High conversion efficiency of an optical parametric amplifier pumped by 1 kHz Ti:Sapphire laser pulses for tunable high-harmonic generation.

We report that high-conversion efficiency of nearly 50% has been realized by combining a commercially available Ti:Sapphire femtosecond, 1 kHz laser system and an optical parametric amplifier (OPA). For an input energy of 2.2 mJ/pulse at 1 kHz and 35 fs duration, the total OPA output energy of the signal plus idler pulses is 1.09 mJ/pulse at a signal wavelength of 1310 nm. We found that the output beam profile is almost flat-top due to high gain saturation in the OPA. Using the signal pulse, we generate high-harmonics in gases and measure the velocity map images of photoelectrons ionized from argon gas as a function of the signal wavelength. We observe that in a particular range of the high-harmonic photon energy, a four-fold photoelectron angular structure is observed in the low kinetic energy region. Our results indicate that the output pulses with the high-conversion efficiency OPA and super Gaussian beam profile can be used for experiments requiring generation of tunable high-harmonics in the extreme ultra-violet region.