ABSTRACT
Active polarization control is highly desirable in photonic systems but has been limited mostly to discrete structures in bulky dielectric media and liquid crystalbased variable retarders. Here, we report electrically reconfigurable polarization conversion across telecommunication wavelengths (1410 to 1575 nanometers) in van der Waals layered materials using tri-layer black phosphorus (TLBP) integrated in a Fabry-Pérot cavity. The large electrical tunability of birefringence in TLBP enables spectrally broadband polarization control. We found that polarization states could be generated over a large fraction of the Poincaré sphere through spectral tuning, and that electrical tuning enables the state of polarization conversion to span nearly half the Poincaré sphere. We observed both linear to circular and cross-polarization conversion with voltage, demonstrating versatility with a high dynamic range.
ABSTRACT
Black phosphorus (BP) offers considerable promise for infrared and visible photonics. Efficient tuning of the bandgap and higher subbands in BP by modulation of the Fermi level or application of vertical electric fields has been previously demonstrated, allowing electrical control of its above-bandgap optical properties. Here, we report modulation of the optical conductivity below the bandgap (5 to 15 µm) by tuning the charge density in a two-dimensional electron gas induced in BP, thereby modifying its free carrier-dominated intraband response. With a moderate doping density of 7 × 1012 cm-2, we were able to observe a polarization-dependent epsilon-near-zero behavior in the dielectric permittivity of BP. The intraband polarization sensitivity is intimately linked to the difference in effective fermionic masses along the two crystallographic directions, as confirmed by our measurements. Our results suggest the potential of multilayer BP to allow new optical functions for emerging photonics applications.
