What really happens at femtosecond junctions?
"Until now, to accurately describe the interaction of two or more converging beams of ultra-short electromagnetic pulses, the FDTD (Finite Difference Time Domain) method was used, using full Maxwell equations. In terms of calculation, FDTD is extremely time-consuming: a single simulation takes supercomputers many days. The situation was exacerbated by the fact that even after the involvement of computational clusters the results obtained in an acceptable time were for small volumes, often only micrometres in size. For these reasons, those dealing with laser opticians used so-called unidirectional methods, especially those using the equation known as NLSE (Nonlinear Schrödinger Equation) and the less known but more accurate UPPE (Unidirectional Pulse Propagation Equation)."
"The latest version of the Hussar program makes it possible to design, for example, a device for time resolved fluorescence. Such devices use the fact that when a femtosecond laser pulses enter the interior of a non-linear crystal together with a week fluorescence signal, a third beam appears, with frequency being the sum of both frequencies of the primary beams. The fluorescence signal can therefore be summed with the gating pulse, which provides an exact information on the time of the fluorescence occurrence. The described processes, however, become particularly efficient when the angle between the interacting impulses is about 20 degrees. Simulating such systems went beyond the capabilities of the existing software. Now, however, using the Hussar program, beams that cross at angles of even 140 degrees can be modelled."
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