Excitonic interactions with intense terahertz pulses in ZnSe/ZnMgSSe multiple quantum wells

We studied the interaction of excitons with terahertz (THz) electric fields in ZnSe/ZnMgSSe multiple quantum wells with THz-pump and optical-probe spectroscopy. The incident THz pulse with a peak electric field of $\sim 70\text{kV}/\text{cm}$ induces strong spectral modulations in the heavy-hole and light-hole excitonic absorption resonances. The dependence of the excitonic absorption resonance energy on the THz field follows the Stark effect for smaller THz fields. For larger THz fields, the peak shift deviates from the Stark effect, which implies that the interaction enters the nonperturbative regime.

Figure 1

(Color) (a) Schematic of THz-pump and optical-probe experimental setup. The linear polarization directions of both the pump and probe pulse are the same and parallel to the MQW layers. (b) Observed THz temporal profile and its Fourier components. (c) Excitonic absorption spectra observed at pump-probe delay times of $-1.8$ (THz pump off) and 0 ps (THz pump on). (d) Differential absorption spectrum of the sample because of the THz pulse, shown as a function of optical energy and delay time between THz-pump and optical-probe pulses.

Figure 2

(Color online) Absorption spectra in the vicinity of the hh and lh excitonic resonances obtained at a delay time of 0 ps for different incident THz fields $E_{\text{THz}}$. Solid and dashed lines correspond to the experimental data and calculated fits, respectively. Inset shows the calculated result of 0 kV/cm (solid line), which is decomposed into the sum of two Gaussian excitons, a broadened 2D continuum, and a background (dashed lines).

Figure 3

(Color online) (a) Dependence on the energy of the absorption resonance of the hh (solid circles) and lh (open circles) excitons on the incident THz field $E_{\text{THz}}$, derived from the fits shown in Fig. 2. Solid and dashed lines are fits of Eq. (2) to the data for the hh and lh excitons, respectively. (b) Field-dependent ionization rate $\gamma \left(E_{\text{THz}}\right)$ of the hh (solid circles) and lh (open circles) excitons. The one-dot-dashed line and two-dot-dashed line show the calculated tunneling ionization rate for the case of 3D and 2D excitons. The solid and dashed lines are guides for the $\sqrt{E_{\text{THz}}}$ dependence.

Figure 4

(Color online) Comparison of the temporal profiles between the absorption change in the vicinity of the hh exciton (at 2.86 eV) and the absolute value of the electric field of the THz pulse.