Ultrafast Two-Dimensional Time-Domain Spectroscopy of Hydrogen-Like Impurity Centers in Germanium

Coherence lifetimes (T
2
) of excited states of a hydrogen-like arsenic donor in germanium (Ge:As) have been determined by nonlinear two-dimensional (2D) ultrafast time-domain terahertz spectroscopy at cryogenic temperatures. The derived coherence times for the 2po and $2p\pm$ donor states are 2 ps and 0.15 ps respectively.