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Ultra-sensitive refractive index sensing enabled by a dramatic ellipsometric phase change at the band edge in a one-dimensional photonic crystal

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Abstract

Surface plasmon polaritons (SPPs) and Bloch surface waves (BSWs) have been widely utilized to design sensitive refractive index sensors. However, SPP- and BSW-based refractive index sensors require additional coupling component (prism) or coupling structure (grating or fiber), which increases the difficulty to observe ultra-sensitive refractive index sensing in experiments. Herein, we realize dramatic ellipsometric phase change at the band edges in an all-dielectric one-dimensional photonic crystal for oblique incidence. By virtue of the dramatic ellipsometric phase change at the long-wavelength band edge, we design an ultra-sensitive refractive index sensor at near-infrared wavelengths. The minimal resolution of the designed sensor reaches 9.28×10−8 RIU. Compared with SPP- and BSW-based refractive index sensors, the designed ultra-sensitive refractive index sensor does not require any additional coupling component or coupling structure. Such ultra-sensitive refractive index sensor would possess applications in monitoring temperature, humidity, pressure, and concentration of biological analytes.

© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

1. Introduction

Over the past decades, refractive index sensing has attracted the great interest of researchers due to its potential applications in monitoring temperature [13], humidity [4,5], pressure [6,7], and concentration of biological analytes [810]. It is known that optical resonances can be utilized to realize sensitive refractive index sensing [1116]. Among them, two kinds of optical resonances have been utilized frequently, i.e., surface plasmon polaritons (SPPs) and Bloch surface waves (BSWs). for SPPs, electromagnetic waves are strongly enhanced around the interfaces between a metal layer and a dielectric layer [17,18]. Assisted by the unique resonant properties of SPPs, the resolution of refractive index sensors can be low as the order of 10−6 RIU (refractive index units) [19]. However, the dispersion relation of SPPs lies below the light cone, which indicates that SPPs cannot be directly excited by the electromagnetic waves incident from the air [1719]. To achieve SPP-based sensitive refractive index sensing, additional coupling components (prisms [2022]) or coupling structures (gratings [23,24] or fibers [25,26]) are required. For BSWs, electromagnetic waves are strongly enhanced around the surfaces of one-dimensional (1-D) photonic crystals (PhCs) [5,2731]. Enabled by the unique resonant properties of BSWs, the resolution of refractive index sensors can be low as the order of 10−5 RIU [32,33]. Nevertheless, similar to SPPs, the dispersion relation of BSWs lies below the light cone [5,2733]. To achieve BSW-based sensitive refractive index sensing, additional coupling components (prisms [34,35] or coupling structures (gratings [3638]) are also required. Although SPPs and BSWs have been widely utilized to achieve sensitive refractive index sensing, the requirement of additional coupling components or coupling structures increases the difficulty to observe ultra-sensitive refractive index sensing in experiments.

Recently, researchers utilized dramatic ellipsometric phase change of Tamm plasmon polaritons (TPPs) to achieve sensitive refractive index sensing [3942]. Ellipsometric phase is the reflection phase difference between transverse magnetic (TM) and transverse electric (TE) polarized electromagnetic waves [43]. For TPPs, electromagnetic waves are strongly enhanced around the interfaces between a metal layer and a 1-D PhC [4448]. Around the TPP wavelength at oblique incidence, the ellipsometric phase changes dramatically with respect to the wavelength. Therefore, the resolution of refractive index sensing can be low as the order of 10−6 RIU [39]. Different from SPPs and BSWs, the dispersion relation of TPPs lies inside the light cone. In other words, TPPs can be directly excited by the electromagnetic waves incident from the air without additional coupling components or coupling structures [4448]. Nonetheless, metal layers with inevitable absorption are required to obtain TPPs. Such inevitable absorption of metal layers limits the resolution of refractive index sensing based on ellipsometric phase change of TPPs [40].

As a kind of lossless micro-structures, all-dielectric 1-D PhCs have received enormous attention since they play an important role in nano-optics [4953]. Owing to the multiple scattering, the reflection phase at band edges in 1-D PhCs changes insensitively with respect to the wavelength while that within the photonic bandgap (PBG) changes smoothly with respect to the wavelength [5456]. To the best of our knowledge, ellipsometric phase at band edges in 1-D PhCs has not been studied yet. In this paper, we realize dramatic ellipsometric phase change at band edges in a 1-D PhC for oblique incidence. Since the axial symmetry of the system for oblique incidence is broken, the wavelengths of band edges in the 1-D PhC for TM and TE polarizations are different. Therefore, at the wavelength of the band edge in the 1-D PhC for TM or TE polarization, the ellipsometric phase changes dramatically with respect to the wavelength. By virtue of the dramatic ellipsometric phase change at the long-wavelength band edge, we achieve ultra-sensitive refractive index sensing at near-infrared wavelengths. The minimal resolution reaches 9.28×10−8 RIU. Compared with SPP-, BSW- and TPP-based sensitive refractive index sensing, ultra-sensitive refractive index sensing achieved in this work does not require any additional coupling component, coupling structure, or metal. According to the continuity relation of tangential component of wave vector, the state of band edge can be directly excited from the electromagnetic wave incident from the air. The designed 1-D PhC is composed of alternating silicon (Si) and silicon oxide (SiO2) layers, which can be easily fabricated via the current electron-beam vacuum deposition [52] or the magnetron sputtering techniques [57]. Such ultra-sensitive refractive index sensor would possess applications in monitoring temperature, humidity, pressure, and concentration of biological analytes.

This paper is organized as follows. In Sec. 2, we realize dramatic ellipsometric phase change at the band edges in an all-dielectric 1-D PhC and explain the underlying reason. In Sec. 3, we utilize the dramatic ellipsometric phase change to achieve ultra-sensitive refractive index sensing. In Sec. 4, we show an application of the refractive index sensing: measuring the concentration of sucrose. Finally, the conclusions are given in Sec. 5.

2. Dramatic ellipsometric phase change at band edge in 1-D PhC

Figure 1 gives the schematic of the proposed ultra-sensitive refractive index sensor. The 1-D PhC (AB)N is composed of alternating Si and SiO2 layers. Simultaneously considering the minimal resolution of refractive index sensing and the difficulty of the fabrication process of the 1-D PhC, the number of periods of the 1-D PhC is selected to be N = 15 (details can be seen from Sec. 3). The incident medium is set to be the air and the exit medium (sensing medium) is set to be the biosolution (nBio = 1.33). Suppose that a plane wave launches into the 1-D PhC with an incident angle θ.

 figure: Fig. 1.

Fig. 1. Schematic of the proposed ultra-sensitive refractive index sensor. The 1-D PhC (AB)15 is composed of alternating Si and SiO2 layers. The incident medium is set to be the air and the exit medium (sensing medium) is set to be the biosolution.

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Figures 2(a) and 2(b) give the measured data (black circles, extracted from Ref. [58]) of the real parts of the refractive indices of Si and SiO2 [i.e., Re(nA) and Re(nB)] as functions of the wavelength, respectively. Then, we utilize the polynomial functions to fit the real parts of the refractive indices of Si and SiO2. The corresponding fitting curves are also shown by the blue solid lines. The fitting polynomial functions can be expressed as

$$Re ({n_\textrm{A}}) ={-} 1.1831{\lambda ^3} + 5.2301{\lambda ^2} - 7.7643\lambda + 7.3651,$$
$$Re ({n_\textrm{B}}) ={-} 0.011934\lambda + 1.4625,$$
where λ represents the wavelength in units of micrometer. Considering the material losses during the fabrication process, the imaginary parts of the refractive indices of Si and SiO2 thin films within the wavelength range of interest are chosen to be κA = Im(nA) = 1×10−4 [59] and κB = Im(nB) = 1.2×10−5 [60], respectively. The thicknesses of Si and SiO2 layers satisfy the quarter-wavelength condition Re(nA)dA = Re(nB)dB = λBrg/4 = c/(4fBrg), where the Bragg wavelength is set to be λBrg = 1140 nm (the corresponding Bragg frequency is fBrg = 263.16 THz). Notice that the Bragg wavelength can be selected to be another value as long as the PBG (along with its band edges) can occur since what we utilize to achieve ultra-sensitive refractive index sensing is the resonance of the band edge of the PBG. Since the quarter-wavelength condition is satisfied, the Bragg frequency is the center frequency of the PBG when the dispersion effects of the refractive indices of Si and SiO2 are ignored [61]. However, here we consider the dispersion effects of the refractive indices of Si and SiO2, the Bragg frequency slightly deviates from the center frequency of the PBG (fcenter = 269.93 THz). Hence, the thicknesses of Si and SiO2 layers can be calculated as dA = 80.1 nm and dB = 196.7 nm, respectively.

 figure: Fig. 2.

Fig. 2. Real parts of the refractive indices of (a) Si and (b) SiO2 as functions of the wavelength. The black circles represent the measured data extracted from Ref. [58]. The blue solid lines represent the polynomial fitting curves.

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According to the transfer matrix method [62], we calculate the reflectance spectrum and the reflection phase spectrum (in deg) of the 1-D PhC (AB)15 at normal incidence in Figs. 3(a) and 3(b), respectively. Owing to the axial symmetry of the system, the TM reflectance spectrum (or the TM reflection phase spectrum) is identical to the TE reflectance spectrum (or the TE reflection phase spectrum) [49]. Clearly, a PBG is opened around the Bragg wavelength λBrg = 1140 nm. The short- and the long-wavelength band edges are located at 867.5 and 1546.0 nm, respectively. Notice that the positions of the band edges are estimated using the reflectance dips nearest the PBG. Around the short- and the long-wavelength band edges, the reflection phase changes intensively due to the multiple scattering. In contrast, with the PBG, the reflection phase changes smoothly. In the following, we will utilize such intensive reflection phase change around the band edge to achieve dramatic ellipsometric phase change at the band edge for the case of oblique incidence.

 figure: Fig. 3.

Fig. 3. (a) Reflectance and (b) reflection phase spectra (in deg) of the 1-D PhC (AB)15 at normal incidence. The refractive index of the biosolution is set to be nBio = 1.33.

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For the case of oblique incidence, the axial symmetry of the system is broken. Therefore, the TE reflectance spectrum (or the TE reflection phase spectrum) becomes distinguishable from the TM reflectance spectrum (or the TM reflection phase spectrum). Figure 4 gives the reflectance spectrum of the 1-D PhC (AB)15 as a function of the incident angle for TM and TE polarizations. The left half represents TM polarization while the right half represents TE polarization. The black color represents unity reflectance while the white color represents zero reflectance. One can see that the TE reflectance spectrum is indeed different from the TM reflectance spectrum at oblique incidence. Specifically, as the incident angle increases from 0° to 60°, the short-wavelength band edge for TM polarization shifts from 867.5 to 819.4 nm while that for TE polarization shifts from 867.5 to 765.6 nm. At the same time, the long-wavelength band edge for TM polarization shifts from 1546.0 to 1222.8 nm while that for TE polarization shifts from 1546.0 to 1472.9 nm. The wavelength difference between the band edges for TM and TE polarizations gives rise to the reflection phase difference between the TM and TE polarizations. Without losing the generality, we only focus on the long-wavelength band edge.

 figure: Fig. 4.

Fig. 4. Reflectance spectrum of the 1-D PhC (AB)15 as a function of the incident angle for TM and TE polarizations. The left half represents TM polarization while the right half represents TE polarization.

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Next, we select two typical incident angles θ1 = 15° and θ2 = 30° to observe the ellipsometric phase. Figures 5(a) gives the reflectance spectra of the 1-D PhC (AB)15 at θ1 = 15° for TM and TE polarizations. As demonstrated, the long-wavelength band edge for TM polarization is located at 1518.3 nm (shown by point A) while that for TE polarization is located at 1539.6 nm (shown by point B). The wavelength difference of the long-wavelength band edge for TM and TE polarizations is 21.3 nm. Figure 5(c) gives the reflection phase spectra (in deg) of the 1-D PhC (AB)15 at θ1 = 15° for TM and TE polarizations. Clearly, around the long-wavelength band edge for TM or TE polarization, the reflection phase changes intensively. In contrast, at other wavelengths, the reflection phase changes smoothly. Two ellipsometric parameters (ellipsometric amplitude Ψ and the ellipsometric phase Δ) satisfy the following equation [39]

$$\frac{{{r_{\textrm{TM}}}}}{{{r_{\textrm{TE}}}}} = \tan \Psi {e^{i\Delta }},$$
where rTM and rTE represent the reflection coefficients for TM and TE polarizations, respectively. Furthermore, the reflection coefficients for TM and TE polarizations can be expressed as
$${r_{\textrm{TM}}} = |{{r_{\textrm{TM}}}} |{e^{i{\varphi _{\textrm{TM}}}}},$$
$${r_{\textrm{TE}}} = |{{r_{\textrm{TE}}}} |{e^{i{\varphi _{\textrm{TE}}}}},$$
where φTM and φTE represent the reflection phases for TM and TE polarizations, respectively. Substituting Eqs. (4) and (5) into Eq. (3), one can obtain the expressions of two ellipsometric parameters
$$\Psi = \textrm{arctan}\frac{{|{{r_{\textrm{TM}}}} |}}{{|{{r_{\textrm{TE}}}} |}},$$
$$\Delta = {\varphi _{\textrm{TM}}} - {\varphi _{\textrm{TE}}}.$$
Therefore, the ellipsometric phase Δ is the reflection phase difference between TM and TE polarizations. Figure 5(e) gives the ellipsometric phase spectrum (in deg) of the 1-D PhC (AB)15 at θ1 = 15°. Interestingly, the ellipsometric phase changes dramatically at both the long-wavelength band edges for TM and TE polarizations. The reason can be explained as follows. At the long-wavelength band edge for TM polarization, the reflection phase for TM polarization changes intensively while that for TE polarization changes smoothly. Hence, the ellipsometer phase Δ = φTMφTE changes dramatically. Similarly, at the long-wavelength band edge for TE polarization, the reflection phase for TM polarization changes smoothly while that for TE polarization changes intensively. Hence, the ellipsometer phase Δ = φTMφTE also changes dramatically.

 figure: Fig. 5.

Fig. 5. (a) Reflectance, (c) reflection phase (in deg) and (e) ellipsometric phase (in deg) spectra of the 1-D PhC (AB)15 at θ1 = 15°. (b) Reflectance, (d) reflection phase (in deg) and (f) ellipsometric phase (in deg) spectra of the 1-D PhC (AB)15 at θ2 = 30°.

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Similarly, Fig. 5(b) gives the reflectance spectra of the 1-D PhC (AB)15 at θ2 = 30° for TM and TE polarizations. As demonstrated, the long-wavelength band edge for TM polarization is located at 1441.1 nm (shown by point A) while that for TE polarization is located at 1522.0 nm (shown by point B). The wavelength difference of the long-wavelength band edge for TM and TE polarizations at θ2 = 30° reaches 80.9 nm, which is larger than that at θ1 = 15°. Figure 5(d) gives the reflection phase spectra (in deg) of the 1-D PhC (AB)15 at θ2 = 30° for TM and TE polarizations. Clearly, around the long-wavelength band edge for TM or TE polarization, the reflection phase changes intensively. In contrast, at other wavelengths, the reflection phase changes smoothly. Figure 5(f) gives the ellipsometric phase spectrum (in deg) of the 1-D PhC (AB)15 at θ2 = 30°. Similar to Fig. 5(e), the ellipsometric phase changes dramatically at both the long-wavelength band edges for TM and TE polarizations. Such dramatic ellipsometric phase change at the long-wavelength band edge can be utilized to achieve ultra-sensitive refractive index sensing.

3. Ultra-sensitive refractive index sensing enabled by dramatic ellipsometric phase change

In this section, we utilize the dramatic ellipsometric phase change at the long-wavelength band edge for TM polarization in Sec. 2 to achieve ultra-sensitive refractive index sensing. It is known that as the first derivative (absolute value) of the ellipsometric phase with respective to the wavelength becomes larger, the sensitivity of the refractive index sensing becomes more sensitive [27]. As the first derivative (absolute value) of the ellipsometric phase with respective to the wavelength becomes larger, the ellipsometric phase at a fixed wavelength changes more dramatically with the refractive index of the biosolution, giving rise to a larger sensitivity and a smaller resolution. The first derivative (absolute value) of the ellipsometric phase with respective to the wavelength can be expressed as

$$F = \left|{\frac{{\textrm{d}\Delta }}{{\textrm{d}\lambda }}} \right|.$$
For a fixed incident angle, we calculate the first derivative (absolute value) F as a function of the wavelength and then obtain the maximal first derivative (absolute value) Fmax. Figure 6 gives the relationship between the maximal first derivative (absolute value) Fmax (in deg/nm) and the incident angle (in deg). As demonstrated, as the incident angle increases from 45.0° to 57.0°, the maximal first derivative (absolute value) Fmax increases from 6.00×102 to 8.79×104. As the incident angle increases from 57.0° to 70.0°, the maximal first derivative (absolute value) Fmax decreases from 8.79×104 to 3.45×102. Therefore, the maximal first derivative (absolute value) reaches its maximum 8.79×104 at θ0 = 57.0°. To achieve ultra-sensitive refractive index sensing, we select the incident angle as θ0 = 57.0° in the following.

 figure: Fig. 6.

Fig. 6. Relationship between the maximal first derivative (absolute value) Fmax (in deg/nm) and the incident angle (in deg).

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To assess the performance of the refractive index sensing, we calculate the ellipsometric phase spectra (in deg) at the incident angle θ0 = 57.0° with different refractive indices of the sensing medium (i.e., the biosolution) nBio = 1.33, 1.335 and 1.34 in Fig. 7(a). Owing to the ultra-large first derivative (absolute value), the ellipsometric phase is ultra-sensitive to the refractive index of the biosolution at the long-wavelength band edge. Specifically, as the refractive index of the biosolution slightly increases from 1.33 to 1.34, the ellipsometric phase at the fixed wavelength λ0 = 1243.35 nm dramatically increases from ‒162.0° to ‒154.0°. Figure 7(b) further gives the ellipsometric phase (in deg) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the refractive index of the biosolution. As the refractive index of the biosolution increases from 1.33 to 1.43, the ellipsometric phase dramatically increases from ‒161.0° to 1.6°. The total ellipsometric phase change reaches 162.6°.

 figure: Fig. 7.

Fig. 7. (a) Ellipsometric phase spectra (in deg) at the incident angle θ0 = 57.0° with different refractive indices of the biosolution nBio = 1.33, 1.335 and 1.34. (b) Ellipsometric phase (in deg) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the refractive index of the biosolution.

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The sensitivity of the refractive index sensor based on ellipsometric change can be determined by [63]

$$S = \left|{\frac{{\textrm{d}\Delta }}{{\textrm{d}{n_{\textrm{Bio}}}}}} \right|.$$
According to Eq. (9), we calculate the sensitivity (in deg/RIU) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the refractive index of the biosolution, as shown in Fig. 8(a). Clearly, as the refractive index of the biosolution increases from 1.33 to 1.354, the sensitivity increases from 4.89×102 to 1.08×104 deg/RIU. As the refractive index of the biosolution continues to increase from 1.354 to 1.43, the sensitivity decreases from 1.08×104 to 5.19×101 deg/RIU. The maximal sensitivity reaches 1.08×104 deg/RIU.

 figure: Fig. 8.

Fig. 8. (a) Sensitivity (in deg/RIU) and (b) resolution (in RIU) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as functions of the refractive index of the biosolution.

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Under the current ellipsometric phase measuring technique, a realistically achievable limit of the ellipsometric phase noise is δΔ = 0.001° [64]. Therefore, the resolution of the refractive index sensing (i.e., δn) can be calculated by [65]

$$\mathrm{\delta }n = \frac{{\mathrm{\delta }\Delta }}{S}.$$
One can see that the minimal resolution δn is inversely proportional to the sensitivity S. Once the realistically achieved limit of the ellipsometric phase noise δΔ is determined, the minimal resolution δn is completely determined by the sensitivity S. According to Eq. (10), we calculate the resolution of the refractive index sensing (in RIU) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the refractive index of the biosolution, as shown in Fig. 8(b). Clearly, as the refractive index of the biosolution increases from 1.33 to 1.354, the resolution decreases from 2.05×10−6 to 9.28×10−8 RIU. As the refractive index of the biosolution continues to increase from 1.354 to 1.43, the resolution increases from 9.28×10−8 to 1.93×10−5 RIU. The minimal resolution reaches 9.28×10−8 RIU.

As we illustrated in Sec. 2, as the number of periods N increases, the resonance of the long-wavelength band edge becomes stronger. Hence, the minimal resolution becomes smaller. Table 1 gives the minimal resolution of the 1-D PhCs (AB)N with different numbers of periods N = 5, 10 and 15. It can be seen that the minimal resolutions for N = 5, 10 and 15 are 6.81×10−7, 2.55×10−7 and 9.28×10−8 RIU, respectively. As the number of periods N increases, the minimal resolution becomes smaller. However, the increase in the number of periods N will increase the difficulty of the fabrication process of the 1-D PhC. Simultaneously considering the minimal resolution and the difficulty of the fabrication process, we finally select the number of periods N = 15.

Tables Icon

Table 1. Minimal resolution of the 1-D PhCs (AB)N with different numbers of periods N = 5, 10 and 15

Finally, we compare the performance of the refractive index sensing in this work with those in the reported works based on various mechanisms (SPPs [25,26], BSWs [32,33] and ellipsometric phase change [39]), as shown in Table 2. One can see that the refractive index sensing achieved in this work does not require any coupling component, coupling structure, or metal while possessing the smallest resolution. The ultra-sensitive refractive index sensing achieved in this work would be utilized in ultra-sensitively monitoring temperature, humidity, pressure, and concentration of biological analytes.

Tables Icon

Table 2. Comparison between the performance of the refractive index sensing achieved in this work and those in the reported works based on various mechanisms

4. Application: measuring concentration of sucrose

In this section, we show an application of the refractive index sensing: measuring the concentration of sucrose. The biosolution in Fig. 1 is selected to be sucrose. Figure 9 gives the measured data (black circles, extracted from Ref. [66]) of the refractive index of sucrose nBio as a function of the concentration. Then, we utilize a linear function (shown by blue solid line) to fit the refractive index of sucrose. The fitting function can be expressed as

$${n_{\textrm{Bio}}} = 0.016286C + 1.334,$$
where C represents the concentration of sucrose in units of g/L. Figure 10(a) gives the ellipsometric phase spectra (in deg) at the incident angle θ0 = 57.0° with different concentrations of sucrose C = 0, 0.3 and 0.6 g/L. As the concentration of sucrose slightly increases from 0 to 0.6 g/L, the ellipsometric phase at the fixed wavelength λ0 = 1243.35 nm dramatically increases from ‒159.7° to ‒147.6°. Figure 10(b) further gives the ellipsometric phase (in deg) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the concentration of sucrose. As the concentration of sucrose increases from 0 to 5 g/L, the ellipsometric phase dramatically increases from ‒159.7° to 0.7°. The total ellipsometric phase change reaches 160.4°. The sensitivity of the concentration of sucrose based on ellipsometric change can be determined by
$$S = \left|{\frac{{\textrm{d}\Delta }}{{\textrm{d}C}}} \right|.$$
According to Eq. (12), we calculate the sensitivity (in deg·L/g) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the concentration of sucrose, as shown in Fig. 11(a). Clearly, as the concentration of sucrose increases from 0 to 1.2 g/L, the sensitivity increases from 1.12×101 to 1.72×102 deg·L/g. As the concentration of sucrose continues to increase from 1.2 to 5 g/L, the sensitivity decreases from 1.72×102 to 1.30×10° deg·L/g. The maximal sensitivity reaches 1.72×102 deg·L/g. Under the current ellipsometric phase measuring technique, a realistically achievable limit of the ellipsometric phase noise is δΔ = 0.001° [64]. Therefore, the resolution of the concentration of sucrose (i.e., δC) can be calculated by
$$\mathrm{\delta }C = \frac{{\mathrm{\delta }\Delta }}{S}.$$
According to Eq. (13), we calculate the resolution of the concentration of sucrose (in g/L) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the concentration of sucrose, as shown in Fig. 11(b). Clearly, as the concentration of sucrose increases from 0 to 1.2 g/L, the resolution decreases from 8.94×10−5 to 5.80×10−6 g/L. As the concentration of sucrose continues to increase from 1.2 to 5 g/L, the resolution increases from 5.80×10−6 to 7.70×10−4 g/L. The minimal resolution reaches 5.80×10−6 g/L.

 figure: Fig. 9.

Fig. 9. Refractive index of sucrose as a function of the concentration. The black circles represent the measured data extracted from Ref. [66]. The blue solid line represents the linear fitting curves.

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 figure: Fig. 10.

Fig. 10. (a) Ellipsometric phase spectra (in deg) at the incident angle θ0 = 57.0° with different concentrations of sucrose C = 0, 0.3 and 0.6 g/L. (b) Ellipsometric phase (in deg) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the concentration of sucrose.

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 figure: Fig. 11.

Fig. 11. (a) Sensitivity (in deg·L/g) and (b) resolution (in g/L) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as functions of the concentration of sucrose.

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5. Conclusion

In summary, we design an ultra-sensitive refractive index sensor based on the dramatic ellipsometric phase change at the long-wavelength band edge in an all-dielectric 1-D PhC. Assisted by the dramatic ellipsometric phase change at the long-wavelength band edge, the minimal resolution of the designed sensor reaches 9.28×10−8 RIU. Compared with conventional SPP-, BSW- and TPP-based sensitive refractive index sensors, the designed ultra-sensitive refractive index sensor does not require any additional coupling component, coupling structure, or metal. Such ultra-sensitive refractive index sensor would possess applications in monitoring temperature, humidity, pressure, and concentration of biological analytes.

Funding

National Natural Science Foundation of China (12104105, 61805064); Science and Technology Program of Guangzhou (202102020571); Shanghai Pujiang Program (20PJ1412200); Start-up Funding of Guangdong Polytechnic Normal University (2021SDKYA033).

Disclosures

The authors declare that there are no conflicts of interest related to this article.

Data availability

Data underlying the results presented in this paper are available from the corresponding authors upon reasonable request.

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Data availability

Data underlying the results presented in this paper are available from the corresponding authors upon reasonable request.

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Figures (11)

Fig. 1.
Fig. 1. Schematic of the proposed ultra-sensitive refractive index sensor. The 1-D PhC (AB)15 is composed of alternating Si and SiO2 layers. The incident medium is set to be the air and the exit medium (sensing medium) is set to be the biosolution.
Fig. 2.
Fig. 2. Real parts of the refractive indices of (a) Si and (b) SiO2 as functions of the wavelength. The black circles represent the measured data extracted from Ref. [58]. The blue solid lines represent the polynomial fitting curves.
Fig. 3.
Fig. 3. (a) Reflectance and (b) reflection phase spectra (in deg) of the 1-D PhC (AB)15 at normal incidence. The refractive index of the biosolution is set to be nBio = 1.33.
Fig. 4.
Fig. 4. Reflectance spectrum of the 1-D PhC (AB)15 as a function of the incident angle for TM and TE polarizations. The left half represents TM polarization while the right half represents TE polarization.
Fig. 5.
Fig. 5. (a) Reflectance, (c) reflection phase (in deg) and (e) ellipsometric phase (in deg) spectra of the 1-D PhC (AB)15 at θ1 = 15°. (b) Reflectance, (d) reflection phase (in deg) and (f) ellipsometric phase (in deg) spectra of the 1-D PhC (AB)15 at θ2 = 30°.
Fig. 6.
Fig. 6. Relationship between the maximal first derivative (absolute value) Fmax (in deg/nm) and the incident angle (in deg).
Fig. 7.
Fig. 7. (a) Ellipsometric phase spectra (in deg) at the incident angle θ0 = 57.0° with different refractive indices of the biosolution nBio = 1.33, 1.335 and 1.34. (b) Ellipsometric phase (in deg) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the refractive index of the biosolution.
Fig. 8.
Fig. 8. (a) Sensitivity (in deg/RIU) and (b) resolution (in RIU) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as functions of the refractive index of the biosolution.
Fig. 9.
Fig. 9. Refractive index of sucrose as a function of the concentration. The black circles represent the measured data extracted from Ref. [66]. The blue solid line represents the linear fitting curves.
Fig. 10.
Fig. 10. (a) Ellipsometric phase spectra (in deg) at the incident angle θ0 = 57.0° with different concentrations of sucrose C = 0, 0.3 and 0.6 g/L. (b) Ellipsometric phase (in deg) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as a function of the concentration of sucrose.
Fig. 11.
Fig. 11. (a) Sensitivity (in deg·L/g) and (b) resolution (in g/L) at the fixed wavelength λ0 = 1243.35 nm and the incident angle θ0 = 57.0° as functions of the concentration of sucrose.

Tables (2)

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Table 1. Minimal resolution of the 1-D PhCs (AB)N with different numbers of periods N = 5, 10 and 15

Tables Icon

Table 2. Comparison between the performance of the refractive index sensing achieved in this work and those in the reported works based on various mechanisms

Equations (13)

Equations on this page are rendered with MathJax. Learn more.

R e ( n A ) = 1.1831 λ 3 + 5.2301 λ 2 7.7643 λ + 7.3651 ,
R e ( n B ) = 0.011934 λ + 1.4625 ,
r TM r TE = tan Ψ e i Δ ,
r TM = | r TM | e i φ TM ,
r TE = | r TE | e i φ TE ,
Ψ = arctan | r TM | | r TE | ,
Δ = φ TM φ TE .
F = | d Δ d λ | .
S = | d Δ d n Bio | .
δ n = δ Δ S .
n Bio = 0.016286 C + 1.334 ,
S = | d Δ d C | .
δ C = δ Δ S .
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