Generation of subfemtosecond ultraviolet pulses by three-color near-infrared ionizing fields.

We propose a method for generating extremely short (few- and even subcycle) ultraviolet (UV) pulses with the use of three-color ionizing fields. We demonstrate that low-order combination frequencies can form a supercontinuum up to the fifth harmonic of the fundamental field. This effect is achieved by using three-color fields with two weak components detuned from half of the frequency of the intense fundamental field, which can be obtained from an optical parametric generator. Our calculations based on the solution of the time-dependent Schrödinger equation for the helium atom show that using a three-color near-infrared ionizing field with a duration of 25 fs can result in generating an extremely short UV pulse with a central wavelength of about 300 nm and a full width at half maximum of the intensity of about 0.9 fs.