Page 353 - High Power Laser Handbook

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

physics, electronics, and lasers. However, one region of the EM spec-
trum has been relatively underutilized to date: the extreme ultravio-
let (EUV) and soft x-ray range, corresponding to wavelengths 10 to
100 times shorter than visible light and with photon energies in the
range of tens to hundreds of electron volts. EUV light is both useful
and difficult to exploit for the same reason—that is, it is ionizing radi-
ation that interacts strongly with matter. This strong interaction
makes EUV light difficult to generate and severely restricts the types
of optics that can be used. However, the development of EUV optical
technologies has strong motivation. With EUV light, it is possible to
make microscopes that can resolve smaller features than is possible
using visible light. In addition, with EUV lithography, it is possible to
write smaller patterns. Furthermore, these wavelengths are well
matched to the primary atomic resonances of most elements, making
possible many element- and chemical-specific spectroscopies and
spectromicroscopies.
The compelling scientific applications of EUV light have led to
the development of several dozen large-scale synchrotron radiation
sources, with more than 10,000 users worldwide. However, synchro-
tron light sources have major disadvantages, especially when uses for
EUV light move from the research lab into manufacturing or analyti-
cal applications. The most obvious disadvantage is the large size and
cost of these sources. Experiments must be constructed at the facility
itself, and any samples must be brought to the facility. Furthermore, a
number of emerging applications of EUV and soft x rays, such as soft
x-ray holography, require coherent light. This need has prompted the
development of large-scale “fourth generation” free-electron lasers.
However, these sources are even larger and often more costly than
synchrotron light sources. The need for small-scale coherent light
sources has motivated research in both x-ray lasers and upconversion
of coherent light from a laser to very short wavelengths. During the
past decade, both types of light sources have been successfully used
for a variety of application experiments, such as nanoscale imaging
and studies of molecular dynamics.
In particular, the process of high-order harmonic generation
(HHG) has proven to be a very useful coherent tabletop x-ray laser
source that can be used for a variety of applications in basic and
applied science (Fig. 12.16). 46,47 In HHG, a very intense femtosecond
laser focused into an atomic gas is upconverted into the EUV or soft
x-ray regions of the spectrum. The HHG process results from a com-
plex laser-atom interaction, in which the light from an incident intense
laser pulse first pulls an electron from an atom through a process of
field ionization and then drives this electron back into its parent
ion. The resulting recollision process coherently emits a short-wave-
length photon whose energy is given by

E = I + . 32 U (12.11)
max p p