Multiscale simulation of high-order harmonic generation: From microscopic to macroscopic

We developed a theoretical method to study the high-order harmonic generation (HHG) from a gas target in an intense laser pulse. We obtain the microscopic HHG by solving the time-dependent Schrodinger equation with a single-active-electron approximation and the macroscopic HHG by summing over all the microscopic HHG with phase matching and self-absorption in two scales. One is in the laser wavelength scale, within which the laser peak intensity does not change while the propagation phase changes. Another is in the laser beam waist scale, within which the laser intensity and Gouy phase shift change. Taking the Ar atom as an example, we calculate the macroscopic HHG energy distribution and divergence. We also discuss the dependency of the macroscopic HHG on the gas pressure and gas jet position.