Generation of high-order harmonics with tunable photon energy and spectral width using double pulses

This work theoretically investigates high-order harmonic generation in rare gas atoms driven by two temporally delayed ultrashort laser pulses. Apart from their temporal delay, the two pulses are identical. Using a single-atom model of the laser-matter interaction it is shown that the photon energy of the generated harmonics is controllable within the range of thousands meV -- a bandwidth comparable to the photon energy of the fundamental field -- by varying the time delay between the generating laser pulses. In addition, by setting the delay to values comparable to the pulse duration, the produced harmonic peaks become sharper than the harmonics that are generated by a single Gaussian pulse. High-order harmonics that are generated by double pulses have advantageous characteristics, which mimick certain properties of an extreme ultraviolet (XUV) monochromator. For example, experiments requiring tunability of one specific harmonic, the combination of a multilayer mirror and the described method can provide radiation, which is otherwise achievable with a single-stage XUV monochromator. With the proposed method, a simpler setup at a much lower cost and comparatively higher spectral yield can be implemented.