Optimal Design of Silicon Photonic Wire Biosensors for High Sensitive Detection

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Advances in Biomedical Engineering Research (ABER) Volume 2, 2014

Optimal Design of Silicon Photonic Wire Biosensors for High Sensitive Detection Yaping Zhang, Qinyang Sun The University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo, 315100, China yaping.zhang@nottingham.edu.cn Abstract Silicon photonic wire biosensors’ homogenous detection sensitivities with TE and TM modes for typical test solutions have been investigated. The highest homogenous detection sensitivity of 0.8486 has been achieved when the silicon photonic wire’s width and core thickness are of 0.5μm and 0.22μm for TM mode homogenous sensing. It was found that the homogenous detection sensitivity of TM mode of a silicon photonic wire is about 7 times of that of a TE mode. Silicon photonic wires’ homogenous detection sensitivities are also compared with that of a Silicon on Silica (SOI) rib waveguide, and it was found that the homogenous detection sensitivity of a silicon photonic wire is generally a few thousand times of that of a SOI rib waveguide for either TE or TM mode.

Keywords Biosensors; Lab on a Chip; Silicon Photonic Wire Detection; Evanescent Field Detection; Detection Sensitivity

Introduction Many research results have shown that silicon photonic wire evanescent field biosensors have the highest sensitivity of detection compared with other waveguide platforms. It is well-known that SOI material and its fabrication technology have been well established for telecommunication applications, which may find wide applications in biosensing. Recently, great efforts have been put into the exploitation of these advanced technologies for their implementations in biosensing application areas, especially for multiple detection and parallel processing of multiple analytes in multiple test solutions simultaneously and instantaneously in real time.

This study has investigated the homogeneous detection sensitivities of silicon photonic wire waveguide ‘lab on a chip’ biosensors with varied silicon photonic wires’ core thickness and ridge width, for the purpose of homogenous sensing applications with TE and TM modes. This has been carried out for typical test solutions with refractive index in the ranges of 1.3∼1.35 and 1.4∼1.485. The homogenous detection sensitivity of a silicon photonic wire has also been compared with that of a SOI rib waveguide.

Optimal Design of Highly Sensitive Silicon Photonic Wire Biosensors The side view of a silicon photonic wire biosensor is shown in Fig. 1. It shows that the guided light extends its tail into a text solution in the form of evanescent field. The sensing surface has been functionalized with antibody which can be combined with the analytes in the test solution and hence sense its concentration.

FIG. 1 SIDE VIEW OF A SILICON PHOTONIC WIRE BIOSENSOR

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Advances in Biomedical Engineering Research (ABER) Volume 2, 2014

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Homogeneous Sensitivity of Detection For the homogeneous sensing, the waveguide sensitivity of detection can be expressed as:

where neff is the propagation mode effective refractive index, and nc is the refractive index of cover solution.

For the optimal design of highly sensitive silicon photonic wire biosensors, investigations on homogeneous detection sensitivity have been carried out in two structures: (a) silicon photonic wire structure Fig. 2(a), (b) SOI rib waveguide structure in Fig. 2(b).

FIG. 2 CROSS-SECTION OF (a) SILICON PHOTONIC WIRE STRUCTURE; (b) SOI RIB WAVEGUIDE STRUCTURE

Homogeneous Detection Sensitivity of a Silicon Photonic Wire Biosensor The homogeneous detection sensitivity of a silicon photonic wire structure has been investigated on silicon photonic wire width W= 0.5μm and 0.8μm and silicon core thickness h= 0.22μm and 0.34μm for both TE and TM modes. The cover medium’s (i.e. test solution’s) refractive index varies from 1.3 to 1.35 and 1.4 to 1.485 respectively.

Simulation results are compared and shown in Fig. 3-5.

FIG. 3 COMPARISON OF HOMOGENEOUS DETECTION SENSITIVITY OF SILICON PHOTONIC WIRE BIOSENSORS: SILICON PHOTONIC WIRE WIDTH = 0.5μm, 0,8μm,ILICON CORE THICKNESS = 0.22μm, OVER MEDIUM INDEX = 1.3 ∼ 1.35 (a) TE MODE AND (b) TM MODE

FIG. 4 COMPARISON OF HOMOGENEOUS DETECTION SENSITIVITY OF SILICON PHOTONIC WIRE BIOSENSORS: SILICON PHOTONIC WIRE WIDTH = 0.5μm, 0,8μm, SILICON CORE THICKNESS = 0.34μm, COVER MEDIUM INDEX = 1.3 ∼ 1.35 (a) TE MODE AND (b) TM MODE

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Advances in Biomedical Engineering Research (ABER) Volume 2, 2014

FIG. 5 COMPARISON OF HOMOGENEOUS DETECTION SENSITIVITY OF SILICON PHOTONIC WIRE BIOSENSORS: SILICON PHOTONIC WIRE WIDTH = 0.5μm, 0,8μm, SILICON CORE THICKNESS = 0.22μm, COVER MEDIUM INDEX = 1.4 ∼ 1.485 (a) TE MODE AND (b) TM MODE

It can been seen from the Figs. 3-5 that for the same test solution (i.e. with the same medium cover index and fixed silicon core thickness), TM mode is more sensitive than TE mode, and narrower silicon photonic wire (i.e. 0.5 μm) is more sensitive than wider silicon photonic wire (i.e. 0.8 μm).

To further investigate the homogeneous detection sensitivity of a silicon photonic wire structure for the optimal design of a highly sensitive silicon photonic wire biosensor, silicon photonic wire structures with silicon core thickness of 0.22μm and 0.34μm are studied, while the silicon photonic wire width is fixed at w = 0.5μm, and the cover medium’s index ranges are set at typical test solution index ranges of 1.3∼1.35 and 1.4∼1.485, for both TE and TM modes. Simulation results are shown in Fig. 6 and Fig. 7 respectively.

It can be observed that for a silicon photonic wire biosensor, the thinner the silicon core thickness is (i.e. h=0.22μm) the higher the detection sensitivity is. The homogeneous detection sensitivity of a TM mode is about 7 times of that of a TE mode.

FIG. 6 COMPARISON OF HOMOGENEOUS DETECTION SENSITIVITY OF SILICON PHOTONIC WIRE BIOSENSORS: SILICON PHOTONIC WIRE WIDTH = 0.5μm, SILICON CORE THICKNESS = 0.22μm, 0.34μmCOVER MEDIUM INDEX = 1.3 ∼ 1.35 (a) TE MODE AND (b) TM MODE

FIG. 7 COMPARISON OF HOMOGENEOUS DETECTION SENSITIVITY OF SILICON PHOTONIC WIRE BIOSENSORS: SILICON PHOTONIC WIRE WIDTH = 0.5μm, SILICON CORE THICKNESS = 0.22μm, 0.34μmCOVER MEDIUM INDEX = 1.4 ∼ 1.485 (a) TE MODE AND (b) TM MODE

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Homogeneous Detection Sensitivity of a SOI Rib Waveguide Biosensor A rib waveguide structure as shown in Fig2b with dimensions: w1 = 2μm, w2 = 6μm, h = 1.75μm and h = 4μm is investigated. The homogeneous detection sensitivity of the SOI rib waveguide biosensor with the above dimensions and cover medium’s index range of 1.3-1.35 is shown in Fig. 8.

FIG. 8 HOMOGENEOUS DETECTION SENSITIVITY OF A SOI RIB WAVEGUIDE BIOSENSOR WITH TE AND TM MODES COVER MEDIUM INDEX = 1.3 ∼ 1.35

Discussion and Conclusions From simulation results, it can be seen that 1) TM mode is more sensitive than TE mode, typically about 7 times of IE mode in sensitivity; 2) the homogenous detection sensitivity for a silicon photonic wire biosensor with a silicon core thickness of 0.22μm is over twice of that of 0.34μm; 3) In the case of TM mode, the homogenous detection sensitivity for h = 0.22μm is around 0.8 where the highest value is 0.8486; 4) the detection sensitivity of a silicon photonic wire is generally a few thousand times of that of a SOI rib waveguide for either TE or TM mode.

To conclude, the optimal design of silicon photonic wire evanescent field biosensor with the highest homogenous detection sensitivity of 0.8486 has been achieved when the silicon photonic wire’s width and core thickness are of 0.5μm and 0.22μm for TM mode homogenous sensing.

ACKNOWLEDGMENT

This work was supported by the University of Nottingham Ningbo China’s Sandpit Project on Biosensor.

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