Plasmonic mode coupling and thin film sensing in metal-insulator-metal structures.

Optical sensors based on surface plasmon resonance (SPR) in the attenuated total reflection (ATR) configuration in layered media have attracted considerable attention over the past decades owing to their ability of label free sensing in biomolecular interaction analysis, and highly sensitive detection of changes in refractive index and thickness, i.e. the optical thickness, of thin film adsorbates (thin film sensing). Furthermore, SPR is highly sensitive to the refractive index of the medium adjacent to the bare metal, and it allows for bulk sensing as well. When deposited at the metal/air interface, an adsorbed layer disturbs the highly localized, i.e. bound, wave at this interface and changes the plasmon resonance to allow for sensing in angular or wavelength interrogation and intensity measurement modes. A high degree of sensitivity is required for precise and efficient sensing, especially for biomolecular interaction analysis for early stage diagnostics; and besides conventional SPR (CSPR), several other configurations have been developed in recent years targeting sensitivity, including long-range SPR (LRSPR) and waveguide-coupled SPR (WGSPR) observed in MIM structures, referred here to by MIM modes, resulting from the coupling of SPRs at I/M interfaces, and Fano-type resonances occurring from broad and sharp modes coupling in layered structures. In our previous research, we demonstrated that MIM is better than CSPR for bulk sensing, and in this paper, we show that CSPR is better than MIM for thin film sensing for thicknesses of the sensing layer (SL) larger than 10 nm. We discuss and compare the sensitivity of CSPR and MIM for thin film sensing by using both experiments and theoretical calculations based on rigorous electromagnetic (EM) theory. We discuss in detail MIM modes coupling and anti-crossing, and we show that when a thin film adsorbate, i.e. a SL), is deposited on top of the outermost-layer of an optimized MIM structure, it modifies the characteristics of the coupled modes of the structure, and it reduces the electric field, both inside the SL and at the SL/air interface, and as a result, it decreases the sensitivity of the MIM versus the CSPR sensor. Our work is of critical importance to plasmonic mode coupling using MIM configurations, as well as to optical bio- and chemical-sensing.

Surface Enhanced Visible Absorption of Dye Molecules in the Near-Field of Gold Nanoparticles.

Surface enhanced absorption is a plasmonic effect parenting to surface enhanced fluorescence and Raman scattering, and it was clearly reported to occur in the infrared region of the spectrum of light. In this paper, we unambiguously show that it also occurs in the visible region of the spectrum by using a dye; i.e. an azo-dye, which exhibits a good light absorption in that region, and gold nanoparticles, which act as plasmonic nanoantennas that capture and re-radiate light, when the azo-dyes and the nanoparticles are incorporated in the bulk of solid films of polymer. In such a configuration, it is possible to use a dye concentration much larger than that of the nanoparticles and absorption path lengths much larger than those of the molecularly thin layers used in surface enhanced effects studies. In addition, the dye undergoes shape and orientation change; i.e. isomerization and reorientation, upon polarized light absorption; and the observation of surface enhanced visible absorption is done by two separate experiments; i.e. UV-visible absorption spectroscopy and photo-induced birefringence, since the signals detected from both experiments are directly proportional to the extinction coefficient of the dye. Both the dye's absorption and photoorientation are enhanced by the presence of the nanoparticles.

Light-tunable Fano resonance in metal-dielectric multilayer structures.

High-Q optical Fano resonances realized in a variety of plasmonic nanostructures and metamaterials are very much promising for the development of new potent photonic devices, such as optical sensors and switches. One of the key issues in the development is to establish ways to effectively modulate the Fano resonance by external perturbations. Dynamic tuning of the Fano resonance applying the mechanical stress and electric fields has already been demonstrated. Here, we demonstrate another way of tuning, i.e., photo-tuning of the Fano resonance. We use a simple metal-dielectric multilayer structure that exhibits a sharp Fano resonance originating from coupling between a surface plasmon polariton mode and a planar waveguide mode. Using a dielectric waveguide doped with azo dye molecules that undergo photoisomerization, we succeeded in shifting the Fano resonance thorough photo-modulation of the propagation constant of the waveguide mode. The present work demonstrates the feasibility of photo-tuning of the Fano resonance and opens a new avenue towards potential applications of the Fano resonance.

Long-range surface plasmons supported by a bilayer metallic structure for sensing applications.

We show, both theoretically and experimentally, that long-range surface plasmons (LRSPs) are supported by asymmetric structure, consisting of a thin silver/gold bilayer metallic film sandwiched between a magnesium fluoride (i.e., MgF2) buffer layer and a sensing medium (water). The geometrical parameters of the structure are optimized to yield efficient excitation of LRSPs by using transfer matrix method based on Fresnel reflection. The excitation of LRSPs was performed by using a custom-made automated optical setup based on angular interrogation with the precision of 0.01°. We demonstrate that the bimetallic asymmetric structure achieves better minimum reflectivity resolution than monometallic (gold) asymmetric structure. Finally, figures of merit are compared for bimetallic, monometallic, and conventional SPR structures, and we found that the bimetallic asymmetric structure provides a higher figure of merit; e.g., more than double for monometallic LRSP configuration and 8 times as compared to the conventional surface plasmon resonance sensor.

[Pediatric multiple sclerosis: is it different from the adult form?]. / La SEP de l'enfant: est-elle différente de celle de l'adulte ?

INTRODUCTION: Multiple sclerosis (MS) is not uncommon in children. The aim of this study was to compare early onset MS (EOMS) with adult onset MS (AOMS). METHODS: A retrospective study including MS cases between 1997 and 2010. EOMS was defined by age at MS onset<18years. Data were collected using the EDMUS database (European Database of Multiple Sclerosis) including: sex, age at onset, disease duration, EDSS, score after relapse. The MSSS and the Progression Index were calculated. Patients with disease duration less than one year were excluded. MS symptoms at onset and at further relapses were also noted. These parameters were compared between the EOMS and the AOMS groups. RESULTS: Two hundred fifty-nine cases were included including 31 EOMS (11.96%). The mean follow-up was 96months. The relapsing-remittent form was significantly more frequent in the pediatric group (94% vs 79%). Mean EDSS and MSSS scores and the percentage of fast progressors (MSSS>5) were lower in the EOMS group. Analysis of neurological symptoms at the first MS attack and further neurological events showed a lower frequency of gait disturbances, motor symptoms and bladder symptoms in the EOMS group compared with the AOMS group. The 10-year mean EDSS score was 1.9 for EOMS and 4.1 for AOMS, after 25years it was 4.5, and 7.27 respectively. CONCLUSION: This study highlights the relative frequency of EOMS in our MS population. However, different severity scores showed less disability progression in EOMS patients compared with AOMS patient; irreversible disability was reached at an early age.

[Multiple sclerosis without treatment: characteristic features of 70 untreated patients in a cohort of 1187 patients]. / Sclérose en plaques sans traitement de fond : analyse des caractéristiques de 70 patients non traités sur une cohorte de 1187 patients.

INTRODUCTION: First treatment protocols for multiple sclerosis (MS) have been established in France for over 15 years. Presently, a large majority of patients are treated, or has been treated in the past years, with one or more disease modifying drugs. However, despite a long-term follow-up, a certain patients remain untreated. OBJECTIVE: The aim of this study was to determine in a large cohort the proportion of patients who never received any medication for MS and to analyze their profiles and reasons for no treatment. PATIENTS AND METHODS: We studied a cohort of 1187 MS patients followed in a French (Alsace) cohort, all included in the EDMUS (European database for Multiple Sclerosis). We then performed a retrospective study on patients followed from at least 5 years (724 patients) and retained those who had never received MS medication. RESULTS: Seventy patients (9.8% of the whole cohort) corresponded to the inclusion criteria. They were 57 women and 13 men, mean age 54.9 years (range 33-81). The mean duration of the disease was 20.6 years (range 5-56). MS was of relapsing remitting type in 46 patients (65.7%), primary progressive in 11 patients (15.7%) and secondary progressive in 13 patients (18.6%). In patients with relapsing remitting disease, the annualized relapse rate was 0.33 (range 0.08-1). Mean EDSS was 3.4 after a mean follow-up of 20.6 years. Progression index was 0.16 without any differences between progressive and relapsing remitting forms (0.15 and 0.16 respectively). Reasons for not treating were: lack of disease activity (65.8%), very slow disease progression (10%), patient's initial decision followed by very slow progression (14.2%), contraindication for treatment in patients with longstanding progressive disease (10%). There were also patients (4.3%) whose initially well-stabilized disease recently became active again, leading to reconsideration about starting treatment. CONCLUSION: After a mean follow-up of 20 years, the proportion of treatment-free patients was around 10%. Most of these patients had a relapsing remitting form with a low rate of relapse or a progressive form with very slow progression.

[Methylprednisolone pulse in treatment of childhood chronic inflammatory demyelinating polyneuropathy]. / Place des bolus de méthylprednisolone dans le traitement des polyradiculonévrites chroniques de l'enfant.

Chronic inflammatory demyelinating polyneuropathy (CIDP) in children is rare and treatment is based primarily on intravenous immunoglobulins or oral corticosteroids. Boluses of methylprednisolone (MP) are a possible alternative. We report 3 cases of CIDP in children with good outcome after MP pulse therapy. One male (7 years of age) and 2 females (4 and 5 years of age) presented with recurring episodes of functional impotence of both lower limbs and walking impairment, partially reversible without treatment. Clinical and electrophysiological data and the analysis of the cerebrospinal fluid were compatible with CIDP. MP pulses were administered: the total number of pulses varied from 5 to 8, very satisfactory progression on the clinical and electrophysiological pattern was noted, without recurrence in the 3 cases. Childhood CIDP presents clinical, electrophysiological outcome, and prognostic particularities, recurring readily, and the outcome is good. Boluses of MP are an alternative for treatment of these neuropathies in childhood.

The anisotropic nanomovement of azo-polymers.

Nanoscale polymer movement is induced by a tightly focused laser beam in an azo-polymer film just at the diffraction limit of light. The deformation pattern that is produced by photoisomerization of the azo dye is strongly dependent on the incident laser polarization and the longitudinal focus position of the laser beam along the optical axis. The anisotropic photo-fluidity of the polymer film and the optical gradient force played important roles in the light induced polymer movement. We also explored the limits of the size of the photo-induced deformation, and we found that the deformation depends on the laser intensity and the exposure time. The smallest deformation size achieved was 200 nm in full width of half maximum; a value which is nearly equal to the size of the diffraction limited laser spot.