Magnetically Tunable Refractive Indices of Fe3O4–Polydimethylsiloxane Composite Films

As a promising smart material, magnetorheological elastomer (MRE) is mainly investigated for its mechanical properties. This study presents the results on MRE’s optical property. The MRE films, i.e., the composite films made of polydimethylsiloxane (PDMS) embedded with 0.5–2.5 wt % Fe₃O₄ nanofillers, were spin-cast onto Si substrates from mixture solutions that were prepared via toluene dilution, mechanical stirring, ultrasonic agitation, and vortex mixing. There are two components in the obtained films, namely, the opaque micron-sized Fe₃O₄ clusters and the homogeneous transparent host medium consisting of PDMS and nanosized Fe₃O₄ particles. The film’s refractive indices were measured using an ellipsometer under an external magnetic field of B = 0, 55, 100, 135, and 180 mT. The experimental results indicate that subjection to a magnetic field can remarkably affect the film’s refractive indices. For the film containing 2.5 wt % Fe₃O₄, the refractive index increases to 2.127 when applying a magnetic field of B = 55 mT, while the refractive index is only 1.666 in the absence of a magnetic field. It is believed that the magnetically tunable refractive indices of the composite films result from the film’s magnetostrictive stresses, which change the nanoparticle concentration in the transparent film host mediums and the dielectric constant of PDMS. As the refractive index could be changed via imposing a magnetic field, the composite films under investigation might find applications in optical microelectromechanical systems and intelligent optical elements.