Higher Harmonic Generation
High Harmonic Generation (HHG) is a nonlinear process used to generate coherent light usually in the extreme ultraviolet (EUV) range of the electromagnetic spectrum. The production of this light can be described in a 3-step model. First, the intense electric field of the laser causes tunneling ionization of the atoms. Second, the newly-freed electron is accelerated in free space away from the ion. Since the field is oscillating in direction, the force on the electron will later be directed back toward the ion. If the time when the electron is ionized relative to the phase of the EM wave is within a certain range, the electron can return to the ion. In the third step, the electron recombines with the ion and gives off a single photon of whatever kinetic energy the electron has (in addition to the ionization potential energy). This process is happening simultaneously for all atoms in the focus, starting with every half-cycle of the light. Each of those half-cycles of light is producing a short burst of broadband light that corresponds to a pulse in the attosecond range. Since those bursts are happening periodically, the spectrum we observe is a set of odd harmonics of the frequency of the driving beam.
The wavelengths that are produce are so short that there are very limited options for manipulating the focusing and polarization properties of the beams. As a part of a collaboration, we are exploring techniques to make this process more efficient while also providing control of the output beams through control of the driving laser.