  Dissipation of electromagnetic energy through absorption is a fundamental
process that underpins phenomena ranging from photovoltaics to photography,
analytical spectroscopy, photosynthesis, and human vision. Absorption is also a
dynamic process that depends on the duration of the optical illumination. Here
we report on the resonant plasmonic absorption of a nanostructured metamaterial
and the non-resonant absorption of an unstructured gold film at different
optical pulse durations. By examining the absorption in travelling and standing
waves, we observe a plasmonic relaxation time of 11 fs as the characteristic
transition time. The metamaterial acts as a beam-splitter with low absorption
for shorter pulses, while as a good absorber for longer pulses. The transient
nature of the absorption puts a frequency limit of ~90 THz on the bandwidth of
coherently-controlled, all-optical switching devices, which is still a thousand
times faster than other leading switching technologies.
