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Chapter I Brief Introduction...
Organic NLO Materials:
Organic materials are molecular materials that consist of chemically
bonded molecular units interacting in the bulk media through weak van der
Waal interactions. The organic molecules which contain both conjugated
bonds and acceptor group on one side and a donor group on the other side
are known as nonlinear optical (NLO) materials [12].
A typical SHG active molecule can be represented as follows:
Donor π – conjugated system acceptor
The π – conjugated molecules with a donor and acceptor will not
display SHG activity if they possess a centre of symmetry. This symmetry
requirement eliminates several materials for consideration of SHG. The π
conjugated systems would be benzene, azobenzene, stilbene, tolans,
biphenyl, hetrocycle, polyens, benzylidene, etc. Some accepter groups are,
for instance, NO2, NO, CN, NH3 + , CF3, COCH3, COOH, COO - , etc. Whereas,
some donor groups are NH2, NHCH3, FCl, Br, CH3, COOCH3, NHCOCH3, O -
etc.
The trade off between optical transparency and SHG efficiency is an
important issue while designing devices using NLO materials. Both the
second order optical nonlinearity and optical transparency are affected by the
nature of conjugated bonds, length of π - conjugation and strength of electron
donor/ acceptor constituents[14,42].
The second order NLO organic materials are most often based on π
conjugated molecules (Chromophores) with strong electron donor and
accepter group at the end of π – conjugate structure. Such molecules must be
ordered in an acentric manner in a material to achieve a macroscopic second
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