Few-cycle mid-infrared pulse generation, characterization, and coherent propagation in optically dense media

The generation of four- to five-cycle mid-infrared pulses using a single-stage potassium niobate optical parametric amplifier (OPA) is demonstrated. The OPA is pumped by a gain-switched Ti:Sapphire regenerative amplifier and is seeded with the near-infrared portion of a white light continuum. The OPA is continuously tunable from 2700 to 4700 nm, and maintains sub-65 fs pulses that are fully characterized using cross-correlation frequency-resolved optical gating (XFROG). These are the shortest near-transform-limited pulses reported over this large infrared spectral range. This apparatus is used to measure ultrafast vibrational responses, specifically, pulse profiles modified by the free-induction decay (FID) of O–H oscillators. Both the intensity and instantaneous phase of these pulses are determined after traversing samples of isotopically diluted water (HDO in D 2 O ) using XFROG, representing a new application of the XFROG technique to the study of (dipolar) molecular responses. Pronounced beating of the trailing edges of FID-modified pulses is observed even below optical densities of one, and the details of these features are found to depend on the chirp of the field used to generate the FID. These results indicate that discretion should be used in the selection of sample conditions for nonlinear infrared spectroscopic measurements.