Magneto optics of single photons emitted from single InAs/GaAs self-assembled quantum dots in a planar microcavity

We fully characterize the fine spectral structure of neutral and negatively charged single microcavity quantum dot excitons, using polarization-sensitive magneto-photoluminescence spectroscopy. We show that the microcavity allows the simultaneous detection of both the bright and dark excitons using Faraday configuration. Thus, we were able to fully determine the fine structure and the g-factors of the neutral and negatively charged single exciton states within the same single quantum dot. Our measurements are in excellent agreement with novel, many carrier model calculations, which take into account Coulomb and exchange interactions among all the confined e–h pair states.     

Fine structure of highly charged excitons in semiconductor quantum dots

An exciton in a symmetric semiconductor quantum dot has two possible states, one dark and one bright, split in energy by the electron-hole exchange interaction. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton. By measuring these splittings, and also those from the singly charged and doubly charged biexcitons, all on the same quantum dot, we show how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.     

Voltage control of the spin dynamics of an exciton in a semiconductor quantum dot

We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L = 2 becomes a bright exciton with L = 1. The spin-flip process is revealed in the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.     

Emission from neutral and charged excitons in a single quantum dot in a magnetic field

We report on optical spectroscopy of self-assembled InAs quantum dots in a magnetic field. We describe how we measure the emission characteristics of a single quantum dot (QD) in high magnetic fields at low temperature using a miniature, fiber-based confocal microscope. Example results are presented on a QD whose charge can be controlled using a field-effect device. For the uncharged, singly and doubly charged excitons we find a diamagnetism and the spin Zeeman effect. In contrast, for the triply-charged exciton we find a fundamentally different behavior. Anti-crossings in magnetic field imply that confined states of the QD are hybridized with Landau-like levels associated with the two-dimensional continuum.     

Probing single-charge fluctuations at a GaAs/AlAs interface using laser spectroscopy on a nearby InGaAs quantum dot

We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with ±5 nm resolution. The results identify and quantify the main source of charge noise in the commonly used optical field-effect devices.     

Voltage-controlled linewidth of excitonic transitions in a single self-assembled quantum dot

We report electron and hole tunnelling phenomena in a single self-assembled quantum dot as a function of the applied electric field. We use absorption spectroscopy which allows us to measure excitonic transitions under conditions where optical recombination cannot be observed due to the high, ionizing, electric field.     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

Controlling the interaction of electron and nuclear spins in a tunnel-coupled quantum dot

We present a technique for manipulating the nuclear spins and the emission polarization from a single optically active quantum dot. When the quantum dot is tunnel coupled to a Fermi sea, we have discovered a natural cycle in which an electron spin is repeatedly created with resonant optical excitation. The spontaneous emission polarization and the nuclear spin polarization exhibit a bistability. For a σ(+) pump, the emission switches from σ(+) to σ(-) at a particular detuning of the laser. Simultaneously, the nuclear spin polarization switches from positive to negative. Away from the bistability, the nuclear spin polarization can be changed continuously from negative to positive, allowing precise control via the laser wavelength.     

Zn(L‐proline)2 as an efficient and reusable catalyst for the multi‐component synthesis of pyran‐annulated heterocyclic compounds

We have developed a simple, straightforward and highly efficient multi‐component one‐pot synthesis of 2‐amino‐3‐cyano‐4H‐pyrans and pyran‐annulated heterocyclic compounds. Zn(L‐proline)₂ has been utilized as a mild and efficient Lewis acid catalyst for the three‐component reaction of aromatic aldehydes, malononitrile and enolizable C‐H acids or activated phenols to synthesize pyran‐annulated scaffolds. 3‐Methyl‐1‐phenyl‐2‐pyrazolin‐5‐one, 4‐hydroxycoumarine, dimedone, 4‐hydroxy‐6‐methyl‐2‐pyrone, resorcinol, 1‐naphthol and 2‐naphthol were used as activated C‐H acids. The products were isolated in high yields via vacuum filtration and without using any chromatography. The Zn(L‐proline)₂ was recovered and reused in another reaction (three times) without significant loss of activity.     

Entangled photon pairs from semiconductor quantum dots

Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the "which path" information contained in the photons' color and that the remanent information in the quantum dot degrees of freedom is negligible.