Near-Field Ultrafast Nanoscopy of Carrier Dynamics in Semiconductor Nanowires

Carrier distribution and dynamics in semiconductor materials often govern their physical properties that are critical to functionalities and performance in industrial applications. Many techniques have been developed to resolve the charge transport and carrier dynamics in semiconductor nanostructures for the design of future electronic and photonic devices. Conventional pump-probe microscopy has limited spatial resolution due to optical diffraction. Recently, ultrafast infrared-terahertz nano-spectroscopy was developed through the integration of scanning near-field optical microscopy and pump-probe optics. However, given the limited photon energy, the efforts have been primarily focused on studying carrier dynamics in narrow bandgap semiconductors or graphene plasmons.<br/> Here, we report near-field ultrafast optical nanoscopy in the visible-near-infrared spectral region to access the carrier dynamics in silicon, one of the most prevalent materials in current semiconductor technology. Our pump beam has a wavelength of 400 nm (3.1 eV), which is sufficient to excite carriers in common optoelectronic semiconductors, including silicon (bandgap of 1.12 eV) and GaAs (bandgap of 1.42 eV). By combining ultrafast nanoscale measurements and theoretical modeling, we unravel the local photocarrier recombination dynamics in silicon nanowires. Moreover, we demonstrate the spatial mapping of carrier lifetime in silicon with a sub-50 nm resolution. Our results provide the capability to probe carrier behaviors in nanoscale materials and devices, which is of great significance to understanding the optoelectronic properties and practical functionality of semiconductor nanostructures.