Excited states of neutral donor bound excitons in GaN

We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygen—the key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200 μeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task.EC/H2020/749565/EU/Heat Transport and its Effects on the Performance of Nanostructured, Photonic Materials/PhotoHeatEffectDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Demented versus non-demented very old inpatients: the same comorbidities but poorer functional and nutritional status

Background: demented patients have been reported to be healthier than other old people of the same age. Objectives: to assess comorbid conditions, functional and nutritional status in medically ill hospitalised patients with normal cognition or affected by dementia of various causes and severities, or mild cognitive impairment (MCI). Design and Setting: a prospective study was carried out, between January and December 2004, in the Rehabilitation and Geriatric Hospital (HOGER). Methods: activities of daily living (ADL), instrumental activities of daily living (IADL) and mini nutritional assessment (MNA) scores were assessed as a function of the status of the patient two weeks before admission to hospital. On admission, cognitive status was assessed by a systematic battery of neuropsychological tests, comorbid conditions were assessed with the Charlson comorbidity index (CCI), and body mass index (BMI) and functional independence measure (FIM) were determined. BMI and FIM were also determined on discharge. Results: we studied 349 patients (mean age 85.2±6.7; 76% women): 161 (46.1%) cognitively normal, 37 (10.6%) with MCI and 151 (43.3%) demented (61 Alzheimer's disease (AD), 62 mixed dementia (MD) and 17 vascular dementia (VaD)). ADL, IADL, FIM and MNA scores on admission decreased with cognitive status, regardless of the type of dementia. Functionality at discharge remained significantly lower in demented patients than in other patients. CCI was high and similar in all three groups (mean 4.6±2.7). Patients with VaD had poorer health than other demented patients, with a higher average comorbidity score, more frequent hypertension, stroke and hyperlipidaemia. Comorbidity did not increase with severity levels of dementia. Conclusions: in this cohort of very old inpatients, demented patients, non-demented patients and patients with MCI had similar levels of comorbidity, but demented patients had a poorer functional and nutritional statu

High-temperature Mott transition in wide-band-gap semiconductor quantum wells

The crossover from an exciton gas to an electron-hole plasma is studied in a GaN/(Al,Ga)N single quantum well by means of combined time-resolved and continuous-wave photoluminescence measurements. The two-dimensional Mott transition is found to be of continuous type and to be accompanied by a characteristic modification of the quantum well emission spectrum. Beyond the critical density, the latter is strongly influenced by band-gap renormalization and Fermi filling of continuum states. Owing to the large binding energy of excitons in III-nitride heterostructures, their injection-induced dissociation could be tracked over a wide range of temperatures, i.e., from 4 to 150K. Various criteria defining the Mott transition are examined, which, however, do not lead to any clear trend with rising temperature: the critical carrier density remains invariant around 1012cm−2

Functional abdominal pain disorders and patient- and parent- reported outcomes in children with inflammatory bowel disease in remission

BACKGROUND: Chronic abdominal pain occurs frequently in pediatric patients with inflammatory bowel disease (IBD) in remission. AIMS: To assess the prevalence and factors associated with Functional Abdominal Pain Disorders among IBD children in remission (IBD-FAPD). METHODS: Patients with IBD for > 1 year, in clinical remission for ≥ 3 months were recruited from a National IBD network. IBD-FAPDs were assessed using the Rome III questionnaire criteria. Patient- or parent- reported outcomes were assessed. RESULTS: Among 102 included patients, 57 (56%) were boys, mean age (DS) was 15.0 (± 2.0) years and 75 (74%) had Crohn's disease. Twenty-two patients (22%) had at least one Functional Gastrointestinal Disorder among which 17 had at least one IBD-FAPD. Past severity of disease or treatments received and level of remission were not significantly associated with IBD-FAPD. Patients with IBD-FAPD reported more fatigue (peds-FACIT-F: 35.9 ± 9.8 vs. 43.0 ± 6.9, p = 0.01) and a lower HR-QoL (IMPACT III: 76.5 ± 9.6 vs. 81.6 ± 9.2, p = 0.04) than patients without FAPD, and their parents had higher levels of State and Trait anxiety than the other parents. CONCLUSIONS: Prevalence of IBD-FAPD was 17%. IBD-FAPD was not associated with past severity of disease, but with fatigue and lower HR-QoL

III-nitride photonic cavities

Owing to their wide direct bandgap tenability, III-nitride (III-N) compound semiconductors have been proven instrumental in the development of blue light-emitting diodes that led to the so-called solid-state lighting revolution and blue laser diodes that are used for optical data storage. Beyond such conventional optoelectronic devices, in this review, we explore the progress made in the past 15 years with this low refractive index material family for the realization of microdisks as well as 2D and 1D photonic crystal (PhC) membrane cavities. Critical aspects related to their design and fabrication are first highlighted. Then, the optical properties of passive PhC structures designed for near-infrared such as their quality factor and their mode volume are addressed. Additional challenges dealing with fabrication pertaining to structures designed for shorter wavelengths, namely the visible to ultraviolet spectral range, are also critically reviewed and analyzed. Various applications ranging from second and third harmonic generation to microlasers and nanolasers are then discussed. Finally, forthcoming challenges and novel fields of application of III-N photonic cavities are commented

A novel class of coherent light emitters: polariton lasers

The specificities of polariton lasers-a new generation of coherent light-emitting devices where the relaxation of the bosonic quasiparticles responsible for the light emission, the polaritons, is stimulated whereas the photon emission process issued from the radiative decay of those polaritons is purely spontaneous-are reviewed. It is shown that the practical realization of such devices able to operate at room temperature would most likely rely on wide bandgap semiconductors (inorganic or organic ones) exhibiting highly stable excitons/polaritons. Using III-nitrides, the most advanced of the candidates to date, an electrically driven multiple quantum well polariton laser would display a threshold current density down to similar to 100 A cm(-2), a value more than one order of magnitude lower than that of state of the art GaN-based edge-emitting laser diodes. For the sake of comparison, the main features of optically pumped polariton lasers available that differ from those of their conventional counterparts, namely vertical cavity surface-emitting lasers, are also discussed

Probing alloy formation using different excitonic species: The particular case of InGaN

Since the 1960s, alloys are grouped into two classes, featuring bound states in the bandgap (I - GaP:N, ZnTe:O, etc.) or additional, non-discrete, band states (II - SiGe, GaAsP, InGaAs, etc.). Therefore, one can observe either a rich and informative zoo of excitons bound to isoelectronic impurities (I), or the typical bandedge emission of a semiconductor that shifts and broadens with rising isoelectronic doping (II). Because no such strongly localized excitons are found in the photoluminescence (PL) spectra of the investigated bulk In(x)Ga(1-x)N epilayers ( 0 ≤ x ≤ 2.4%, 100 nm thick, growth on bulk GaN substrates), we suggest to utilize shallow impurities as a tool to study the distribution of isoelectronic centers. By micro-PL, we directly observe an entire hierarchy of bound excitons related to silicon-indium complexes as individual, energetically sharp emission lines appear (full width at half-maximum ≈ 300μeV). In order to exemplify our approach, we determine nanoscopic parameters of the InGaN alloy like the exciton diffusion length. We expect that our spectroscopic analysis represents a general pathway for studying mixed crystal alloys associated to class II, which approaches the high level of spectroscopic sophistication evidenced in literature for class I alloys

Continuous Wave Blue Lasing in III-Nitride Nanobeam Cavity on Silicon

IIIV photonics on silicon is an active and promising research area. Here, we demonstrate room-temperature (RT) lasing in short-wavelength III-nitride photonic crystal nanobeam cavities grown on silicon featuring a single InGaN quantum well (QW). In the low-absorption QW region, high quality factors in excess of 104 are measured, while RT blue lasing under continuous-wave optical pumping is reported in the high-absorption wavelength range, hence the high QW gain region. Lasing characteristics are well accounted for by the large spontaneous emission coupling factor (beta > 0.8) inherent to the nanobeam geometry and the large InGaN QW material gain. Our work illustrates the high potential of III-nitrides on silicon for the realization of low power nanophotonic devices with a reduced footprint that would be of prime interest for fundamental lightmatter interaction studies and a variety of lab-on-a-chip applications including biophotonics