Relativistic mid-wavelength infrared pulses generated in intense-laser mass-limited target interactions

Infrared spectroscopy, ultra-fast x-ray high harmonic generation, and time-resolved imaging of molecular structures benefit from the availability of intense mid-infrared wavelength pulses. Here we present a new approach to generating these, in which an intense short laser pulse is incident upon a near-critical density, spherical, mass-limited carbon target. After the laser pulse interaction, the carbon ions produced form a central force field. Plasma electrons accelerated by the laser return to the positively charged carbon target under the action of this field. The energy of these electrons is reduced and is lower than their energy in the laser field. These low-energy refluxing electrons start to oscillate with a rotating figure-of-eight motion around the positively charged carbon target and emit relativistically intense mid-infrared pulses with the wavelength in the range 1 to 4 mu m.