N. Akopian

Optical evidence for lack of polarization in (112¯0) oriented GaN∕(AlGa)N quantum structures

We apply continuous and time resolved photoluminescence spectroscopy for studying GaN∕AlGaN multiquantum wells structures grown on nonpolar a-plane GaN templates. We found that (a) the energy of the emission from the nonpolar samples decreases slightly with the quantum well width, in a manner explained by the quantum size effect only; (b) the energy differences between the absorption and the emission peaks are independent of the well width; and (c) the decay time of the photoluminescence is only slightly dependent on the quantum well width and is quite similar to that of bulk GaN. These observations are markedly different from measurements obtained from conventional polar [0001] oriented quantum well samples. They clearly demonstrate the absence of an electric field in the nonpolar samples. Our observations are favorably compared with an eight bands k∙P model calculations.

Polarization sensitive spectroscopy of charged quantum dots

We present an experimental and theoretical study of the polarized photoluminescence spectrum of single semiconductor quantum dots in various charge states. We compare our high resolution polarization sensitive spectral measurements with a many-carrier model which we developed for this purpose. The model considers both the isotropic and anisotropic exchange interactions between all participating electron-hole pairs. With this addition, we calculate both the energies and polarizations of all optical transitions between collective, quantum dot confined charge-carrier states. We succeed in identifying most of the measured spectral lines. In particular, the lines resulting from singly, doubly, and triply negatively charged excitons and biexcitons. We demonstrate that lines emanating from evenly charged states are linearly polarized. Their polarization direction does not necessarily coincide with the traditional crystallographic direction. It depends on the shells of the single carriers, which participate in the recombination process.

Emission characteristics of quantum dots in planar microcavities

The emission properties of single quantum dots in planar microcavities are studied experimentally and theoretically. Fivefold enhanced spontaneous emission outside the microcavity is found for dots in resonance with the cavity mode, relative to detuned dots. Using high-power excitation we obtain the in-plane cavity dispersion. Near-field images of the emission show spatial distributions of several micrometers for resonant dots, which decrease in size with the detuning from resonance. These experimental findings are explained using a simple and intuitive model.

Correlated and entangled pairs of single photons from semiconductor quantum dots

Entangled photon pairs are emitted from a biexciton decay cascade of single quantum dots when spectral filtering is applied. We show this by experimentally measuring the density matrix of the polarization state of the photon pair emitted from a continuously pumped quantum dot. The matrix clearly satisfies the Peres criterion for entanglement. By applying in addition a temporal window, the quantum dot becomes an entangled light source.

Fermi edge singularity observed in GaN/AlGaN heterointerfaces

We observe sharp spectral lines, at energies which are higher than the bulk GaN band gap, in the photoluminescence and photoluminescence excitation spectra of GaN/AlGaN heterointerfaces grown by molecular beam epitaxy. The spectra and their temperature dependence are in accord with the Fermi edge singularity expected for two dimensional electron gas systems. The associated localized hole energy in the AlGaN interface side was extracted directly from the spectra.

Distilling entanglement from random cascades with partial "which path" ambiguity

We develop a framework to calculate the density matrix of a pair of photons emitted in a decay cascade with partial “which path” ambiguity. We describe an appropriate entanglement distillation scheme which works also for certain random cascades. The qualitative features of the distilled entanglement are presented in a two dimensional “phase diagram”. The theory is applied to the quantum tomography of the decay cascade of a biexciton in a semiconductor quantum dot. Agreement with experiment is obtained.

Entangled photon pairs from radiative cascades in semiconductor quantum dots

We use polarization tomography to demonstrate for the first time entanglement between photon pairs from the biexciton radiative cascade. The entangled pair violates Bellpsilas inequality and satisfies Peres criterion by more than 3 standards deviations of the experimental uncertainty.

Polarization correlations between single photons emitted by quantum dots in planar microcavities

We measured the second order correlation function between single photons emitted from optically excited single self‐assembled quantum dots embedded in planar microcavities. We clearly distinguish between radiative recombination processes in the presence of a charge in the dot to recombination in its absence. We observed strong correlations of the linear polarization states of photons emitted by the recombining neutral biexcitons and the polarization states of consecutive recombining neutral excitons.