Experimental Demonstration of Negative Index of Refraction in Magnetic Semiconductors

Homogeneous negative index materials have been introduced as an alternative to conventional metamaterials designs. Based on direct experimental evidence, we demonstrate that the magnetic semiconductor, Cr-doped indium oxide, possesses a negative refractive index near 28.0<formula formulatype="inline"><tex Notation="TeX">$~\mu\hbox{m}$</tex> </formula>. This effect is based on the coexistence of the magnon mode with the plasmonic mode, with simultaneous negative permeability and permittivity responses. Thin films of In<formula formulatype="inline"><tex Notation="TeX">$_{2-x}$</tex> </formula>Cr<formula formulatype="inline"><tex Notation="TeX">$_{\rm x} O_{3}$</tex> </formula> are fabricated, and the magnetic measurements clearly demonstrate ferromagnetism with a high saturation magnetization and a Curie temperature which is much higher than room temperature. The refractive index is extracted from combined transmittance and reflectance data and is compared with the theoretical prediction. Moreover, a direct experimental method is used to demonstrate negative refraction in this material.