Near-field observation of surface plasmon polaritons launched by V-shaped nanorods on a gold surface.

By scanning near-field optical microscopy, we study the propagation of surface waves created by V-shaped nanorods deposited on a gold thin film. The nanorods launch surface plasmon polaritons that interfere with the incident light, producing interference patterns. The angle of the V-shaped rods varies from 110° to 180° (straight rod). We observe that the near-field distribution strongly depends on the angle of the V. For angles close to straight rods, a hot spot is visible, whereas for a narrower angle, the surface plasmon waves are launched in specific directions. The experimental results are in good qualitative agreement with numerical simulations performed with a simple analytical model that considers the rods as a sum of isolated surface plasmon sources.

Mapping and quantifying electric and magnetic dipole luminescence at the nanoscale.

We report on an experimental technique to quantify the relative importance of electric and magnetic dipole luminescence from a single nanosource in structured environments. By attaching a Eu^{3+}-doped nanocrystal to a near-field scanning optical microscope tip, we map the branching ratios associated with two electric dipole and one magnetic dipole transitions in three dimensions on a gold stripe. The relative weights of the electric and magnetic radiative local density of states can be recovered quantitatively, based on a multilevel model. This paves the way towards the full electric and magnetic characterization of nanostructures for the control of single emitter luminescence.

Role of thermal heating on the voltage induced insulator-metal transition in VO2.

We show that the main mechanism for the dc voltage or dc current induced insulator-metal transition in vanadium dioxide VO(2) is due to local Joule heating and not a purely electronic effect. This "tour de force" experiment was accomplished by using the fluorescence spectra of rare-earth doped micron sized particles as local temperature sensors. As the insulator-metal transition is induced by a dc voltage or dc current, the local temperature reaches the transition temperature indicating that Joule heating plays a predominant role. This has critical implications for the understanding of the dc voltage or dc current induced insulator-metal transition and has a direct impact on applications which use dc voltage or dc current to externally drive the transition.

Near-field analysis of surface waves launched at nanoslit apertures.

With the aim of analyzing the properties of the waves that are scattered by nanoslits on metallic surfaces, we provide a direct observation of the near-field in a slit-doublet experiment at optical wavelengths. We show that two distinct waves are involved: a surface plasmon polariton and another wave with a free-space character. From the recorded data, we have extracted the amplitudes and phases of these waves, their damping characteristic lengths and their relative weights as a function of the separation distance from the slit. The analysis is fully supported by a quantitative agreement with vector-theory computational results.

Analysis of the measured signals in apertureless near-field optical microscopy.

We present an analytical model able to explain the optical signal recorded during our experimental approach curves in the infrared at a wavelength lambda=10.6 microm, with a home-made apertureless near-field scanning optical microscope ANSOM. This model uses classical electrodynamics to calculate the scattering cross section of the oscillating tip, considered as a dipole, and its dielectric image in the sample as a function of the tip-sample separation from the near-field to the far-field regime. The dipoles are placed in a non-uniform electric field because of the standing wave arising from the interference between the incident and the specular laser beams. We also added a background field coming from a scatterer on the surface in order to account for zeroing of the optical signal for particular tip-sample separation and interference patterns.

Apertureless near-field scanning optical microscope working with or without laser source.

An apertureless near-field scanning optical microscope (ANSOM), used indifferent configurations, is presented. Our versatile home-made setup, based on a sharp tungsten tip glued onto a quartz tuning fork and working in tapping mode, allows to perform imaging over a broad spectral range. We have recorded optical images in the visible (wavelength, lambda = 655 nm) and in the infrared (lambda = 10.6 microm), proving that the setup routinely achieves an optical resolution of <50 nm regardless of the illumination wavelength. We have also shown optical images recorded in the visible (lambda = 655 nm) in an inverted configuration where the tip does not perturb the focused spot of the illumination laser. Approach curves as well as image profiles have revealed that on demodulating the optical signal at higher harmonics, we can obtain an effective probe sharpening which results in an improvement of the resolution. Finally, we have presented optical images recorded in the infrared without any illumination, that is, the usual laser source is replaced by a simple heating of the sample. This has shown that the ANSOM can be used as a near-field thermal optical microscope (NTOM) to probe the near field generated by the thermal emission of the sample.

Apertureless scanning near-field fluorescence microscopy in liquids.

We show that apertureless scanning near-field optical microscopes that use sharp vibrating conical tips can be operated in liquid environments. We have investigated the damping of the tip oscillation as a function of its shape and as a function of its depth under the liquid surface. The degradation of the quality factor from 150 in air down to 15 in liquid does not impede to perform topographic and optical measurements with a very good sensitivity. As an example of application, we present near-field fluorescence images of dye-doped polystyrene spheres immersed in a liquid.

Upconversion fluorescence imaging of erbium-doped fluoride glass particles by apertureless SNOM.

We have imaged fluorescent erbium-doped fluoride glass particles by apertureless scanning near-field optical microscopy. The optical excitation has been performed at lambda = 780 nm whereas fluorescence emission has been collected around lambda = 550 nm. This process, called upconversion by energy transfer, involves two erbium ions and is not linear. Besides an improvement of the lateral resolution, we have observed on some particles that the fluorescence is not homogeneously distributed, but is rather localized in some zones brighter than others. By making tip approach curves, we have also observed that the amount of fluorescence intensity scattered by the tip is increasing when the tip is approaching the sample surface.

Apertureless scanning near-field magneto-optical microscopy of magnetic multilayers.

We imaged magnetic domains in Pt/Co/Pt multilayers using an apertureless scanning near-field optical microscope operating in reflection mode. As the magneto-optical effects are weak for this kind of structure, a polarization modulation technique with a photoelastic modulator was used to reveal the contrast between magnetic domains. In the case of a Pt/Co/Pt trilayer structure, a strong improvement in lateral resolution is observed compared with far-field magneto-optical images and good sensitivity is achieved. In the case of a Pt/[Co/Pt]Pt multilayer structure, stripe domains of 200 nm width could be resolved, in good agreement with images obtained by magnetic force microscopy on the same structure.

Polarization effects in apertureless scanning near-field optical microscopy: an experimental study.

Strong electric-field enhancements at the apex of a tungsten tip illuminated by an external light source were recently predicted theoretically. We present an experimental study of the dependence of this effect on the polarization angle of the incident light. It is shown that the intensity of the light scattered by the tungsten tip of an apertureless scanning near-field optical microscope is 2 orders of magnitude higher when the incident light is p polarized than when it is s polarized. This experimental result is in good agreement with theoretical predictions and provides an easy way to test the quality of the tips.

Combined nitric oxide inhalation, prone positioning and almitrine infusion improve oxygenation in severe ARDS.

PURPOSE: To determine the efficacy and side effects of prone positioning (PP) and nitric oxide (NO) inhalation, alone, associated, or combined with i.v. almitrine for the treatment of hypoxaemia in severe acute respiratory distress syndrome (ARDS). METHODS: Over a period of 20 months, 27 consecutive critically ill patients with severe ARDS (Murray score > 2.5, PaO2/FiO2 < 170 after alveolar recruitment) were prospectively and randomly included. They inhaled NO for two hours at concentrations of 5 and 10 ppm for one hour each (H0-H2). One hour later, they were returned to the prone position for four hours (H3-H7). During the last two hours in this position (H5-H7), they were assigned to further inhalation of 10 ppm NO (Group B, n = 9) or to no further inhalation (Group A, n = 9). In group C (n = 9), the procedure for group B was combined with perfusion of 16 mg.kg-1.min-1 almitrine throughout the study. RESULTS: Compared with control values, two hours NO inhalation improves PaO2/FiO2 and shunt effect by +28% and -9%, PP by +88% and -27%, PP + almitrine by +132% and -28%, NO + almitrine by +153 and -28%, PP + NO by +94% and -29%, NO + PP + almitrine by +327 and -48%. NO inhalation reduces pulmonary vascular resistance. Other haemodynamic parameters remain unchanged, whatever the treatment. NO inhalation improves PaO2/FiO2 by over 20% in 50% of the patients and PP is effective in 78% of the cases. CONCLUSION: Prone Position improves PaO2/FiO2 significantly more than NO alone but less than PP + almitrine or NO + almitrine. The best results are obtained with the association of NO + Prone position + Almitrine.

Haemodynamic conditions enhancing gas embolism after venous injury during laparoscopy: a study in pigs.

This study was designed to determine the conditions that promote carbon dioxide embolism after venous injury during laparoscopy in pigs. Injury to an iliac vein was filmed during laparoscopy in the presence of a pneumoperitoneum created at increasing pressures from 0 to 30 mm Hg in 5-mm Hg increments. At intraperitoneal pressures less than 20 mm Hg, there was a parallel increase in femoral venous pressures, resulting in haemorrhage, with persistent blood flow to the inferior vena cava. At intraperitoneal pressures of 20-30 mm Hg, there was collapse of the femoral vein, occurring earlier in the presence of hypovolaemia. Between these two states (haemorrhage and collapse), there was a point of equilibrium which allowed retrograde venous penetration of carbon dioxide bubbles. During release of the pneumoperitoneum, these bubbles were exteriorized through the area of the injury, but some passed into the inferior vena cava where their presence was detected by an oesophageal Doppler probe.

Effect of injection pressure on the success rate of peribulbar block.

The aim of this prospective double-blind study was to evaluate the effect of the pressure on injection of local anaesthetic during peribulbar anaesthesia. Fifty patients scheduled for cataract surgery under peribulbar anaesthesia, with a two injection site technique, were randomly assigned to receive a mixture of 5 ml of etidocaine 1%, 4 ml of bupivacaine 0.5% and hyaluronidase 50 IU, injected under a constant pressure of either 140 g.cm-2 (group 1) or 250 g.cm-2 (group 2). After orbital compression, the degree of akinesia of the extra-ocular and orbicular muscles was graded by clinical assessment. A significantly higher rate of satisfactory akinesia of the extra-ocular muscles was found in group 2 (72% vs 28% in group 1) (p < 0.01). No significant relationship was found between the time taken to administer the anaesthetic mixture at constant pressure and the quality of the block.

Enhanced nociceptive behaviour following conditioning injection of formalin in the perioral area of the rat.

The possible existence of long-term modifications in response to a transient nociceptive conditioning stimulation was investigated in the rat in three experiments. (1) A nociceptive conditioning stimulus was delivered in the form of a s.c. formalin injection (conditioning injection) in the left upper lip. Evaluation of the nociceptive behaviour triggered by another formalin injection (testing injection) made in the controlateral right upper lip was carried out in distinct groups of rats 7, 14 or 28 days after the conditioning. An enhanced nociceptive response at day 7 and 14 and a return to the baseline at day 28 were observed. (2) A similar protocol was developed with formalin used for both conditioning and testing but an anaesthetic blockade of the infraorbital nerve was performed just before the conditioning injection to suppress the initial barrage. The change observed at day 7 was suppressed by the nerve block. (3) A conditioning nociceptive stimulus was applied either ipsilaterally to the right lower lip or to the tail. An increased nociceptive response was observed when the conditioning stimulus was applied to the same side as the test stimulus but no increase in the formalin test response was detected when the conditioning stimulus was applied to the tail. These results indicated that, after a single formalin injection in the left upper lip, a hyperexcitability developed that depended on the initial barrage, lasted for at least 2 weeks, was no longer present at 4 weeks and might rely on a segmental mechanism. The hypothesis of a central sensitization triggered by an initial barrage and maintained by an ongoing input induced from the periphery is discussed.

Sustained hyperalgesia can be induced in the rat by a single formalin injection and depends on the initial nociceptive inputs.

The present study was undertaken to find out if a hyperalgesia can be observed 1 week after a conditioning stimulation of the orofacial area of the rat. Sprague-Dawley rats received a left infraorbital infiltration with either bupivacaine or saline and then, 30 min thereafter, an injection of either saline or 10% formalin solution in the left upper lip. Four groups of animals were thus made up depending on their conditioning treatment. Seven days later, an algesimetric test initiated by a contralateral orofacial formalin injection was carried out. The duration of lip rubbing was significantly increased in formalin-conditioned groups. The hyperalgesia observed at 7 days was suppressed by an infraorbital nerve block carried out at the time of the conditioning stimulus. These results tend to indicate that a nociceptive message of short duration induces a trace in the central nervous system which can be retained for 1 week.

Intrathecal midazolam versus intrathecal morphine in orofacial nociception: an experimental study in rats.

The antinociceptive properties of intrathecal midazolam (15 and 30 micrograms) and intrathecal morphine (30 micrograms) were compared using an adaptation of the formalin test to the orofacial region in rats. Both midazolam and morphine were effective in reducing the nociceptive response of the second phase of this biphasic pain test. Only midazolam (30 micrograms) reduced also the first phase. These results confirm the existence of analgesic properties of midazolam in the case of long-lasting pain.

Masseteric facilitation induced by electrical stimulation of rat orofacial tissues.

The effects of a conditioning electrical shock applied to the periodontium of the lower incisor or the glabrous area of the lower lip on the jaw-closing reflex in the anesthetized, non-paralysed rat were studied. The masseteric reflex was triggered by stimulation of the mesencephalic nucleus as a test shock and was recorded from the masseter muscle. There was facilitation of the jaw-jerk reflex, which culminated at an interval of 10-15 ms between the conditioning and the test shocks. This facilitation was not suppressed by digastric excision or by blocking a possible rebound closing reflex evoked by jaw opening. No inhibitory influence was observed. This facilitatory effects relies on an A alpha input and on cell bodies making up the mesencephalic nucleus. The direct excitatory electrical events observed in the masseter muscle after periodontal or labial stimulation proved to be due to the diffusion of the bioelectrical activity generated in the neighbouring jaw-opening muscles.

Localisation of the first-order neurone of the jaw opening reflex elicited by periodontal stimulation.

The localisation of the first-order neurone of the jaw-opening reflex (JOR), provoked by periodontal stimulation, was investigated in the rat. A section of the mandibular part of the trigeminal ganglion was carried out without impairing the motor root. It suppressed the reflex triggered by the stimulation of the lower incisor. Bilateral destruction of the mesencephalic nucleus and tract does not modify the JOR. These results suggest that the first-order neurone of the reflex is located in the trigeminal ganglion.