Page 29 - Visions of the Future Chemistry and Life Science

18 G. ROBERTS

1.5.1 Attosecond laser pulses
Of course, even when the world’s fastest laser pulses are available, there is
always a feeling that what is really required is pulses that are faster still!
Laser pulses with durations in the attosecond regime would open up the
possibility of observing the motions of electrons in atoms and molecules
on their natural time scale and would enable phenomena such as atomic
and molecular ionisation (Section 1.2) and the dynamics of electron orbits
about nuclei to be captured in real time.
There are several proposals actively being pursued around the world to
generate laser pulses that are significantly shorter than the shortest avail-
able today (the current world record is 4.5fs). The physics of each scheme
is well understood and the technology required to implement them in exis-
tence; what is tricky is that the proposals are not so easy to apply in the
laboratory. To reach the attosecond regime, laser pulses must be composed
of very many different frequencies, as required by the time–energy uncer-
tainty principle, and they must be coherent. A usable source of attosecond
pulses must also be intense enough to result in experimentally detectable
changes in light absorption or emission, and they must be separated in time
by at least one millionth of a second so that the changes they induce can
be recorded by modern electronic circuitry.
One scheme which has generated considerable optimism is that sug-
gested by Corkum and colleagues at the National Research Council in
Ottawa, Canada, which takes advantage of the high harmonic frequencies
simultaneously generated when an intense femtosecond laser pulse ionises
a gas of helium or neon in a narrow waveguide to construct the broad spec-
trum of colours necessary to support attosecond laser emission. These har-
monics are just like the overtones of a musical note: they are generated by
oscillations of the electrons liberated by ionisation in the laser field and are
formed coherently, that is with their amplitudes in phase with one
another. Figure 1.1 presents a schematic illustration of the mechanism by
which a high-harmonic photon is emitted in an atom. At the present time
researchers have succeeded in generating up to the 297th harmonic in
helium of the original 800 nm light from a 25fs titanium:sapphire laser by
this approach, yielding a harmonic spectrum which extends into the X-ray
region as far as 2.7 nm, and current research focusses on exploiting this
broadband emission to construct a usable attosecond laser. In addition to
providing a possible source of attosecond light, high-order harmonic gen-