Resonant high harmonic generation from laser ablated plasma plumes

HHG is an excellent table top source of highly coherent extreme ultraviolet and soft X-ray laser pulses.

[1] HHG process can be very easily as well as intuitively explained by a simple three-step model originally proposed by Paul Corkum in 1993.

Step 1: The outermost electron undergoes tunnel ionization upon interaction with the ultrashort laser pulse.

Step 2: This tunnel ionized electron undergoes acceleration under the effect of laser pulse electric field.

Step 3: When the electric field the ultrashort laser pulse undergoes the reversal of direction, this accelerated electron returns and recombines radiatively with the parent ion emitting high harmonics.

[2] In Corkum's three-step model, the electron is treated as a free particle having no effect of the coulomb potential.

Then in the fourth step, this trapped electron recombines radiatively with the parent ion (ground state) emitting resonantly enhanced high harmonic.

Three step model of high-harmonic generation
HHG spectra from tin ablation irradiated by a femtosecond laser pulse.
Schematic of experimental setup for HHG measurement from the laser ablation plume pumped by femtosecond laser pulse.
Schematic diagram of four-step model to explain the resonant harmonic generation