  Interband Cascade Lasers (ICL) have matured into a versatile technological
platform in the mid-infrared spectral domain. To broaden their applicability
even further, ongoing research pursues the emission of ultrashort pulses. The
most promising approach is passive mode-locking with a fast saturable absorber.
However, despite vast attempts no passive mode-locked ICL has been demonstrated
up to date. In this study we perform pump-probe measurements on the ICL gain
and show that its dynamics are mainly ($\sim$70%) governed by a fast recovery
time of 2 ps. Our findings explain why passive mode-locking has not succeeded
so far, shedding a new light on ICL dynamics as we propose strategies to
overcome the current limitations.
