Ultrafast Switching of Whispering Gallery Modes in Quantum Dot Superparticles.

Microscopic dielectric structures can leverage geometry and photophysics to confine light, acting as microresonators. However, the use of light to reversibly manipulate the spectral pattern of photonic resonances on ultrafast time scales has hardly been explored. Here, we use femtosecond light pulses to drive reversible changes in the photonic resonances of optical microresonators over a broad spectral range. We employ pump-probe microscopy to investigate the dynamic modulation of the photonic response of whispering-gallery microresonator superparticles self-assembled from colloidal quantum dots. Our findings provide crucial insight into the photophysics of semiconductor superstructures, paving the way to their prospective application as ultrafast optical switches for photonics, optoelectronics, and communication technologies. In particular, we demonstrate that ultrafast photoexcitation can initiate ultrafast excitation transfer between neighboring superparticles, forming a dimer, and induce electronically and thermally driven changes in the refractive index of individual superparticles, dynamically modulating their resonances on distinctive time scales.