At high enough intensities, light-matter interactions become nonlinear. At long wavelengths, various nonlinear phenomena are critical for applications spanning attosecond pulse generation to super-resolution imaging. One of the simplist nonlinear interactions is the generation of optical harmonics from an intense beam of ligth interacting with matter. Second harmonic generation was first measured shortly after the advent of the laser, from a ruby laser focused onto a quartz crystal (and is often considered the birth of nonlinear optics). The process can be thought of as the merging of two photons from the laser into a single photon at twice the energy. Today, harmonic generation is ubiquitous. The generation of high-order harmonics in atomic, molecular and more recently solids remains an active field of study here at PULSE and many other places around the world. Generally speaking such driven nonlinear interactions are much weaker in the x-ray regime than the optical, owing to the short wavelength and high-frequency of the radiation. Even the generation of x-ray second harmonic in matter was effectively not possible before the advent of the x-ray free-elecrtron laser.
Our group explores nonlinear x-ray interactions with an eye towards their use for measurements of chemical and materials dynamics. We are particularly interested In the hard x-ray regime, where the interaction of multiple photons is expected to give simultaneous and near instantaneous atomic-scale structural and spectroscopic information. Far from resonance, the linear x-ray matter interaction can be well approximated by the scattering off a collection of free-electrons, and indeed most experiments even on the FEL can be thought of as interacting one-photon at a time. Thus, x-ray nonlinearities are nominally expected to be extremely weak compared to what is typical in the optical regime, especially off resonant. Here we are studying several fundamental interactions including both resonant and non-resonant two-photon scattering (second harmonic generation, nonlinear Compton scattering, x-ray (hyper) Raman scattering) as well as the mixing of x-ray and optical radiation. In the latter case, we plan on using nonlinear wave-mixing for studying a diverse range of phenomena ranging from the mechanism behind solid-state high-harmonic genartion (with Shambhu Ghimire et al.) and as a bulk probe of topological properties of quantum materials (with Mariano Trigo et al.).
Under construction. Contact David Reis for more information.