Direct Imaging of Ultrafast Charge Carrier Dynamics in Semiconducting Nanowires Using Two-Photon Excitation and Spatially-Separated Pump-Probe Microscopy

The increasing use of nanoscale materials in scientific research and device design places a greater emphasis on characterizing the heterogeneity of nanostructures. When designing electronic components around the use of individual nanoparticles, it is important to understand variability between seemingly identical particles produced in the same synthesis. To do this, we have developed an ultrafast optical microscope capable of studying single nanostructures with spatial resolution of hundreds of nanometers. Emission images of zinc oxide needle-like nanowires show a modulated pattern along the long axis of the wire that are attributed to the coupling of the optical field into structurally dependent resonance modes. Simulations suggest that these are size dependent hybrid modes, containing character of both whispering gallery and Fabry-Perot modes. By incorporating transient absorption pump-probe techniques into the microscope design, we can observe the recombination dynamics of excited carriers on femtosecond timescales following excitation. Due to the high resolution of the instrument, it is possible to observe the dynamics at different locations within a single nanostructure. This technique is used to study the correlation between the decay kinetics of silicon nanowires and doping density for a variety of surface treatments. The motion of excited carriers in silicon nanowires was directly imaged by holding the pump beam in a particular location and scanning the probe beam over the entire structure. The resulting images show free carriers spreading out from the area of excitation, leaving the immobile trapped carriers behind.