Page 354 - High Power Laser Handbook

322 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 323

(a)

(b)

Figure 12.16 In the process of high-harmonic generation, coherent x-ray
beams are generated through a coherent electron ionization and recollision
process. (a) The classical picture of strong-field ionization. (b) A representation
of the quantum equivalent.

2
where I is the ionization potential of the atom, and U ∝ Iλ is the
p
p
ponderomotive potential or energy gained in the driving field. The
dynamics of the recollision process occur on attosecond timescales; an
understanding of this process has led to the birth of the field of atto-
second science. 48–53 The HHG radiation is actually emitted as a
sequence of attosecond bursts; under the correct conditions, single iso-
lated attosecond pulses can result. The extremely short duration of
54
the EUV and soft x-ray light emitted by HHG makes it possible to
observe extremely fast processes in atomic, molecular, and solid-state
systems.
The HHG process is powered by high-power ultrashort pulse
lasers. Although the required intensities of up to 10 W/cm are com-
2
15
parable to those used in laser fusion, the pulse energy required to
obtain this intensity is modest because femtosecond duration pulses
are used. The high-power laser used to drive the HHG process can
easily fit into a fraction of a standard optical table, essentially provid-
ing a robust and practical way of implementing a tabletop EUV or
soft x-ray laser. Much of the recent rapid progress in the use of HHG
has been due to the development of a new generation of tabletop-
scale, solid-state, ultrashort-pulse lasers capable of generating femto-
second pulses with very high peak and average power. In the longer
term, the further development of HHG-based light sources at shorter
wavelengths in the “water window” region of the soft x-ray spectrum
(corresponding to photon energies of greater than 300 eV) will allow