Synthesis and shaping of picosecond pulses by frequency conversion of femtosecond pulses in engineered quadratic media

Several spectroscopic techniques require the synchronization between tunable picosecond pulses. Picosecond optical parametric oscillators offer wide tunability and high optical power, but they suffer from a complex architecture. Conversely femtosecond sources are more compact and reliable. In order to exploit them it would be of interest to assess an efficient "spectral compressor" device able to generate narrowband picosecond pulses starting from femtosecond ones. A recently proposed scheme consists in exploiting a second-harmonic-generation process in the presence of high group-velocity-mismatch between the fundamental frequency (FF) and the second harmonic pulse (SH). Such a process exhibit a high energy transfer from the broadband FF pulse to the narrowband SH one. In this paper we experimentally show that engineered quasi-phase-matching structures can be used to generate with high efficiency SH pulses with transform-limited picosecond duration and predetermined shape. As representative examples we report the synthesis of Gaussian and super-Gaussian SH pulses, and also of a pair of phase-coherent picosecond pulses with given carrier-frequency detuning.