  Molecules with their axes sharply confined in space, available through
laser-induced alignment methods, are essential for many current experiments,
including ultrafast molecular imaging. Most of these applications require both
that the aligning laser field is turned-off, to avoid undesired perturbations,
and that the molecules remain aligned sufficiently long that reactions and
dynamics can be mapped out. Presently, this is only possible for small, linear
molecules and for times less than 1 picosecond. Here, we demonstrate strong,
field-free alignment of large molecules inside helium nanodroplets, lasting
tens of picoseconds. Molecular alignment in either one or three dimensions is
created by a slowly switched-on laser pulse, made field-free through rapid
pulse truncation, and retained thanks to the impeding effect of the helium
environment on molecular rotation. We illustrate the opportunities that
field-free aligned molecules open by measuring the alignment-dependent
strong-field ionization yield of a thiophene oligomer. Our technique will
enable molecular-frame experiments, including ultrafast excited state dynamics,
on a variety of large molecules and complexes.
