1D reconfigurable bistable photonic device composed of phase change material for detection of reproductive female hormones

We are proposing a 1D photonic bio-sensing structure (PQ)^N GDG(PQ)^N capable of detecting reproductive hormones of female as depicted in figure 1. The proposed photonic structure (PS) is composed of two identical 1D PSs (PQ)^N and the modified defect layer D whose both sides are coated with two identical buffer layers G made up of phase change chalcogenide material Ge₂Sb₂Te₅. This modified three layer assembly GDG is embedded between 1D PSs (PQ)^2N to configure proposed 1D photonic bio-sensing structure (PQ)^N GDG(PQ)^N . The proposed architecture integrated with GST material induces switchable spectral response and also prevent direct exposure of Si material layer with air and water which in turn restricts the possibility of oxidation of silicon in presence of blood sample to be investigated. The detection of reproductive hormones in the blood samples of female patient has been done by pouring it inside cavity region D of air which is sandwiched between buffer layers of GST material. The letters P and Q are used to represent SiO₂ and Si material layers respectively of 1D PS (PQ)^N . The letter N symbolizes the period number of the structure which is fixed to 5. The entire 1D photonic bio-sensing structure has been kept in air. The thickness of layers P, Q, G and D of the proposed design are optimized to d_P = 380 nm, d_Q = 260 nm, d_G =  20 nm and d_D = 320 nm respectively which yields wider PBG and high sensitivity value of the proposed design. The investigations of proposed research work has been carried out in the region of investigation 600 nm to 770 nm in which the refractive indices of materials SiO₂ and Si are considered as n_P = 1.45 and n_Q = 3.3 respectively.

Phase change material exhibits unconventional remarkably different optical properties between amorphous and crystalline state under the influence of appropriate thermal, optical or electrical stimuli. In this work we have analyzed the performance of proposed design by considering the experimentally obtained complex refractive index values of Ge₂Sb₂Te₅ material under amorphous and crystalline phases both with the help of spectroscopic ellipsometry technique. In this experimental technique thin film of GST material of thickness 20 nm is grown over silicon wafer and by applying Tauc-Lorentz model the complex refractive index values of GST material in terms of wavelength dependent real and imaginary parts are obtained for amorphous and crystalline phases both. Conceptual visualization of this statement has been shown in figure 2 which depicts real and imaginary parts of complex refractive index of GST material in amorphous and crystalline phases both. Here notations naGST, ncGST and KaGST, KcGST are being used to represent real and imaginary parts of complex refractive index of GST material in amorphous and crystalline phases respectively.

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There are several methods by which phase transition between amorphous to crystalline and back in GST materials can be achieved. For example thermal heating through laser pulse of duration micro or nano seconds can be used for robust switching between amorphous to crystalline phases of GST material. Other conventional methods like optical or electrical stimuli may also be used for switching between amorphous and crystalline phases of GST material. The large value of thermal stability and high switching speed makes the GST material as a promising candidate for designing of rewritable and nonvolatile optical devices which in turn may be useful to realize reconfigurable and switchable photonic devices. In this study the phenomenon of phase change in GST material can be easily achieved by applying laser pulse of short during only over buffer layers made up of phase change material. Moreover the optical properties of materials SiO₂ and Si used in this design along with blood samples containing reproductive hormones will not be affected by the application of laser pulse of short duration on Ge₂Sb₂Te₅ material for switching between aGST and cGST phases.

The experimentally obtained complex refractive index values of Ge₂Sb₂Te₅ material under amorphous and crystalline phases [35] have been used to develop theoretical relations giving real and imaginary values of Ge₂Sb₂Te₅ material under amorphous and crystalline phases with the help of polynomial curve fitting of degree 3 and 4 to ensure higher accuracy between experimental and theoretical results. The following expressions 4 and 5 are giving theoretical values of complex refractive index of Ge₂Sb₂Te₅ material under amorphous and crystalline phases used in this study respectively.

$$\begin{eqnarray}&&na=naGST+iKaGST\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&nc=ncGST+iKcGST\end{eqnarray} \tag{ 5 }$$

Here naGST and KaGST are representing real and imaginary parts of complex refractive index of GST material in amorphous phase whose curve fitting expressions are as under

$$\begin{eqnarray}&&naGST=-0.51929{\lambda }^{4}+4.3531{\lambda }^{3}-12.984{\lambda }^{2}+15.789\lambda -2.0315\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&KaGST=-0.29346{\lambda }^{3}+2.3145{\lambda }^{2}-5.9547\lambda +4.9925\end{eqnarray} \tag{ 7 }$$

Similarly ncGST and KcGST are representing real and imaginary parts of complex refractive index of GST material in crystalline phase as under

$$\begin{eqnarray}&&ncGST=1.2751{\lambda }^{3}-8.1973{\lambda }^{2}+16.168\lambda -2.9464\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&KcGST=-0.72443{\lambda }^{4}+5.461{\lambda }^{3}-13.331{\lambda }^{2}+9.7612\lambda +1.8907\end{eqnarray} \tag{ 9 }$$

Figure 2 shows the variation of naGST, KaGST, ncGST and KcGST with respect to wavelength. There is a pronounced contrast of complex refractive index properties of GST material between amorphous and crystalline phases in near infrared region. This property of change in refractive index of GST material under amorphous and crystalline phases opens new gateway for designing of reconfigurable and switchable photonic biosensors with ultra high sensitivity based on GST materials.

In this section we describe the refractive index formulae to determine refractive index values of progesterone and estradiol reproductive hormones in females. These values are dependent on their respective concentration level in blood samples. Progesterone is a type of steroid hormone which needs to be investigated during menstrual cycle disorder as well as early stages of pregnancy. The role of progesterone in females is more significant during reproductive process such as menstruation and parturition. This hormone has the ability to release egg from ovary of women when it is fertilize by sperm. Besides this the production of milk in mammary glands located inside breasts of females begins when progesterone hormone combines with other hormones. The progesterone is also responsible for developing fetus in uterus during late pregnancy. Moreover estradiol is a type of estrogen which is considered to be female sex hormone. Its main purpose is to establish and then retain the reproductive system. The increase in estradiol level causes the ripeness and release of the eggs during the menstrual cycle. These hormones are also responsible for the thickening of the uterus lining during implantation of fertilized egg. This hormone is very essential in males' sexual function which includes libido, erectile function, and spermatogenesis.

The refractive index of progesterone and estradiol hormones depending upon their concentration levels in blood sample has been theoretically obtained with the help of expressions 10 and 11 respectively as given below.

$$\begin{eqnarray}&&{n}_{p}=4.2882\times {10}^{-8}\,{c}^{3}-2.2847\times {10}^{-6}\,{c}^{2}+4.5962\times {10}^{-5}c+1.3352\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{n}_{e}=2.678\times {10}^{-4}{c}^{3}-1.7684\times {10}^{-3}{c}^{2}+3.7489\times {10}^{-3}c+1.3333\end{eqnarray} \tag{ 11 }$$

Here c is the concentration level of progesterone and estradiol hormones in blood sample in unit of nmol l⁻¹. We have used experimentally obtained refractive index values of progesterone and estradiol hormones which have been obtained with the help of Atago PAL-RI portable refractometer. In order to get more realistic results, the concentration dependent refractive index values of progesterone and estradiol hormones obtained theoretically with the help of relations 10 and 11 respectively have been synchronized with experimentally obtained refractive index values of progesterone and estradiol hormones. For this purpose cubic curve fitting has been applied on the experimentally obtained discrete refractive index values of progesterone and estradiol hormones to get the relations 10 and 11 respectively. The theoretical cubic curve fitting data along with experimentally obtained data of refractive index values of progesterone and estradiol hormones are plotted in figures 3(a) and (b) respectively.

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Figures 3(a) and (b) show the refractive index variation of cubic curve fitting and experimental data corresponding to blood sample containing progesterone and estradiol hormones of different concentrations respectively. Refractive index of progesterone hormone varies between 1.3352 to 1.3358 as its concentration level in blood sample changes from 0 to 31 nmol l⁻¹ as shown in figure 3(a). Beside this the refractive index values of estradiol hormone swiftly changes from 1.3332 to 1.3359 as we change concentration level in blood sample from 0 to 1.3 nmol l⁻¹ respectively. Further increase in the concentration level of estradiol hormone from 1.3 nmol l⁻¹ to 3.1 nmol l⁻¹ causes a little change in the refractive index value of estradiol hormone which varies from 1.3355 to 1.3359 as depicted in figure 3(b). From figures 3(a) and (b) it can be clearly seen that the cubic curve fitting data obtained theoretically by using relations 10 and 11 are well in agreement with the experimentally obtained data from portable refractometer Atago PAL-RI.

The transmission characteristics of 1D photonic biosensor (PQ)⁵ GDG(PQ)⁵at normal incidence has been investigated in this section under the influence of amorphous and crystalline phases of GST material. The transfer matrix method has been applied with the help of MATLAB software to get the results of this study. The structural parameters of the proposed biosensor are in accordance with the parameters as defined in section 3.1. To carry out the investigations first blood sample containing progesterone hormones of concentrations 0 nmol l⁻¹, 60 nmol l⁻¹, 80 nmol l⁻¹ and 100 nmol l⁻¹ have been poured one by one into defect layer region D of air. The transmission spectra of 1D photonic bio-sensing structure (PQ)⁵ GDG(PQ)⁵ under crystalline and amorphous phases of GST material corresponding to different blood samples containing progesterone hormone of concentrations 0 nmoll⁻¹ , 60 nmol l⁻¹, 80 nmol l⁻¹ and 100 nmol l⁻¹ are being plotted in figures 4 (a) and (b) respectively.

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Figure 4(a) depicts that under crystalline phase of GST material four distinguishable defect modes corresponding to blood samples of concentrations 0 nmol l⁻¹, 60 nmol l⁻¹, 80 nmol l⁻¹ and 100 nmol l⁻¹ are found between 692 nm to 696 nm inside photonic band gap. The reason of these distinguishable defect modes in side PBG is due to minute change in the concentration dependent effective refractive indices of blood samples containing progesterone hormone as described in relation 10. The new location of the defect mode shifted to higher wavelength side is dependent upon the concentration of progesterone hormones in blood sample which varies from 0 nmol l⁻¹ to 100 nmol l⁻¹. The standing wave concept of laser cavity can also be used to understand the presence of defect mode inside PBG as noticed in figure 3. According to standing wave concept of laser cavity only those wavelengths inside cavity can be come out which satisfy the relation δ = kλ = n_eff L. Here notations δ, k, λ, n_eff and L have been used to represent optical path difference, an integer, wavelength inside cavity, effective refractive index and cavity length respectively. Moreover the transmittance of the defect modes corresponding to various blood samples of different concentration slightly varies above 90% which is good for any photonic biosensing application. The transformation of GST from aGST to cGST phases under the above mentioned circumstances exhibits significant change in defect mode location inside PBG with moderate reduction in its transmittance as shown in figure 4(b). It also shows that under amorphous phase of GST material defect modes of concentrations 0 nmol l⁻¹, 60 nmol l⁻¹, 80 nmol l⁻¹ and 100 nmol l⁻¹ have repositioned their location between 718 nm to 722 nm. Beside this the concentration dependent transmittance of these defect modes also vary from 70% to 76% which is more than enough for photonic bio-sensing applications. This is due to fact that under crystalline phase KcGST is higher in contrast to KaGST as evident from figure 2 which signifies optical loss. Another observation is that under crystalline phase full width half maximum of defect mode increases which in turn affects the system performance. Thus the application of GST material in fabrication of modern photonic devices can also be used for designing of switchable and reconfigurable photonic biosensors.

Next we have studied how defect mode position and intensity inside PBG will be affected under amorphous and crystalline phases of GST material when cavity is loaded with blood samples containing estradiol hormone of concentration 0 nmol l⁻¹, 6 nmol l⁻¹, 8 nmol l⁻¹ and 10 nmol l⁻¹ one by one. For this reason we have plotted transmission spectra of the proposed bio-sensing design when the cavity is loaded with blood samples containing estradiol hormone of concentration 0 nmol l⁻¹, 6 nmol l⁻¹, 8 nmol l⁻¹ and 10 nmol l⁻¹ under the influence of amorphous and crystalline phases of GST material as shown in figures 5(a) and (b) respectively.

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Under crystalline phase of GST material four closely spaced distinguishable defect modes corresponding to blood samples containing estradiol hormones of concentration 0 nmol l⁻¹, 6 nmol l⁻¹, 8 nmol l⁻¹ and 10 nmol l⁻¹ are found between wavelength 690 nm to 710 nm inside PBG of the structure under consideration as presented in figure 5(a). These defect modes shifts toward higher wavelength side as concentration of blood sample increases but their intensity slightly decreases from 98% to 90% as depicted in figure 5(a). The switching of GST material from crystalline phase to amorphous phase results lateral shift in the position of closely spaced distinguishable defect modes inside PBG between wavelength range 710 nm to 735 nm as shown in figure 5(b). Interestingly we have noticed that under amorphous phase the intensity of these defect modes increases from 72% to 89% depending upon the concentration of estradiol hormone in blood sample from 0 nmol l⁻¹ to 10 nmol l⁻¹ contrary to the findings of same blood sample under crystalline phase as shown in figure 5(a). Moreover full width hall maximum of each defect mode under amorphous phase is reduced marginally as compared to the results of figure 5(a). Thus the application of phase change GST materials can be used to improve the performance of reconfigurable photonic bio-sensing devices.

Further the effect of variation in the thickness of defect layer region loaded with blood sample containing progesterone hormone of concentration 20 nmol l⁻¹, on the performance of the device under cGST and aGST phases of Ge₂Sb₂Te₅ material has been investigated. For this purpose transmission spectra of proposed bio-sensing design at θ₀ = 0° with defect layer thicknesses 320 nm, 340 nm, 360 nm and 380 nm under the influence of crystalline and amorphous phases of GST material have been plotted in figures 6(a) and (b) respectively.

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Figure 6(a) depicts four evenly spaced narrow defect modes of intensity more than 95% which are located between wavelength range 690 nm to 720 nm inside PBG of the structure. As the thickness of cavity region increases from 320 nm to 380 nm in steps of 20 nm, position of the narrow defect mode inside PBG evenly shifts toward higher wavelength side between 690 nm to 720 nm inside PBG. Moreover this increase in thickness of cavity region results the variation in intensity from 98% to 95% which is quite good for photonic bio-sensing application. Next we switch GST material from crystalline to amorphous phase and keeping other parameters of the design identical to evaluate the performance of the design under the influence of amorphous phase of GST material. The transmission spectra of proposed design loaded with blood sample containing progesterone hormone of concentration 20 nmol l⁻¹ corresponding to defect layer thicknesses 320 nm, 340 nm, 360 nm and 380 nm under amorphous phase has been plotted in figure 6(b). It shows that under the influence of amorphous phase of GST material, PBG is almost stagnant to that of figure 6(a). On the other hand defect mode positions corresponding thicknesses 320 nm, 340 nm, 360 nm and 380 nm are shifted to new positions inside PBG and are located between wavelength range 712 nm to 750 nm as shown in figure 6(b). The increase in the thickness of defect layer region results corresponding increase in its intensity which varies from 74% to 100% corresponding to thickness variation from 320 nm to 380 nm respectively. Beside this the increase in the thickness also increases the full width half maximum of defect mode inside PBG which in turn alters the performance of the design.

In this section we have estimated the performance of the proposed photonic biosensor by changing the thickness of cavity region from 320 nm to 380 nm insteps of 20 nm when the cavity is loaded with blood sample containing estradiol hormone of concentration 7 nmol l⁻¹ under crystalline and amorphous phases of GST material. For this purpose we have plotted transmission spectra corresponding to crystalline and amorphous phases of GST material as shown in figures 7(a) and (b) respectively.

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Figure 7(a) shows evenly spaced distinguishable defect modes of transmittance which varies from 98% to 92% corresponding to thickness variation from 320 nm to 380 nm respectively under crystalline phase of GST material. These defect modes are located between wavelength range 690 nm to 718.5 nm inside PBG and have almost identical full width half maximum. Further we switch Ge₂Sb₂Te₂ material from cGST to aGST phases keeping other parameters identical and observe that the effect of this change on the performance of the device in comparison to performance under crystalline phase as shown in figure 7(a). The transmission spectra showing performance of the design under amorphous phase of GST corresponding to defect layer thicknesses 320 nm, 340 nm, 360 nm and 380 nm, has been plotted in figure 7(b) at normal incidence. It shows that under amorphous phase of GST, position of PBG is almost stagnant but the defect modes inside this PBG are repositioned to new locations between wavelength range 720 nm to 750 nm. The intensity of these defect modes gradually increases from 74% to 100% as thickness of defect layer increases from 320 nm to 380 nm respectively. Besides this their FWHM also increases as evident from figure 7(b). Thus the performance of the photonic biosensor is almost identical when the cavity region is loaded separately with blood samples containing progesterone and estradiol reproductive hormones of certain concentration under crystalline and amorphous phase of GST material.

In this section the effect of change in angle of incidence from 0° to 30° in steps of 10° corresponding to TE polarized electromagnetic wave on the performance of the proposed biosensor has been studied. For this purpose we have loaded the cavity region of thickness 320 nm with blood sample containing progesterone hormone of concentration 20 nmol l⁻¹.

Figures 8(a) and (b) show the transmittance spectra of proposed structure under crystalline and amorphous phase of GST material at normal incidence respectively. Under crystalline phase of GST material figure 8(a) shows that the increase in angle of incidence from 0° to 30° results the shifting of defect modes towards lower wavelength side between wavelength range 694 nm to 670 nm inside the PBG. The gradual increase in the incident angle also decreases the intensity of the defect mode which varies between 94% to 89. The increase in the angle of incidence corresponding to TE polarized waves improves the geometrical path travelled by the light wave inside defect layer loaded with blood sample containing women reproductive hormones. This improvement in the geometrical path ensures much better interaction between light wave and blood sample which in turn improves the performance of the proposed bio-sensing design. Next we switch Ge₂Sb₂Te₅ material from cGST phase to aGST phase keeping all other structural parameters constant. Under these circumstances the transmission spectra has been plotted in figure 8(b). This switching between phases results change in the positions of angle of incident dependent defect modes inside PBG as shown in figure 8(b). Under amorphous phase of GST the intensity of defect modes inside PBG is nearly 72%. This value of intensity is enough for bio-sensing applications. Thus the variation in angle of incidence can also be used to improve the performance as well as conformation of investigations carried by proposed design under cGST and aGST phases of Ge₂Sb₂Te₅ material which induces the reconfigurable property into our proposed photonic bio-sensing design.

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3.8.1. Effect of change in angle of incidence corresponding to TE polarized electromagnetic wave on the performance of the design when the cavity of thickness 320 nm is loaded with estradiol sample of concentration 7 nmol l⁻¹ under cGST and aGST phases of Ge₂Sb₂Te₅ material

Here the effect of change in angle of incidence from 0° to 30° in steps of 10° corresponding to TE polarized electromagnetic wave on the performance of the proposed biosensor loaded with blood sample containing estradiol hormone of concentration 7 nmol l⁻¹ has been studied. For this purpose we have fixed the defect layer thickness to 320 nm. Figures 9(a) and (b) show the transmittance spectra of proposed structure under crystalline and amorphous phases of GST material at normal incidence respectively.

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Under crystalline phase of GST material figure 8(a) shows four defect modes of decreasing intensity corresponding to angle of incidence 0°, 10°, 20° and 30°. These defect modes are located between wavelength range 660 nm to 700 nm inside PBG under cGST phase of Ge₂Sb₂Te₅ material. On the other hand under amorphous phase of GST material the positions of these angle dependent defect modes inside PBG relocate their positions between wavelength range 700 nm to 730 nm. The intensity of these defect modes is almost stagnant which is above 70%. Thus we have similar findings of the proposed design loaded with two different blood samples containing female reproductive hormones under the influence of cGST and aGST phases of Ge₂Sb₂Te₅ material.

3.8.2. Performance evaluation of the proposed design

The evaluation of the performance and efficiency of the proposed design has been done by calculating sensitivity (S), quality factor (Q) and figure of merit (FOM) values because these are most common parameters to evaluate the performance of any biosensor [6, 13–15]. The sensitivity is the ratio of change in the central wavelength of defect mode (Δλ) due to change in the refractive index of cavity region (Δn) as defined below

$$\begin{eqnarray}&&S=\displaystyle \frac{{\rm{\Delta }}\lambda }{{\rm{\Delta }}n}\end{eqnarray} \tag{ 12 }$$

For getting accuracy in biosensor measurements the quality factor of the biosensor should be as high as possible and it can be calculated with the help of following relation

$$\begin{eqnarray}&&Q=\displaystyle \frac{{\lambda }_{peak}}{FWHM}\end{eqnarray} \tag{ 13 }$$

where λ_peak and FWHM are used to represent central wavelength and full width half maximum of transmission peak inside cavity region.

The figure of merit (FOM) of biosensor design is defined as the ratio of sensitivity to FWHM of defect mode inside PBG. FOM can be obtained by using following relation

$$\begin{eqnarray}&&FOM=\displaystyle \frac{S}{FWHM}\end{eqnarray} \tag{ 14 }$$

The evaluation of the performance of the proposed 1D photonic biosensor (PQ)⁵ GDG(PQ)⁵ loaded with blood samples containing progesterone and estradiol reproductive hormones of different concentration under crystalline and amorphous phases of GST has been done by calculating S, Q and FOM values of designs depending upon change in internal and external parameters of the design. The numerical values of S, Q and FOM of design under crystalline and amorphous phases have been obtained by using relations (12) to (14) and are listed in table 1 to table 8. Tables 1–4 are summarizing the calculated results of the design corresponding to blood sample with progesterone hormone of concentration 0, 60 nmol l⁻¹, 80 nmol l⁻¹ and 100 nmol l⁻¹ at incident angle 0°and defect layer thickness 320 nm and 380 nm under crystalline and amorphous phases of GST. On the other hand tables 5–8 are summarizing above mentioned findings for blood sample containing estradiol hormone of concentration 0, 6 nmol l⁻¹, 8 nmol l⁻¹ and 10 nmol l⁻¹ under crystalline and amorphous phases of GST. The comparison between the results of table 1 and table 2 yields that under given circumstances with progesterone sample at d_D = 320 nm the performance of the proposed design in terms S, Q and FOM values can be improved significantly by switching amorphous to crystalline phase of GST material. The similar findings have been worked out with d_D = 380 nm as per data mentioned in tables 3 and 4 corresponding to crystalline and amorphous phases respectively. We have also evaluated the performance of the devise when cavity is loaded with blood samples containing estradiol hormones of different concentration at d_D = 320 nm and 380 nm under amorphous and crystalline phases of GST material. The results associated with these findings are presented in tables 5–8. Additionally the order of limit of detection of our design is 10^–5. It helps to estimate the smallest change in the refractive index of analyte which could be significantly detected by proposed structure. Thus we can conclude that GST can be a better choice for designing 1D defective photonic structures suitable for refractive index based sensing applications. Moreover the GST based photonic devices are reconfigurable and externally tunable.

Table 1. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing progesterone hormones of different concentration with ${d}_{D}=320\,{\rm{nm}}$ at θ₀ = 0° under crystalline phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Progesterone	0	1.33520	718.33	0.05	—	14367	—
	60	1.33899	718.85	0.05	137.02	14377	2740.5
	80	1.34621	719.54	0.06	62.67	11992	1044.5
	100	1.35983	721.00	0.05	59.28	14420	1185.6

Table 2. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing progesterone hormones of different concentration with ${d}_{D}=320\,{\rm{nm}}$ at θ₀ = 0° under amorphous phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Progesterone	0	1.33520	692.83	0.06	—	11547	—
	60	1.33899	693.14	0.06	81.69	11552	1361.5
	80	1.34621	693.77	0.06	57.22	11563	953.7
	100	1.35983	694.97	0.06	48.72	11583	812.0

Table 3. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing progesterone hormones of different concentration with ${d}_{D}=380\,{\rm{nm}}$ at θ₀ = 0° under crystalline phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Progesterone	0	1.33520	715.98	0.06	—	11933	—
	60	1.33899	716.40	0.06	110.67	11940	1844.7
	80	1.34621	717.14	0.05	67.212	14342.8	1344.2
	100	1.35983	718.61	0.05	59.68	14372.2	1193.7

Table 4. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing progesterone hormones of different concentration with ${d}_{D}=380\,{\rm{nm}}$ at θ₀ = 0° under amorphous phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Progesterone	0	1.33520	743.16	0.2	—	3715.8	—
	60	1.33899	743.37	0.21	55.33	3639.9	263.5
	80	1.34621	743.85	0.26	43.59	2860.9	167.7
	100	1.35983	744.625	0.31	31.50	2402.0	101.6

Table 5. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing estradiol hormones of different concentration with ${d}_{D}=320\,{\rm{nm}}$ at θ₀ = 0° under crystalline phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Estradiol	0	1.33330	692.65	0.06	—	11544.2	—
	6	1.34997	694.11	0.05	87.85	13882.2	1757.04
	8	1.38723	697.41	0.05	61.194	13948.2	1223.88
	10	1.46175	704.20	0.05	52.86	14084.0	1057.23

Table 6. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing estradiol hormones of different concentration with ${d}_{D}=320\,{\rm{nm}}$ at θ₀ = 0° under amorphous phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Estradiol	0	1.33330	718.11	0.07	—	10258.7	—
	6	1.34997	719.96	0.1	110.98	7199.6	1109.78
	8	1.38723	723.9	0.1	73.06	7239.0	730.62
	10	1.46175	731.01	0.11	55.35	6645.6	503.21

Table 7. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing estradiol hormones of different concentration with ${d}_{D}=380\,{\rm{nm}}$ at θ₀ = 0° under crystalline phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Estradiol	0	1.33330	715.8	0.06	—	11930.0	—
	6	1.34997	717.55	0.06	104.98	11959.7	1749.65
	8	1.38723	721.49	0.09	73.062	8016.6	811.80
	10	1.46175	729.12	0.11	59.40	6628.4	540.01

Table 8. The performance of proposed design (PQ)⁵ GDG(PQ)⁵ loaded with blood sample containing estradiol hormones of different concentration with ${d}_{D}=380\,{\rm{nm}}$ at θ₀ = 0° under amorphous phase of GST.

Types of hormones	C (nmol l−1)	Refractive index	${\lambda }_{d}$(nm)	${\lambda }_{FWHM}$ (nm)	S (nm/RIU)	$Q$	FOM (RIU)
Estradiol	0	1.33330	743.07	0.28	—	2653.8	—
	6	1.34997	744.2	0.31	67.79	2400.7	218.67
	8	1.38723	746.46	0.39	41.91	1914.0	107.46
	10	1.46175	750.37	0.58	30.44	1293.7	52.48

Finally we have compared our results with some excellent piece of research work carried by distinguished photonic workers in table 9. This comparison highlights the usefulness of our proposed design in contrast to the previous research works on refractive index based sensing mechanism having smaller sensitivity. The sensitivity of our structure can be easily tailored conveniently by changing incident angle as well as thickness of defect layer region. Beside this sensitivity can also be tailored by switching states of Ge₂Sb₂Te₅ material from aGST to cGST phases and back. The presence of GST material does not require phase matching conditions similar to the devices based on surface plasmon resonance (SPR) to achieve tunable sensitivity. The order of quality factor and figure of merit values of the proposed photonic biosensor are similar to majority of schemes cited below in table 9. The experimental realization of the proposed design based on refractive index sensing technology has been shown in figure 1. Here we required a wide band source of light through which light can be launched into proposed structure with the help of single mode fiber and directional couplers. The blood sample flow chamber should be mounted near the cavity layer with IN and OUT facility for sample to be poured. The interaction between light and blood sample under investigation results the change in position of defect mode inside PBG. This shifting of defect mode indie PBG of transmission spectra is analyzed with the help of optical spectrum analyzer and final results will be displayed in computer with the help of software. It is evident from table 9 that the proposed biosensor possesses relatively good sensitivity and much better performance to detect reproductive hormones in females in contrast to the resent bio-sensing research work.

Table 9. Comparison of numeric values of sensitivity, quality factor and figure of merit of proposed design with previous research work of similar kind for evaluating the performance of the proposed bio-sensing design (PQ)⁵GDG(PQ)⁵.

Year	S (nm/RIU)	Q-factor	FOM (RIU)	Frequency Range	References
2016	34.11	Not mentioned	1.1 × 103	THz	[38]
2017	17	3 × 104	2.23 × 102	Visible to NIR	[39]
2019	25.75–51.49	Not mentioned	Not mentioned	NIR	[40]
2019	32–43.13	Not mentioned	Not mentioned	NIR	[41]
2019	53.0–90.9	Not mentioned	Not mentioned	NIR	[42]
2020	10	3 × 102	15.1	Visible	[43]
2021	71–75	Not mentioned	Not mentioned	NIR	[44]
This work	137.02–30.44	(0.12–1.43) × 104	(0.2–2.7) × 103	Visible	...