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We have observed a fixed wavelength emission at 178.7 nm in sodium vapour under 578.7 nm two-photon resonance excitation. The proposed non-linear wave mixing scheme is described by ω178.7 nm = 2ωL + ω465.7 nm; where ω178.7 is the 178.7 nm photon frequency, ωL is the laser-photon frequency, and ω465.7 is the 465.7 nm photon frequency. This 465.7 nm emission comes from another six-wave mixing process involving two hyper-electronic Raman scattering photons. The excitation spectrum of the 178.7 nm emission has a typical multiwave mixing pattern with a competing effect appearing at higher temperatures under two-photon resonance excitation. Numerical analysis indicates that this vacuum ultraviolet emission has a poor phase-match condition that will depress the emission intensity to a certain extent. This makes the observation more difficult compared with other reported four-wave mixing generated emissions. Fortunately, on the one hand, it is enhanced by quasi-auto-ionization resonance when the 3s–5s transition is coupled to the sodium continuum by a 330.2 nm photon. On the other hand, its wavelength sits so close to the sodium Cooper minimum that weak absorption will not suppress this vacuum ultraviolet emission further.

With 150KeV Mg+ ion implantation, the optical and structural characteristics of GaN films were studied. Post-implant annealing up to 1000°C was performed in N2 ambient with a rapid thermal annealing (RTA) system, without an encapsulation layer. We observed a green band photoluminescence from Mg-implanted GaN. This green band photoluminescence should be associated with Mg induced defect-clustering in GaN. We also use the x-ray diffraction method to study the correlation between structure defects and implantation. We observed an extra shoulder peak at the small angle side of the GaN[0004] diffraction peak. The origin of this shoulder may be attributed to implanted magnesium induced GaN lattice strain.