**5. An optimised structure for higher spectral sensitivity**

The analysis done so far gives some insight into the effects the structural parameters have on sensor performance. These results are summarised in Table 1.

Fig. 8. Shift in resonant wavelength of the loss spectrum for a variation in tAu of the proposed SPR sensor. Analyte refractive index (na =1.33), d01/Λ = 0.15, d1/Λ = 0.2, d2/Λ = 0.35.

According to Table 1, d01, tAu and d1 can be considered as loss control parameters. With regards to spectral sensitivity, the main candidates to consider are tAu and d1. Of these two parameters, d1 appears relatively easier to control as compared to tAu. It will therefore be more convenient to fix tAu to an appropriate value and optimise d1 to achieve the desired sensitivity. It must also be noted that there is a limit to which d1 can be increased due to the associated high confinement losses. To minimise the confinement losses, d01 can optimised

A Novel Compact Photonic Crystal Fibre Surface

**Confinement**

tolerance possible.

**6. Conclusion** 

parameters; Λ = 1.5 μm, d1/Λ = 0.50, d01/Λ = 0.20, tAu = 40 nm.

 **Loss (dB/cm)**

Plasmon Resonance Biosensor for an Aqueous Environment 93

**= 1.5 m**

**na=1.33 na=1.34**

**Wavelength (nm)**

Figure 10 shows the shift in resonant wavelength for a change in analyte refractive index from na = 1.33 to na = 1.34 for the optimised structure. It indicates an improvement in the spectral sensitivity, which is now approximately 4000 nm / RIU. If the assumption is made that 0.1nm change in the position of a resonance peak can be detected reliably, the resulting sensor resolution is 2.5×10-5 RIU, which is better than the 3×10-5 RIU and 1×10-4 RIU reported by (Hassani and Skorobogatiy 2006) and (Hautakorpi, Mattinen, and Ludvigsen 2008)respectively. The optimisation procedure presented so far can give an indication of the manufacturing tolerances acceptable, to maintain the estimated sensor sensitivity. Of particular interest is the fabrication tolerance of the air holes of the PCF SPR sensor structure. It can be concluded from Fig. 6, 9 and 10 that the maximum allowable change in both d01 /Λ and d1/Λ to maintain the estimated sensitivities, is 50%. Hence, the proposed design can maintain its estimated sensitivity of 4000 nm / RIU provided the holes are fabricated within the 50% tolerance assuming all other conditions remain constant. The current state of advanced PCF fabrication technologies, make fabrication within this

Since sensor length is inversely proportional to the modal loss, optimization of the PCF structural parameters allows design of PCF SPR sensors of widely different lengths (from millimetre to meter), while having comparable sensitivities. Due to the relatively high loss of our proposed PCF SPR sensor, its length is limited to the centimetre scale. Therefore, the proposed sensor should be rather considered as an integrated photonics element than a fibre.

The design and optimisation of a novel PCF SPR biosensor has been presented in this chapter. The loss spectra, phase matching conditions and sensitivity of the proposed

biosensor have been presented using a full – vector FEM with PML.

Fig. 10. Shift in resonant wavelength of the optimised PCF SPR biosensor. Structural

500 550 600 650 700 750 800

to keep much of the field inside the core without compromising much sensitivity. By taking all these factors into consideration, the final set of device parameters to maximise sensitivity whilst maintaining an appreciable confinement loss are; Λ = 1.5 μm, d1/Λ = 0.50, d01/Λ = 0.20, tAu = 40 nm.

Fig. 9. Shift in resonant wavelength of the loss spectrum for a variation in d1 of the proposed SPR sensor. Analyte refractive index (na=1.33), d01/Λ = 0.15, tAu = 40 nm, d2/Λ = 0.35.


Table 1. Summary of influence of structural parameters on properties of the proposed PCF SPR biosensor.↑ represents an increase in a parameter or property whilst ↓ represents a decrease.

to keep much of the field inside the core without compromising much sensitivity. By taking all these factors into consideration, the final set of device parameters to maximise sensitivity whilst maintaining an appreciable confinement loss are; Λ = 1.5 μm, d1/Λ = 0.50,

**Wavelength (nm)**

Fig. 9. Shift in resonant wavelength of the loss spectrum for a variation in d1 of the proposed SPR sensor. Analyte refractive index (na=1.33), d01/Λ = 0.15, tAu = 40 nm, d2/Λ = 0.35.

d1↑ ↑ ↑ ↑

tAu↑ ↑ ↑ ↓

Table 1. Summary of influence of structural parameters on properties of the proposed PCF SPR biosensor.↑ represents an increase in a parameter or property whilst ↓ represents a

↑ - But it has less influence on it as compared to tAu and d1.

**Parameter Sensitivity** <sup>λ</sup> S (λ) **Resonant Wavelength** 

↑ - But has less influence on it due to the fact that there is a relatively small change in λpeak for a slight increase in d01.

500 550 600 650 700 750 800

**d1/=0.25, na=1.34**

**( λpeak) Confinement Loss** 

↓

**d1/=0.15, na=1.33 d1/=0.15, na=1.34 d1/=0.20, na=1.33 d1/=0.20, na=1.34 d1/=0.25, na=1.33**

**= 1.5 m**

d01/Λ = 0.20, tAu = 40 nm.

d01↑

decrease.

**Confinement**

0

20

40

60

80

100

120

140

160

180

 **Loss (dB/cm)**

Fig. 10. Shift in resonant wavelength of the optimised PCF SPR biosensor. Structural parameters; Λ = 1.5 μm, d1/Λ = 0.50, d01/Λ = 0.20, tAu = 40 nm.

Figure 10 shows the shift in resonant wavelength for a change in analyte refractive index from na = 1.33 to na = 1.34 for the optimised structure. It indicates an improvement in the spectral sensitivity, which is now approximately 4000 nm / RIU. If the assumption is made that 0.1nm change in the position of a resonance peak can be detected reliably, the resulting sensor resolution is 2.5×10-5 RIU, which is better than the 3×10-5 RIU and 1×10-4 RIU reported by (Hassani and Skorobogatiy 2006) and (Hautakorpi, Mattinen, and Ludvigsen 2008)respectively. The optimisation procedure presented so far can give an indication of the manufacturing tolerances acceptable, to maintain the estimated sensor sensitivity. Of particular interest is the fabrication tolerance of the air holes of the PCF SPR sensor structure. It can be concluded from Fig. 6, 9 and 10 that the maximum allowable change in both d01 /Λ and d1/Λ to maintain the estimated sensitivities, is 50%. Hence, the proposed design can maintain its estimated sensitivity of 4000 nm / RIU provided the holes are fabricated within the 50% tolerance assuming all other conditions remain constant. The current state of advanced PCF fabrication technologies, make fabrication within this tolerance possible.

Since sensor length is inversely proportional to the modal loss, optimization of the PCF structural parameters allows design of PCF SPR sensors of widely different lengths (from millimetre to meter), while having comparable sensitivities. Due to the relatively high loss of our proposed PCF SPR sensor, its length is limited to the centimetre scale. Therefore, the proposed sensor should be rather considered as an integrated photonics element than a fibre.
