High-harmonic Generation

 

 

 Coherent, high-brightness radiation at discrete wavelengths, λN = λ0/N (N an odd integer), with λN down to 20 nm or so, can be produced by focusing an intense laser into a noble gas target. The mechanism behind this process, known as High Harmonic Generation (HHG), is a topic of considerable basic and applied physics. The physical mechanism responsible for the VUV and soft X-ray emission is now well understood. Atomic electrons, driven by the strong electric field of the laser pulse, collide violently with their respective, positively charged nuclei. During the collision, the laser field transfers significant energy to the electron. The electron then leaves the atom with high kinetic energy or dumps the energy in the form of a high frequency photon (the HHG process).

Construction of an apparatus to produce VUV and soft X-rays via HHG is now underway in the CAMOS lab by the research group of Professor Bob Jones.  In the HHG techniques being used, a hollow-core fiber serves as a wave-guide for both the harmonic and fundamental (pump) radiation, resulting in a high-quality harmonic spatial mode and eliminating fundamental/harmonic phase-mismatches that are unavoidable in focused pump configurations. The harmonic beam is characterized using a standard X-ray spectrograph, in this case a McPherson 1 meter grazing incidence spectrometer. Following HHG, selection of a single harmonic for a particular pump probe study is readily accomplished using absorptive filters, reflective dielectric mirrors, grazing incidence diffraction gratings, or any combination of these.