Probing the Free-carrier Absorption in Multi-Layer Black Phosphorus

TL;DR

This study investigates the carrier relaxation and free-carrier absorption in black phosphorus using time-resolved measurements, revealing transition dynamics, carrier lifetimes, and the impact of probe energy on absorption behavior.

Contribution

It provides new insights into the inter-band and intra-band transition dynamics and quantifies carrier lifetimes and recombination coefficients in black phosphorus.

Findings

Pauli blocking observed when probe energy exceeds bandgap

Sign change in differential transmission below bandgap due to FCA

Carrier lifetime estimated at 1.3 ns, with radiative recombination coefficient of 5.9×10^{-10} cm^3/s

Abstract

We study the carrier relaxation dynamics in thin black phosphorus (bP) using time-resolved differential transmission measurements. The inter-band and intra-band transitions, relaxation, and carrier recombination lifetimes are revealed by tuning the mid-infrared probe wavelength above and below the bandgap of black phosphorus. When the probe energy exceeds the bandgap, Pauli blocked inter-band transitions are observed. The differential transmission signal changes sign from positive to negative when the probe energy is below the bandgap, due to the absence of inter-band transitions and enhancement in the free-carrier absorption (FCA). The minority carrier lifetime and radiative recombination coefficient are estimated 1.3 ns, and 5.9$\rm \times 10^{-10}$ $\rm cm^{3}/s$, respectively. The overall recombination lifetime of bP is limited by radiative recombination for excess carrier densities larger than 5$\rm \times 10^{19}$ $\rm cm^{-3}$.