Extending harmonic order with a half-cycle-delayed pulse

摘要

WHEN a dissociative system is subjected to an intense laser pulse, it will be photo-ionized. In the photo-ionization process, the dissociative system will develop a time-dependent dipole moment and emit an odd harmonic of the incident laser frequency.In the past ten years, optical harmonic generation has attracted much attention. This is because high optical harmonics will undoubtedly lead to the development of bright, short-pulse and versatile sources of coherent radiation in the vacuum ultraviolet(UV) and X-ray ranges. In addition, its ultrashort characteristic will be important for investigating many ultrafast phenomena. But people always hope the harmonic radiation could extend to the "water window" waveband for many applications in biology. Therefore, some ideas are proposed to shorten the emission wavelength such as choosing higher ionization potential system, and better phase-matching. However, to the best of our knowledge, until now the shortest harmonic wavelength for a gas target is 6.7 nm (the 37th harmonic of 248.6 nm) in helium. It is still longer than the attractive "water window" waveband.