Stable interferometric platform for phase modulation of seeded free-electron lasers.

We present a compact phase modulation setup designed for high laser intensities sufficient to drive highly nonlinear processes, such as high-gain harmonic generation in seeded free-electron lasers. This paves the way for all-extreme-ultravioloet coherent nonlinear spectroscopy. The high linearity, phase stability, and sensitivity of the setup are demonstrated by probing the quantum interference of electronic wave packets in the deep ultraviolet region (268 nm) combined with photoion time-of-flight mass spectrometry.