  Successful phase-matching methods for Third Harmonics Generation (THG)
include phase-matching in birefringent crystal and quasi-phase-matching (QPM)
in crystal with periodically poled domains. However, these methods are not
feasible in some isotropic materials (e.g. fused silica and photosensitive
silicate glass). It was known that volume-grating in isotropic materials can
independently generate frequency-converted waves. One of disadvantages of
single-layer volume-grating is that the brightness of harmonic emission can not
be enhanced by increasing the grating thickness. In this paper, a THG device
with stratified sub-gratings was designed to enhance THG in isotropic
materials: several sub-gratings were arranged parallel, and the grating-figures
misalignment between neighboring sub-gratings was pre-fabricated. In terms of
extension of interaction length in THG, our multi-layer sub-grating is formally
equivalent to the multi-layer periodically poled crystal (e.g. lithium niobate)
in conventional QPM approach. According to the calculation results, the N-layer
(N >2) can, in principle, generate TH output intensity of N*N times stronger
than single-layer volume-grating does, also compared to N times stronger than
N-layer without figures-misalignment. The effect of random fabrication error in
grating thickness on normalized conversion efficiency was discussed.
