Large defect-induced sub-bandgap photoresponse in semiconductor nanowires via waveguiding excitation

A large defect-induced sub-bandgap photoresponse over a broad spectral range is observed in semiconductor single nanowires via optical waveguiding excitation. Using an evanescent coupling technique, the excitation sub-bandgap light is efficiently transferred from a silica fiber taper into a CdS single nanowire (bandgap ∼ 2.46 eV), and is tightly confined and guided through the whole length of the nanowire, which significantly enhances the light-defect interaction compared with the conventional irradiation excitation scheme. Under 593 nm wavelength (∼2.09 eV) waveguiding excitation with an input power of 10 pW level at room temperature, a 350 nm diameter 150 μm-length CdS nanowire shows a responsivity of 250 A W-1, offering a sub-bandgap photosensitivity five orders of magnitude larger than by irradiation excitation. These results may open opportunities for noninvasive characterization of defect states in semiconductor nanowires, as well as for enabling novel sub-bandgap nanowire devices.