Tuning Two-Dimensional Hyperbolic Plasmons in Black Phosphorus

The optical response of a two-dimensional material is given by its in-plane optical-conductivity tensor, which can be anisotropic. In the extreme, a $h{{0}{0ex}}y{{0}{0ex}}p{{0}{0ex}}e{{0}{0ex}}r{{0}{0ex}}b{{0}{0ex}}o{{0}{0ex}}l{{0}{0ex}}i{{0}{0ex}}c$ $m{{0}{0ex}}a{{0}{0ex}}t{{0}{0ex}}e{{0}{0ex}}r{{0}{0ex}}i{{0}{0ex}}a{{0}{0ex}}l$'s tensor includes opposite signs in the two directions, offering the potential for a wide variety of applications. This work shows that atomically thin black phosphorus can be efficiently tuned to become hyperbolic over the entire visible spectrum into the ultraviolet, via bias voltage, strain, number of layers, or optical pumping. This approach will enable actively modulated nanophotonics and optoelectronics for sensing, wavefront control, nanoscale heat transfer, and enhancement of spontaneous-emission rate.