Abstract
The concept of photonic bandgap fiber-based surface plasmon resonance sensor operating with low refractive index analytes is developed. Plasmon wave on the surface of a thin metal film embedded into a fiber microstructure is excited by a leaky Gaussian-like core mode of a fiber. We demonstrate that by judicious design of the photonic crystal reflector, the effective refractive index of the core mode can be made considerably smaller than that of the core material, thus enabling efficient phase matching with a plasmon, high sensitivity, and high coupling efficiency from an external Gaussian source, at any wavelength of choice from the visible to near-IR. To our knowledge, this is not achievable by any other traditional sensor design. Moreover, unlike the case of total internal reflection waveguide-based sensors, there is no limitation on the upper value of the waveguide core refractive index, therefore, any optical materials can be used in fabrication of photonic bandgap fiber-based sensors. Based on numerical simulations, we finally present designs using various types of photonic bandgap fibers, including solid and hollow core Bragg fibers, as well as honeycomb photonic crystal fibers. Amplitude and spectrum based methodologies for the detection of changes in the analyte refractive index are discussed. Furthermore, sensitivity enhancement of a degenerate double plasmon peak excitation is demonstrated for the case of a honeycomb fiber. Sensor resolutions in the range 7 * 10(-6) -5 * 10(-5) RIU were demonstrated for an aqueous analyte.
Highlights
Propagating at the metal/dielectric interface, surface plasmons [1] are extremely sensitive to changes in the refractive index of the dielectric
The high refractive index defect created by the first layer of the reflector attracts a localized state that causes anticrossing with the core mode, forcing the core mode to cross over the dispersion relation of the analyte, and enabling phase matching with the plasmon
We have presented a novel approach to the design of photonic crystal fiberbased Surface Plasmon Resonance (SPR) sensors for measuring changes in low refractive index analytes
Summary
Propagating at the metal/dielectric interface, surface plasmons [1] are extremely sensitive to changes in the refractive index of the dielectric. In photonic bandgap fiber-based SPR sensors, fundamental Gaussian-like leaky core mode can be phase matched with a plasmon at any desired wavelength of operation, enabling sensing anywhere from the visible to mid-IR. The lowest loss leaky core mode typically exhibits a Gaussian-like intensity distribution in the waveguide core region, enabling its convenient excitation by the Gaussian beam of an external light source Using such a leaky mode for sensing gives the additional advantage of an effectively single mode propagation regime. With a proper choice of the reflector layer thicknesses, one can position reflector bandgap to be most efficient for guiding the core modes having refractive indices around that of the plasmon In such a sensor, the core guided Gaussian-like mode (thick blue curve in the inset) is mostly confined to the waveguide core region, while the plasmon (thick dashed curve in the inset) is localized at the gold-water interface.
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