Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 44
originates from charge separation, or orientation, however, experimental evidence
indicates the former [45].
Electric field poling of glasses at elevated temperatures has induced non-zero χ2 in a
number of systems such as silicates and tellurites. Similar effects have been observed
using corona poling, electron bombardment, and UV laser exposure. This is though to be
due to space charge left behind after mobile species, such as alkali or hydrogen ions, have
moved out of the poled region [45].
If the incident beam is non-resonant, i.e. detuned from any fundamental frequency of
components in the glass, third-order non-linear effects will dominate. Equation (2.12)
shows the relation between the non-linear refractive index, n2, to the real part of χ3 [45].
12π
n 2 = Re χ 3 ( 1111)(
−ϖ
, ϖ , ϖ , −ϖ
)
(2.12)
n
where n is the linear refractive index. This phenomena results in intensity dependent self-
focusing. These non-linearities are electronic in origin, and due to fluctuations in electron
cloud density, with sub-ps (10 -12 s) response times [45]. The non-linear refractive index
can be approximated by equation (2.13).
n
2
2 ⎛ n + 2 ⎞ 2 2
⎜ ( n −1)
d n
3 ⎟
=
⎝ ⎠
(2.13)
nE
2
s