Broadband terahertz study of excitonic resonances in the high-density regime in $\mathrm{GaAs}∕{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

We report the first terahertz study of the intraexcitonic $1s\text{−}2p$ transition at high excitation densities in $\mathrm{GaAs}∕{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with increasing density, after ultrafast photoexcitation at the near-infrared exciton line. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite terahertz dielectric response. Comparison with near-infrared absorption changes reveals a significantly enhanced energy shift and broadening of the intraexcitonic resonance.

Figure 1

(a) Solid line: near-infrared, equilibrium absorption spectrum of the quantum well sample. Dashed curve: spectrum of the pump pulses. (b) Corresponding absorption spectra at delay time $\Delta t=2.5\mathrm{ps}$ after resonant $1s\mathrm{HH}$ excitation, for several densities $n$ (where $n_0\approx 1\times {10}^{10}{\mathrm{cm}}^{-2}$). The lattice temperature is $T_{\mathrm{L}}=6\mathrm{K}$. (c) and (d) Ultrafast dynamics of the THz conductivity $\Delta {\sigma }_1\left(\omega \right)$ after resonant $1s\mathrm{HH}$ excitation for initial pair densities (c) $n\approx 2\times {10}^{10}{\mathrm{cm}}^{-2}$ and (d) $n\approx 14\times {10}^{10}{\mathrm{cm}}^{-2}$.

Figure 2

(Color online) Photoinduced THz conductivity spectra $\Delta {\sigma }_1\left(\omega \right)$ (left panels) and dielectric function changes $\Delta {ϵ}_1\left(\omega \right)$ (right panels) for various excited $e\text{−}h$ pair densities $n$ (given in units of $n_0={10}^{10}{\mathrm{cm}}^{-2}$). Experimental data (solid dots) are measured at pump-probe delay $\Delta t=2.5\mathrm{ps}$. Dashed lines, quasi-2D exciton model; solid lines, sum of exciton and Drude components, as explained in the text. At the two lowest densities, the free carriers vanish and the models coincide, while the model curve at the highest density shows a pure Drude response. Curves in each panel are shifted vertically for clarity, and are equally scaled.

Figure 3

Pair densities from the two-component model explained in the text. (a) Total excited pair density as a function of pump power. (b) Free carrier density as a function of the total pair density. Lines are a guide to the eye.

Figure 4

(Color online) (a) Excitonic level spacing $E_{1s\to 2p}$ (filled squares) measured via THz spectroscopy, and density-induced energy shift $\Delta E_{\mathrm{NIR}}$ (open circles) of the interband resonance. (b) Width ${\Gamma }_{\mathrm{X}}$ (filled squares) of the intraexcitonic $1s\to 2p$ resonance as fitted via the two-component model of Eq. (2), corresponding to the solid curves in Fig. 2. Open circles: width ${\Gamma }_{\mathrm{NIR}}$ (FWHM) of the near-infrared band-edge absorption. Solid and dashed lines: linear fits to the data.