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A kind of surface plasmon resonance sensor based on grapefruit photonic crystal fiber (PCF) filled with different numbers of silver nanowires has been studied in this paper. The surface plasmon resonance modes and the sensing properties are investigated comprehensively using the finite element method (FEM). The simulation results show that the intensity sensitivity is related to nanowire numbers and the distance between two nanowires. The optimum value obtained is 2,400 nm/RIU, corresponding to a resolution of 4.51 × 10^{−5} RIU with a maximum distance of 2 μm. To a certain extent, the PCF filled with more nanowires is better than with just one. Furthermore, the air holes of grapefruit PCF are large enough to operate in practice. Moreover, the irregularity of the filled nanowires has no effect on sensitivity, which will be very convenient for the implementation of experiments.

Surface plasmon resonance (SPR) is one of the most promising optical techniques which refers to the excitation of surface plasmon polaritons. Surface plasmons can be excited by light when the phase matching condition is met between the exciting light and the surface plasmons [

Photonic crystal fiber (PCF) has a lot of peculiar properties, such as endless single-mode characteristic, high birefringence coefficient, controllable dispersion, high nonlinear coefficient,

The sensing mechanism of PCF-based SPR sensors is through coupling the leaky core mode to the plasmon to achieve resonance sensing [

Metal nanoparticles and nanowires (columns), and other nanostructures can also be used to generate surface plasmon resonance [

We utilize the self-designed grapefruit PCF made of silica in the simulation [_{a} = 1.33) in this paper.

We use the finite element method (FEM) to solve the light field mode and calculate the attenuation constant of the fundamental mode of fiber with different mosaic structures of silver nanowires (optical field distribution of the fundamental mode is shown in _{0} is the wave number (k_{0} = 2ð/ë), _{eff}

Therefore the optical fiber transmission loss coefficient can be expressed with the power attenuation coefficient á as:

This article uses the mode power attenuation coefficient _{a}_{a} = 1.33 (red solid curve). It is noteworthy that the resonance peak shift of peak I is 1 nm, and that of peak II is 12 nm. It is obvious that the peak II is more sensitive than peak I. This can be explained by noting that the silver nanowire surface supports several waveguide modes which result in several peaks. Peak I is the coupling between the high-order mode and the core guided mode and peak II is the coupling between the fundamental waveguide mode and the core guided mode. In the next section we only focus on the peak II for sensing.

The relationship between wavelength and attenuation constant of the fundamental mode of the peak II is shown in _{peak}) towards the longer wavelength.

If the spectral variation of 0.1 nm can be accurately detected, we can get the corresponding refractive index resolution as [

The power detection sensitivity for the refractive index variation _{a}

We can get the maximal sensitivity at 647 nm, and equals 178 RIU^{−1}. It is typically a safe assumption that a 1% change in the transmitted intensity can be detected reliably, which leads to a sensor resolution of 5.62 × 10^{−5} RIU.

In consideration of the interaction between the two silver nanowires, we investigate the SPR sensors based on PCFs filled with two silver nanowires of different relative distances. Numerical simulation shows that when the number of the nanowires increases to two, the transmission loss coefficient and the sensitivity will change for the interaction of electromagnetic field between the two silver nanowires. Then the relative distance between two silver nanowires has changed. In order to make it feasible for real operation, we place the silver nanowires in the edge of the circle (as shown in

As shown in ^{−1} to 180 RIU^{−1}) with the increase of relative distance between two silver nanowires within 2 ìm (

When the number of the nanowires increases to three, the transmission loss coefficient and the sensitivity will change for the interaction of electromagnetic field between the silver nanowires. The results show that the spectral and intensity sensitivity (183 RIU^{−1}) of the grapefruit PCF filled with more silver nanowires (within three) is better than the one filled with less. The sensor resolution reaches 5.46 × 10^{−5} RIU. The comparisons of attenuation spectra of the fundamental mode and the amplitude detection sensitivity curves are shown in

Meanwhile, as shown in

Based on practical considerations, such as that it can be more close to the condition of that the holes filled with three silver nanowires irregularly during the actual operation, we carried out the related simulation experiment of the surface plasmon resonance sensors based on grapefruit PCF irregularly filled with three silver nanowires, and the resonant mode obtained is shown in

The numerical results show that it is the same Δ_{peak} (=12 nm) as the regular ones. The relationship between wavelength and attenuation constant of the fundamental mode of the grapefruit PCF filled with three silver nanowires irregularly is shown in _{a}^{−1}, which is also similar to the surface plasmon resonance sensors based on grapefruit PCF regularly filled with three silver nanowires (183 RIU^{−1}), as shown in

In this paper we have analyzed surface plasmon resonance sensors based on photonic crystal fibers filled with silver nanowires of different numbers and different distances between two silver nanowires through the finite element method (FEM) by using the COMSOL Multiphysics software. Numerical results show that the sensitivity will increase from 109 RIU^{−1} to 180 RIU^{−1} with the increase of relative distance between two silver nanowires within 2 ìm (è ≈ 20°), and then it tends to be stable with the continued increase. With the appropriate distance between two silver nanowires, when the number increases to three, the spectral and intensity sensitivity is better (from 178 RIU^{−1} to 183 RIU^{−1}) due to the interaction of electromagnetic field between the silver nanowires. The sensitivity will remain relatively stable with the continuous increasing of the numbers of silver nanowires and the PCF filled with more nanowires is better than the one. Thus by adjusting the distance between the two silver nanowires and the numbers of silver nanowires, we can tune the resonance wavelength to a desired value. Moreover, the irregularity of the filled nanowires has no effect on sensitivity, which will be very convenient for the practical implementation of the experiment.

This work was supported by the National Basic Research Program of China (973 Program). (Grant NO. 2010CB327801) and National Natural Science Foundation of China (Grant NO. 10874128).

(

Relationship between wavelength and attenuation constant of the fundamental mode of the grapefruit PCF occupied by one silver nanowire of 300 nm radius, and the fundamental mode at peak I (the origin of the resonance peak is 310 nm) and peak II (the origin of the resonance peak is 635 nm); the red and blue curves represent the refractive indices of the samples which are 1.33 and 1.335, respectively.

(^{−1}

(^{−1})

The changes of sensitivity of different relative distances between two silver nanowires in the PCF.

(^{−1})

The relationship between the sensitivity and different numbers of PCFs filled with silver nanowires.

(_{eff}_{a} = 1.335).

(^{−1})