Waveguide-coupled Surface Plasmon Resonance Sensors Research Articles

Waveguide-Coupled Surface Plasmon Resonance Sensor for Both Liquid and Gas Detections

In this paper, we propose a novel surface plasmon resonance sensor for both liquid and gas detections, which is based on the two dielectric planar waveguide (PWG)-coupled plasmonic structure. Two sharp Fano-type resonances, which arise from the electromagnetic coupling between surface plasmon polariton (SPP) and PWG modes, appear in the reflection curve, and they can be used to monitor the minute changes of the refractive indices of gas and liquid due to their high imaging sensitivities (> 9.0 × 103 RIU− 1). The Fano resonance angles and the figures of merit for imaging sensitivity can be effectively manipulated by the thicknesses of guiding layers, and the sensor system can measure over a wide range of the refractive indices of sensing media.

Tunable Fano resonances and improved sensitivity in waveguide-coupled surface plasmon resonance sensors with a bimetallic layer

A multilayer structure consisting of Au–Ag bimetal and dielectric layers which allows coupling between surface plasmon polaritons and waveguide modes is studied by calculating reflectivity curves of incident light. The coupling results in asymmetric resonance modes, which can be attributed to Fano resonances (FR). The variation trend of FR is investigated in detail by fitting numerical results with an analytic model when changing both waveguide widths and silver thicknesses. It is demonstrated that changing the thickness ratio of Au–Ag bilayers can tune the coupling strength and the line shape of FR. We build the resonant mode diagram according to the different asymmetric line shape which can be depicted by the sign of the fitting Fano constant q. At a critical Ag thickness, the coupling gives rise to a q = 0 symmetrical resonance state (plasma induced transparency). Finally, the best optimized thickness combination (Ag/Au) in the structure is found when the refractive index of the sensing layer medium changes. The sensitivity of the optimized sensor is twice as high as that of the single Au film structure.

Characteristics of multiple Fano resonances in waveguide-coupled surface plasmon resonance sensors based on waveguide theory

We observe and analyze multiple Fano resonances and the plasmon-induced transparency (PIT) arising from waveguidecoupled surface plasmon resonance in a metal-dielectric Kretschmann configuration. It is shown that the simulation results for designed structures agree well with those of the dispersion relation of waveguide theory. We demonstrate that the coupling between the surface plasmon polariton mode and multi-order planar waveguide modes leads to multiple Fano resonances and PIT. The obtained results show that the number of Fano resonances and the linewidth of resonances depend on two structural parameters, the Parylene C and SiO2 layers, respectively. For the sensing action of Fano resonance, the figure of merit for the sensitivity by intensity is estimated to be 44 times higher than that of conventional surface plasmon resonance sensors. Our research reveals the potential advantage of sensors with high sensitivity based on coupling between the SPP mode and multi-order PWG modes.

Hafnium dioxide as a dielectric for highly-sensitive waveguide-coupled surface plasmon resonance sensors

Hafnium dioxide has been recognized as an excellent dielectric for microelectronics. However, its usefulness for the surface plasmon based sensors has not yet been tested. Here we investigate its usefulness for waveguide-coupled bi-metallic surface plasmon resonance sensors. Several Ag/HfO2/Au multilayer structure sensors were fabricated and evaluated by optical measurements and computer simulations. The resulting data establish correlations between the growth parameters and sensor performance. The sensor sensitivity to refractive index of analytes is determined to be Sn=∂θSPR∂n≥470. The sensitivity data are supported by simulations, which also predict 314 nm for the evanescent field decay length in air.

Waveguide-coupled surface plasmon resonance sensor structures: Fano lineshape engineering for ultrahigh-resolution sensing

We analyze in detail the plasmon-induced transparency and Fano resonance exhibited by a waveguide-coupled surface plasmon resonance sensor structure. It is shown that the results of electromagnetic calculations made for the structure agree very well with those of mechanical calculations made for two coupled harmonic oscillators. This implies that an analogy holds between the present electromagnetic system and the coupled-oscillator system. The analogy established allows us to conclude that the plasmon-induced transparency and Fano resonance are caused by the coupling between a surface plasmon polariton and a planar waveguide mode. Sensing action of the Fano resonance is also analyzed in detail. From the calculation of the figure of merit for the sensitivity by intensity, it is shown that there is an optimum condition for the coupling of the modes to achieve a maximum sensitivity. Under the optimum condition, the figure of merit is found to be three orders of magnitude higher than that of a conventional surface plasmon sensor.

Fano resonance and plasmon-induced transparency in waveguide-coupled surface plasmon resonance sensors

A planar multilayer structure that allows coupling between surface plasmon polaritons and waveguide modes is proposed. Calculated reflectivity curves exhibit sharp resonances due to the Fano resonance and plasmon-induced transparency arising from the coupling. Electric field profiles obtained at the resonances demonstrate the hybrid nature of the modes excited. When the Fano resonance is used for sensing, the sensitivity with intensity modulation is enhanced by two orders of magnitude relative to that of conventional surface plasmon resonance sensors.

A Highly Sensitive Voltage Interrogation Method Using Electro-Optically Tunable Waveguide Coupled Surface Plasmon Resonance Sensors

We present a highly sensitive detection method using electro-optically (EO) tunable waveguide coupled surface plasmon resonance (WCSPR) sensors by theoretical simulation. In this method, light is focused through a prism onto metal surface of the WCSPR sensor and the reflection is collected with a bicell photodetector. Time varied scanning DC electrical signal is applied on the waveguide layer of the WCSPR sensor and the ratio of the differential signal to the sum signal of the photodetector is calculated after each change. For each analyte, we get voltage-ratio curve after voltage scanning. Taking the ratio for a reference analyte calculated with null DC voltage applied as resonant ratio, different resonant voltages can be found in the voltage-ratio curves for different analytes. Theoretical simulation shows a high linearity between the resonant voltages and the refractive index (RI) of analytes. A sensitivity as high as 2.34 x 10(5) V/RIU can be expected in this method.

Sensitive voltage interrogation method using electro-optically tunable SPR sensors

A novel voltage interrogation method using electro-optically tunable waveguide-coupled surface plasmon resonance sensors is demonstrated. Before measurements, we use a bicell photodetector to detect the reflectance from the sensor and take the differential signal from the photodetector as the resonance condition. For different analytes, by scanning the DC voltage on the waveguide layer of the sensor chip, the resonance condition can be maintained the same. Under this condition, we record the values of this voltage, namely the resonant voltage. Theoretical calculations and experimental results show the resonant voltage has a highly linear and sensitive response to analyte's refractive index. This method is simple in configuration, and complicated signal processing algorithms can be avoided.

Hybrid differential interrogation method for sensitive surface plasmon resonance measurement enabled by electro-optically tunable SPR sensors

A novel detection method enabled by electro-optically tunable waveguide-coupled surface plasmon resonance sensors is demonstrated. Both the WCSPR response of sensor and the interrogation light are varied simultaneously in this hybrid scheme. Modulation and demodulation of the sensor's response are achieved by applying a high-frequency AC electrical signal and electrically filtering the detected signal. Scanning the incident angle at a lower speed yields an angular dependent response. Theoretical analyses and experimental results show that the angular-dependent signal is closely related to the derivative of the SPR reflectivity with a sharp, linear jump near the minimum of the SPR peak. Thus, simple linear-fitting and zero-finding algorithms can be used to locate the SPR angle, and sophisticated data processing algorithms and electronic hardware can be avoided.

Colloidal gold submonolayer-coated thin-film glass plates for waveguide-coupled surface plasmon resonance sensors.

A novel optical sensor based on spectroscopic measurement of the plasmon absorption of a colloidal gold submonolayer immobilized upon a planar waveguide has been developed for label-free detection of biomolecular interactions at the sensor's surface. We fabricated the sensor by locally modifying a 50-microm-thick glass plate with hemoglobin (Hb) and then self-assembling gold colloids from the aqueous solution onto the Hb-modified area of the glass plate. Polychromatic light from a xenon-arc lamp was launched into the thin-film glass plate by use of a broadband fiber-coupling method. With the use of a CCD detector to monitor the light beam emitted from an end face of the glass plate, the plasmon absorption spectrum of the colloidal gold submonolayer was determined to depend on the polarization states of the guided light and to change with the refractive index of the medium in contact with the colloids. In addition to simplicity of fabrication and the ease of use, the sensor yields a larger absorbance sensitivity than the normal transmission measurement.