Ultrafast resonant terahertz response of excitons in semiconductor quantum dots

Exciton formation and carrier-carrier interactions in semiconductor quantum dots have been investigated by means of terahertz time-domain spectroscopy. The formation of excitons leads to a blueshift of the terahertz absorption as compared to the bare electronic intersublevel absorption. We show that this is a consequence of Coulomb interactions among carriers in the quantum dots. Time-resolved studies reveal the time scale on which photogenerated electron-hole pairs in the GaAs barriers transform into confined excitons in the dots.

Figure 1

(Color online) Frequency dependence of the absorption coefficient (a) and the refractive index (b) measured with terahertz pulses polarized along the [110] direction $(T=5\mathrm{K})$. The dashed line in (b) represents the variation of the GaAs matrix refractive index. The dash-dotted lines are fits according to a Lorentz model. Inset: spectrum of the ultrabroadband terahertz transient (logarithmic scale). The strong absorption at $\sim 75\mathrm{meV}$ stems from the Si cryostat windows.

Figure 2

(Color online) Photoinduced absorption (a) and refractive index (b) change versus frequency for $815\mathrm{nm}$ cw excitation and two different polarizations of the FIR probe. The Gaussian on the bottom of (a) represents the energetic position of the [110] ISL transition. The two bars indicate the calculated energetic positions of the [110] $X^0$ and $X^{-}$ absorptions. The dash-dotted line in (b) shows the calculated index variation due to the SP bleaching only. (c) and (d) show temperature-dependent measurements. The dashed lines are results from a Drude model to fit the background. The curves are offset for clarity. Inset: temperature dependence of the $X$ (circles) and $SP$ (squares) amplitudes. The PL peak intensity is shown as a solid line.

Figure 3

(Color online) Buildup of the exciton absorption $X$ (shaded area). Absorption coefficient (a) and refractive index change (b) versus frequency for different pump-probe delays after photoexcitation. The dashed lines result from a Drude fit of the background. The curves are offset for clarity. Inset: time dependence of the $X$ amplitude (symbols—measurement; line—exponential fit).