Diffraction and focusing of spectral energy in multiphoton processes

Abstract

We demonstrate an analogy between a two-photon process driven by a chirped pulse and Fresnel diffraction from a slit: in both situations interference between different ‘‘paths’’ leading to the same ‘‘final state’’ determines the resulting ‘‘diffraction pattern.’’ On the basis of this analogy, a spectral Fresnel zone plate was designed for ‘‘focusing of spectral energy’’: at the two-photon level the spectral energy can be concentrated in an effective bandwidth that is much smaller than the bandwidth of the original excitation pulse. To show this effect, two experiments were performed with femtosecond laser pulses with a well-controlled power spectrum and chirp: frequency doubling in a nonlinear crystal and two-photon excitation of Rydberg states in rubidium.