Tunable mid-infrared dual-band and broadband cross-polarization converters based on U-shaped graphene metamaterials.

We propose a tunable dual-band reflective cross-polarization converter composed of periodically arranged single layer U-shaped graphene nanostructures in mid-infrared region. The proposed dual-band reflective cross-polarization converter can convert the polarization state of an incident wave from the linear polarization state to its cross polarization state at the operating frequencies of 34.67 and 44.13 THz with the high-efficiency polarization conversion ratio (PCR) approaching 100%. Furthermore, as a complementary structure, a reflective cross-polarization converter with a hollow-carved U-shaped graphene sheet shows a broadband polarization conversion performance with a bandwidth of 1 THz and the PCR over 90%. The bandwidth of this broadband converter can be further extended to 2 THz after certain geometric parameter optimization. More importantly, both the dual-band and broadband cross-polarization converters not only can dynamically tune their PCR peak frequencies and magnitudes by adjusting the chemical potential and relaxation time of graphene without changing the geometric structure but also have good angular stability with high PCR in a wide range of incident angle up to 55°. These polarization converters may have great potential applications in mid-infrared spectroscopy, radiometer, sensor, and other photonic devices.