  Atomically thin semiconductor heterostructures provide a two-dimensional (2D)
device platform for creating high densities of cold, controllable excitons.
Interlayer excitons (IEs), bound electrons and holes localized to separate 2D
quantum well layers, have permanent out-of-plane dipole moments and long
lifetimes, allowing their spatial distribution to be tuned on demand. Here, we
employ electrostatic gates to trap IEs and control their density. By
electrically modulating the IE Stark shift, electron-hole pair concentrations
above $2\times10^{12}$ cm$^{-2}$ can be achieved. At this high IE density, we
observe an exponentially increasing linewidth broadening indicative of an IE
ionization transition, independent of the trap depth. This runaway threshold
remains constant at low temperatures, but increases above 20 K, consistent with
the quantum dissociation of a degenerate IE gas. Our demonstration of the IE
ionization in a tunable electrostatic trap represents an important step towards
the realization of dipolar exciton condensates in solid-state optoelectronic
devices.
