  The concept of stimulated emission of bosons has played an important role in
modern science and technology, and constitutes the working principle for
lasers. In a stimulated emission process, an incoming photon enhances the
probability that an excited atomic state will transition to a lower energy
state and generate a second photon of the same energy. It is expected, but not
experimentally shown, that stimulated emission contributes significantly to the
zero resistance current in a superconductor by enhancing the probability that
scattered Cooper pairs will return to the macroscopically occupied condensate
instead of entering any other state. Here, we use time- and angle-resolved
photoemission spectroscopy to study the initial rise of the non-equilibrium
quasiparticle population in a Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ cuprate
superconductor induced by an ultrashort laser pulse. Our finding reveals
significantly slower buildup of quasiparticles in the superconducting state
than in the normal state. The slower buildup only occurs when the pump pulse is
too weak to deplete the superconducting condensate, and for cuts inside the
Fermi arc region. We propose this is a manifestation of stimulated
recombination of broken Cooper pairs, and signals an important momentum space
dichotomy in the formation of Cooper pairs inside and outside the Fermi arc
region.
