Sub-5 fs pulse generation from a noncollinear optical parametric amplifier

Transform-limited visible pulses with as short as sub-4 fs duration have been generated for the first time by several improvements made to our previous noncollinear optical parametric amplifier (NOPA). All of the signal–idler group-velocity matching and pump–signal pulse-front matching and angular dispersion matching of the parametric amplification processes and the final fine tuning of dispersion compensation using deformable mirror after a pair of chirped mirrors are essentially important to generate such short pulses. We demonstrated the generation of a continuous, simultaneously phase-matched 250 THz parametrically amplified spectrum. Resulting visible–near-IR signal-wave pulses have been compressed to a 4 fs duration using a micro-machined flexible mirror. Feedback for the iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by a second-harmonic generation frequency-resolved optical gating. Tunable operation with bandwidth-limited sub-10 fs pulses in the visible (550–700 nm) and near infrared (900–1300 nm) was also performed by changing the seed delay with respect to the pump after increasing the seed chirp. The NOPA is one of the most useful light sources for ultrafast spectroscopy at the present stage on an extremely short timescale. Application to the vibrational wavepacket dynamics in terms of dynamic mode coupling is discussed. The future prospect of the NOPA is described at the end.