J. Kalden

Direct observation of correlations between individual photon emission events of a microcavity laser

Lasers are recognized for coherent light emission, the onset of which is reflected in a change in the photon statistics. For many years, attempts have been made to directly measure correlations in the individual photon emission events of semiconductor lasers. Previously, the temporal decay of these correlations below or at the lasing threshold was considerably faster than could be measured with the time resolution provided by the Hanbury Brown/Twiss measurement set-up used. Here we demonstrate a measurement technique using a streak camera that overcomes this limitation and provides a record of the arrival times of individual photons. This allows us to investigate the dynamical evolution of correlations between the individual photon emission events. We apply our studies to micropillar lasers with semiconductor quantum dots as the active material, operating in the regime of cavity quantum electrodynamics. For laser resonators with a low cavity quality factor, Q, a smooth transition from photon bunching to uncorrelated emission with increasing pumping is observed; for high-Q resonators, we see a non-monotonic dependence around the threshold where quantum light emission can occur. We identify regimes of dynamical anti-bunching of photons in agreement with the predictions of a microscopic theory that includes semiconductor-specific effects.

Highly ordered catalyst-free and mask-free GaN nanorods on r-plane sapphire.

Self-organized and highly ordered GaN nanorods were grown without catalyst on r-plane sapphire using a combination of molecular beam epitaxy and metal-organic vapor-phase epitaxy. AlN nucleation centers for the nanorods were prepared by nitridation of the sapphire in a metal-organic vapor-phase epitaxy reactor, while the nanorods were grown by molecular beam epitaxy. A coalesced two-dimensional GaN layer was observed between the nanorods. The nanorods are inclined by 62 degrees towards the [Formula: see text]-directions of the a-plane GaN layer. The high degree of ordering and the structural perfection were confirmed by micro-photoluminescence measurements.

Electroluminescence from a single InGaN quantum dot in the green spectral region up to 150 K.

We present electrically driven luminescence from single InGaN quantum dots embedded into a light emitting diode structure grown by metal-organic vapor-phase epitaxy. Single sharp emission lines in the green spectral region can be identified. Temperature dependent measurements demonstrate thermal stability of the emission of a single quantum dot up to 150 K. These results are an important step towards applications like electrically driven single-photon emitters, which are a basis for applications incorporating plastic optical fibers as well as for modern concepts of free space quantum cryptography.

Fine tuning of quantum-dot pillar microcavities by focused ion beam milling

The targeted fine tuning of semiconductor pillar microcavities by postfabrication focused ion beam milling is described for the example of ZnSe-based structures with CdSe quantum dots embedded. Using the sensitive dependence of the spectral position of the modes on the cavity diameter, the modes are precisely blueshifted by a reduction of the pillar diameter with an accuracy below 100nm. The microcavities can be tuned to match the emission energy of individual quantum dots at a certain temperature, which results in a strongly enhanced luminescence intensity of the dots.

Light-emitting diode based on mask- and catalyst-free grown N-polar GaN nanorods.

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.

A CdSe quantum dot based resonant cavity light-emitting diode showing single line emission up to 90 K

A II-VI wide-bandgap resonant cavity light-emitting diode is presented. The active region consists of CdSe quantum dots embedded in ZnSSe/MgS barriers, resulting in improved quantum efficiency at elevated temperatures. The resonant cavity is formed by a 14-period bottom distributed Bragg reflector and the semiconductor to air interface on top of the structure. Temperature dependent micro-electroluminescence measurements reveal emission of a single quantum dot up to 90 K. The turn-on voltages are 6 V at 4 K and 4 V at room temperature. These results are promising for the realization of green surface-emitting devices in general, and especially for an electrically driven prospective single photon source operating at room temperature.

Optical properties of InGaN quantum dots in monolithic pillar microcavities

The integration of InGaN quantum dots into GaN-based monolithic microcavities grown by metal-organic vapor-phase epitaxy is demonstrated. Microphotoluminescence spectra reveal distinct spectrally sharp emission lines around 2.73 eV, which can be attributed to the emission of single InGaN quantum dots. The samples are structured into airpost pillar microcavities. The longitudinal and transversal mode spectra of these cavities are in good agreement with theoretical calculations based on a vectorial transfer-matrix method. Quality factors up to Q=280 have been achieved.

Optical Properties and Modal Gain of InGaN Quantum Dot Stacks

We present investigations of the optical properties of stacked InGaN quantum dot layers and demonstrate their advantage over single quantum dot layer structures. Measurements were performed on structures containing a single layer with quantum dots or threefold stacked quantum dot layers, respectively. A superlinear increase of the quantum dot related photoluminescence is detected with increasing number of quantum dot layers while other relevant GaN related spectral features are much less intensive when compared to the photoluminescence of a single quantum dot layer. The quantum dot character of the active material is verified by microphotoluminescence experiments at different temperatures. For the possible integration within optical devices in the future the threshold power density was investigated as well as the modal gain by using the variable stripe length method. As the threshold is 670 kW/cm2 at 13 K, the modal gain maximum is at 50 cm–1. In contrast to these limited total values, the modal gain per quantum dot is as high as 10–9cm–1, being comparable to the IIVI and III-As compounds. These results are a promising first step towards bright low threshold InGaN quantum dot based light emitting devices in the near future (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Modal gain and its diameter dependence in single-ZnO micro- and nanowires

Nanowires can successfully be used as building blocks for nanoscaled laser devices. Calculations predict an extremely large modal gain for nanowires made up of semiconductors such as GaN or ZnO. We determine experimentally the modal gain of single-ZnO nano- and microwires to approach 5000 cm?1 under particular size conditions. We demonstrate the distinct and sensitive dependence of the modal gain on the wire diameter and discuss optimizations for lasing of these wires.

Polarized light emission from CdSe/ZnSSe quantum-dot monolithic pillar microcavities

Small-size II-VI micropillars with asymmetrical cross sections are presented as a way to achieve more than 95% of the emitted light from single quantum dots having one particular linear polarization state. We show that the detected PL intensity of the QD is increased by optimizing the spectral overlap between QD emission and resonator mode. The polarization of the emitted light is defined by the polarization of the resonator mode. Moreover, the internal mode structure in photonic molecules is investigated by studying their far field pattern. The observed field distribution opens the possibility of coupling individual quantum dots to each other via the mediation of the electromagnetic field.