Chirping of an Optical Transition by an Ultrafast Acoustic Soliton Train in a Semiconductor Quantum Well

Acoustic solitons formed during the propagation of a picosecond strain pulse in a GaAs crystal with a $\mathrm{ZnSe}/\mathrm{ZnMgSSe}$ quantum well on top lead to exciton resonance energy shifts of up to 10 meV, and ultrafast frequency modulation, i.e., chirping, of the exciton transition. The effects are well described by a theoretical analysis based on the Korteweg–de Vries equation and accounting for the properties of the excitons in the quantum well.

Figure 1

(a) Experimental scheme. (b) Calculated strain pulse as initially generated in the metal film for excitation density $W=10\mathrm{mJ}/{\mathrm{cm}}^2$, and (c) its transformation to a soliton train after propagation through 0.1 mm of GaAs. The time in (b) is shifted toward that in (c) by the sample traversal time $t_0$. (d) The measured reflectivity spectrum of the ZnSe QW, normalized to the off-resonance background. The vertical dashed line in (d) shows the energy position of the heavy-hole exciton resonance.

Figure 2

Spectral-temporal contour plots of the reflectivity normalized to the off-resonance background, measured for pump fluences of (a) $1\mathrm{mJ}/{\mathrm{cm}}^2$, (b) $5.1\mathrm{mJ}/{\mathrm{cm}}^2$, and (c) $10.2\mathrm{mJ}/{\mathrm{cm}}^2$. Black lines are calculated contours of equal reflectance changes for corresponding pump fluences. Insets show the spectral profiles of reflectivity: blue dashed line is the stationary spectrum, red and black lines are, respectively, the measured and calculated spectra at the specified time, for the corresponding fluence. Black arrows in (b) and (c) indicate the arrival time of individual solitons at the QW center.