  Nonlinear frequency conversion in optics can originate the coherent light at
the wavelength where it is hard or unlikely to achieve by directly lasing and
is a fundamental topic in science and engineering covering both classical and
quantum regions. The critical requirement for efficient nonlinear frequency
conversion is phase-matching, a momentum conservation relation between the
fundamental and harmonic light. It is dreaming of a technique for compensating
the phase-mismatching in universal nonlinear materials and wavelength ranges,
since the phase-matching was proposed in 1962. Here, an additional periodic
phase (APP) concept was proposed for the phase-matching in nonlinear optics and
experimentally demonstrated the APP phase-matched second harmonic generation
with a conventional crystal quartz at the wavelength deep to vacuum-ultraviolet
177.3 nm. This study may not only develop a universal way to resuscitate the
nonlinear optical materials for efficient nonlinear frequency conversion, but
also may revolutionize the nonlinear photonics and their further applications.
