Coupled Plasmon-waveguide Resonance Research Articles

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.

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.

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.

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.

Theoretical modeling of a coupled plasmon waveguide resonance sensor based on multimode optical fiber

A coupled plasmon waveguide resonance (CPWR) sensor based on metal/dielectric-coated step index multimode optical fiber is proposed. Theoretical simulations using the four-layer Fresnel equations based on a bi-dimensional optical fiber model were implemented on four structures: Ag–ZnO, Au–ZnO, Ag–TiO2 and Au–TiO2. By controlling the thickness of dielectric layer, we managed to manipulate the CPWR resonance wavelengths. When a CPWR resonance dip is in the short wavelength region, it is insensitive to the change of surrounding refractive index (SRI) and can be used as a reference to improve the sensing accuracy of surface plasmon resonance (SPR) mode. With the increase of the thickness of the dielectric layer, the CPWR resonance dips shift to longer wavelength and the corresponding sensitivities increase. When the 1st CPWR resonance wavelength is near 1550 nm and SRI is around 1.333, the sensitivities of four structures reach 1360.61 nm/RIU, 1375.76 nm/RIU, 1048.48 nm/RIU and 1015.15 nm/RIU, respectively. The values are close to that of the conventional SPR optical fiber sensor while the spectral bandwidths of the optical fiber CPWR sensors are narrower.

Theoretical Investigation on Coupled Plasmon Waveguide Resonance in Real and Complex Domain for High Precision Nanoplasmonic Sensing

Numerical investigations on modified multilayer coupled plasmonic structure, termed as coupled plasmon waveguide resonance (CPWR) structure, have been presented in real as well as complex plane domain. Coupling of waveguide resonance and plasmonic resonance results in dual resonance. Reflectance, field enhancement, and phase-dependent resonant behavior have been analyzed in angular interrogation for three different metal layers in order to optimize the structure with proper choice of metal layers. The resonance parameters have also been theoretically investigated in differential angular regime for better comparison of the shape of different resonance curves. Complex amplitude reflection and transmission co-efficients are analyzed with circular graphical plots in the complex plane. Simultaneous observations of the real and imaginary parts as well as amplitude and phase of the reflection and transmission co-efficients have been demonstrated. Different conventional optical polymers have been considered as waveguide layer materials and their respective real and complex plane analyses demonstrate the comparison for the proper choice of the waveguide layer. Moreover, the resonance parameters and the differential plots in real as well as complex plane for coarse and precise measurement have also been investigated for nanoplasmonic sensing applications to provide improved resolution.

Transverse magnetic-reflected polarizer for application in surface plasmon resonance configuration

We developed a wideband transverse magnetic (TM) −reflected polarizer composed of BK7 prism and Ag/Cr/TiO2 film. Based on the polarization-dependent loss of coupled plasmon-waveguide resonance (CPWR), BK7 prism/Ag/Cr/TiO2/air layered structure can reflect the TM-components of lightwave and severely attenuate the transverse electric (TE) −components in a wavelength range from 600nm to 900nm at incident angles greater than the critical angle of total reflection. In terms of its practical application, a compact, low cost surface plasmon resonance (SPR) configuration with an integrated polarizer for wavelength modulation is presented and experimentally demonstrated.

Plasmon-Waveguide Resonances with Enhanced Figure of Merit and Their Potential for Anisotropic Biosensing in the Near Infrared Region

The TM and TE guided modes in the coupled plasmon-waveguide resonance configuration are investigated in the spectral domain. Here we use the modes dispersion to study their capability for sensing in the near infrared region. It is shown that the spectral widths of the guided modes are, at least, one order of magnitude smaller than the conventional surface plasmon resonance counterpart. The enhanced sensitivity and figure of merit of the guided modes reveal their potential for sensing in the spectral interrogation method where the traditional configurations are inherently limited. Moreover, the high resolution associated with the narrow resonances and the polarization dependence make these modes very promising for anisotropic biosensing in the spectral interrogation approach. The extremely high figure of merit, large penetration depth, and propagation distance in the near infrared region open the possibility of combining the plasmon-waveguide configuration with absorption spectroscopy techniques for molecular recognition.

A BIOSENSOR USING COUPLED PLASMON WAVEGUIDE RESONANCE COMBINED WITH HYPERSPECTRAL FLUORESCENCE ANALYSIS

We developed a biosensor that is capable for simultaneous surface plasmon resonance (SPR) sensing and hyperspectral fluorescence analysis in this paper. A symmetrical metal-dielectric slab scheme is employed for the excitation of coupled plasmon waveguide resonance (CPWR) in the present work. Resonance between surface plasmon mode and the guided waveguide mode generates narrower full width half-maximum of the reflective curves which leads to increased precision for the determination of refractive index over conventional SPR sensors. In addition, CPWR also offers longer surface propagation depths and higher surface electric field strengths that enable the excitation of fluorescence with hyperspectral technique to maintain an appreciable signal-to-noise ratio. The refractive index information obtained from SPR sensing and the chemical properties obtained through hyperspectral fluorescence analysis confirm each other to exclude false-positive or false-negative cases. The sensor provides a comprehensive understanding of the biological events on the sensor chips.

Multi-Channel Hyperspectral Fluorescence Detection Excited by Coupled Plasmon-Waveguide Resonance

We propose in this paper a biosensor scheme based on coupled plasmon-waveguide resonance (CPWR) excited fluorescence spectroscopy. A symmetrical structure that offers higher surface electric field strengths, longer surface propagation lengths and depths is developed to support guided waveguide modes for the efficient excitation of fluorescence. The optimal parameters for the sensor films are theoretically and experimentally investigated, leading to a detection limit of 0.1 nM (for a Cy5 solution). Multiplex analysis possible with the fluorescence detection is further advanced by employing the hyperspectral fluorescence technique to record the full spectra for every pixel on the sample plane. We demonstrate experimentally that highly overlapping fluorescence (Cy5 and Dylight680) can be distinguished and ratios of different emission sources can be determined accurately. This biosensor shows great potential for multiplex detections of fluorescence analytes.

Coupled plasmonic assisted progressive multiple resonance for dielectric material characterization

In this paper, we report the theoretical analysis of different types of plasmonic structures with the main emphasis on the coupled plasmon waveguide resonance due to several advantages offered by the same compared to other structures. Multiple resonance dips are found to increase with the increase in the dielectric layer thickness and this progressive nature of the dips can be efficiently utilized to characterize the dielectric material within a composite multilayer plasmonic structure. Supporting simulation results carried out in the MATLAB environment are also provided to validate the proposed application.

A sensitivity comparison of optical biosensors based on four different surface plasmon resonance modes

Current surface plasmon resonance (SPR) modes based on the attenuated total reflection (ATR) method can broadly be categorized as: conventional SPR, long-range SPR (LRSPR), coupled plasmon-waveguide resonance (CPWR), and waveguide-coupled SPR (WCSPR). Although the features of optical biosensors are dependent upon their particular SPR mode, a common requirement for all biosensors utilized for biomolecular interaction analysis (BIA) is a high degree of sensitivity. The current paper presents a theoretical analysis and comparison of the sensitivity and resolution of these four types of SPR biosensors when employed in three of the most prevalent detection methods, namely angular interrogation, wavelength interrogation, and intensity measurement. This study develops a detailed understanding of the influences of various biosensor design parameters in order to enhance the sensitivity and detection limit capabilities of such devices.

Optical Anisotropy in Lipid Bilayer Membranes: Coupled Plasmon-Waveguide Resonance Measurements of Molecular Orientation, Polarizability, and Shape

The birefringence and linear dichroism of anisotropic thin films such as proteolipid membranes are related to molecular properties such as polarizability, shape, and orientation. Coupled plasmon-waveguide resonance (CPWR) spectroscopy is shown in the present work to provide a convenient means of evaluating these parameters in a single lipid bilayer. This is illustrated by using 1–10 mol % of an acyl chain chromophore-labeled phosphatidylcholine (PC) incorporated into a solid-supported PC bilayer deposited onto a hydrated silica surface. CPWR measurements were made of refractive index and extinction coefficient anisotropies with two exciting light wavelengths, one of which is absorbed by the chromophore and one of which is not. These results were used to calculate longitudinal and transverse molecular polarizabilities, the orientational order parameter and average angle between the longitudinal axis of the lipid molecule and the membrane normal, and the molecular shape factors of the lipid molecules. The values thereby obtained are in excellent agreement with parameters determined by other techniques, and provide a powerful tool for analyzing lipid-protein, protein-protein, and protein-ligand interactions in proteolipid films.

Plasmon resonance spectroscopy: probing molecular interactions at surfaces and interfaces

Surface plasmon resonance (SPR) spectroscopy can be applied to a wide variety of interfacial systems. It involves resonant excitation by polarized light of electronic oscillations (plasmons) in a thin metal film. These generate a surface‒localized evanescent electromagnetic field that can be used to probe the optical properties perpendicular to the film plane of materials immobilized at the surface. Spectra depend on three parameters: refractive index (n), absorption coefficient (k) and thickness (t). Maxwell's equations provide an analytical relationship between these properties and SPR spectra, allowing their evaluation. An extension of this methodology, called coupled plasmon‒waveguide resonance (CPWR or PWR), is able to characterize film propertiesbothperpendicular and parallel to the surface plane. In a PWR device, the metal film is covered with a dielectric coating that acts as an optical amplifier, provides protection for the metal layer, and possesses a surface that allows various molecular immobilization strategies. The exceptionally narrow line widths of PWR spectra yield enhanced sensitivity and resolution. The application of this technology to several biomembrane systems will be described, demonstrating its ability to observe both binding and structural events occurring during membrane protein function.

Interaction of Phosphatidylserine Synthase from E. coli with Lipid Bilayers: Coupled Plasmon-Waveguide Resonance Spectroscopy Studies

The interaction of phosphatidylserine (PS) synthase from Escherichia coli with lipid membranes was studied with a recently developed variant of the surface plasmon resonance technique, referred to as coupled plasmon-waveguide resonance spectroscopy. The features of the new technique are increased sensitivity and spectral resolution, and a unique ability to directly measure the structural anisotropy of lipid and proteolipid films. Solid-supported lipid bilayers with the following compositions were used: 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC); POPC-1-palmitoyl-2-oleoyl- sn-glycero-3-phosphate (POPA) (80:20, mol/mol); POPC-POPA (60:40, mol/mol); and POPC-1-palmitoyl-2-oleoyl- sn-glycero-3-[phospho- rac-(1-glycerol)] (POPG) (75:25, mol/mol). Addition of either POPA or POPG to a POPC bilayer causes a considerable increase of both the bilayer thickness and its optical anisotropy. PS synthase exhibits a biphasic interaction with the bilayers. The first phase, occurring at low protein concentrations, involves both electrostatic and hydrophobic interactions, although it is dominated by the latter, and the enzyme causes a local decrease of the ordering of the lipid molecules. The second phase, occurring at high protein concentrations, is predominantly controlled by electrostatic interactions, and results in a cooperative binding of the enzyme to the membrane surface. Addition of the anionic lipids to a POPC bilayer causes a 5- to 15-fold decrease in the protein concentration at which the first binding phase occurs. The results reported herein lend experimental support to a previously suggested mechanism for the regulation of the polar head group composition in E. coli membranes.

Coupled Plasmon-Waveguide Resonance Spectroscopy Studies of the Cytochrome b6f/Plastocyanin System in Supported Lipid Bilayer Membranes

The incorporation of cytochrome (cyt) b 6 f into a solid-supported planar egg phosphatidylcholine (PC) bilayer membrane and complex formation with plastocyanin have been studied by a variant of surface plasmon resonance called coupled plasmon-waveguide resonance (CPWR) spectroscopy, developed in our laboratory. CPWR combines greatly enhanced sensitivity and spectral resolution with direct measurement of anisotropies in refractive index and optical extinction coefficient, and can therefore probe structural properties of lipid-protein and protein-protein interactions. Cyt b 6 f incorporation into the membrane proceeds in two stages. The first occurs at low protein concentration and is characterized by an increase in total proteolipid mass without significant changes in the molecular order of the system, as demonstrated by shifts of the resonance position to larger incident angles without changing the refractive index anisotropy. The second stage, occurring at higher protein concentrations, results in a decrease in both the mass density and the molecular order of the system, evidenced by shifts of the resonance position to smaller incident angles and a large decrease in the membrane refractive index anisotropy. Plastocyanin can bind to such a proteolipid system in three different ways. First, the addition of plastocyanin before the second stage of b 6 f incorporation begins results in complex formation between the two proteins with a K D of ∼10 μM and induces structural changes in the membrane that are similar to those occurring during the second stage of complex incorporation. The addition of larger amounts of plastocyanin under these conditions leads to nonspecific binding to the lipid phase with a K D of ∼180 μM. Finally, the addition of plastocyanin after the completion of the second phase of b 6 f incorporation results in tighter binding between the two proteins ( K D ≈ 1 μM). Quantitation of the binding stoichiometry indicates that two plastocyanin molecules bind tightly to the dimeric form of the cyt b 6 f complex, assuming random insertion of the cytochrome into the bilayer. The structural basis for these results and formation of the proteolipid membrane are discussed.

Coupled plasmon-waveguide resonators: a new spectroscopic tool for probing proteolipid film structure and properties

A variant of surface plasmon resonance (SPR) spectroscopy has been developed that involves a coupling of plasmon resonances in a thin metal film and waveguide modes in a dielectric overcoating. This new technique is referred to as coupled plasmon-waveguide resonance (CPWR) spectroscopy. It combines a greatly enhanced sensitivity (due to increased electromagnetic field intensities at the dielectric surface) and spectral resolution (due to decreased resonance linewidths), with the ability to directly measure anisotropies in refractive index and optical absorption coefficient in a dielectric film adsorbed onto the surface of the overcoating. Experimental data obtained with an egg phosphatidylcholine bilayer are presented to document these properties.