Ultrashort optical spectroscopy (UOS) is the only experimental technique capable of probing the properties of complex materials at the fundamental timescales of electronic and lattice motion. Recently, UOS has moved beyond conventional "pump-probe" experiments, which typically give indirect information on material properties, to alternative approaches aimed at directly accessing specific material parameters with high temporal and spatial resolution. This has given new insight into materials such as high-temperature superconductors, semiconductor nanostructures, and perovskite photovoltaics that cannot be obtained through other methods.
In this talk, I will first give an overview of UOS, with a particular focus on variants that have gained recent interest, such as ultrafast optical microscopy and time-resolved second harmonic generation (TR-SHG). I will then discuss some examples from my group's recent work in more depth, beginning with the first ultrafast optical microscopy experiments on single silicon nanowires. These experiments enabled us to track photoexcited electrons and holes as they propagate along a nanowire, in a "textbook" case of classic one-dimensional carrier diffusion. A second example will describe our use of TR-SHG to unravel the fundamental timescales governing magnetoelectric (ME) coupling in a hybrid ferroelectric/ferromagnet oxide heterostructure. In this work, we were able to not only show that ME coupling is governed by spin-lattice relaxation in the ferromagnetic layer, but also that ultrashort optical pulses can be used to control ME coupling in this system. Finally, I will discuss our development of a new UOS technique, ultrafast wide-field optical microscopy, which can rapidly acquire microscopic wide-field images with femtosecond temporal resolution, with many potential applications in physics, chemistry, and biology. Overall, advanced UOS techniques have already opened up new avenues throughout science by directly probing previously inaccessible material parameters, and the rapid developments in this field makes it nearly certain that UOS will gain more prominence in the years to come.