  Diode lasers enable one to continuously cover the 730 to 1100 nm range as
well as the 370 to 550 nm range by frequency doubling, but a large part of the
electro-magnetic spectrum spanning from green to red remains accessible only
through expensive and unpractical optically pumped dye lasers. Here we devise a
method to multiply the frequency of optical waves by a factor 3/2 with a
conversion that is phase-coherent and highly efficient. Together with harmonic
generation, it will enable one to cover the visible spectrum with semiconductor
lasers, opening new avenues in important fields such as laser spectroscopy and
optical metrology.
