Efficient broadband 400 nm noncollinear second harmonic generation of chirped femtosecond laser pulses in BBO and LBO

摘要

Regarding the field of ultra-short pulses, frequency doubling has many attractive features like extending the wavelength range, improving the pulse contrast or obtaining even shorter pulses. However, fs second harmonic generation (SHG) faces specific difficulties mainly due to the phase-matching (PM) condition to be fulfilled on a very broad spectrum and to the high electric field associated. For short pulse durations, phase mismatch, whose first order approximation corresponds to group-velocity mismatch (GVM), leads to temporal broadening and reduces the conversion efficiency (CE). At high intensities, third order nonlinear phenomena (self-phase modulation, cross-phase modulation, self-focusing …) can decrease the CE and alter the SHG beam quality. In order to reduce these effects, one can use a thin crystal at moderate power density at the expense, however, of the CE. These problems have been extensively studied these last years and several solutions proposed. In particular, noncollinear (NC) geometries generally make it possible to suppress the GVM by adjusting incidence angles of fundamental pulses on a nonlinear crystal. It also has several other specific advantages such as the direct separation of the three interacting waves. Nevertheless, raw application of the NC geometry concept usually gives a very low CE due to poor spatial and temporal overlap. High energy, high efficiency fs SHG is thus far from trivial and still remains a scientific and technical challenge. Smith [1] and later Liu et al [2] have detailed an analysis of the three wave-mixing ultra-broadband PM condition using the concept of pulse-front tilt (PFT). We applied their model to design an original set-up for high power achromatic fs NC SHG with PFT in BetaBarium Borate (BBO) and Lithium Triborate (LBO). The fundamental wave is delivered by a classical multipass CPA Ti:Sa system (carrier wavelength∼800nm, energy per pulse up to 100 mJ, 50 fs pulse duration, 20 Hz repetition ra- e, see [3] for more details). Two set-ups were designed in order to simultaneously match the pulse-fronts of the three waves as well as their GV. The first one uses prisms and is well-suited for moderate input fundamental power, whereas the second uses gratings at high input energy levels. For input fundamental energies in the mJ range, BBO and LBO gave similar results. The measured optimized CE was around 65 % with a very homogeneous spatial profile and a SH wave spectral width around 8 nm (corresponding to a FTL pulse duration of around 30 fs). At higher energies, more than 22 mJ at 400 nm has been proved with CE around 40% and a pulse duration of 45 fs (UV Wizzler measurement [4]) behind a grating compressor.