  Many of the fundamental optical and electronic properties of atomically thin
transition metal dichalcogenides are dominated by strong Coulomb interactions
between electrons and holes, forming tightly bound atom-like excitons. Here, we
directly trace the ultrafast formation of excitons by monitoring the absolute
densities of bound and unbound electron-hole pairs in monolayers of WSe$_2$
following femtosecond non-resonant optical excitation. To this end,
phase-locked mid-infrared probe pulses and field-sensitive electro-optic
sampling are used to map out the full complex-valued optical conductivity of
the non-equilibrium system and to discern the hallmark low-energy responses of
bound and unbound pairs. While free charge carriers strongly influence the
infrared response immediately after above-bandgap injection, up to 60% of the
electron-hole pairs are bound as excitons already on a sub-picosecond
timescale, evidencing extremely fast and efficient exciton formation. During
the subsequent recombination phase, we still find a large density of free
carriers in addition to excitons, indicating a non-equilibrium state of the
photoexcited electron-hole system.
