  Intense laser pulses have recently emerged as a tool to tune between
different orders in complex quantum materials. Among different light-induced
phenomena, transient superconductivity far above the equilibrium transition
temperature in cuprates is particularly attractive. Key to those experiments
was the resonant pumping of specific phonon modes, which was believed to induce
superconducting phase coherence by suppressing the competing orders or
modifying the structure slightly. Here, we present a comprehensive study of
photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We
find that upon photo-excitations, Josephson plasma edge in superconducting
state is initially removed accompanied by quasiparticle excitations, and
subsequently reappears at frequency lower than the static plasma edge within
short time. In normal state, an enhancement or weaker edge-like shape is indeed
induced by pump pulses in the reflectance spectrum accompanied by simultaneous
rises in both real and imaginary parts of conductivity. We compare the
pump-induced effects between near- and mid-infrared excitations and exclude
phonon pumping as a scenario for the photo-induced effects above. We further
elaborate the transient responses in normal state are unlikely to be explained
by photo-induced superconductivity.
