Demonstrating the decoupling regime of the electron-phonon interaction in a quantum dot using chirped optical excitation

Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultrashort laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations.

Figure 1

(a) Scheme of the n-i-p structure with an embedded layer of quantum dots (QDs). (b) Scheme of the folded $4f$ pulse shaper to control the chirp introduced into an ultrashort, transform-limited laser pulse. The unchirped pulse is directed with mirrors (M) onto a grating, focused onto a folding mirror (FM) and back-reflected with a slight angle overshooting M1. The chirped pulse is then sent to the microscope (not shown) and excites the QD (TEM image, $28\times 8{\mathrm{nm}}^2$) which emits a resonance fluorescence photon. (c) Response of the QD to broadband excitation as a function of $V_g$. A clear Coulomb blockade is observed. The excitonic transitions are identified. The excitation pulses (linear polarization, 938.22 nm center wavelength) were positively chirped. (d) Detected resonance fluorescence signal after broadband excitation as a function of the detection wavelength. The peak arises from emission from the $|{\mathrm{X}}^{1-}\rangle \to |{\mathrm{e}}^{1-}\rangle$ transition. The gate voltage was $V_g=0.3\mathrm{V}$ at which a single electron resides in the QD. (e) Rabi rotations driven on the $|{\mathrm{X}}^{1-}\rangle \leftrightarrow |{\mathrm{e}}^{1-}\rangle$ transition with a 2 ps long, transform-limited pulse. Blue points show the detected resonance fluorescence signal; red curve is a damped sine fit.

Figure 2

(a) Experimentally measured ${\mathrm{X}}^{1-}$ resonance fluorescence signal as a function of the square-root of the excitation power, and (b) calculated occupation of the $|{\mathrm{X}}^{1-}\rangle$ as a function of pulse area for different chirp parameters as indicated.

Figure 3

(a) Time evolution of the instantaneous eigenenergies of the coupled electronic-light system for a chirp of $\alpha =0.3{\mathrm{ps}}^2$ and pulse area $\mathrm{\Theta }=3\pi$ (solid lines) and $\mathrm{\Theta }=10\pi$ (dashed lines). The black dashed lines indicate the uncoupled energies. (b) The phonon spectral density.

Figure 4

Calculation of the $|{\mathrm{X}}^{1-}\rangle$ occupation for temperatures of 4 K, 50 K, and 100 K (solid, dashed, dotted lines) as a function of pulse area for positive chirp $\alpha =0.70{\mathrm{ps}}^2$ (red lines) and negative chirp $\alpha =-0.66{\mathrm{ps}}^2$ (blue lines).