Tip-enhanced two-photon excited fluorescence spectroscopy of monolayer black phosphorus

Abstract

The optical properties of monolayer black phosphorus (BP) in two-photon excited fluorescence (TPEF) are pivotal for photoluminescence applications at the single molecular level. In this study, we theoretically examine the TPEF properties of monolayer BP using a tip-enhanced spectroscopy system. We detail the influence of parameters such as the tip radius, the distance between the tip and the BP film, the incidence angle and the tip half-cone angle on the fluorescence excitation and emission enhancement of monolayer BP. This examination employs the finite element method. Our findings reveal that the signal enhancement in TPEF is substantially improved, approximately two orders of magnitude greater than that in single-photon excited fluorescence (SPEF), attributed to the high-order nonlinear coupling effect. By optimising the system configuration, we determined that the TPEF enhancement factor with monolayer BP can reach as high as eight orders of magnitude under an excitation wavelength of 800 nm. The highest spatial resolutions achievable for TPEF and SPEF spectroscopy are also 4.4 nm and 5.3 nm, respectively. These results provide an in-depth understanding of the TPEF enhancement mechanism and are invaluable for designing and applying BP-based optoelectronic devices.