A route for the top-down fabrication of ordered ultrathin GaN nanowires.

We introduce a facile route for the top-down fabrication of ordered arrays of GaN nanowires with aspect ratios exceeding 10 and diameters below 20 nm. Highly uniform thin GaN nanowires are first obtained by lithographic patterning a bilayer Ni/SiNxhard mask, followed by a combination of dry and wet etching in KOH. The SiNxis found to work as an etch stop during wet etching, which eases reproducibility. Arrays with nanowire diameters down to (33 ± 5) nm can be achieved with a uniformity suitable for photonic applications. Next, a scheme for digital etching is demonstrated to further reduce the nanowire diameter down to 5 nm. However, nanowire breaking or bundling is observed for diameters below ≈20 nm, an effect that is associated to capillary forces acting on the nanowires during sample drying in air. Explicit calculations of the nanowire buckling states under capillary forces indicate that nanowire breaking is favored by the incomplete wetting of water on the substrate surface during drying. The observation of intense nanowire photoluminescence at room-temperature indicates good compatibility of the fabrication route with optoelectronic applications. The process can be principally applied to any GaN/SiNxnanostructures and allows regrowth after removal of the SiNxmask.

Compact acousto-optic multimode interference device in (Al,Ga)As.

Multimode interference (MMI) devices are key components in modern integrated photonic circuits. Here, we present acoustically tuned optical switches on an (Al,Ga)As platform that enable robust, compact and fast response systems improving on recently demonstrated technology. The device consists of a 2 × 2 MMI device fine-tuned in its center region by a focused surface acoustic wave (SAW) beam working in the low GHz range. In this way, we can tune the refractive index profile over a narrow modulation region and thus control the optical switching behaviour via the applied SAW intensity. Direct tuning of the MMI device avoids losses and phase errors inherent to arrayed waveguide based switches, while also reducing the dimensions of the photonic circuit.

A modified Shockley equation taking into account the multi-element nature of light emitting diodes based on nanowire ensembles.

In this work we study how the multi-element nature of light emitting diodes (LEDs) based on nanowire (NW) ensembles influences their current voltage (I-V) characteristics. We systematically address critical issues of the fabrication process that can result in significant fluctuations of the electrical properties among the individual NWs in such LEDs, paying particular attention to the planarization step. Electroluminescence (EL) maps acquired for two nominally identical NW-LEDs reveal that small processing variations can result in a large difference in the number of individual nano-devices emitting EL. The lower number of EL spots in one of the LEDs is caused by its inhomogeneous electrical properties. The I-V characteristics of this LED cannot be described well by the classical Shockley model. We are able to take into account the multi-element nature of such LEDs and fit the I-V characteristics in the forward bias regime by employing an ad hoc adjusted version of the Shockley equation. More specifically, we introduce a bias dependence of the ideality factor. The basic considerations of our model should remain valid also for other types of devices based on ensembles of interconnected p-n junctions with inhomogeneous electrical properties, regardless of the employed material system.

Acoustically driven arrayed waveguide grating.

We demonstrate compact tunable phased-array wavelength-division multiplexers driven by surface acoustic waves (SAWs) in the low GHz range. The devices comprise two couplers, which respectively split and combine the optical signal, linked by an array of single-mode waveguides (WGs). Two different layouts are presented, in which multi-mode interference couplers or free propagating regions were separately employed as couplers. The multiplexers operate on five equally distributed wavelength channels, with a spectral separation of 2 nm. A standing SAW modulates the refractive index of the arrayed WGs. Each wavelength component periodically switches paths between the output channel previously asigned by the design and the adjacent channels, at a fixed applied acoustic power. The devices were monolithically fabricated on (Al,Ga)As. A good agreement between theory and experiment is achieved.

Compatibility of the selective area growth of GaN nanowires on AlN-buffered Si substrates with the operation of light emitting diodes.

AlN layers with thicknesses between 2 and 14 nm were grown on Si(111) substrates by molecular beam epitaxy. The effect of the AlN layer thickness on the morphology and nucleation time of spontaneously formed GaN nanowires (NWs) was investigated by scanning electron microscopy and line-of-sight quadrupole mass spectrometry, respectively. We observed that the alignment of the NWs grown on these layers improves with increasing layer thickness while their nucleation time decreases. Our results show that 4 nm is the smallest thickness of the AlN layer that allows the growth of well-aligned NWs with short nucleation time. Such an AlN buffer layer was successfully employed, together with a patterned SiOx mask, for the selective-area growth (SAG) of vertical GaN NWs. In addition, we fabricated light-emitting diodes (LEDs) from NW ensembles that were grown by means of self-organization phenomena on bare and on AlN-buffered Si substrates. A careful characterization of the optoelectronic properties of the two devices showed that the performance of NW-LEDs on bare and AlN-buffered Si is similar. Electrical conduction across the AlN buffer is facilitated by a high number of grain boundaries that were revealed by transmission electron microscopy. These results demonstrate that grainy AlN buffer layers on Si are compatible both with the SAG of GaN NWs and LED operation. Therefore, this study is a first step towards the fabrication of LEDs on Si substrates based on homogeneous NW ensembles.

High-temperature, continuous-wave operation of terahertz quantum-cascade lasers with metal-metal waveguides and third-order distributed feedback.

Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (≤ 15 µm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters.

Synchronized photonic modulators driven by surface acoustic waves.

Photonic modulators are one of the most important elements of integrated photonics. We have designed, fabricated, and characterized a tunable photonic modulator consisting of two 180°-dephased output waveguide channels, driven by a surface acoustic wave in the GHz frequency range built on (Al,Ga)As. Odd multiples of the fundamental driven frequency are enabled by adjusting the applied acoustic power. A good agreement between theory and experimental results is achieved. The device can be used as a building block for more complex integrated functionalities and can be implemented in several material platforms.

Lateral distributed-feedback gratings for single-mode, high-power terahertz quantum-cascade lasers.

We report on terahertz quantum-cascade lasers (THz QCLs) based on first-order lateral distributed-feedback (lDFB) gratings, which exhibit continuous-wave operation, high output powers (>8 mW), and single-mode emission at 3.3-3.4 THz. A general method is presented to determine the coupling coefficients of lateral gratings in terms of the coupled-mode theory, which demonstrates that large coupling strengths are obtained in the presence of corrugated metal layers. The experimental spectra are in agreement with simulations of the lDFB cavities, which take into account the reflective end facets.

Splitting and lasing of whispering gallery modes in quantum dot micropillars.

We have studied the whispering gallery mode (WGM) resonances of GaAs/AlGaAs microcavity pillars containing InAs quantum dots. High quality factor WGMs are observed from a wide range of pillars with diameters from 1.2 to 50 µm. Multimode lasing with sub-milliwatt thresholds and high beta-factors approaching unity is observed under optical pumping in a 4 µm diameter pillar. Mode splitting is observed in WGMs from pillars with diameters of 5 µm, 20 µm and 50 µm.We develop a model in which the mode splitting in the larger pillars is caused by resonant scattering from the quantum dots themselves. The model explains why splittings are observed in all of the larger pillars and that the splitting decreases with increasing wavelength. Numerical simulations by COMSOL confirm that the model is plausible. This mechanism of splitting should be general for all circular resonant structures containing quantum dots such as microdisks, rings, toroids, and microspheres.

Anomalous n-type electrical behaviour of Pd-contacted CNTFET fabricated on small-diameter nanotube.

A Pd-contacted dopant-free CNTFET with small-diameter (0.57 nm) carbon nanotube showing an anomalous n-type electrical characteristic is reported for the first time. This observed behaviour is attributed to a carbon nanotube work function higher than (or close to) palladium as well as a large hole-to-electron effective mass ratio of approximately 2.5 predicted by hybridization in small-diameter nanotubes. A variation of the conduction type with temperature is also observed and is attributed to an increase of the palladium work function and decrease of the CNT work function with increasing temperature.

Quantum dot dipole orientation and excitation efficiency of micropillar modes.

The relative intensity of photonic modes in microcavity pillars with embedded self-assembled quantum dots is shown to be a sensitive function of quantum dot dipole orientation and position. This is deduced from a comparison of experiment and calculated intensities of light emission for many nominally identical pillars. We are able to obtain the overall degree of in-plane polarization of the quantum dot ensemble and also to obtain information on the degree of polarization along the growth axis.

Nuclear spin switch in semiconductor quantum dots.

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a thresholdlike enhancement or reduction of the local nuclear field by up to 3 T can be generated by varying the pumping intensity. The excitation power threshold for such a nuclear spin "switch" is found to depend on both the external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of the spin transfer from electrons to the nuclei of the dot.

Ethnopharmacological survey of plants used in the traditional treatment of hypertension and diabetes in south-eastern Morocco (Errachidia province).

This survey was undertaken in the Errachidia province in south-eastern Morocco in order to inventory the main medicinal plants used in folk medicine to treat arterial hypertension and diabetes mellitus. Four hundred individuals who knew about and/or had used the medicinal plants for the indicated diseases, including some herbal healers, were interviewed throughout different regions of the province. The inventory of medicinal plants is summarized in a synoptic table, which contains the scientific, vernacular and common name of the plant, its ecological distribution, the part of the plant and the preparation used and the therapeutic indication. Extensive investigations have brought to light 64 medicinal plants belonging to 33 families; of these, 45 are used for diabetes, 36 for hypertension, and 18 for both diseases. Of these plants, 34% grow in the wild, 44% are cultivated, and 22% are not indigenous to the area and are brought from other parts of Morocco or from outside the country. The survey shows that 78% of the patients regularly use these medicinal plants. In this region, the most frequently used plants to treat diabetes include Ajuga iva, Allium cepa, Artemisia herba-alba, Carum carvi, Lepidium sativum, Nigella sativa, Olea europaea, Peganum harmala, Phoenix dactylifera, Rosmarinus officinalis, and Zygophyllum gaetulum, and those to treat hypertension include Ajuga iva, Allium cepa, Allium sativum, Artemisia herba-alba Asso, Carum carvi, Nigella sativa, Olea europea, Rosmarinus officinalis, Origanum majorana, Peganum harmala, and Phoenix dactylifera. The local people recognize the toxic plants and are very careful in using such plants, which are Citrullus colocynthis, Datura stramonium, Nerium oleander, Nigella sativa, Peganum harmala and Zygophyllum gaetulum. Our survey shows that traditional medicine in the south-eastern Moroccan population has not only survived but has thrived in the transcultural environment and intermixture of many ethnic traditions and beliefs.

Enhanced all-optical tuning of leaky eigenmodes in photonic crystal waveguides.

We demonstrate all-optical switching in an active two-dimensional photonic crystal waveguide, observing as large as 16 nm blueshifts of a leaky eigenmode at 839 nm and switching ratios of almost 70%. These results are larger than those previously observed in similar experiments performed on passive photonic crystal waveguides; the enhancement is due to resonant photogeneration of carriers by In(0.12)Al(0.2)Ga(0.68)As quantum wells in the core of the waveguide. The effective change in the refractive index of the structure is approximately10(-2), with a rise time of approximately1 ps and a decay time of approximately10 ps, potentially allowing high-speed switching and fast modulation rates.

Comparative study of the determination of bupivacaine in human plasma by gas chromatography-mass spectrometry and high-performance liquid chromatography.

A comparison was made between high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) as methods for determining bupivacaine in human plasma. Both methods utilized pentycaine as an internal standard, were found to be linear in the range 5-320 ng and had acceptable precision and accuracy. The precision for the HPLC method was better than that for the GC-MS method. The limits of detection of the HPLC and GC-MS methods were ca. 1.0 and 0.1 ng, respectively. Good agreement between the HPLC and GC-MS methods was obtained for the analysis of samples taken from a patient receiving bupivacaine topically. For most purposes the HPLC method would be slightly better. However, for samples containing interfering peaks GC-MS provides a higher degree of resolution from such interferants.

Physiochemical and immunological comparisons between angiotensin I-converting enzymes purified from different mammalian species.

Angiotensin I-converting enzyme (ACE) was purified from lungs of pig, rat, monkey and human for comparison of its physicochemical, enzymatic and immunological properties. The protocol involved three chromatographic steps after detergent extraction, i.e. DEAE-Sphérodex ion exchange, lisinopril-Sepharose affinity and Superose 12 HPLC, plus Mono-Q HPLC for monkey ACE. Purified ACE's presented numerous homologies: in particular, closely similar specific activities, catalytic efficiencies, Km's, optimal pH and chloride activations; the molecular weights were about 170 kDa by SDS-PAGE and 320 kDa by gel-filtration on Superose 12; the isoelectric points were about 4.5-4.7. Specific polyclonal antibodies recognized the antigen (porcine ACE) as well as rat, monkey and human ACEs. In contrast, three monoclonal antibodies (F02.4.1, F01.1.3 and F03) produced against porcine ACE showed some differences: they only reacted with pig enzyme and only one (F0.2.4.1) was anticatalytic. Moreover, the cross-reactivity judged on ELISA with porcine ACE characterized different epitopes specific for the porcine enzyme. In particular, the binding of F02.4.1 was not diminished by previous treatment with saturating concentrations of synthetic competitive ACE inhibitors. Thus, the extrapolation to human of data obtained on animal models should be possible at least for pharmacological and medical trials.

Purification and analysis of lung and plasma angiotensin I-converting enzyme by high-performance liquid chromatography.

We purified angiotensin I-converting enzyme (ACE) from pig and human lung and plasma for comparison of some physicochemical properties between the endothelial membrane-bound form and the soluble form of the enzyme. After affinity chromatography on Sepharose CL-4B/lisinopril, gel-filtration HPLC on Superose 12 achieved homogeneity for both forms as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Whatever the source of ACE, the molecular weight was 300 +/- 40 kDa after calibration of Superose 12 with standard globular proteins and 172 +/- 4 kDa by SDS-PAGE, with or without reduction, a result suggesting interactions between the glycopolypeptide chain and the chromatographic gel possibly related to the overall shape and sugar content of the enzyme. Ion-exchange HPLC analysis on TSK-DEAE showed that the membrane-bound and soluble forms of ACE are not isoenzymes, although isoelectrofocusing did show that the isoelectric point of soluble ACE was lower than those of tissue ACE, suggesting a different glycosylation. No significant difference between porcine and human ACE appeared. HPLC methods seem to be of particular interest for the purification of ACE with a high yield and for the analysis of its putative differently glycosylated isoforms.

1,25-Dihydroxycholecalciferol effects on plasma calcitonin levels and calcitonin mRNA in normal or partially vitamin D-depleted rats.

The physiological effect of 1,25-(OH)2D3 on the regulation of calcitonin (CT) secretion was studied by measuring plasma CT levels and CT mRNAs extracted from thyroid glands of normal (D+) or partially vitamin D-depleted rats (D-). In both groups, acute 1,25-(OH)2D3 administration of 0.1 microgram/kg b.w. yielded an early drop in plasma calcium concentrations (around 0.6-1 mg/dl) with a maximum decrease 15 min after treatment. In spite of this hypocalcemia, a significant rise in plasma CT levels was observed within 5 min in D+ animals and within 30 min in D- animals after injection of the vitamin D metabolite. Nevertheless, the increased CT secretion was not associated with a marked and sustained rise in CT mRNA levels measured by dot-blot hybridization or CT mRNA activity evaluated by translation assay. By contrast to the observations made previously using supra-physiological doses of the vitamin D metabolites, no clear-cut effect on CT mRNA levels was found with lower doses. If we hypothesized that 1,25-(OH)2D3 plays a physiological role in CT secretion, our results suggest that this rapid control could be exerted at a post-translational level may be via an increase in the cytoplasmic ionized calcium concentration of C-cells.