B. Gayral

Strong Purcell effect for InAs quantum boxes in three-dimensional solid-state microcavities

A strong enhancement of the spontaneous emission rate (Purcell effect) has been observed for self-assembled InAs/GaAs quantum boxes inserted in GaAs-based pillar microcavities (/spl times/5) and microdisks (/spl times/15) using time-resolved as well as c.w. photoluminescence experiments. We show that the magnitude of the Purcell effect can be quantitatively understood by considering both the Purcell figure of merit F/sub p/ of such cavities (F/sub p//spl Gt/1) and the spatial/spectral distribution of the inhomogeneous collection of atom-like emitters. These results open the way to the development of single-photon devices such as photon-guns or photon-turnstiles, able to emit photons one-by-one in a deterministic way.

Cavity-quantum electrodynamics using a single InAs quantum dot in a microdisk structure

We investigate cavity-quantum electrodynamics (QED) effects in an all-semiconductor nanostructure by tuning a single self-assembled InAs quantum dot (QD) into resonance with a high quality factor microdisk whispering gallery mode (WGM). The stronger temperature dependence of the QD single-exciton (1X) resonance allows us to change the relative energy of the WGM and the 1X transitions by varying the sample temperature. The two coupled resonances exhibit crossing behavior due to the weak coupling cavity-QED regime. We demonstrate exciton lifetime reduction by 6 due to the Purcell effect by tuning the QD into resonance with the WGM. Our experiments also show that single-exciton lifetime is independent of temperature up to 50 K.

InAs quantum dots: artificial atoms for solid-state cavity-quantum electrodynamics

Abstract Spontaneous emission (SE) control in the solid state has been extensively studied for about 10 years as a route toward optoelectronic devices with improved properties or novel functionalities. Thanks to their atom-like emission and to their relatively large oscillator strength, InGaAs/GaAs semiconductor quantum boxes (QBs) open revolutionary opportunities for the application of the concepts of cavity quantum electrodynamics in monolithic microcavities. Such emitters experience in particular a very large enhancement of their spontaneous emission rate (Purcell effect) when inserted in high Q /low volume micropillars (×5) or microdisks (×15). This result opens unique opportunities for the development of solid-state photon guns, able to emit single-photon pulses in a deterministic way. QBs in microdisks should also allow in the near future to achieve a strong coupling regime for a single solid-state emitter.

Photon correlation spectroscopy of a single quantum dot

It is by now widely accepted that various quantum dot ~QD! structures exhibit features in transport 1,2 or optical spectroscopy 3‐5 that indicate full three-dimensional confinement of carriers. Identification of QD’s as artificial atoms has been strengthened by the recent observation of strong photon antibunching in single-exciton emission, 6,7 which is the typical signature of an anharmonic quantum system: after a photon is emitted from a single two-level ~anharmonic! emitter, the system is necessarily in the radiatively inactive ground state and a second photon cannot be emitted immediately after the first one. Even though the coherence properties of QD single-exciton emission closely follow those of atoms, the overall spectral features of single QD’s are significantly more complicated. Since the size of QD’s is roughly two orders of magnitude larger than those of atoms, multiparticle excitations give rise to emission peaks with energies comparable to that of a single exciton. Of primary importance in QD spectroscopy is the biexciton state, which corresponds to a doubly-excited QD with completely filled lowest electronand hole-energy levels. When the biexciton state decays by radiative recombination, the final state is a single-exciton state and the generated photon is shifted as compared to the single exciton emission due to Coulomb interaction between the carriers. Biexciton emission in QD spectroscopy has been traditionally identified using the ~quadratic! pump-power dependence of the corresponding peak. In this paper, we demonstrate that photon-correlation measurements provide a powerful tool for characterizing the multiexciton spectral features of QD’s. Our measurements provide a strong support for the identification of a biexciton emission peak, by demonstrating its strong correlations with the subsequent single-exciton emission. We observe that biexciton intensity autocorrelation exhibits bunching together with antibunching or only antibunching under continuous-wave ~cw! excitation depending on the excitation level. In contrast, we find strong antibunching under pulsed excitation. The large difference between the levels of antibunching under continuous wave and pulsed excitations points out to the importance of excitation mechanism and the role of free carriers in QD physics. The lack of polarization correlation between biexciton and single-exciton emissions indicates that spin dephasing is likely to play a key role under nonresonant excitation. We also observe that a third emission peak in QD spectra exhibits strong correlations with both exciton and biexciton fluorescence: we argue that these correlation signatures suggest the identification of this additional line as a charged-exciton emission. Our self-assembled InAs QD’s were grown by molecularbeam epitaxy using the partially covered island technique. 8 Growth resulted in typically lens-shaped QD’s with a base diameter of 40‐50 nm and a height of 3 nm, having their single-excitonic emissions between 925 nm and 975 nm in the spectrum. In our sample, the QD’s were embedded in the center of a 200-nm-thick GaAs microdisk structure located above a 0.5-mm-thick Al0.65Ga0.35As post. The diameter of the disks was 5-mm and the average number of QD’s within the disks was less than one. Details of the microdisk processing can be found elsewhere. 9 Our experimental setup consisted of a combination of a low-temperature diffractionlimited scanning optical microscope and a Hanbury Brown

Structural and optical properties of InGaN/GaN nanowire heterostructures grown by PA-MBE

The structural and optical properties of InGaN/GaN nanowire heterostructures grown by plasma-assisted molecular beam epitaxy have been studied using a combination of transmission electron microscopy, electron tomography and photoluminescence spectroscopy. It is found that, depending on In content, the strain relaxation of InGaN may be elastic or plastic. Elastic relaxation results in a pronounced radial In content gradient. Plastic relaxation is associated with the formation of misfit dislocations at the InGaN/GaN interface or with cracks in the InGaN nanowire section. In all cases, a GaN shell was formed around the InGaN core, which is assigned to differences in In and Ga diffusion mean free paths.

Exciton and Biexciton Luminescence from Single GaN/AlN Quantum Dots in Nanowires

We present a microphotoluminescence study of single GaN/AlN quantum dots embedded in single nanowires. At low excitation power, single exciton lines with full width at half-maximum as narrow as 1 meV are observed. The study of the excitation power dependence of the emission allows us to identify the biexciton transitions with binding energies ranging from 20 to 40 meV.

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection. At 100 mA DC current, a 1 mm2 chip that integrates around 10(7) nanowires emits an output power on the order of 10 µW. However, the emission of the nanowire-based LED shows a spotty and polychromatic emission. By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire. Detailed μ-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation. By combining both μ-electroluminescent and μ-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs.

Optical properties of GaN quantum dots grown on nonpolar (11-20) SiC by molecular-beam epitaxy

We report on nonpolar GaN quantum dots embedded in AlN, grown on (11-20) 6H–SiC by plasma-assisted molecular-beam epitaxy. These dots are aligned in the growth plane and present a constant aspect ratio of 10. Their optical properties were studied as a function of GaN coverage. Especially, the variation of their emission energy as compared to that of (0001) GaN quantum dots is a clear fingerprint of the reduced internal electric field present in these nonpolar nanostructures. Time-resolved spectroscopy confirmed this result by revealing lifetimes in the few 100 ps range in contrast to the much longer ones obtained for the (0001) GaN quantum dots.

Growth mechanism and properties of InGaN insertions in GaN nanowires.

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base.

Probing exciton localization in nonpolar Ga N ∕ Al N quantum dots by single-dot optical spectroscopy

We present an optical spectroscopy study of nonpolar