Violet phosphorus with high-index and low-loss: A promising candidate for nanophotonics applications

The recent synthesis and characterization of violet phosphorus (VP) demonstrate that it is the most stable allotrope of phosphorus, which could become a promising 2D layered semiconductor with potential applications in optoelectronic and electronic devices. However, experiments exploring the complex refractive index or permittivity have yet to be performed, hindering further application access. Due to the small size of the VP crystal sample available by the current growth method, the measurement for the complex refractive index is challenging. In this work, coherent acoustic phonon spectroscopy was performed first in the a-b plane using polarization-resolved pump-probe spectroscopy, revealing negligible anisotropy in the refractive index in the VIS-NIR regime (400-1000 nm). Then, the non-polarized reflectance spectra of VP films with different thicknesses on Si in the VIS-NIR regime were collected. The complex refractive index spectra were obtained by fitting the reflectance spectra with Fresnel's law considering multiple reflections in thin films. It was found that VP has high refractive indices and low extinction coefficients in the spectral range of 560-1000 nm, based on which a metasurface with VP nanodisks could achieve a high reflection band in the VIS-NIR regime by simulation. These results unveil the striking potential of VP for nanophotonics applications.