  Generating energetic femtosecond mid-IR pulses is crucial for ultrafast
spectroscopy, and currently relies on parametric processes that, while
efficient, are also complex. Here we experimentally show a simple alternative
that uses a single pump wavelength without any pump synchronization and without
critical phase-matching requirements. Pumping a bulk quadratic nonlinear
crystal (unpoled LiNbO$_3$ cut for noncritical phase-mismatched interaction)
with sub-mJ near-IR 50-fs pulses, tunable and broadband ($\sim 1,000$
cm$^{-1}$) mid-IR pulses around $3.0~\mu\rm m$ are generated with excellent
spatio-temporal pulse quality, having up to 10.5 $\mu$J energy (6.3%
conversion). The mid-IR pulses are dispersive waves phase-matched to near-IR
self-defocusing solitons created by the induced self-defocusing cascaded
nonlinearity. This process is filament-free and the input pulse energy can
therefore be scaled arbitrarily by using large-aperture crystals. The technique
can readily be implemented with other crystals and laser wavelengths, and can
therefore potentially replace current ultrafast frequency-conversion processes
to the mid-IR.
