  The field of attosecond science was first enabled by nonlinear compression of
intense laser pulses to a duration below two optical cycles. Twenty years
later, creating such short pulses still requires state-of-the-art few-cycle
laser amplifiers to most efficiently exploit 'instantaneous' optical
nonlinearities in noble gases for spectral broadening and parametric frequency
conversion. Here, we show that nonlinear compression can in fact be much more
efficient when driven in molecular gases by pulses substantially longer than a
few cycles, due to enhanced optical nonlinearity associated with rotational
alignment. We use 80-cycle pulses from an industrial-grade laser amplifier to
simultaneously drive molecular alignment and supercontinuum generation in a
gas-filled capillary, producing more than two octaves of coherent bandwidth and
achieving >45-fold compression to a duration of 1.7 cycles. As the enhanced
nonlinearity is linked to rotational motion, the dynamics can be exploited for
long-wavelength frequency conversion and compressing picosecond lasers.
