Composition variation in Al-based dilute nitride alloys using apertureless scanning near-field optical microscopy.

We use apertureless scanning near-field optical microscopy to study the phase separation in chemical beam epitaxy grown Al0.1Ga0.9NxAs1-x alloys. Pits attributed to nitrogen-clustering observed on the Al0.1Ga0.9NxAs1-x surface grown at 420 °C become larger at higher growth temperatures, and 3D islands appear on the surface at 565 °C. Atomic force microscopy phase measurements reveal a composition difference between the islands and the pits, whereas the sample grown at 420 °C appears to be homogeneous. Confocal Raman spectra show that all the N atoms are bonded to Al instead of Ga. Using apertureless scanning near-field optical microscopy, the luminescence of a gold tip is mapped over the surface of the sample grown at 565 °C. We extract the shift of the tip's surface plasmon resonance and determine the variation in the refractive index between the islands and the pits to be close to 0.2. Numerical simulations of the tip luminescence while in contact with the sample predict a similar variation of ∼0.3 in the refractive indices between AlGaAs islands and AlN pits, a substantially smaller value than the difference in the bulk refractive indices of the two media (∼1.8), which we attribute to a convolution of material distribution in an uneven topography. The excellent agreement between simulation and experiments supports the hypothesis of nitrogen-clustering in the pits.

Fine structure of excitons in InAs/GaAs coupled auantum dots: a sensitive test of electronic coupling.

Exciton fine structure in InAs/GaAs coupled quantum dots has been studied by photoluminescence spectroscopy in magnetic fields up to 8 T. Pronounced anticrossings and mixings of optically bright and dark states as functions of magnetic field are seen. A theoretical treatment of the mixing of the excitonic states has been developed, and it traces observed features to structural asymmetries. These results provide direct evidence for coherent coupling of excitons in quantum dot molecules.

Exciton dephasing in quantum dot molecules.

We have measured the exciton dephasing time in InAs/GaAs quantum dot molecules having different interdot barrier thicknesses in the temperature range from 5 to 60 K, using a highly sensitive four-wave mixing heterodyne technique. At 5 K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed. These results show how the quantum-mechanical coupling of the electronic wave functions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.

Coupling and entangling of quantum states in quantum dot molecules.

We demonstrate coupling and entangling of quantum states in a pair of vertically aligned, self-assembled quantum dots by studying the emission of an interacting electron-hole pair (exciton) in a single dot molecule as a function of the separation between the dots. An interaction-induced energy splitting of the exciton is observed that exceeds 30 millielectron volts for a dot layer separation of 4 nanometers. The results are interpreted by mapping the tunneling of a particle in a double dot to the problem of a single spin. The electron-hole complex is shown to be equivalent to entangled states of two interacting spins.

Hidden symmetries in the energy levels of excitonic 'artificial atoms'

Quantum dots or 'artificial atoms' are of fundamental and technological interest--for example, quantum dots may form the basis of new generations of lasers. The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure). Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot. The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms. We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle. In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system. Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states.

Red-Emitting Semiconductor Quantum Dot Lasers

Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at approximately 707 nanometers was observed at 77 kelvin with a threshold current of 175 milliamperes for a 60-micrometer by 400-micrometer broad area laser. An external efficiency of approximately 8.5 percent at low temperature and a peak power greater than 200 milliwatts demonstrate the good size distribution and high gain in these high-quality QDs.

Spatially resolved visible luminescence of self-assembled semiconductor quantum dots.

Ensembles of defect-free InAIAs islands of ultrasmall dimensions embedded in AIGaAs have been grown by molecular beam epitaxy. Cathodoluminescence was used to directly image the spatial distribution of the quantum dots by mapping their luminescence and to spectrally resolve very sharp peaks from small groups of dots, thus providing experimental verification for the discrete density of states in a zero-dimensional quantum structure. Visible luminescence is produced by different nominal compositions of InxAI(1-x)As-AIyGa(1-y)As.