  Strong-field mid-infrared pump--terahertz (THz) probe spectroscopy has been
proven as a powerful tool for light control of different orders in strongly
correlated materials. We report the construction of an ultrafast broadband
infrared pump--THz probe system in reflection geometry. A two-output optical
parametric amplifier is used for generating mid-infrared pulses with GaSe as
the nonlinear crystal. The setup is capable of pumping bulk materials at
wavelengths ranging from 1.2 $\mu$m to 15 $\mu$m and beyond, and detecting the
subtle, transient photoinduced changes in the reflected electric field of the
THz probe at different temperatures. As a demonstration, we present 15 $\mu$m
pump--THz probe measurements of a bulk EuSbTe$_{3}$ single crystal. A $0.5\%$
transient change in the reflected THz electric field can be clearly resolved.
The widely tuned pumping energy could be used in mode-selective excitation
experiments and applied to many strongly correlated electron systems.
