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Chapter I Brief Introduction...
first Nonlinear term as +βE 2
. If a field –E is applied, the polarization is still
predicted to be +βE 2
. Yet, if the medium is centro-symmetric the polarization
should be –βE 2
. This contradiction can be resolved only if β=0. Thus the
centro-symmetric medium has zero β-value. In the case of third order NLO
susceptibility, if +E field produces polarization +γE 3
and –E field produces –
γE 3
, so the second order hyper-polarizability (γ) is the first non-zero nonlinear
term in centro-symmetric media [15].
1.2 Various Types of Second-Order NLO Effects
In a medium several second-order NLO effects can be observed
experimentally which are summerized as follows [16].
Second Harmonic Generation (SHG)
The process of transformation of light with frequency into light with
double frequency 2 is referred as SHG [17]. In figure (1.1), the two photons
with momenta P1 and P2 are absorbed and photon with momentum P is
emitted. This process is spontaneous and involves three photon transitions,
for instance, two photons with the same energy h are absorbed
spontaneously to emit a photon with energy 2h.
Figure 1.1 Second harmonic generation.
Frequency doubling or SHG is a special case of sum frequency
generation if the two input wavelengths are the same. In figures (1.2) to (1.5)
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