Multifunctional and bidirectional tunable terahertz device for polarization conversion and absorption based on black phosphorus

In this work, a multifunctional and bidirectional tunable terahertz (THz) device for polarization conversion and absorption is proposed. The device can modulate both forward and backward incident electromagnetic waves. When x/y-polarized and circularly polarized (CP) waves are backward incident, the polarization conversion rate (PCR) exceeds 90% in the range of 0.69–1.62 THz with a relative bandwidth of 80.5%. The ellipticity reaches 1 at 0.65 THz and 1.64 THz. When u/v-polarized and CP waves are forward incident, PCR exceeds 90% in the frequency range of 1.71–3.07 THz with a relative bandwidth of 56.9%. The absolute ellipticity values reach 1 at 0.86 THz, 1.1 THz, and 1.63 THz. When the forward incident wave is x-polarized, the device exhibits absorption rates of 98.9% at 0.96 THz and 97.7% at 1.67 THz. Meanwhile, linear dichroism reaches 0.75 at 0.96 THz. Importantly, all functions can be turned on or off by the carrier density of black phosphorus. The proposed device has potential applications in detection, sensors, imaging, and multifunctional terahertz systems.