Tunable intraband optical conductivity and polarization-dependent epsilon-near-zero behavior in black phosphorus

TL;DR

This study demonstrates tunable intraband optical conductivity in black phosphorus by adjusting charge density, revealing polarization-dependent epsilon-near-zero behavior, which could enable novel photonic functionalities.

Contribution

It introduces a method to modulate BP's intraband response via charge density tuning, highlighting polarization effects linked to anisotropic effective masses.

Findings

Observation of polarization-dependent epsilon-near-zero behavior in BP.

Demonstration of intraband optical conductivity tuning through charge density control.

Link between polarization sensitivity and anisotropic effective fermionic masses.

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 band-gap (5-15 um) by tuning the charge density in a two-dimensional electron gas (2DEG) induced in BP, thereby modifying its free carrier dominated intraband response. With a moderate doping density of 7x10^12/cm2 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.