Ultrashort Laser Pulse Phenomena, 2nd Edition

3

Light–Matter Interaction

The generation and application of fs light pulses revolve around light–matter

interaction. In the preceding chapter we considered situations in which the prop-

agating field does not change the material response. The result was a linear

dependence of the output field on the input field, a feature attributed to linear

optical elements. The medium could be described by a transfer function

˜

H(),

which for a material, is completely determined by a complex dielectric constant

˜(). The real part of ˜() is responsible for dispersion, determining phase and

group velocity, for example. The imaginary part describes (frequency-dependent)

loss or gain. In many cases these linear optical media can be considered as host

materials for sources of nonlinear polarization. It is the latter which will be

discussed in this section.

Let us consider a pulse propagating through a (linear) optical material, for

instance, glass. In addition to the processes described above, we will have to

consider nonlinear optical interaction if the electric field strength is high. This

can result from high pulse power and/or tight focusing. The decomposition of the

polarization according to Eq. (1.68), P = P

L

+ P

NL

, accounts then for different

optical properties of one and the same material. Another possible situation is

that the host material affects the pulse through the linear optical response only,

but contains additional substances interacting nonlinearly with the pulse. This,

for example, can occur if the light pulse is at resonance with a dopand material,

which considerably increases the interaction strength. Examples of such compos-

ite materials are glass doped with ions, dye molecules dissolved in a (transparent)

solvent, and resonant gas molecules surrounded by a (nonresonant) buffer gas.

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