Picosecond Interferometry for Investigating the Birefringent Optical Properties of Black Phosphorus*

Black phosphorus (BP) is an emerging two-dimensional semiconducting material with great potential for nanoelectronic and nanophotonic applications. Many theoretical studies have predicted the anisotropic optical properties of BP, but the direct experimental quantification remains challenging since the ease of BP’s degradation and the indirect nature of conventional approaches. This work reports a direct investigation of the birefringent optical constants of micrometer-thick BP samples with picosecond (ps) interferometry. In this ps-interferometry approach, an ultrathin (~5 nm) platinum layer is deposited for launching acoustic waves, which also naturally protects the BP flake from degradation. The birefringent optical constants of BP for light polarization along the two primary crystalline orientations, zigzag and armchair, are directly obtained via fitting the attenuated Brillouin scattering signals. A bi-exponential model is proposed to analyze the BS signals for a random incident light polarization. The BP experimental results and the associated measurement sensitivity analysis demonstrate the reliability and accuracy of the ps-interferometry approach for capturing the polarization-dependent optical properties of birefringent materials.