Exciton formation dynamics in (Ga,In)As quantum wells

We investigate the ultrafast dynamics of exciton formation in (Ga,In)As quantum wells using optical pump–terahertz probe spectroscopy, complemented by time-resolved photoluminescence. Terahertz spectroscopy directly probes intraexcitonic transitions and thus, in principle, distinguishes between unbound electron-hole plasma and Coulomb-bound exciton populations. At low excitation densities, this separation is effective: the Drude response of the electron-hole plasma remains spectrally distinct from narrow intraexcitonic resonances. However, at higher excitation densities, spectral broadening and overlap obscure this distinction. Following low-density nonresonant excitation, we initially observe a pure Drude-like plasma response that gradually transitions into an intraexcitonic resonance. In contrast, at higher excitation densities, conventional analysis based on the Drude-Lorentz model suggests questionably high exciton fractions up to 32% immediately after excitation. It remains uncertain whether these fractions represent genuine instantaneous exciton formation or arise from spectral overlap and analytical ambiguity. To resolve this, we introduce a differential probing technique utilizing exciton ionization induced by strong THz fields. By comparing responses obtained at high and low THz field strengths, we isolate the exciton contribution from the pure plasma contributions. This approach reveals intrinsic exciton formation dynamics governed by two characteristic timescales: a fast component in the vicinity of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>10</a:mn> <a:mspace width="0.16em"/> <a:mi>ps</a:mi> </a:mrow> </a:math> and a slower process around <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>250</c:mn> <c:mspace width="0.16em"/> <c:mi>ps</c:mi> </c:mrow> </c:math> . Notably, these timescales remain largely independent of excitation density, contrasting sharply with the strong density dependence suggested by conventional methods. Our results establish a robust method to disentangle exciton and plasma dynamics, providing clearer insight into exciton formation under nonresonant excitation in semiconductor quantum wells.