My research focuses on understanding and controlling what happens when light hits nanoparticles. Semiconductor nanocrystals and metal nanoparticles both interact strongly with light in a way that can be tuned by changing the size, shape, and composition of the particles. Coupling the nanoparticles to one another can lead to new optical properties that are qualitatively different from the properties of any of the components. These opportunities to engineer optical response make the nanoparticles and assemblies of the nanoparticles attractive as the building blocks for a wide range of applications, including the conversion of sunlight into useful energy, detection of biomolecules, and nanoscale photonics. In all of these applications, the nanoparticles absorb energy from incoming light and then transform it, on ultrafast time scales, into a different form of energy: electrical current, mechanical motion, or an altered optical signal. Development thus requires an understanding of the processes that occur after light is absorbed by nanoparticles, on time scales from femtoseconds to nanoseconds. I study these processes using ultrafast laser pulses and optical microscopy.