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We report a comprehensive experimental study and detailed model analysis ofthe terahertz dielectric response and density kinetics of excitons and unboundelectron-hole pairs in GaAs quantum wells. A compact expression is given, inabsolute units, for the complex-valued terahertz dielectric function ofintra-excitonic transitions between the 1s and higher-energy exciton andcontinuum levels. It closely describes the terahertz spectra of resonantlygenerated excitons. Exciton ionization and formation are further explored,where the terahertz response exhibits both intra-excitonic and Drude features.Utilizing a two-component dielectric function, we derive the underlying excitonand unbound pair densities. In the ionized state, excellent agreement is foundwith the Saha thermodynamic equilibrium, which provides experimentalverification of the two-component analysis and density scaling. During excitonformation, in turn, the pair kinetics is quantitatively described by a Sahaequilibrium that follows the carrier cooling dynamics. The terahertz-derivedkinetics is, moreover, consistent with time-resolved luminescence measured forcomparison. Our study establishes a basis for tracking pair densities viatransient terahertz spectroscopy of photoexcited quasi-two-dimensionalelectron-hole gases.

Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases