  Transparent materials do not absorb light but have profound influence on the
phase evolution of transmitted radiation. One consequence is chromatic
dispersion, i.e., light of different frequencies travels at different
velocities, causing ultrashort laser pulses to elongate in time while
propagating. Here we experimentally demonstrate ultrathin nanostructured
coatings that resolve this challenge: we tailor the dispersion of silicon
nanopillar arrays such that they temporally reshape pulses upon transmission
using slow light effects and act as ultrashort laser pulse compressors. The
coatings induce anomalous group delay dispersion in the visible to
near-infrared spectral region around 800 nm wavelength over an 80 nm bandwidth.
We characterize the arrays' performance in the spectral domain via white light
interferometry and directly demonstrate the temporal compression of femtosecond
laser pulses. Applying these coatings to conventional optics renders them
ultrashort pulse compatible and suitable for a wide range of applications.
