Single Semiconductor Nanostructure Extinction Spectroscopy

While emission-based, single-particle microscopies and spectroscopies have been highly successful in revealing the properties of matter hidden by ensemble averages, their limits have now become apparent. To address recognized future needs and, in particular, the need to go beyond fluorescent specimens, single-particle extinction techniques have been developed. Motivating this has been the desire to acquire information about the electronic structure of nanoscale materials difficult to obtain otherwise using either ensemble or emission-based, single-particle measurements. These techniques are, however, nontrivial since single nanostructures attenuate only 0.0001-1% of the incident light. This Review Article describes the challenges associated with overcoming the low signal-to-noise ratios inherent to low-dimensional semiconductor nanostructure extinction measurements. It simultaneously describes the fundamental operating principles and achievements of photothermal heterodyne imaging (PHI) and spatial modulation spectroscopy (SMS), two of the most popular approaches to measuring single-particle extinction. It then reviews what exactly we have learned about the fundamental physics of a model system, viz., low-dimensional CdSe, via single-particle extinction measurements. The Review Article finally describes the development of a new single-particle extinction methodology, infrared photothermal heterodyne imaging, which portends future successes in revealing the detailed physics of nanostructures beyond both ensemble averages and corresponding single-particle, emission-based insights.