Atomically Thin Electro-Optic Polarization Modulator

Polarization of light is crucial for a wide variety of classical and quantum optics-based applications such as chemical and biological imaging, birefringence sensing and polarization encoding of qubits and implementation of optical quantum computing. Here<b>¹</b>, we demonstrate electronically reconfigurable, diverse broadband polarization conversion at telecom wavelengths with atomically thin van der Waals heterostructures, using trilayer black phosphorus (TLBP) integrated in a Fabry-Pérot cavity. The quasi-1D exciton-dominated over-unity birefringence coupled with the inherent crystal symmetry-dictated anisotropy in TLBP enables a broadband operation window of the entire telecom E, S and C bands (1360-1580 nm), for a critically coupled resonant optical cavity. The strong light-matter interaction of TLBP with the optical cavity mode further enhances the intrinsic optical anisotropy 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 active 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) conversion schemes, 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. A two-dimensional control map can thus be employed to program arbitrary polarization on the Poincaré sphere by an appropriate combination of the device orientation and applied voltage, for the same device. Furthermore, we highlight strategies to realize ultra-high efficiency devices with lossless cavities and detuned (to the TLBP exciton) working wavelengths. Our finding of atomically thin devices for high dynamic range polarization modulation highlights the potential of BP as a promising element to miniaturizing polarization-sensitive electro-optic modulators, metasurfaces and spatial light modulators.<br/><i>1. “Broadband electro-optic polarization conversion with atomically thin black phosphorus”</i> – Biswas et al., Science 374, 448–453 (2021).