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Chapter I Brief Introduction...
In a dielectric medium when an electric field is applied, the charges are
displaced slightly from their usual position. This small movement of positive
charge in one direction and negative charge in other, results in a collection of
induced dielectric dipole moment [11]. A light wave consists of electric and
magnetic fields which vary sinusoidally at optical frequencies. The motion of
charged particles in a dielectric medium in response to an optical electrical
field also oscillates and forms oscillatory dipoles. At very low fields, the
induced polarization is directly proportional to the electric field [12].
P = 0 (1)
E (1.1)
Where, E is the magnitude of the applied electric field, (1)
is the linear
susceptibility of the material, P is the polarization and 0 is the permittivity of
free space.
The NLO phenomena occur at sufficiently intense fields. As the applied
field strength increases (e.g. in lasers) the polarization response of the
medium is no longer linear as shown by the equation (1.1). The induced
polarization (P) becomes a function of the applied field and given by [13],
P = 0 (1)
E + 0 (2)
E 2
+ 0 (3)
E 3
+ ………... (1.2)
Each term is modified by the susceptibility, a tensor quantity. The
susceptibility tensor for second-order effects (2) = ij relates each of the three
components of the polarization – Px, Py and Pz to the nine products of the two
applied light fields, for a total 27 terms. For convenience, the second order
susceptibility is converted into nonlinear optical coefficient dijk. Different
definitions and dimensions are used; therefore, absolute value of dijk should
be used with care. The most common second order effect and the most
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