Imaging and Spectroscopy of Single Metal Nanostructure Absorption.

The highly tunable optical properties of metal nanoparticles make them an ideal building block in any application that requires control over light, heat, or electrons on the nanoscale. Because of their size, metal nanoparticles both absorb and scatter light efficiently. Consequently, improving their performance often involves shifting the balance between absorption and scattering to promote desirable features of their optical properties. Scattering by single metal nanoparticles is commonly characterized using dark-field scattering spectroscopy, but routine methods to characterize pure absorption over a broad wavelength range are much more complex. This article reviews work from our lab using photothermal imaging in combination with dark-field scattering and electron microscopy to separate radiative and nonradiative properties of single nanoparticles and their assemblies. We present both initial work using different laser wavelengths to explore pure absorption free from scattering contributions based on the heat released into the environment as well as the development of photothermal spectroscopy over a broad wavelength range, making it possible to resolve details that are otherwise hidden in ensemble measurements that most of the time also do not separate radiative and nonradiative properties.