Temporal characterization of attosecond wave forms in the sub-optical-cycle regime

We present a temporal characterization of sub-optical-cycle extreme ultraviolet radiation generated in a hollow-core waveguide. This generation scheme permits the use of relatively long 13-fs driving laser pulses to generate sub-optical-cycle bursts of high-order harmonic light. Using two-color cross-correlation and phase retrieval techniques, we extract the extreme ultraviolet wave form and show that it consists of chirped 470-as bursts, spaced by ∼1.3 fs, within a 1.4-fs intensity envelope. The radiation is spectrally narrow and energy tunable, making it a useful tool to investigate state-selective molecular and materials dynamics.