  We experimentally demonstrate wide-band (>100 THz) frequency down-conversion
of near-infrared (NIR) femtosecond-scale pulses from an Er:fiber laser to the
mid-infrared (MIR) using four-wave-mixing (FWM) in photonic-chip
silicon-nitride waveguides. The engineered dispersion in the nanophotonic
geometry, along with the wide transparency range of silicon nitride, enables
large-detuning FWM phase-matching and results in tunable MIR from 2.6-3.6 um on
a single chip with 100-pJ-scale pump-pulse energies. Additionally, we observe >
20 dB broadband parametric gain for the NIR pulses when the FWM process is
operated in a frequency up-conversion configuration. Our results demonstrate
how integrated photonic circuits could realize multiple nonlinear optical
phenomena on the same chip and lead to engineered synthesis of broadband,
tunable, and coherent light across the NIR and MIR wavelength bands from
fiber-based pumps.
