Ultrashort Laser Pulse Phenomena, 2nd Edition

202 Light–Matter Interaction

which leads to the definition of the two photon absorption coefficient β

2

:

β

2

=

3

4

ω



cn

0

χ

(3)

i

. (3.165)

In terms of intensities, the expression for the attenuation of beam 1 in presence

of beam 2 is:

dI

1

dz

=−β

2

(

I

1

+ 2I

2

)

I

1

. (3.166)

Again, it is only when the two counter-propagating beams have equal intensity

that the relative attenuation of both beams is equal.

3.7. CONTINUUM GENERATION

One of the most impressive (and simplest) experiments with ultrashort light

pulses is the generation of a white light continuum. At the same time contin-

uum generation with laser pulses is one of the most complex and difficult to

analyze processes