Trajectory interferences in a semi-infinite gas cell

We observe ring structures in high-order harmonics generated in the loose focusing geometry of a semi-infinite gas cell. Phase matching and absorption considerations indicate that the measured signal is restricted to the harmonics generated in the exit plane. We therefore attribute the ring structure to interferences in the single-atom response in the transverse exit plane of the interaction region. Calculations using the stationary phase approximation were used to analyze the contributions of the long and the short trajectories, respectively, and confirm our experimental findings. The simple setup makes quantum path interference measurements feasible without the need for special filtering mechanisms.