Optical Interferometric Imaging of Sub-50 nm Semiconductor Nanoparticles

摘要

Optically detecting individual nanoparticles (NPs) form a technical basis of numerous bio-chemical imaging techniques that are based upon the optical emission or scattering from NPs. For plasmonic NPs such as the ones made of silver or gold, strong scattering resonances at visible frequencies enable facile detection of individual particle as small as 30 nm. For strongly emitting semiconductor quantum dots, fluorescence detection enables easy detection at single photon as well as at single particle level. On the other hand, for other classes of NPs that are neither strongly scattering nor fluorescing (such as iron oxides, silicons, or polymeric NPs), sensitive optical detection is not straightforward. In this note, we demonstrate a simple yet sensitive optical detection of weakly scattering NPs. The technique is based on the coherent interference of scattering from NPs and the reflection from air-substrate interface, which optically "amplifies" the scattering signal. The detection sensitivity of the current method is compared with that of the conventional Raman-based detection.