Charge and spin control of ultrafast electron and hole dynamics in single CdSe/ZnSe quantum dots

Abstract

We study the dynamics of photoexcited electrons and holes in single negatively charged CdSe/ZnSe quantum dots with two-color femtosecond pump-probe spectroscopy. An initial characterization of the energy level structure is performed at low temperatures and magnetic fields of up to 5 T. Emission and absorption resonances are assigned to specific transitions between few-fermion states by a theoretical model based on a configuration interaction approach. To analyze the dynamics of individual charge carriers, we initialize the quantum system into excited trion states with defined energy and spin. Subsequently, the time-dependent occupation of the trion ground state is monitored by spectrally resolved differential transmission measurements. We observe subpicosecond dynamics for a hole excited to the D shell. The energy dependence of this D-to-S shell intraband transition is investigated in quantum dots of varying size. Excitation of an electron-hole pair in the respective p shells leads to the formation of singlet and triplet spin configurations. Relaxation of the p-shell singlet is observed to occur on a time scale of a few picoseconds. Pumping of p-shell triplet transitions opens up two pathways with distinctly different scattering times. These processes are shown to be governed by the mixing of singlet and triplet states due to exchange interactions enabling simultaneous electron and hole spin flips. To isolate the relaxation channels, we align the spin of the residual electron by a magnetic field and employ laser pulses of defined helicity. This step provides ultrafast preparation of a fully inverted trion ground state of the quantum dot with near unity probability, enabling deterministic addition of a single photon to the probe pulse. Therefore our experiments represent a significant step towards using single quantum emitters with well-controled inversion to manipulate the photon statistics of ultrafast light pulses.

Electrical control of optically pumped electron spin in a single GaAs/AlAs quantum dot fabricated by nanohole infilling

“…Second, there is no measurable delay between the contrapolarized and copolarized emissions for a bias voltage of 0.87 V (compared to the 70-ps delay of the contrapolarized emission at 0.78 V). The fraction of emission that is contrapolarized, however, remains significant A fast hole spin relaxation time in the P orbital would, however, lead to rapid filling of both the |T 0 , ⇑ P h and |T 0 , ⇓ P h states after excitation with positive circularly polarized light [21], providing a possible route to lower the degree of circular polarization. This hole spin relaxation in the P shell prior to thermal relaxation to the S shell would also contribute to decreasing the amplitude of the negative degree of circular polarization at 0.78 V [see Fig.…”

Controlling the phase diagram of finite spin- 12 chains by tuning the boundary interactions

“…Furthermore, most of the existing quantum technologies are based on few-body or few-level systems (see Refs. [16][17][18][19] to only name a few of the latest works). How to utilize the many-body features (such as quantum phase transitions, elementary excitations, and other collective phenomena) is still under hot debate (e.g., Refs.…”

Multicolor femtosecond pump-probe system with single-electron sensitivity at low temperatures and high magnetic fields

“…10(c)]. 8,10 For positive time delays, we observe a striking feature with two spectral components X − σ − and X − σ + at energies of the Zeeman components of X − . The corresponding PL spectrum is depicted in Fig.…”

“…3). 10,17 Ideally, the time shift tS should be set to a value short compared to the interpulse distance 1/frep (25 ns in our system) but long enough to make sure that the time delay in the reference phase always remains negative when varying the time delay between the pump and the probe.…”

“…[7][8][9] Moreover, we have already demonstrated quantum-dot based operation of a single-photon addition to a bright multimode coherent state. 10 While we are currently developing strategies to extend such functionality efficiently toward few-photon coherent inputs, 11 detailed investigations on single quantum systems are often challenged by weak signals potentially distorted by thermal artifacts. 8,10 Here, we present an instrumental platform specifically designed for high-precision three-color pump-probe spectroscopy on single quantum emitters inside a low-temperature magnetic cryostat.…”