Page 16 - Visions of the Future Chemistry and Life Science

6 G. ROBERTS

matically increased by the process of chirped-pulse amplification. In this
technique, the weakly intense ultrafast pulses are first stretched in time to
between 100 and 1000 ps (a picosecond (ps) is 1000 fs), then amplified by
about a million times in one or more further Ti:sapphire laser crystals, and
finally recompressed to femtosecond durations. A typical peak power
achievable with an amplified Ti:sapphire laser today is a hundred billion
watts for a laser beam area of one square centimetre (the highest is just over
a thousand million billion watts per square centimetre), which contrasts
with an incident power of about 0.001watts received through the iris of a
human eye looking directly into the sun. For further details concerning the
physics which underpins the operation of ultrafast lasers and their amplifi-
cation, the interested reader is referred elsewhere for information (see
Further reading).
For studies in molecular physics, several characteristics of ultrafast
laser pulses are of crucial importance. A fundamental consequence of the
short duration of femtosecond laser pulses is that they are not truly mono-
chromatic. This is usually considered one of the defining characteristics of
laser radiation, but it is only true for laser radiation with pulse durations
of a nanosecond (0.000 000 001s, or a million femtoseconds) or longer.
Because the duration of a femtosecond pulse is so precisely known, the
time-energy uncertainty principle of quantum mechanics imposes an
inherent imprecision in its frequency, or colour. Femtosecond pulses must
also be coherent, that is the peaks of the waves at different frequencies
must come into periodic alignment to construct the overall pulse shape
and intensity. The result is that femtosecond laser pulses are built from a
range of frequencies: the shorter the pulse, the greater the number of fre-
quencies that it supports, and vice versa.
The second requirement for investigations in ultrafast photophysics is
one of wide wavelength coverage. The capacity for wavelength tuning is an
essential ingredient in studies of molecular dynamics due to the different
energy gaps that separate the quantum levels of molecules: vibrational res-
onances are excited with infrared light for example, whilst electronic states
that correspond to different arrangements of the molecular electrons are
reached by light in the visible and ultraviolet spectrum. The high output
power of chirped-pulse amplified femtosecond lasers renders them ideal for
synchronous pumping of optical parametric devices, whereby photons of
light at one frequency are converted through their self-interactions in non-
centrosymmetric media into photons at different frequencies. Today, the