We demonstrate that conformal encapsulation using atomic layer deposition of GaAs nano-cavity resonator made of photonic crystal cavity prevents photo-induced oxidation. This improvement allows injecting a large quantity of energy in the resonator without any degradation of the material, thus enabling spectral stability of the resonance. We prove second harmonic and third harmonic generation over more than one decade of pump power variation, thanks to this encapsulation, with a total efficiency (ηSHG = 8.3 × 10-5 W-1 and ηTHG = 1.2 × 10-3 W-2 ) and a large net output energy for both operations (PSHGout=0.2nW and PTHGout=8pW).