Insight from Picosecond-Pulsed Second-Harmonic Generation in Periodically Poled LiTaO3

The past few decades have seen tremendous development in the field of non-linear laser frequency conversion encompassed under optical technology. This has been mainly earmarked by the technical breakthrough in the production of periodically poled crystal structures thereby allowing for practical operation under low temperatures by employing quasi-phase-matching of the interacting laser waves as suggested in the classical work. Notable progress in practical application of these crystal structures has helped edge closer towards greater efficiency operation at higher input intensities. Unfortunately, several shortcomings and limitations are associated with both high-intensity and short-pulsed operation in nonlinear laser-frequency conversion. Therefore, development of theoretical and application-oriented computational models that allow for a complex multiscale analysis of the main laser-frequency conversion mechanism in the presence of various laser-induced side effects is the surest way of ensuring progress in application and optimization of high-efficiency nonlinear optical devices.

Oleg Louchev and Satoshi Wada at Center for Advanced Photonics, RIKEN in Japan in collaboration with Vladislav Panchenko at Crystallography and Photonics of Russian Academy of Sciences developed a two-temperature model which allows for comprehensive simulation of the mechanism of short-pulsed photoionization. Their studies revealed that the incorporation of an extended set of recombination-kinetics-related energy-release and heat-exchange processes followed by short-pulsed photoionization by two-photon absorption of the second harmonic allows accurate simulation of the electron-lattice relaxation dynamics and electron-lattice temperature evolution in lithium tantalate crystal in nonlinear laser-frequency conversion. Their work is now published in the research journal, Physical Review Applied.

To conduct the numerical study, the researchers begun by detailing the experimental data of the temperature-controlled high-frequency picosecond second-harmonic generation in periodically poled lithium tantalate. The research team then introduced a modification to the two-temperature model thereby allowing the calculation of all the parameters which were seen significant for the laser-beam frequency conversion and propagation in nonlinear optical devices. Eventually, they made a detailed computational study so as to evaluate the effect of the main important parameters.

The authors found that the two-photon ionization with the recombination mechanism via electron-ion lattice interaction followed by a direct transfer of the recombination energy to the lattice was the main laser matter energy-transfer pathway responsible for the majority of the crystal lattice heating continuing for approximately 50 picoseconds after laser-pulse termination and competing with effect of electron-phonon energy transfer from the free electrons.

The Oleg Louchev and colleagues study presented a workable modification of the two-temperature model for treating short-pulsed laser-matter interaction in nonlinear optical crystals based on the estimation of single pulse temperature increase obtained from the experimental data of the temperature-controlled picosecond-pulsed second-harmonic generation in periodically poled stoichiometric lithium tantalate crystal. It has been seen that the time delay is due to a recombination bottleneck which hinders faster relaxation to thermal equilibrium in photo-ionized dielectric crystal.

In conclusion, this work puts forward that the effect of energy transfer to the lattice during photoionization and relaxation can play a substantial role in other processes of laser-matter interaction including modification of the optical properties, ablation, phenomena of optical breakdown, damage, and short-pulsed filamentation.