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.

黑磷（BP）为红外和可见光子学提供了广阔前景。先前已经证明了通过费米能级的调制或垂直电场的施加可以有效地调节BP中的带隙和更高的子带，从而可以对其带隙以上的光学特性进行电控。在这里，我们报告了通过调节BP诱导的二维电子气中的电荷密度，从而调节了带隙（5至15μm）以下的光导率，从而改变了其以自由载流子为主的带内响应。中等掺杂浓度为7×10 ¹² cm ^-2，我们能够在BP的介电常数中观察到极化相关的epsilon-near-zero行为。正如我们的测量所证实的，带内极化灵敏度与沿两个晶体学方向的有效铁离子质量的差异密切相关。我们的结果表明，多层BP有望为新兴的光子学应用提供新的光学功能。