报告题目：Second Harmonic Generation in Disordered Nonlinear Crystals: Application to Ultra-short Laser Pulse Characterization.

  摘要：This work is devoted to the study of second harmonic generation in nonlinear ferroelectric crystals formed by a random distribution of domains with inverted quadratic nonlinear susceptibility (such as the Strontium Barium Niobate and Calcium Barium Niobate crystals) and its application to the single-shot characterization of ultrashort laser pulses. The basic principle of operation is related to the unique type of SH emission in those kinds of crystals where the second harmonic signal is emitted transversally to the beam propagation direction. Using the transverse second harmonic generation from these crystals we measure the pulse duration, the chirp parameter and the temporal profile in a single-shot configuration. This method has been implemented both in transverse auto-correlation and transverse cross-correlation schemes for the measurement of pulses with durations in the range from several tens up to several hundreds of femtoseconds. The main advantages gained with the developed techniques against other traditional methods include the removal of the requirement of thin nonlinear crystals for harmonic generation, the possibility to get automatic phase matching without angular alignment or temperature control over a very wide spectrum and a simplified operation process. Different types of pulses have been measured in different conditions and the limits of validity of the technique have been explored.

　　Since this work relies strongly upon the characteristics of emission of the second harmonic signal by these random crystals, an important part of this work has been focused on the characterization of the distribution of domains of the random nonlinear ferroelectric crystals and its relation with the angular emission of the second harmonic signal. The domain distribution of the nonlinear polarization implies an associated distribution of reciprocal lattice vectors, which can compensate the phase mismatch in the nonlinear interaction. Any change in the domain distribution would have a direct impact in the second harmonic generated and in its intensity angular distribution. Based on these fundamental concepts we demonstrate an indirect non-destructive optical method for the characterization of nonlinear domain statistics based on the analysis of the second harmonic generation intensity angular distribution. This method has been implemented experimentally and tested in crystals with different types of distributions. To gain a deeper insight on these processes, numerical simulations have been performed using a split-step fast-Fourier transform beam propagation method. It has been demonstrated that the analysis of the dependence of the second harmonic generation angular emission with the fundamental beam wavelength can be used to obtain relevant information about complicated domain structures. This method could be used for real time monitoring of the unknown domain distribution during the poling or crystal growing process.