Polarization dynamics in black phosphorus-based van der Waals heterostructures*

Polarization of light plays an essential role in a wide variety of classical and quantum optics-based applications. We demonstrate electronically reconfigurable, broadband polarization conversion at telecom wavelengths with atomically thin van der Waals heterostructures, using hBN-encapsulated trilayer black phosphorus (TLBP) integrated in a Fabry-Pérot cavity. The strong light-matter interaction of TLBP with the optical cavity mode further enhances the intrinsic material (BP) birefringence in the complex reflectivity plane and allows generation of polarization states over the entire Poincaré sphere via spectral and input azimuth tuning. In addition, the extreme susceptibility of the exciton binding energy and its Bohr-radius to electrical doping provides near-complete suppression of its dipole oscillator strength, enabling electrostatic control of the polarization state across nearly half the Poincaré sphere. Both linear to circular (tunable quarter-waveplate) and linear cross-polarization (tunable half-waveplate) dynamics, among numerous other trajectories, were achieved (for example at ~1444 nm) via electrical gating (up to ~8x10¹²/cm²) demonstrating superior control over the reflected ellipticity and azimuthal angles. Specifically, for the aforementioned two cases, the ellipticity was tuned from 0^o to 44^o and the azimuthal was tuned from 24^o to 90^o, outperforming most existing polarization modulators. We also discuss spatial polarization dependence through Stokes mapping of our devices, which reveals both dielectric disorder and stacking order differences in TLBP.