Acoustically regulated carrier injection into a single optically active quantum dot

We study the carrier injection into a single InGaAs/GaAs quantum dot regulated by a radio frequency surface acoustic wave. We find that the time of laser excitation during the acoustic cycle programs both the emission intensities and time of formation of neutral $\left(X^0\right)$ and negatively charged $\left(X^{-}\right)$ excitons. We identify underlying, characteristic formation pathways of both few-particle states in the time-domain experiments and show that both exciton species can be formed either with the optical pump or at later times by injection of single electrons and holes “surfing” the acoustic wave. All experimental observations are in excellent agreement with calculated electron and hole trajectories in the plane of the two-dimensional wetting layer which is dynamically modulated by the acoustically induced piezoelectric potentials. Taken together, our findings provide insight on both the onset of acoustoelectric transport of electrons and holes and their conversion into the optical domain after regulated injection into a single quantum dot emitter.

Figure 1

(a) Schematic of our experiment consisting of an IDT generating the SAW which interacts with a single QD. (b) Type-II band edge modulation induced by the SAW. Since the size of the QD is almost three orders of magnitude smaller than the acoustic wavelength, the QD acts as a pointlike, optically active deep trap for e's (orange) and h's (blue). (c) Time-integrated emission of a single QD without (black) and with SAW applied (red) and stroboscopic excitation as sketched in (b). In addition to an overall increase of the PL intensity, we observe pronounced intensity exchange between different excitonic complexes confined in the QD. (d)–(g) Four different $s$-shell exciton configurations observed in the spectra.

Figure 2

Stroboscopic, time-integrated PL spectra of a single QD (a) when optically generating carriers at four characteristic times during the acoustic cycle. (b)–(e) Schematic of the corresponding excitation conditions and motion of e's (orange) and h's (blue) in the type-II band edge modulation. The center and right panels show the time evolution in steps of $T_{\mathrm{SAW}}/4$ for a SAW propagating from left to right. (f) and (g) $X^0$ and $X^{-}$ emission lines at e and h reflow, respectively. Clear spectral shifts due to dynamic acoustic tuning are resolved for both lines. The dashed lines in (f) and (g) indicate a spectral tuning range of $\sim$$0.4\mathrm{meV}$. Line colors correspond to the excitation conditions (b) and (d).

Figure 3

Time-resolved PL of the (a) $X^0$ and (b) $X^{-}$ emission recorded at the four different stroboscopic excitation conditions shown in the center. Black and gray arrows mark formation and recombination with the optical pump. The color code of arrows marking time-delayed formation processes is the same as in Figs. 4 and 5. Dashed and solid color arrows in (a) and (b) mark suppression of $X^0$ and enhancement of $X^{-}$ emission due to conditional $X^0\to X^{-}$ formation.

Figure 4

Overview of different formation pathways for $X^0$ and $X^{-}$ at the time of laser excitation (formation with pump, black/gray) or after a temporal delay (color coded). Conditional formation pathways $X^0\to X^{-}$ are highlighted by the dashed arrows.

Figure 5

Calculated e (orange) and h (blue) dynamics for the four characteristic excitation conditions (a)–(d) with the SAW propagating along the horizontal axis from left to right and time increasing from top to bottom. The position of the QD is in the center of each main panel. A cross section of the e and h densities at the position of the QD is plotted in the right-hand side panels. Horizontal dashed lines mark SAW-controlled, time-delayed arrival of carriers at the QD positions and arrows mark the experimentally observed signatures.