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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.
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