  We demonstrate the generation of nanosecond mid-infrared pulses via fast
modulation of thermal emissivity, enabled by the absorption of visible pump
pulses in unpatterned silicon and gallium arsenide. The free-carrier dynamics
in these materials result in nanosecond-scale modulation of thermal emissivity,
leading to nanosecond pulsed thermal emission. To our knowledge, the nanosecond
thermal-emissivity modulation demonstrated in this work is three orders of
magnitude faster than what has been previously demonstrated. We also indirectly
observed sub-nanosecond thermal pulses from hot carriers in semiconductors. The
experiments are well described by our multiphysics model. Our method of
converting visible pulses into the mid infrared using modulated emissivity
obeys different scaling laws and can have significant wavelength tunability
compared to approaches based on conventional nonlinearities.
