Sophie Meuret

Bright UV Single Photon Emission at Point Defects in h-BN

To date, quantum sources in the ultraviolet (UV) spectral region have been obtained only in semiconductor quantum dots. Color centers in wide bandgap materials may represent a more effective alternative. However, the quest for UV quantum emitters in bulk crystals faces the difficulty of combining an efficient UV excitation/detection optical setup with the capability of addressing individual color centers in potentially highly defective materials. In this work we overcome this limit by employing an original experimental setup coupling cathodoluminescence within a scanning transmission electron microscope to a Hanbury-Brown-Twiss intensity interferometer. We identify a new extremely bright UV single photon emitter (4.1 eV) in hexagonal boron nitride. Hyperspectral cathodoluminescence maps show a high spatial localization of the emission (∼80 nm) and a typical zero-phonon line plus phonon replica spectroscopic signature, indicating a point defect origin, most likely carbon substitutional at nitrogen sites. An additional nonsingle-photon broad emission may appear in the same spectral region, which can be attributed to intrinsic defects related to electron irradiation.

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.

InGaN nanowires with high InN molar fraction: growth, structural and optical properties.

The structural and optical properties of axial GaN/InGaN/GaN nanowire heterostructures with high InN molar fractions grown by molecular beam epitaxy have been studied at the nanoscale by a combination of electron microscopy, extended x-ray absorption fine structure and nano-cathodoluminescence techniques. InN molar fractions up to 50% have been successfully incorporated without extended defects, as evidence of nanowire potentialities for practical device realisation in such a composition range. Taking advantage of the N-polarity of the self-nucleated GaN NWs grown by molecular beam epitaxy on Si(111), the N-polar InGaN stability temperature diagram has been experimentally determined and found to extend to a higher temperature than its metal-polar counterpart. Furthermore, annealing of GaN-capped InGaN NWs up to 800 °C has been found to result in a 20 times increase of photoluminescence intensity, which is assigned to point defect curing.

A polarity-driven nanometric luminescence asymmetry in AlN/GaN heterostructures

Group III Nitrides nanowires are well suited materials for the design of light emitting devices. The internal electric field created by spontaneaous and piezoelectric polarizations in these materials poses some difficulties, but also possible solutions, towards this goal. Here, we report on the high spatial asymmetry of the cathodoluminescence intensity across a GaN quantum well embedded in an AlN nanowire, when a 60 keV, 1 nm wide electron beam is scanned over this heterostructure. This asymmetry is remarkable between positions at different sides of the quantum well. We interpret this asymmetry as originating from the different drift directions of carriers due to the internal electric field. This interpretation is corroborated by the direct determination of the polarity with convergent beam electron diffraction. A precise knowledge of hole mobility and diffusion coefficients would allow an estimate of the electric field in the AlN segment of the nanowire.

Efficient green emission from wurtzite AlxIn1-xP nanowires

Direct band gap III–V semiconductors, emitting efficiently in the amber–green region of the visible spectrum, are still missing, causing loss in efficiency in light emitting diodes operating in this region, a phenomenon known as the “green gap”. Novel geometries and crystal symmetries however show strong promise in overcoming this limit. Here we develop a novel material system, consisting of wurtzite AlxIn1–xP nanowires, which is predicted to have a direct band gap in the green region. The nanowires are grown with selective area metalorganic vapor phase epitaxy and show wurtzite crystal purity from transmission electron microscopy. We show strong light emission at room temperature between the near-infrared 875 nm (1.42 eV) and the “pure green” 555 nm (2.23 eV). We investigate the band structure of wurtzite AlxIn1–xP using time-resolved and temperature-dependent photoluminescence measurements and compare the experimental results with density functional theory simulations, obtaining excellent agreement. Our work paves the way for high-efficiency green light emitting diodes based on wurtzite III-phosphide nanowires.

Nanoscale Relative Emission Efficiency Mapping Using Cathodoluminescence g(2) Imaging

Cathodoluminescence (CL) imaging spectroscopy provides two-dimensional optical excitation images of photonic nanostructures with a deep-subwavelength spatial resolution. So far, CL imaging was unable to provide a direct measurement of the excitation and emission probabilities of photonic nanostructures in a spatially resolved manner. Here, we demonstrate that by mapping the cathodoluminescence autocorrelation function g(2) together with the CL spectral distribution the excitation and emission rates can be disentangled at every excitation position. We use InGaN/GaN quantum wells in GaN nanowires with diameters in the range 200–500 nm as a model system to test our new g(2) mapping methodology and find characteristic differences in excitation and emission rates both between wires and within wires. Strong differences in the average CL intensity between the wires are the result of differences in the emission efficiencies. At the highest spatial resolution, intensity variations observed within wires are the result of excitation rates that vary with the nanoscale geometry of the structures. The fact that strong spatial variations observed in the CL intensity are not only uniquely linked to variations in emission efficiency but also linked to excitation efficiency has profound implications for the interpretation of the CL data for nanostructured geometries in general.

Correlative electron energy loss spectroscopy and cathodoluminescence spectroscopy on three-dimensional plasmonic split ring resonators

We present the surface plasmon resonance modes in three-dimensional (3D) upright split ring resonators (SRR) as studied by correlative cathodoluminescence (CL) spectroscopy in a scanning electron microscope (SEM) and electron energy loss spectroscopy (EELS) in a transmission electron microscope. We discuss the challenges inherent in studying the plasmon modes of a 3D nanostructure and how meeting these challenges benefits from the complementary use of EELS and SEM-CL. With the use of EELS, we detect a strong first order mode in the SRR; with comparison to simulations, we are able to identify this as the well-known magnetic dipole moment of the SRR. Combining the EELS spectra with SEM-CL on the same structure reveals the higher order modes present in this 3D nanostructure, which we link to the coupling and hybridization of rim modes present in the two upright hollow pillars of the split ring.

Optical Spectroscopy at High Spatial Resolution with Fast Electrons

1. Laboratoire de Physique des Solides, UMR 8502 CNRS and Université Paris-Sud, Orsay, France 2. Laboratoire Aimé Cotton, CNRS, Université Paris Sud and ENS Paris-Saclay, Orsay, France 3. CEA, INAC-PHELIQS, « Nanophysique est semiconducteurs group », Grenoble, France 4. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan 5. Instituto de Fiısica “Gleb Wataghin” Universidade Estadual de Campinas – Unicamp, Campinas, Sao Paulo, Brazil

Probing Plasmon-NV0 Coupling at the Nanometer Scale with Photons and Fast Electrons

The local density of optical states governs an emitters’ lifetime and quantum yield through the Purcell effect. It can be modified by a surface plasmon electromagnetic field, but such a field has a spatial extension limited to a few hundreds of nanometers, complicating the use of optical methods to spatially probe emitter–plasmon coupling. Here we show that a combination of electron-based imaging, spectroscopies, and photon-based correlation spectroscopy enables measurement of the Purcell effect with nanometer and nanosecond spatiotemporal resolutions. Due to the large variability of radiative lifetimes of emitters in nanoparticles we relied on a statistical approach to probe the coupling between nitrogen-vacancy centers in nanodiamonds and surface plasmons in silver nanocubes. We quantified the Purcell effect by measuring the nitrogen-vacancy excited state lifetimes in a large number of either isolated nanodiamonds or nanodiamond-nanocube dimers and demonstrated a significant lifetime reduction for dimers.

High-spatial-resolution detection of new single-photon emission sources

There is currently a large scientific effort being made to identify, characterize, and control new single-photon emission sources (SPEs), with the aim of extending their spectral range.1 SPEs— light sources that are emitted as single particles or photons—are intrinsically small and, to a first approximation, are necessarily a two-level system.2 Furthermore, the typical size of active structures in modern light emitters is in the nanometer range (e.g., for quantum wells) or smaller (for point defects).2 The identification of new SPEs therefore requires a technique that permits high spatial resolution and optical characterization. Although pure optical or scanning probe techniques can be used to achieve high spatial resolution and optical characterization of SPEs, it is not possible to meet both these requirements with a single technique. It is thought that hexagonal boron nitride (h-BN) is a promising candidate for extending the spectral region of available SPEs into the far-UV (presenting an excitonic emission at 5.8eV).1 Moreover, common h-BN samples produce more complex emissions than have generally been attributed to the presence of structural defects.3 Despite a large number of experimental studies, it has not yet been possible to attribute specific emission features for the proper identification of defective structures. Light emission from these structural defects occurs at extremely localized regions, with lateral sizes of about 80nm and the localized regions indicate the possible existence of single defects.4, 5 The large spatial distribution is attributed to charge diffusion caused by the lack of potential barriers.6, 7 The occurrence of Figure 1. Schematic diagram of the scanning transmission electron microscopy-cathodoluminescence (STEM-CL) experimental setup. A focused electron beam, with diameter of 1nm, is scanned over a sample. The light emitted from the excited sample is then collected by a parabolic mirror and coupled through a window to the outside of the microscope vacuum. The emitted light can be analyzed by using either an optical spectrometer or a Hanbury Brown and Twiss (HBT) interferometer (see Figure 2). ADF: Annular dark-field imaging.