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We present results of calculations and experiments on photo-current spectroscopy of modulation doped InAs self-assembled quantum dots. We derive an expression for the transition probability describing the dependence of the photo-current on the parameters of the dot, the number of carriers, and the polarization and energy of incident photons. The results of preliminary calculations are compared with experiments, which clearly show the dependence on photon energy and polarization. [Copyright &y& Elsevier]

Abstract: We measure the dephasing time of the exciton ground state transition in InGaAs quantum dots (QD) and quantum dot molecules (QDM) using a sensitive four-wave mixing technique. In the QDs we find experimental evidence that the dephasing time is given only by the radiative lifetime at low temperatures. We demonstrate the tunability of the radiatively limited dephasing time from 400ps up to 2ns in a series of annealed QDs with increasing energy separation of 69–330meV from the wetting layer continuum. Furthermore, the distribution of the fine-structure splitting and of the biexciton binding energy is measured. decreases from 96 to with increasing annealing temperature, indicating an improving circular symmetry of the in-plane confinement potential. The biexciton binding energy shows only a weak dependence on the confinement energy, which we attribute to a compensation between decreasing confinement and decreasing separation of electron and hole. In the QDM we measured the exciton dephasing as function of interdot barrier thickness in the temperature range from 5 to 60K. At 5K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed, showing how the quantum mechanical coupling in the molecules affects the exciton lifetime and acoustic-phonon interaction. [Copyright &y& Elsevier]

Abstract: The optical performance of InAs/InGaAsP quantum dot (QD) lasers grown on (100) InP was studied for three different material structures. The most efficient QD laser structure, having a threshold current of ∼107mA and an external differential quantum efficiency of 9.4% at room temperature, was used to form the active region of a grating-coupled external cavity tunable laser. A tuning range of 110nm was demonstrated, which was mainly limited by the mirror and internal losses of the uncoated laser diode. Rapid state-filling of the QDs was also demonstrated by observing the evolution of the spectra with increasing injected current. [Copyright &y& Elsevier]

Abstract: During the recent years semiconductor nanostructures have attracted considerable interest with respect to potential applications in quantum information processing. In particular, quantum dot molecules have been suggested to provide the building block of a quantum computer: forming quantum gates due to coherent coupling of two dots. The characteristic dependence of the splitting of ‘bonding’ and ‘anti-bonding’ states suggests coherent coupling of two InAs/GaAs quantum dots. Anti-crossings in the fine structure of excitons due to mixing of optically bright and dark states have been observed in Faraday configuration. In Voigt configuration the diamagnetic shift of the quantum dot molecule is enhanced compared to a single quantum dot. These findings altogether demonstrate the coherent coupling of exciton states in quantum dot molecules. [Copyright &y& Elsevier]

Self-assembled InAs/GaAs quantum dot molecules have been studied spectroscopically in order to obtain a comprehensive picture of the exciton states confined in them. Photoluminescence studies show a characteristic splitting of the quantum dot shells, which clearly hints at a tunnel coupling of the dots. This coupling was confirmed by exciton fine structure studies, where distinct anticrossings are observed in its magnetic field dispersion. These avoided crossings would not occur if the two dots were decoupled. Four-wave mixing studies show a strong shortening of the exciton dephasing in the molecules as compared to quantum dots which appears to be problematic for application of excitons as the genuine bits in quantum information processing. Finally, we demonstrate that the molecule coupling can be controlled by applying an electric field along the molecule axis. [Copyright &y& Elsevier]

We summarize here results of calculations and experiments on electron and valence hole states in a single pair of vertically stacked and electronically coupled InAs self-assembled quantum dots. In perfectly aligned quantum dots one can relate an electron–hole complex to a pair of entangled qubits. The information carried by individual qubit is related to the quantum dot index (isospin) of individual carrier. The quality of fabricated quantum dot molecules is identified from the exciton fine structure in a magnetic field. [Copyright &y& Elsevier]

We have performed magneto-photoluminescence spectroscopy of self-assembled In(Ga)As/GaAs single quantum dots. Both strength and orientation of the magnetic field have been varied. Symmetry of the dot structures as well as its breaking cause characteristic features in the optical spectra, which are determined by the electron–hole exchange and the Zeeman interaction of the carriers. The symmetry breaking can be either dot inherent due to geometry asymmetries or it can be obtained by applying a magnetic field with an orientation different from the dot symmetry axis. [Copyright &y& Elsevier]

The exciton states in self-assembled InAs/GaAs quantum dot molecules have been studied by optical spectroscopy and the results have been compared to detailed theoretical calculations. We demonstrate that excitons in symmetric molecule structures form entangled quantum states with the degree of entanglement controlled by tunneling and Coulomb interaction. [Copyright &y& Elsevier]

We discuss key issues related to quantum dot infrared photodetectors. These are the normal incidence response, the dark current, and the responsivity and detectivity. It is argued that the present devices have not fully demonstrated the potential advantages. The dominant infrared response in devices so far is polarized in the growth direction. The observed dark currents are several orders of magnitude higher than those for quantum well photodetectors; while ideally they should be lower. The areas that need improvements are pointed out. [Copyright &y& Elsevier]