Observation of the Intraexciton Autler-Townes Effect in $\mathrm{GaAs}/\mathrm{AlGaAs}$ Semiconductor Quantum Wells

The near-infrared transmission of a semiconductor multiple quantum well is probed under intense terahertz illumination. We observe clear evidence of the intraexcitonic Autler-Townes effect when the terahertz beam is tuned near the $1s\mathrm{\text{−}}2p$ transition of the heavy-hole exciton. The strongly coupled effective two-level system has been driven with terahertz field strengths of up to $10\mathrm{kV}/\mathrm{cm}$ resulting in a Rabi energy of $\approx 0.6$ times the transition energy. The induced near-infrared spectral changes at low intensities are qualitatively explained using a basic two-level model.

Figure 1

(a) Autler-Townes splitting on resonance (${\omega }_{\mathrm{THz}}={\omega }_{21}$) where the original states split by the Rabi frequency $\Omega$. (b) Energy band diagram of a $\mathrm{GaAs}/\mathrm{AlGaAs}$ quantum well with the electron ($e$) and heavy-hole (HH) and light-hole (LH) states. The THz pump beam is tuned to the $1s\mathrm{\text{−}}2p$ transition of the $e\mathrm{\text{−}}\mathrm{HH}$ exciton. (c) Experimental geometry.

Figure 2

Absorption at 10 K for different time delays between NIR probe and FEL pump pulses at a THz photon energy of 10.5 meV and a THz peak intensity of $500\mathrm{kW}/{\mathrm{cm}}^2$. The traces in the waterfall plot (bottom) are equally spaced in time. A few traces are interpolated between adjacent ones (dashed lines). Absorption peaks are labeled (lower red line). Above, the corresponding contour plot is shown. The right-hand inset displays the THz pulse intensity envelope.

Figure 3

Absorption spectra for different THz photon energies. Photon energies are tuned (a) below, (b) near, and (c) above resonance, as sketched in the insets. The low-temperature (10 K) spectra are vertically offset and labeled by THz peak intensities (in $\mathrm{kW}/{\mathrm{cm}}^2$). Vertical dashed lines mark the undriven $e\mathrm{\text{−}}\mathrm{HH}(1s)$ and $e\mathrm{\text{−}}\mathrm{LH}(1s)$ exciton resonances. Note that the given absolute THz peak intensities are estimated to be accurate within $\pm 30\%$ for different THz photon energies, whereas at the same photon energy relative intensities are accurate to $\pm 10\%$.

Figure 4

Experiment-model comparison. (a) Measured peak positions for different THz photon energies at a THz peak intensity $I_{\mathrm{THz}}\approx 130\mathrm{kW}/{\mathrm{cm}}^2$. The absorption strength is indicated by a gray scale (more absorption for dark points). Error bars denote deviations based on several measurements and the uncertainty in the THz peak intensity. Solid red lines represent the calculation according to Eq. (1) (see text). (b) Separation between the split peaks for $\hslash {\omega }_{\mathrm{THz}}=10.5\mathrm{meV}$ near resonance. Vertical error bars are related to the error in the fitted peak positions, the horizontal ones to the uncertainty in the THz peak intensity.