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.

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

A strong increase of spontaneous radiative emission from colloidally synthesized CdSe/CdS/PMMA hybrid particles is achieved when manipulated into plasmonic bullseye resonators with the tip of an atomic force microscope (AFM). This type of antenna provides a broadband resonance, which may be precisely matched to the exciton ground state energy in the inorganic cores. Statistically analyzing the spectral photoluminescence (PL) of a large number of individual coupled and uncoupled CdSe/CdS/PMMA quantum dots, we find an order of magnitude of intensity enhancement due to the Purcell effect. Time-resolved PL shows a commensurate increase of the spontaneous emission rate with radiative lifetimes below 230 ps for the bright exciton transition. The combination of AFM and PL imaging allows for sub-200 nm localization of the particle position inside the plasmonic antenna. This capability unveils a different coupling behavior of dark excitonic states: even stronger PL enhancement occurs at positions with maximum spatial gradient of the nearfield, effectively adding a dipolar component to original quadrupole transitions. The broadband maximization of light-matter interaction provided by our nanoengineered compound systems enables an attractive class of future experiments in ultrafast quantum optics.

A robust algorithm based on cross-correlations and lucky imaging reliably allows the correction of spectrally diffused datasets. This step enables the resolution-limited analysis of the emission fine structure of semiconductor quantum dots (QDs). Bright and dark excitonic transitions are resolved with optimum signal-to-noise ratio, allowing for a precise determination of the angular direction of linear polarization of the different lines. The angular phases between polarization directions are intrinsically connected to the orientations of emission dipoles. This fact provides a tool for accurate numerical computation of the azimuth ϕ and polar angle θ of the QD with respect to the optical axis. Our in-situ characterization of QD fine structure and orientation represents a precise and non-invasive method without requiring specialized equipment beyond a standard luminescence setup. In this way, important information is provided whenever efficient coupling of a quantum emitter to the electromagnetic field is targeted by various nano-and micro-optic strategies.

We present an ultrafast spectroscopy system designed for temporal and spectral resolution of transient transmission changes after excitation of single electrons in solid-state quantum structures. The system is designed for optimum long-term stability, offering the option of hands-off operation over several days. Pump and probe pulses are generated in a versatile Er:fiber laser system where visible photon energies may be tuned independently from 1.90 eV to 2.51 eV in three parallel branches. Bandwidth-limited pulse durations between 100 fs and 10 ps are available. The solid-state quantum systems under investigation are mounted in a closed-cycle superconducting magnet cryostat providing temperatures down to 1.6 K and magnetic fields of up to 9 T. The free-standing cryomagnet is coupled to the laser system by means of a highbandwidth active beam steering unit to eliminate residual low-frequency mechanical vibrations of the pulse tube coolers. High-NA objective lenses inside the sample chamber are employed for focusing femtosecond laser pulses onto the sample and recollection of the transmission signal. The transmitted probe light is dispersed in a grating monochromator equipped with a liquid nitrogen-cooled CCD camera, enabling a frame rate of 559 Hz. In order to eliminate spurious background effects due to low-frequency changes in the thermal equilibrium of the sample, we operate with a lock-in scheme where, instead of the pump amplitude, the pump-probe timing is modulated. This feature is provided without any mechanical action by an electro-optic timing unit inside the femtosecond Er:fiber system. The performance of the instrument is tested with spectrally resolved pump-probe measurements on a single negatively charged CdSe/ZnSe quantum dot under a magnetic field of 9 T. Selective initialization and readout of charge and spin states is carried out via two different femtosecond laser pulses. High-quality results on subpicosecond intraband relaxation dynamics after single-electron excitation motivate a broad variety of future experiments in ultrafast quantum optics and few-fermion quantum dynamics.