of any photonic gap or pseudogap. As optical extinction is gradually introduced, it is possible to reproduce experimental results found in the literature and which have recently been the subject of an intense debate. Band structure calculations demonstrate that extinction is extraordinarily amplified in the high-energy range and is responsible for the features so far observed in that range in real crystals.