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Original author : Rosli Omar ---- my labmate
Original location: PRG_UTM @ communityzero ---> no more free,
hate them so i migrate here
Introduction
Most polymers are inherently centrosymmetric and thus are not electro-optic.
EO polymers require highly optically nonlinear chromophores/molecules,
which can be incorporated into polymer host. Example: FTC( 2-dicyanomethylen-3-cyano-4-{2-[trans-(4-N,
N-diacetoxyethyl-amino)phenylene-3,4-dibutylthien-5]-vinyl}-5,5-dimethyl-2,5-dihydrofuran)
chromophore doped into a PPMA host has an r33?55pm/V@1060nm. For
a useful macroscopic EO effect, these non-linear chromophores/molecules
have to be arranged in a non-centrosymmetric structure by a poling
electric field.
Why Poling?
Poling is used to remove the inherent centrosymmetry to achieve
the EO effect.
Effect of Poling on the EO property.
Dipolar alignment of the side chains is achieved by poling the polymer
in the region of Tg. The resulting polymer is electro-optic and
an applied field (using the poling electrodes) can be used to change
the birefringence of the polymer and so rotate the light passing
through the film. The material then can be used to upconvert the
optical frequency of a laser wave (e.g. optical second harmonic
generation (SHG)), or to manipulate the material´s refractive
index via the linear electro-optic or Pockels effect, which is a
key mechanism for EO switches and devices.
How to Pole?
Each polymer is first dissolved in an organic solvent, such as chloroform.
The poling field is then applied as the solvent evaporated and the
polymer solidified, and thus a non-centrosymmetry is induced as
the polymer is aligned. The two methods of poling currently employed
are contact electrode poling and corona poling.
1. In the contact electrode poling process, chromophores are aligned
by applying a strong electric-field (a few tens of volts per micron
to develop an appreciable net polar order) while the sample is heated
close to the glass transition temperature (Tg) of the active material.
Cooling down the sample while maintaining the electric field will
‘freeze’ the chromophores into their aligned position.
A major consequence of poling is the introduction of material birefringence.
The polymers are poled at temperatures of Tg because the polymer
chromophores are free to rotate.
2. Using the Corona poling techniques on non-linear optical (NLO)
doped polymers. Corona poling consists of a needle that is placed
a few centimeters from a thin film. In corona poling, a corona needle
at potentials at about V is placed near the surface of a polymer
film that is spin-casted on a grounded, transparent-coated substrate,
which forms a corona discharge. The charge deposits on the surface
and results in a large magnitude electric field across the film.
The needle potential may be positive or negative relative to the
substrate electrode. It has been reported, however, that negative
potentials result in a better molecular alignment with films using
indium tin oxide (ITO) for electrodes.
The poling process strongly demands highest material purity and
clean room processing. Furthermore, it is essential to know the
right poling parameters such as poling temperature and maximum achievable
electric field.
Chromophores Meaning:
1. A chemical group capable of selective light absorption resulting
in the coloration of certain organic compounds
2. The structural feature of a molecule responsible for its absorption
of UV or visible light to give a colored compound. Thus, chromophoric.
3. A group of atoms, generally in an organic compound, that absorbs
light of characteristic wavelengths, thus imparting colour to the
compound. Typical examples are the azo (-N=N-) and nitroso (-N=O)
groups. In dyes, groups called auxochromes (colour enhancers) help
to modify the colour conferred by the chromophore, as well as the
solubility and related properties of the dye molecule. A group derived
from sulphonic acid (-SO3H) is a typical auxochrome.
4. A group causing coloration in a dye. Chromophores are generally
groups of atoms having delocalized electrons.
5. A group in a molecule that causes absorption of electromagnetic
radiation, most usually used in the ultraviolet region. The carbonyl
group and the diazo group (See diazo dyes) are good chromophores.
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