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

Souvik Biswas1, William S. Whitney2, Meir Y. Grajower1, Kenji Watanabe3, Takashi Taniguchi3, Hans A. Bechtel4, George R. Rossman5, Harry A. Atwater1

1 Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States

2 Department of Physics, California Institute of Technology, Pasadena, California 91125, United States

3 National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan

4 Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

5 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, United States


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 Ám) 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 7x1012 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.