Plasmon Waveguide Resonance Research Articles

Interaction of full-length Tau with negatively charged lipid membranes leads to polymorphic aggregates.

The Tau protein is implicated in various diseases collectively known as tauopathies, including Alzheimer's disease and frontotemporal dementia. The precise mechanism underlying Tau pathogenicity remains elusive. Recently, the role of lipids has garnered interest due to their implications in Tau aggregation, secretion, uptake, and pathogenic dysregulation. Previous investigations have highlighted critical aspects: (i) Tau's tendency to aggregate into fibers when interacting with negatively charged lipids, (ii) its ability to form structured species upon contact with anionic membranes, and (iii) the potential disruption of the membrane upon Tau binding. In this study, we examine the disease-associated P301L mutation of the 2N4R isoform of Tau and its effects on membranes composed on phosphatidylserine (PS) lipids. Aggregation studies and liposome leakage assays demonstrate Tau's ability to bind to anionic lipid vesicles, leading to membrane disruption. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) reveals the accumulation of Tau on the membrane surface without protein insertion, structuration, or lipid removal. Plasmon waveguide resonance (PWR) demonstrates a strong binding of Tau on PS bilayers with an apparent Kd in the micromolar range, indicating the deposition of a thick protein layer. Atomic force microscopy (AFM) real-time imaging allows the observation of partial lipid solubilization and the deposition of polymorphic aggregates in the form of thick patches and fibrillary structures resembling amyloid fibers, which could grow from a combination of extracted anionic phospholipids from the membrane and Tau protein. This study deepens our understanding of full-length Tau's multifaceted interactions with lipids, shedding light on potential mechanisms leading to the formation of pathogenic Tau assemblies.

Active Differential Fiber Coupled Plasmon Waveguide Resonance Sensor Based on the Mode Competition Effect

We proposed an active differential intensity (DI) fiber coupled plasmon waveguide resonance (CPWR) sensor based on the mode competition effect in the C-band, aiming to enhance the sensitivity. The sensing head is a fiber probe with a sensing layer of ITO/Au/ITO/TiO2 film, enabling the excitation of CPWR in the C-band. The narrow CPWR spectrum allows DI interrogation by tracking the intensity of light at two wavelengths. The fiber probe is inserted into a dual-wavelength fiber laser to adjust the intra-cavity loss at the two lasing wavelengths. By using the differential modulation of the reflectivity from the refractive index variations, the mode competition effect is triggered. The powers at two lasing wavelengths change oppositely, enlarging the power difference and then resulting in enhanced sensitivity. The average sensitivity is up to 5702 dB/RIUs, which is 10 times higher than that of conventional DI interrogation. The sensitivity enhancement mechanism based on the mode competition effect provides a new technical approach for enhancing the sensitivity of DI SPR sensors.

Plasmon waveguide resonance for tag-free selective Raman detection of biochemical composition in single-cell

Highly selective and non-radioactive Raman detection of the biochemical composition of living single-cell is a key method of evaluating complex biological reaction processes. Conventional selective Raman detection relies on tagging technology to separate the Raman signals from the different cell structures, which involves both a complex synthesis process and a long incubation time. To address this, we present a tag-free Raman selective detection with plasmon waveguide resonance (PWR). PWR biochemical composition detection exhibits a high longitudinal spatial resolution based on the surface and bulk response of low- and high-order modes. Hence, compared with conventional selective Raman detection with tagging technology, it can distinguish Raman signals from cell structures more effectively, without an antibody and label, providing effective tag-free selective detection with high spatial resolution. In addition, rapid selective detection can be achieved using the PWR Raman scattering biochemical composition detection, thereby improving the stability of a living single cell without a long incubation time. Moreover, the electromagnetic field of the evanescent field can be significantly amplified by the proposed PWR biochemical composition detection, which also enhances the Raman signal with high sensitivity without requiring a complicated fabrication process. The proposed PWR has potential applications in drug release and living-cell analysis.

Coupling plasmon-waveguide resonance and multiple plasma modes in hyperbolic metamaterials for high-performance sensing

A sensor based on plasmon-waveguide resonance (PWR) exhibits an impressive narrow linewidth and has attracted extensive attention in plasmon label-free sensing. However, the low surface electric field intensity limits the detection ability of biomolecules, where the refractive index changes are restricted at the sensor surface. In this study, we study the coupling of PWR and multiple plasma modes in a hyperbolic metamaterial (HMM), combining narrow linewidth and electric field enhancement advantages. The PWR-HMM sensor includes a gold film, lossless dielectric layer, and metal/dielectric multilayer HMM array composed of 2-layer Au/Al2O3 stacks. The evanescent field of PWR is used to excite multiple plasma modes in the HMM. The figure of merit of the proposed structure reaches 5417/RIU owing to the existence of lossless dielectric layer, which is 11.7 times than the conventional gold film structure. The maximum bulk sensitivity of the PWR-HMM sensor was 43 000 nm/RIU. In comparison with PWR, the surface electric field intensity and the surface sensitivity of PWR-HMM increase by four and two times, respectively. Furthermore, comparing the sensing performance of the PWR-HMM sensor and PWR-nanoparticle (NP) sensor (coupling PWR and localized surface plasmon resonance), it was found that PWR-HMM has 20% higher surface sensitivity than the PWR-NP. A sensing mechanism coupling PWR and multiple plasma modes in the HMMs opens a gate to significantly improve the PWR sensors performance, which is expected to be used to resolve urgent issues in biological, medical and clinical applications.

Label-free detection of transport kinetics and inhibitor binding of membrane transport proteins with a two-mode plasmonic sensor

Membrane transporters translocate molecules across cell membranes and are involved in a variety of cellular processes. However, there is still no label-free technique for measuring the transport kinetics of non-charged species through membrane transporters. Here, we develop a new method based on surface plasmon resonance (SPR) and plasmon-waveguide resonance (PWR) to measure transporter function and ligand binding simultaneously. The PWR/SPR-combined sensor chip comprises a silica layer with micron-sized pores on a thin gold film, and the geometry allows the appearance of two independent plasmonic modes. The electromagnetic enhancement regions of the SPR-associated mode and the PWR-associated mode are localized inside the pore directly above the gold film and outside the pore above the silica layer, respectively, and can be used to detect refractive index or environmental changes in the corresponding regions. We deposited cell membrane vesicles onto the sensor chip to form pore-spanning cell membrane patches containing transporters of interest. Only species transported across the membrane can enter into the pore region and be detected by the SPR-associated mode, while ligand binding above the membrane can be detected by the PWR-associated mode. We used glucose transporters in HeLa cell membranes to demonstrate that the platform can be used to study how inhibitor binding can influence transport behaviors. More importantly, we successfully measured the glucose transport kinetics across the membrane. The obtained kinetic parameters of glucose transporters were comparable to those reported in studies using radiolabeled glucose, suggesting that this new label-free method can accurately characterize the membrane transport kinetics.

The Optimization of Metal Nitride Coupled Plasmon Waveguide Resonance Sensors Using a Genetic Algorithm for Sensing the Thickness and Refractive Index of Diamond-like Carbon Thin Films

We theoretically designed the Kretschmann configuration coupled plasmon-waveguide resonance (CPWR) sensors, composed of thin films of metal nitrides. The thicknesses of the layers of the CPWR sensors were optimized using a genetic algorithm. The optimized CPWR sensors were applied to simultaneously measure the thickness and refractive index (RI) of diamond-like carbon (DLC) films. The field profiles and the sensitivity of the CPWR sensors in response to thin DLC films were studied using the finite-different time-domain technique and the transfer matrix method. The genetic algorithm method predicted that the two-mode CPWR sensors could simultaneously analyze the thickness and RI of the DLC films as thin as 1.0 nm at a wavelength of 1550 nm. The simulations showed that the angular sensitivity toward the refractive index changes of the DLC films of the optimized CPWR sensors was comparable to that of traditional CPWR sensors.

All-Optical Logic Gates Using a Plasmonic MIM Waveguide and Elliptical Ring Resonator

All-optical logic gates OR, XOR, AND, and NOT based on two-dimensional (2D) plasmonic metal-insulator-metal (MIM) coupled with an elliptical ring resonator (ERR) are presented, simulated, and investigated by using the numerical method of the FEM (finite elements method). The results are compared and validated with the finite difference time domain (FDTD) method. The proposed logic gates are achieved with the same structure using the constructive and destructive optical interferences between a control signal and input signal(s). Their characterization was mainly done for two spectral regions, visible and near-infrared. A high-intensity contrast ratio (CR) between the logic states (“1” and “0”) can be achieved (28 dB) at these spectral regions. We introduce a new parameter, “gap-threshold ratio (GTR),” to characterize the gap between the maximum and minimum of the transmitted signal intensity for all logic gates. The suggested value of transmission threshold between logic “0” and logic “1” states is \(T_{th}=0.2\). A comparison of the two parameters, CR and GTR, with previous works shows that the proposed structure gives very good results for all logic gates configurations. The proposed all-optical logic gates configuration can be a key component in optical processing and telecommunication devices.

Differential Refractive Index Sensor Based on Coupled Plasmon Waveguide Resonance in the C-Band.

We proposed a differential fiber-optic refractive index sensor based on coupled plasmon waveguide resonance (CPWR) in the C-band. The sensor head is a BK7 prism coated with ITO/Au/ITO/TiO2 film. CPWR is excited on the film by the S-polarized components of an incident light. The narrow absorption peak of CPWR makes it possible to realize dual-wavelength differential intensity (DI) interrogation by using only one incident point. To implement DI interrogation, we used a DWDM component to sample the lights with central wavelengths of 1529.55 and 1561.42 nm from the lights reflected back by the sensor head. The intensities of the dual-wavelength lights varied oppositely within the measurement range of refractive index, thus, a steep slope was produced as the refractive index of the sample increased. The experimental results show that the sensitivity is 32.15/RIUs within the measurement range from 1.3584 to 1.3689 and the resolution reaches 9.3 × 10−6 RIUs. Benefiting from the single incident point scheme, the proposed sensor would be easier to calibrate in bio-chemical sensing applications. Moreover, this sensing method is expected to be applied to retro-reflecting SPR sensors with tapered fiber tip to achieve better resolution than wavelength interrogation.

Plasmon Waveguide Resonance: Principles, Applications and Historical Perspectives on Instrument Development.

Plasmon waveguide resonance (PWR) is a variant of surface plasmon resonance (SPR) that was invented about two decades ago at the University of Arizona. In addition to the characterization of the kinetics and affinity of molecular interactions, PWR possesses several advantages relative to SPR, namely, the ability to monitor both mass and structural changes. PWR allows anisotropy information to be obtained and is ideal for the investigation of molecular interactions occurring in anisotropic-oriented thin films. In this review, we will revisit main PWR applications, aiming at characterizing molecular interactions occurring (1) at lipid membranes deposited in the sensor and (2) in chemically modified sensors. Among the most widely used applications is the investigation of G-protein coupled receptor (GPCR) ligand activation and the study of the lipid environment’s impact on this process. Pioneering PWR studies on GPCRs were carried out thanks to the strong and effective collaboration between two laboratories in the University of Arizona leaded by Dr. Gordon Tollin and Dr. Victor J. Hruby. This review provides an overview of the main applications of PWR and provides a historical perspective on the development of instruments since the first prototype and continuous technological improvements to ongoing and future developments, aiming at broadening the information obtained and expanding the application portfolio.

Fluorescence Enhancement via Dual Coupling of Dye Molecules with Silver Nanostructures

We demonstrate the enhancement of fluorescence emitted from dye molecules coupled with two surface plasmons, i.e., silver nanoparticles (AgNPs)-induced localized surface plasmons (LSP) and thin silver (Ag) film supported surface plasmons. Excitation light is illuminated to a SiO2 layer that contains both rhodamine 110 molecules and AgNPs. AgNPs enhances excitation rates of dye molecules in their close proximity due to LSP-induced enhancement of local electromagnetic fields at dye excitation wavelengths. Moreover, the SiO2 layer on one surface of which a 50 nm-thick Ag film is coated for metal cladding (air on the other surface), acts as a waveguide core at the dye emission wavelengths. The Ag film induces the surface plasmons which couple with the waveguide modes, resulting in a waveguide-modulated version of surface plasmon coupled emission (SPCE) for different SiO2 thicknesses in a reverse Kretschmann configuration. We find that varying the SiO2 thickness modulates the fluorescent signal of SPCE, its modulation behavior being in agreement with the theoretical simulation of thickness dependent properties of the coupled plasmon waveguide resonance. This enables optimization engineering of the waveguide structure for enhancement of fluorescent signals. The combination of LSP enhanced dye excitation and the waveguide-modulated version of SPCE may offer chances of enhancing fluorescent signals for a highly sensitive fluorescent assay of biomedical and chemical substances.

High-Performance coupled plasmon waveguide resonance optical sensor based on SiO2:Ag film

A coupled plasmon waveguide resonance (CPWR)-based optical sensor takes advantage of the evanescent wave at a waveguide boundary to excite surface plasmon polaritons, which can link waveguide electromagnetic energy to a surface plasmon wave to enhance the propagating surface plasmon resonance. Herein, a novel optical sensor having a BK7 prism/Ag/SiO2/SiO2:Ag structure and that couples the localised surface plasmon resonance of Ag nanoparticles in a SiO2 film was designed and fabricated. The results showed that the proposed sensor exhibited higher sensitivity (~1396.85 nm RIU−1) and figure of merit (~11.36 RIU−1) values than a traditional CPWR optical sensor (i.e., 368.50 nm RIU−1 and 1.72 RIU−1, respectively). Moreover, the resonance wavelength could be tuned to the near-infrared region by adjusting the thickness of the SiO2 interlayer or the Ag content of the top SiO2 layer. Simulations and experimental data confirmed that the CPWR-based sensor exhibited superior optical sensing performance.

All optic-fiber coupled plasmon waveguide resonance sensor using ZrS2 based dielectric layer.

We developed an all optic-fiber coupled plasmon waveguide resonance (CPWR) sensor using a zirconium disulfide (ZrS2) based dielectric layer. The dielectric constants of ZrS2 were obtained using first-principles calculations. The theoretical model of the proposed sensor was established based on the transfer matrix method, leading to the optimization of the parameters in the sensor. The sensor was fabricated by depositing a gold layer of 35 nm on the fiber core and immobilizing the ZrS2 layer on the gold layer via physical adsorption method. An experimental setup was implemented for measuring the refractive index. The sensor with two cycles showed the best performance, with a sensitivity of higher than 8000 nm/RIU.

Functionalized silicon dioxide self-referenced plasmonic chip as point-of-care biosensor for stroke biomarkers NT-proBNP and S100β

Surface plasmon resonance (SPR) biosensors are often used in the detection of solid, liquid or gaseous samples in diagnostics, pharmaceutics and military defense. Plasmon waveguide resonance (PWR) mode is obtained when a dielectric waveguide layer is added to the metal film. In this study, a self-referenced PWR (SRPWR) silicon dioxide (SiO2) chip was examined. The self-referenced measurement is important to compensate for temperature fluctuations, other instabilities and allows RI signal measurement without an additional reference sample, thus minimising the sample volume needed. The chip was fabricated with a multi-layer of metals and dielectrics, consisting of a 420 nm SiO2 layer, a 40 nm Ag layer and another 480 nm SiO2 layer. This chip was shown to give one internal plasmon excited on the bottom interface SiO2/Ag, which is used as self-reference in the detection. The top layer acts as a waveguide layer and can be designed to give modes with ultrahigh penetration depth. A direct assay was developed, where the recognition molecule (specific antibody) was immobilized onto the SiO2 plasmonic chip surface, via a covalent coupling protocol based on 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde. The SRPWR biosensor was developed for the sensing of two chosen stroke biomarkers: NT-proBNP and S100β, which are sensitive and specific for stroke diagnostics. For both biomarkers, a linear decreasing pattern in the RI signal was recognized with the increasing biomarkers concentrations. Biomarkers detection was conducted in deionized water and validation was done in spiked porcine plasma. The SiO2 based plasmonic chip demonstrates a limit-of-detection of less than 1 ng/mL that is clinically relevant for both stroke biomarkers.

Direct Monitoring of GPCR Reconstitution and Ligand-Binding Activity by Plasmon Waveguide Resonance.

The study of G-protein-coupled receptor (GPCR ) mechanisms of activation and signaling often the isolation, purification, and reconstitution of GPCRs in model lipid membranes. GPCR reconstitution from a detergent-micelle system into model membranes is usually a laborious process whose success is tested after the whole process is over by rather indirect methods such as SDS-PAGE and western blotting or by biophysical approaches. Following that, protein activity is measured in yet a different experimental setup. Overall, the whole procedure is tedious, long, and often requires high quantities of material and the use of labeled proteins. Herein, a protocol is described to follow, in a single experimental setup, both GPCR reconstitution from detergent micelles into planar lipid bilayers and its ligand-binding activity using plasmon waveguide resonance. Alternatively, receptor/ligand interactions can also be investigated from cell membrane fragments overexpressing the receptor of interest, bypassing isolation and reconstitution procedures. The method is direct, sensitive, and does not rely on the use of any labeled material.

Cholesterol impacts chemokine CCR5 receptor ligand-binding activity.

The chemokine CCR5 receptor is target of maraviroc, a negative allosteric modulator of CCR5 that blocks the HIV protein gp120 from associating with the receptor, thereby inhibiting virus cellular entry. As noted with other G-protein-coupled receptor family members, the role of the lipid environment in CCR5 signaling remains obscure and very modestly investigated. Controversial literature on the impact of cholesterol (Chol) depletion in HIV infection and CCR5 signaling, including the hypothesis that Chol depletion could inhibit HIV infection, lead us to focus on the understanding of Chol impact in the first stages of receptor activation. To address this aim, the approach chosen was to employ reconstituted model lipid systems of controlled lipid composition containing CCR5 from two distinct expression systems: Pichiapastoris and cell-free expression. The characterization of receptor/ligand interaction in terms of total binding or competition binding assays was independently performed by plasmon waveguide resonance and fluorescence anisotropy, respectively. Maraviroc, a potent receptor antagonist, was the ligand investigated. Additionally, coarse-grained molecular dynamics simulation was employed to investigate Chol impact in the receptor-conformational flexibility and dynamics. Results obtained with receptor produced by different expression systems and using different biophysical approaches clearly demonstrate a considerable impact of Chol in the binding affinity of maraviroc to the receptor and receptor-conformational dynamics. Chol considerably decreases maraviroc binding affinity to the CCR5 receptor. The mechanisms by which this effect occurs seem to involve the adoption of distinct receptor-conformational states with restrained structural dynamics and helical motions in the presence of Chol.

Dendron-Functionalized Surface: Efficient Strategy for Enhancing the Capture of Microvesicles.

SummaryMicrovesicles (MVs) are used by various types of cells in the human body for intercellular communication, making them biomarkers of great potential for the early and non-evasive diagnosis of a spectrum of diseases. An integrated analysis including morphological, quantitative, and compositional studies is most desirable for the clinical application of MV detection; however, such integration is limited by the currently available analysis techniques. In this context, exploiting the phosphatidylserine (PS) exposure of MVs, we synthesized a series of dendritic molecules with PS-binding sites at the periphery. PS-dendron binding was studied at the molecular level using NMR approaches, whereas PS-containing membrane-dendron interaction was investigated in an aqueous environment using plasmon waveguide resonance spectroscopy. As a proof of concept, polyethylene terephthalate surface was functionalized with the synthetic dendrons, forming devices that can capture MVs to facilitate their subsequent analyses.

Structural insights into the AapA1 toxin of Helicobacter pylori

BackgroundWe previously reported the identification of the aapA1/IsoA1 locus as part of a new family of toxin-antitoxin (TA) systems in the human pathogen Helicobacter pylori. AapA1 belongs to type I TA bacterial toxins, and both its mechanism of action towards the membrane and toxicity features are still unclear. MethodsThe biochemical characterization of the AapA1 toxic peptide was carried out using plasmid-borne expression and mutational approaches to follow its toxicity and localization. Biophysical properties of the AapA1 interaction with lipid membranes were studied by solution and solid-state NMR spectroscopy, plasmon waveguide resonance (PWR) and molecular modeling. ResultsWe show that despite a low hydrophobic index, this toxin has a nanomolar affinity to the prokaryotic membrane. NMR spectroscopy reveals that the AapA1 toxin is structurally organized into three distinct domains: a positively charged disordered N-terminal domain (D), a single α-helix (H), and a basic C-terminal domain (R). The R domain interacts and destabilizes the membrane, while the H domain adopts a transmembrane conformation. These results were confirmed by alanine scanning of the minimal sequence required for toxicity. ConclusionOur results have shown that specific amino acid residues along the H domain, as well as the R domain, are essential for the toxicity of the AapA1 toxin. General significanceUntangling and understanding the mechanism of action of small membrane-targeting toxins are difficult, but nevertheless contributes to a promising search and development of new antimicrobial drugs.

Improved Detection of Plasmon Waveguide Resonance Using Diverging Beam, Liquid Crystal Retarder, and Application to Lipid Orientation Determination.

Plasmon waveguide resonance (PWR) sensors exhibit narrow resonances at the two orthogonal polarizations, transverse electric (TE) and transverse magnetic (TM), which are narrower by almost an order of a magnitude than the standard surface plasmon resonance (SPR), and thus the figure of merit is enhanced. This fact is useful for measuring optical anisotropy of materials on the surface and determining the orientation of molecules with high resolution. Using the diverging beam approach and a liquid crystal retarder, we present experimental results by simultaneous detection of TE and TM polarized resonances as well as using fast higher contrast serial detection with a variable liquid crystal retarder. While simultaneous detection makes the system simpler, a serial one has the advantage of obtaining a larger contrast of the resonances and thus an improved signal-to-noise ratio. Although the sensitivity of the PWR resonances is smaller than the standard SPR, the angular width is much smaller, and thus the figure of merit is improved. When the measurement methodology has a high enough angular resolution, as is the one presented here, the PWR becomes advantageous over other SPR modes. The possibility of carrying out exact numerical simulations for anisotropic molecules using the 4 × 4 matrix approach brings another advantage of the PWR over SPR on the possibility of extracting the orientation of molecules adsorbed to the surface. High sensitivity of the TE and TM signals to the anisotropic molecules orientation is found here, and comparison to the experimental data allowed detection of the orientation of lipids on the sensor surface. The molecular orientations cannot be fully determined from the TM polarization alone as in standard SPR, which underlines the additional advantage of the PWR technique.

Study of G-Protein Coupled Receptor Signaling in Membrane Environment by Plasmon Waveguide Resonance.

Here we describe an experimental technique, termed plasmon waveguide resonance (PWR) spectroscopy that enables the characterization of molecular interactions occurring at the level of anisotropic thin films as lipid membranes and therein inserted or interacting molecules. PWR allows one to characterize such molecular interactions at different levels: (1) acquire binding curves and calculate dissociation constants; (2) obtain kinetic information; (3) obtain information about associated anisotropy changes and changes in membrane thickness; (4) obtain insight about lateral homogeneity (formation of domains). Points 1, 2, and 4 can be directly obtained from the data. Point 3 requires spectral fitting procedures so that the different optical parameters characterizing thin films as proteolipid membranes, namely refractive index and extinction coefficient for both p- (TM component of light that is parallel to the incident light) and s- (TE component of light that is perpendicular to the incident light) polarizations and thickness, can be determined. When applied to membrane proteins as the G-protein coupled receptor (GPCR) family, both ligand-induced conformational changes of the receptor can be followed as well as interactions with effectors (e.g., G-proteins). Additionally, by either altering the lipid composition in cellular membranes or specifically controlling its composition in the case of lipid model membranes with reconstituted proteins, the role of the lipid environment in receptor activation and signaling can be determined. Additionally, the eventual partition of receptors in different lipid microdomains (e.g., lipid rafts) can be followed. Such information can be obtained ex cellulo with mammalian cell membrane fragments expressing the protein of interest and/or in vitro with lipid model systems where the protein under investigation has been reconstituted. Moreover, PWR can also be applied to directly follow the reconstitution of membrane proteins in lipid model membranes. The measurements are performed directly (no labeling of molecular partners), in real time and with very high sensitivity. Here we will discuss different aspects of GPCR activation and signaling where PWR brought important information in parallel with other approaches. The utility of PWR is not limited to GPCRs but can be applied to any membrane protein. PWR is also an excellent tool to characterize the interaction of membrane active molecules (as cell penetrating, antimicrobial, viral and amyloid peptides) with lipids. A brief section is dedicated to such applications, with particular emphasis on amyloid peptides. To finalize, as PWR is a homemade technology, ongoing instrument developments aiming at breaking current experimental limitations are briefly discussed, namely, the coupling of PWR with electrochemical measurements and the expansion of measurements from the visible to the infrared region.

Thickness determination in anisotropic media with plasmon waveguide resonance imaging.

This paper describes a simple procedure to determine the local thickness of a thin anisotropic layer. It also discriminates between isotropic and anisotropic regions, provided a smoothness hypothesis on the refractive index distribution is satisfied. The procedure is based on the analysis of surface plasmon resonance (SPR) data acquired in an imaging mode. The general arrangement of the setup is the Kretschmann configuration. We show, on an azobenzene modified polymer layer, good agreement between atomic force microscopy and optical measurements of thickness variation.