The biochemical sensor based on liquid-core photonic crystal fiber filled with gold, silver and aluminum

Highlights

• The biochemical sensor possesses the high sensitivity of −8383.9 nm/RIU.

• The sensitivity of aluminium-filling PCF sensor is much higher.

• The influences of structural parameters of PCF on the sensitivity is small.

Abstract

A highly sensitive SPR-PCF based biochemical sensor has been proposed based on finite element method simulations. Two metal wires are assumed to fill into two air holes in the y direction and the liquid analyte with refractive index higher than background material is injected into the central air hole. The liquid analyte supports liquid-core mode which couples to SPP mode as the phase matching condition is satisfied. High sensitivity of fiber sensor is achieved by the direct interaction between transmitted light and liquid analyte. The fiber sensor possesses the sensitivities of −8383.9 nm/RIU, −8428.6 nm/RIU and –8776.8 nm/RIU by filling gold, silver and aluminium respectively into the air holes of the PCF as the refractive index of liquid analyte varies from 1.454 to 1.478. The influences of the structural parameters of the PCF on the resonance wavelength and confinement loss are also analyzed.

Introduction

Photonic crystal fibers (PCFs) are of significantly potential due to the periodic arrangement of air holes on its cross section which go through the whole fiber along the direction of light propagation [1]. Compared with conventional optical fibers, PCF possesses many distinctive characteristics, such as flexible structural design, large mode area, high birefringence and low confinement loss [2], [3], [4], [5], [6], [7]. In recent years, PCFs have been broadly put into sensing fields because air holes existed provides a platform to integrate fiber and functional materials. Typical fiber materials are usually a bit sensitive to environment, it has been shown that the sensitivity of fiber sensors can be improved by filling sensitive materials into air holes of PCF [8], [9], [10]. In 1950s, R. H. Ritchie introduced surface plasmons theoretically for the first time [11]. Since then the surface plasmon resonance (SPR) technology has been gradually applied to the fiber devices [12], [13], [14]. The coupling resonance occurs between fiber mode and surface plasmon polariton (SPP) mode when the phase matching condition is satisfied by filling or coating plasma materials into air holes or on the fiber surface. H. W. Lee et al. proposed a novel fiber-splicing technique to pump molten gold into the air holes of PCF and a hollow channel of modified step index fiber, meanwhile dips were observed at the resonance wavelength in the transmission spectra [15]. Shuyan Zhang et al. studied the novel characteristics of metal-filled dual-core PCF based on supermode theory and coupled-mode theory, and found the coupling length between the two cores was reduced by the coupling resonance between core mode and SPP mode [16].

Chemical sensor is a device that can respond to chemical component in a small, specific and reversible way, and can produce a measurable signal proportional to the corresponding concentration. Biosensor is a highly selective monitor which uses the biological active units (such as enzyme, antibody, nucleic acid, cell, etc.) as the biological sensitive units. Because of the close relationship between chemical sensor and biosensor, they are often called biochemical sensors. The measurement of chemical refractive index is an important part of biochemical sensor. The maximum confinement loss appears at the resonance wavelength which is greatly sensitive to ambient variety and can be used to detect biochemical samples. Md. Rabiul Hasan et al. originally proposed a refractive index sensor employing niobium nanofilm and aluminum oxide (Al₂O₃) film coated on the PCF surface, which possesses the sensitivities of 3000 and 8000 nm/RIU at the refractive indices of 1.36, 1.40 respectively and the average sensitivity is just 5143 nm/RIU [17]. Ahmmed A. Rifat et al. coated copper and graphene layer on the surface of PCF and made the sensor simpler practically with the sensitivity of 2000 nm/RIU in the detected range of 1.33–1.37 [18]. Zhenkai Fan et al. discussed the sensing characteristics of PCF with metal film coated on the surface of two air holes and the fiber sensor possesses the average sensitivity of 7017 nm/RIU as refractive index of analyte varies from 1.40 to 1.42 [19]. Zipeng Guo et al. realized a wide-range refractive index sensor based on PCF coated with gold film selectively and the sensitivity is −1931.03 nm/RIU in the refractive index range 1.35–1.46 [20]. Tianshu Li et al. studied H-Shaped PCF based refractive index sensor with the sensitivity of 2770 nm/RIU as the refractive index of analyte varies from 1.33 to 1.36 [21]. Meanwhile, many biochemical sensors based on D-shaped PCF coated with metal film have been proposed with high sensitivities [22], [23], [24], [25], [26], [27]. While the metal film on the surface of fiber or air holes is easy to fall off, and it is also easily oxidized, therefore the fiber sensors are not stable [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Fiber sensors based on PCF filled with metal wire are more stable compared with the structures described above, while the fiber sensors reported possess low sensitivity. Xujun Xin et al. proposed a gold-filled PCF sensor and the average refractive index sensitivity is only −4125 nm/RIU in the sensing range of 1.45–1.49 [28]. Nan Chen et al. utilized the dual-optofluidic-channel PCF filled with gold wire to realize a refractive index sensor with low sensitivity of 5500 nm/RIU [29]. Therefore, realizing a highly sensitive sensor is significant by filling metal wire into the air holes of PCF.

A SPR based biochemical sensor has been proposed based on metal-wire filled PCF whose core is made up of liquid analyte. The sensitivity of fiber sensor can be improved by the direct interaction between transmitted light and liquid analyte. The influences of gold, silver and aluminum on the sensing characteristics were analyzed. We also discussed the effects of structural parameters of PCF on the resonance wavelength and confinement loss. The fiber sensor possesses the high sensitivities of −8383.9 nm/RIU, −8428.6 nm/RIU and −8776.8 nm/RIU based on the PCF filled with gold, silver and aluminum respectively as the refractive index of liquid analyte varies from 1.454 to 1.478.