Coherent phase-matched VUV generation by field-controlled bound states

The generation of high-order harmonics(1) and attosecond pulses(2) at ultrahigh repetition rates (> 1 MHz) promises to revolutionize ultrafast spectroscopy. Such vacuum ultraviolet (VUV) and soft X-ray sources could potentially be driven directly by plasmonic enhancement of laser pulses from a femtosecond oscillator(3,4), but recent experiments suggest that the VUV signal is actually dominated by incoherent atomic line emission(5,6). Here, we demonstrate a new regime of phase-matched below-threshold harmonic generation, for which the generation and phase matching is enabled only near resonance structures of the atomic target. The coherent VUV line emission exhibits low divergence and quadratic growth with increasing target density up to nearly 1,000 torr mm and can be controlled by the sub-cycle field of a few-cycle driving laser with an intensity of only similar to 1x10(13) Wcm(-2), which is achievable directly from few-cycle femtosecond oscillators with nanojoule energy(7).