  Alignment and orientation of molecules by intense, ultrashort laser fields
are crucial for a variety of applications in physics and chemistry. These
include control of high harmonics generation, molecular orbitals tomography,
control of molecular photoionization and dissociation processes, production of
"molecular movies" with the help of X-ray free-electron laser sources and
ultrafast electron diffraction of relativistic electrons. While the dynamics of
laser-induced molecular alignment has been extensively studied and
demonstrated, molecular orientation is a much more challenging task, especially
for asymmetric-top molecules. Here we report the first experimental
demonstration of a field-free, all-optical three-dimensional orientation of
asymmetric-top molecules by means of phase-locked cross-polarized two-color
laser pulses. In addition to the conventional integrated orientation factor, we
report the differential degree of orientation which is not amenable to optical
measurements, but is readily accessible in our angle-resolved imaging
technique. Our scheme applies to a wide class of asymmetric molecules and opens
new ways towards controlling their orientation, eventually leading to direct
imaging of structure of gas-phase molecules using advanced free electron laser
beams with extremely high spatiotemporal resolution.
