KEYWORDS: Excitons, Semiconductors, Nanowires, Microscopy, Femtosecond phenomena, Near field scanning optical microscopy, Near field optics, Visualization, Spatial resolution, Temporal resolution
Excitonic excitations play an important role in the optical response of low-dimension nanoscale semi-conducting materials. During its lifetime, excitons may diffuse or migrate in particular directions, thus constituting a form of excitation information transfer on the nanoscale. The details of the spatio-temporal evolution of excitons remain unclear, because it has been challenging to directly visualize this process with nanometer spatial resolution and femtosecond temporal resolution. Here we describe pump-probe measurements at the nanoscale, using the photo-induced force microscopy (PiFM) and near-field scanning optical microscopy (NSOM) at ambient conditions. We analyze the spatial and temporal characteristics of the excitons in quasi-1D semiconductor nanowires, and provide unprecedented views of their evolution.
KEYWORDS: Near field scanning optical microscopy, Scattering, Light scattering, Atomic force microscopy, Photodiodes, Nickel, Absorbance, Chemistry, Microscopy, Signal detection
Scattering scanning near field optical microscopy (s-SNOM) is a useful tool for providing optical resolution well below the diffraction limit with chemical selectivity. s-SNOM relies on recording the scattering light of a scanning probe tip, coupling the near-field interaction to a far field photo-detector. Photo-induced force microscopy (PiFM) is a much newer technique that also provides chemical resolution well below the diffraction limit. In PiFM, the signal arises from measuring the light induced force on a scanning probe tip of a sample interacting with laser light. It measures and records all information in the near field, with no need for a photo-detector. In this presentation, we describe results comparing and contrasting s-SNOM and PiFM displaying the strengths and weaknesses of both methods.
Raman spectroscopy can provide useful chemical information of nanostructures and molecules. We combine Raman spectroscopy with atomic force microscopy, through dual color photo-induced force microscopy (PiFM). In this modality, images with Raman contrast can be generated with a spatial resolution well below 10 nm at ambient temperature and pressure. Here we utilize this technique to visualize molecules on surfaces with high spatial and temporal resolution. Compared to previous Raman sensitive PiFM measurements, we employ femtosecond pulses and show that this technique is highly sensitive to the stimulated Raman scattering interaction in the molecule.
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