Charge and spin readout scheme for single self-assembled quantum dots

We propose an all optical spin initialization and readout concept for single self-assembled quantum dots, and demonstrate its feasibility. Our approach is based on a gateable single dot photodiode structure that can be switched between charge and readout modes. After optical electron generation and storage, we propose to employ a spin-conditional absorption of a circularly polarized light pulse tuned to the single negatively charged exciton transition to convert the spin information of the resident electron to charge occupancy. Switching the device to the charge readout mode then allows us to probe the charge state of the quantum dot $\left(1e,2e\right)$ using nonresonant luminescence. The spin orientation of the resident electron is then reflected by the photoluminescence (PL) yield of doubly $\left(X^{2-}\right)$ and singly $\left(X^{-1}\right)$ charged transitions in the quantum dot. To verify the feasibility of this spin readout concept, we have applied time gated photoluminescence to confirm that selective optical charging and efficient nonperturbative measurement of the charge state can be performed on the same dot. The results show that, by switching the electric field in the vicinity of the quantum dot, the charging rate can be switched between a regime of efficient electron generation $\left(\Gamma ⪢{10}^6{\text{s}}^{-1}{\text{W}}^{-1}{\text{cm}}^2\right)$ and a readout regime, where the charge occupancy and, therefore, the spin state of the dot can be tested via PL over millisecond timescales, without altering it. Our results show that such a quasicontinuous, nonperturbative readout of the charge state of the dot allows increasing the dark time available for undisturbed spin manipulation and storage into the millisecond range, while still providing sufficient signal for high fidelity readout. Consequently, our readout scheme would allow the investigation of spin relaxation and decoherence mechanisms over the long timescales, predicted by theory.

Figure 1

(Color online) (a) Schematic of the sample structure. We distinguish three electric-field regimes: (b) The luminescence regime at low electric fields. (c) The hole tunneling at moderate electric fields leads to the charging regime. (d) The reset regime, where electrons tunnel to the back contact.

Figure 2

(Color online) Photoluminescence intensity for the neutral exciton $\left(X^0\right)$, negatively charged exciton $\left(X^{-1},X^{-2}\right)$, and biexciton $\left(2X^0,2X^{-1}\right)$ emissions from a single QD as a function of emitted photon energy, and applied electric field (a) with dc bias applied and (b) with application of a reset pulse (see inset). With the introduction of a reset pulse, charge accumulation is prevented and the charge neutral QD transitions become visible.

Figure 3

(Color online) (a) Schematic of the pulse sequence, $V_{\text{reset}}$ is changed during the experiment. (b) Photoluminescence spectra as a function of emitted photon energy for reset voltages ranging from $V_{\text{reset}}=0.5\text{V}$ (blue) to $V_{\text{reset}}=-1.5\text{V}$ (red). (c) Relative luminescence intensities of the charge neutral ($X^0+2X^0$, black), the singly charged ($X^{-}+2X^{-}$, red), and the doubly charged ($X^{2-}$, green) excitonic states. For electric fields $\left|F\right|>90\text{kV}/\text{cm}$, electron tunneling is fast and the neutral charge states can be observed.

Figure 4

(Color online) (a) Schematic representation of the pulse sequence, $\Delta t$ is varied during the experiment. [(b)–(g)] Time evolution of the relative intensities of the charge neutral ($X^0+2X^0$, black), the singly charged ($X^{-}+2X^{-}$, red), and the doubly charged ($X^{2-}$, green) excitonic states. The rows show measurements with constant illumination power [(b)–(d)] $P_{\text{readout}}=1\text{W}/{\text{cm}}^2$ and [(e)–(g)] $P_{\text{readout}}=15\text{W}/{\text{cm}}^2$; whereas columns depict constant electric field [(b) and (e)] $F_{\text{readout}}=21\text{kV}/\text{cm}$, [(c) and (f)] $F_{\text{readout}}=14\text{kV}/\text{cm}$, and [(d) and (g)] $F_{\text{readout}}=7\text{kV}/\text{cm}$. The solid lines are fits to the data based on the rate equation model.

Figure 5

(Color online) Charging rates (a) ${\Gamma }_{ch1}$ and (b) ${\Gamma }_{ch2}$ as a function of power density for different applied electric fields. The charging rates increase linearly with incident power for all electric fields observed. (c) Charging rates ${\Gamma }_{ch1}$ (black, squares) and ${\Gamma }_{ch2}$ (red, circles) as function of applied field normalized for incident power density. The rates vary exponentially on the applied electric field since the charging mechanism is governed by hole tunneling. The inset shows the three levels used, i.e., $N_0$, $N_1$, and $N_2$, in our rate equation analysis of the optical charging.

Figure 6

(Color online) Schematic of the spin readout scheme. The individual steps are: (a) reset by fast electron tunneling, (b) charging with resonant excitation followed by hole tunneling, (c) spin manipulation with, e.g., microwave pulses, (d) spin to charge conversion by spin-conditional resonant absorption, and (e) charge readout via nonresonant photoluminescence.