
THG Microscopy of Cells and Tissues 71
number of emitters within the focal volume. is allows a direct comparison of the THG signal that is only
due to the quasi-phase matching. In both cases, we notice a large enhancement of the harmonic signal for
a particular sample periodicity. e theoretical value for the QPM resonance period can be expressed as
δe l
c
= 2.
(3.28)
It can be noted that the B-THG resonant period corresponds closely to the theoretical value
l
c
BTHG
≈ 70nm and the maximum B-THG is obtained for
δ
e ≈ 150 nm), as the phase and intensity dis-
tribution can be approximated as constant at this scale.
While this geometry demonst ...