Nonlinear X-Ray Optics

Nonlinear X-ray Optics

At high radiation intensities, it possible for light to interact with light via a medium through nonlinear interactions: photons can interact with each other, which can lead to a change in their energy and momentum. However, nonlinear interactions in the X-ray range are virtually unexplored due to their extremely small cross sections. Only recent advances in the development of X-ray sources, namely the realization of the X-ray free-electron laser (XFEL), has opened the door to this research field. We investigate fundamental nonlinear X-ray matter interactions using the coherent, high-intensity pulses of XFELs. Recently, we have performed experiments at both so far existing XFELs, located at SLAC National Laboratory and the SPring8 Research Center in Japan. In particular, we have observed the mixing of optical light with X-rays, nonlinear Compton scattering in the X-ray range, X-ray second harmonic generation and two-photon X-ray absorption.

Schematic of the setup used for the NL Compton experiment. The FEL beam (green) gets focused to a 100nm spot by reflective mirrors onto a solid beryllium target. Photons get nonlinearly scattered at electrons in the Be sample and emit radiation at the sum frequency (blue). The nonlinear radiation is angularly resolved using an array of 2D detectors.

Nonlinear X-ray Compton Scattering

The most fundamental nonlinear light-matter interaction is the the simultaneous scattering of two photons at an electron into a single photon at the sum of their frequencies. For X-ray wavelengths, the quantum aspect of the process becomes measurable in form of a red-shift in the wavelength of the nonlinearly scattered photon [Compton scattering]. Using pulses of the LCLS X-ray free-electron laser, we have recently succeeded in observing nonlinear X-ray Compton scattering for the first time. For X-ray wavelengths, the quantum aspect of the process becomes of importance and the interaction is in the regime of nonlinear quantum electrodynamics (NLQED), an area of physics, which has been experimentally rarely tested so far.

X-ray Second Harmonic Generation

Second harmonic generation (SHG) of visible light was first demonstrated in 1961 and meanwhile has led to numerous applications in many fields of science. Using XFEL radiation, we have recently observed the first second harmonic generation at X-ray wavelengths. We used a diamond crystal in a Bragg geometry for the 2nd harmonic in order to fulfill the phase matching condition.

X-ray Two-Photon Absorption

Two-photon absorption is one of the most fundamental nonlinearities and has become a powerful tool for chemical and biological applications. In the X-ray regime, it could lead to novel atomic-specific probes of electronic structure and dynamics, by providing access to high-angular momentum states using hard X-ray pulses and as an instantaneous probe of electron dynamics. Recently, we demonstrated the first, non-resonant, non-sequential, two-photon hard X-ray absorption at the SACLA FEL.

X-ray Optical Mixing

We have recently demonstrated the mixing of optical with X-ray radiation, i.e. the sum-frequency generation of the optical and X-ray photons. The process is, in essence, optically modulated X-ray diffraction: X-rays inelastically scatter from optically induced charge oscillations. This method allows to investigate the microscopic details of optical interactions on an atomic scale, more specifically, the optically induced microscopic field can be determined through the close relation to the induced charge.

Relevant Publications: