Tuning the properties of colloidal magneto-photonic crystals by controlled infiltration with superparamagnetic magnetite nanoparticles

Abstract

The performance of magnetic-field sensors and optical isolators is largely determined by the efficiency of the active materials. This efficiency could be dramatically increased by integrating Faraday materials in photonic crystals. For this purpose, monodisperse nanospheres were self-assembled into a colloidal photonic crystal and magnetic functionality was introduced by dipping the photonic crystal in a suspension containing superparamagnetic nanoparticles. Reflection and absorbance measurements of these magneto-photonic crystals revealed clear relationships between the time spent in suspension and the position and strength of the photonic band gap. When additional magnetic material was introduced, the band gap was red shifted and the strength of the band gap was decreased. Using Bragg's law and the Maxwell-Garnet approximation for effective media, the filling fraction of the magneto-photonic crystals was calculated from the observed red shift. While superparamagnetic nanoparticles did confer magneto-optical properties to the photonic crystal, they also increased the absorption, which can be detrimental as the Faraday effect is measured in transmission. Therefore a trade-off exists in the optical regime between the amount of Faraday rotation and the absorption. By carefully controlling the filling fraction, this trade-off was investigated and optimized for photonic crystals with different band gaps. Both polystyrene and silica photonic crystals were filled with superparamagnetic nanoparticles. In case of the polystyrene photonic crystals, it was found that the maximum achievable filling fraction was influenced by the size of the polystyrene nanospheres. Smaller polystyrene nanospheres gave rise to smaller pore diameters and a faster onset of pore blocking when filled with superparamagnetic nanoparticles. As a result, the maximum achievable filling fraction was also lower. Pore blocking was found to be negligible in silica photonic crystals. Together with a higher mechanical strength, this makes silica photonic crystals more suited for the fabrication of colloidal magneto-photonic crystals. In this paper, a nanoscale engineering approach is described to carefully control the filling fraction of magneto-photonic crystals. This allows fine-tuning the absorption and the position and strength of the photonic band gap. By tailoring the properties of magneto-photonic crystals, the means for application-specific designs and a better description of Faraday effects in 3D magneto-photonic crystals are provided.