Sub-Two Cycle Compression of Yb Laser Pulses using Rotational Nonlinearity Enhancement in Molecular Gases*

Few-cycle pulse generation has largely been achieved through the nonlinear propagation of short (\textless 10-cycle) pulses in noble gas-filled capillaries, where the ``instantaneous'' optical nonlinearity can be efficiently exploited to generate octave spanning bandwidths. At much longer input pulse durations (\textgreater 100-cycle), generating comparable bandwidths using noble gases requires employing larger levels of nonlinearity, prolonged interaction lengths, or multiple compression stages. By instead using linear molecular gases, where the field-driven alignment of the molecular ensemble boosts the optical nonlinearity, multi-octave supercontinua supporting sub-cycle durations can be generated. We demonstrate the enhanced supercontinuum generation by propagating 280 fs pulses in N$_{\mathrm{2}}$, N$_{\mathrm{2}}$O, and CO$_{\mathrm{2}}$ filled hollow-core fiber. We compress pulses below two-optical cycles and use them to generate high-order harmonics.