CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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Here E is the electric field of light,
( n)
is the susceptibility of nth nonlinearity and
magnitude decreases for the higher nth order terms. Therefore it is clear from the equation
that harmonic generation is present only when light intensities are very high. For many
crystals used for SHG is seen for light intensities of the order of GW/cm 2 .The second term in
the equation is responsible for second harmonic generation.
Let’s consider the second order term
P
(2) 2
2
0
E
(2.16)
For an electromagnetic wave E E0 cos( t kr)
,
(2) 2 2 1 (2) 2 (2) 2
P2 0
E0 cos ( tkr) (
0
E0 0
E0
cos 2( t kr))
(2.17)
2
Additionally harmonic is generated only if the phases of the two waves match. This condition
is given by k( ) k(2 )
. This condition is satisfied in uniaxial birefringent crystals like
BBO that have two different refractive indices n o and n e for the ordinary and the
extraordinary waves where n (2 , ) n ( )
. In our set up we use pulse energy of
e
o
~100μJ/pulse at 800nm which is focused on 0.5mm BBO crystal to generate ~10μJ/pulse of
second harmonic at 400nm.
2. 8. 4. White Light Generation- 450-1100nm Probe
To monitor transients after a pump pulse excitation, we use a probe pulse with
wavelengths ranging from visible to near IR region. This is done by creating white light
using a part of 800nm amplified output. The phenomenon responsible for generation of white
light is self phase modulation. The 800nm pulse with time duration of 50fs and energy of few