Guided-mode resonance based humidity sensing using a multilayer dielectric structure

Research Article Vol. 28, No. 20 / 28 September 2020 / Optics Express 28954 Guided-mode resonance based humidity sensing using a multilayer dielectric structure MICHAL GRYGA,*DALIBOR CIPRIAN,AND PETR HLUBINA Department of Physics, Technical University Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic *michal.gryg[email protected] Abstract: We report on a highly sensitive measurement of the relative humidity of air, which utilizes a guided-mode resonance (GMR) of a multilayer dielectric structure (MDS) and the spectral interference of sand p-polarized waves reflected from the MDS. We employ the MDS represented by four bilayers of TiO 2 /SiO 2 with a termination layer of TiO 2 and demonstrate that the GMR shows up as a shallow and asymmetric dip. The GMR enables us to measure the relative humidity (RH) of air with sensitivities of 0.031–0.114 nm/%RH. In addition, by employing a birefringent crystal of mica, which modifies the phase difference between the polarized waves, the GMR is transformed into the resonance with a sharp dip, and the measured sensitivity is enhanced to 0.120 nm/%RH at 81 %RH. We also determined the sensitivity to the refractive index and the figure of merit as high as 8000 nm/refractive index unit (RIU) and 702 RIU −1 , respectively. The results demonstrate that the GMR based sensor employing the MDS and the spectral interference of polarized waves with their phase difference appropriately adjusted enables a highly sensitive, hysteresis-free humidity measurement, characterized by a high FOM. Humidity sensors employing dielectric multilayers thus represent an effective alternative to available sensors, with advantages such as better mechanical and chemical stability. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement 1. Introduction Relative humidity (RH) belongs to physical quantities that are important in many fields, including health monitoring, pharmaceutical manufacturing, agricultural production, food manufacturing and storage, meteorological services, and so on. Simplest RH sensors are electrical sensors [1,2] utilizing changes in electric capacity or resistance of a sensing element with the RH. Unfortunately, the electrical humidity sensors usually show hysteresis when subjected to a highhumidity environment for a long time. In addition, response of such sensors could be affected by electromagnetic interferences. Consequently, optical humidity sensors [2,3], especially fiber optic sensors are preferred due to their advantages such as remote measurement, high sensitivity, and electromagnetic immunity. A variety of optical humidity sensors have been proposed and confirmed [2–16], with parameters and characteristics such as response time, measurement range, hysteresis, linearity, and sensitivity presented in review papers [3,4]. The sensors include configurations with long period gratings [4], fiber structures with lossy mode resonances [5,6] or multilayer films [7], interferometers [8,9], side-polished fibers [10], waveguides [11,12], photonic crystal-based probes [13], polymer fibers with Bragg gratings [15,16], and so on. Among these sensors, surface plasmon resonance (SPR) based fiber sensors [10,14] are characterized by extreme sensitivity to changes in refractive index of the surrounding medium at a metal-dielectric interface. As an example, a maximum sensitivity of 0.833 nm/%RH at the resonance around 1400 nm has been obtained [14]. In addition, because the SPR effect is manifested when the resonance condition is fulfilled, it is accompanied by the amplitude and phase changes of reflected light wave, and different physical quantities such as the resonant wavelength [10], phase or intensity [14] can be measured to detect the SPR effect [17], which can be utilized in a number of sensing applications, #399816 https://doi.org/10.1364/OE.399816 Journal © 2020 Received 9 Jun 2020; revised 29 Jul 2020; accepted 6 Aug 2020; published 15 Sep 2020 Research Article Vol. 28, No. 20 / 28 September 2020 / Optics Express 28955 including humidity sensing. Similarly, fiber implementations of refractive index sensors such as photonic crystal fiber (PCF)-based sensors [18–21] have been proposed and realized, reaching sensitivities of 70,000 nm/refractive index unit (RIU) [18] and 167,911%/RIU [19] using a PCF two-core coupler detecting wavelength and intensity shifts, respectively. A sensitivity of 8800 nm/RIU was also demonstrated experimentally using four-wave mixing that does not require any post-processing [20], and the highest experimentally demonstrated sensitivity reached 30,000 nm/RIU [21]. In this paper we present a highly sensitive measurement of the RH of air, which is based on the guided-mode resonance (GMR) of a multilayer dielectric structure (MDS) and the spectral interference of sand p-polarized waves reflected from the MDS, which includes four bilayers of TiO2/SiO2with a termination layer of TiO2[22]. The GMR of the MDS shows up as a shallow and asymmetric dip, and it enables to measure the RH of air with sensitivities of 0.031–0.114 nm/%RH. Moreover, by employing a birefringent crystal of mica to modify the phase difference between the polarized waves, their spectral interference is utilized to transform the GMR into the resonance with a sharp dip, and the measured sensitivity to the RH of air is enhanced to 0.120 nm/%RH at 81 %RH. The corresponding sensitivity to the refractive index and the figure of merit (FOM) are as high as 8000 nm/RIU and 702 RIU −1 , respectively. Thus, the GMR based sensor employing the MDS and the spectral interference of polarized waves with their phase difference appropriately adjusted enables a highly sensitive, hysteresis-free humidity measurement. In addition, humidity sensors employing dielectric multilayers, which are easy to fabricate using commercially available vacuum deposition technology [23], represent an effective alternative to available sensors, with advantages such as better mechanical and chemical stability. 2. Background Let us consider an MDS as shown in Fig. 1(a), represented by a system of four bilayers of TiO 2 /SiO 2 (i = j = 1,.., 4) with geometrical thicknesses t 0i= 166.7, 182.8, 179.0, and 183.9 nm, and t 1j= 79.9, 89.2, 86.6, and 87.2 nm, respectively, and a termination layer of TiO 2 of thickness t 05 = 177.4 nm with a rough layer of thickness t 06 = 13.5 nm [22]. For the MDS, the GMR can be utilized in refractive index sensing and thus in a relative humidity measurement. A standard approach in the sensing is based on measurement of the reflectance of a por s-polarized wave, and resolving a shift of the corresponding reflectance dip when the refractive index of the surrounding medium is changed. For the MDS, which we analyzed in detail in a previous paper [22], some reflectance responses are with shallow dips. To increase the depth of the dips, the spectral interference of the polarized waves reflected from the MDS can be used, and this approach is employed in the relative humidity sensing. The interference is attained when both the polarizer and analyzer are oriented 45 ◦ with respect to the plane of incidence [24], and the corresponding reflectance RPA45(λ)is expressed as RPA45(λ)=1 4{Rs(λ)+Rp(λ)+2qRs(λ)Rp(λ)cos[δsp(λ)+∆sp(λ)]} (1) where R s(λ) and R p(λ) are reflectances of sand p-polarized waves, respectively, δsp(λ) is their phase difference, and ∆sp(λ) is an additional phase term, which can be adjusted by a birefringent crystal used at the input of the setup. To model the spectral reflectances of the MDS, a transfer matrix method can be used, and the reflectance response in the Kretschmann configuration with a coupling prism made of BK7 glass can be evaluated. In these calculations, the refractive index of the external medium (air) is considered to be n = 1. Taking into account the dispersion of materials of the structure [22], and assuming that the extinction coefficients for TiO 2 and SiO 2 layers are κTiO2= 1.6 × 10 −3 and κSiO2= 3.4 × 10 −4 , respectively, we model the theoretical reflectance R PA45(λ) as a function Research Article Vol. 28, No. 20 / 28 September 2020 / Optics Express 28956 Fig. 1. An SEM photo of a multilayer dielectric structure (a), theoretical spectral reflectances R PA45(λ) for air and the angle of incidence θ= 41.9 ◦ when ∆sp(λ)= 0 and ∆sp(λ)= 0.4 π , respectively (b). of the wavelength λ for the angle of incidence θ= 41.9 ◦ , when we consider for the phase term ∆sp(λ)constant values of 0 and 0.4π, respectively. The corresponding reflectances R PA45(λ) are shown in Fig. 1(b), and it is clearly seen that owing to the phase change an abrupt change in the reflectance is transformed into a dip, as it is evident for a sharp dip near a wavelength of 810 nm. Consequently, using a birefringent crystal, an abrupt change in the reflectance, for example at an edge of a band gap [22], can be transformed into a dip with a sufficient depth, and this new approach can be utilized in a GMR based humidity sensing employing a multilayer structure. 3. Experimental setup Fig. 2. Experimental setup for measuring the reflectance response of the multilayer structure when the relative humidity of air is changed. An experimental setup used in the relative humidity sensing is shown in Fig. 2. It employs of a white-light source (WLS) (halogen lamp HL-2000, Ocean Optics) with launching optics, an Research Article Vol. 28, No. 20 / 28 September 2020 / Optics Express 28957 input optical fiber and a collimating lens (CL). The collimated beam of 1 mm diameter passes through a linear polarizer (P) (LPVIS050, Thorlabs) oriented 45 ◦ with respect to the plane of incidence so that both pand s-polarized components are generated. Next, a birefringent crystal (BC) of mica [25] with a thickness of 4 mm is included, and its optical axis is parallel to the cleavage planes and is perpendicular to the beam axis. The beam passing through these components is coupled to the MDS on a glass slide using an equilateral BK7 prism (Ealing, Inc.) with index-matching fluid (Cargille, n D= 1.516). The light reflected from the MDS, which was primarily prepared as interference filter by a technique of sputtering (Meopta, Czech Republic), passes through a linear analyzer (A) (LPVIS050, Thorlabs) oriented 45 ◦ or 90 ◦ with respect to the plane of incidence so that the reflectances R PA45(λ) or R s(λ) are measured. The light is launched directly into a read optical fiber (ROF) (M15L02, Thorlabs) of a spectrometer (USB4000, Ocean Optics), connected via USB to a personal computer (PC). A part of the setup for the adjustment of the relative humidity of air [13] is also shown in Fig. 2. The MDS is attached via O ring to a sensing chamber (volume approximately 22 mL) hosting an electrical humidity and temperature sensor (HTS) (HTU21D, Arduino) connected to a controller board (Arduino UNO). The relative humidity of air in the chamber is adjusted by means of a two-line peristaltic pump (PP) (BT100M, 2xYZ1515x, Baoding Chuang Rui Precision Pump Co., Ltd.) with the first line connected to the input of a humidifier (H), and the second line with the output of the humidifier connected to the chamber. Controlling the flow of air in the lines by means of the PP regulator, the relative humidity of air in the chamber can be varied approximately in a range of 22–82 %RH, but it can be enlarged. The lowest relative humidity of air is attained when dry air flows directly through the chamber. 4. Experimental results and discussion In the relative humidity measurements at a temperature of 22.5 ◦ C we applied an approach presented above, which is based on the spectral interference of sand p-polarized waves reflected from the MDS. In Fig. 3(a) is shown the measured reflectance ratio R PA45(λ)/ R s(λ) as a function of the wavelength λ for the external angle of incidence α= 23.6 ◦ (see Fig. 2) and the relative humidity of air ranging from 28.4 %RH to 77.8 %RH. We have revealed a quick response to the relative humidity changes due to the surface wave based sensing principle similar to the SPR. Figure 2indicates that the response near the edge of the band gap is with a shallow and asymmetric dip and is due to the s-polarized guided mode of the MDS. The response near a wavelength of 780 nm is with a sufficient depth to determine the position of the dip, the resonance wavelength, as a function of the relative humidity of air. The resonance wavelength shifts toward longer wavelengths as the relative humidity of air increases, as shown in Fig. 4(a). To resolve a GMR with a sharp dip of maximum depth as outlined in section 2, the phase difference between sand p-polarized waves reflected from the MDS is modified by using a birefringent crystal of mica, whose optical axis is oriented to attain a desired additional phase term ∆sp(λ) [see Eq. (1)]. In other words, an abrupt change in the reflectance ratio (a short-wavelength edge of the band gap), as shown in Fig. 3(a), is transformed into a sharp dip. As an example, Fig. 3(b) shows the measured reflectance ratio R PA45(λ)/ R s(λ) for the angle of incidence α= 26.9 ◦ and the relative humidities 30.9–81.4 %RH, when the birefringent crystal of mica was rotated to attain the dip with maximum depth. A simple spectral-domain interferometric measurement [26] within a spectral region of 500–1000 nm revealed that the additional phase is approximately in a range of 0.89–1.83 rad. It is clearly seen that a sharp resonance dip near a wavelength of 805 nm is resolved for the guided mode with the shifted response (GM-SR), and the full width at half maximum (FWHM) of the dip is approximately 11.4 nm and it is nearly constant for all values of the relative humidity of air. The resonance wavelength shifts toward longer wavelengths as the relative humidity of air increases, as shown in Fig. 4(a). For the greater angle α(smaller angle of incidence θ) the resonance is red shifted and vice versa. Research Article Vol. 28, No. 20 / 28 September 2020 / Optics Express 28958 Fig. 3. Measured reflectance ratio R PA45(λ)/ R s(λ) as a function of the wavelength for different values of the RH of air, α= 23.6 ◦ (a), α= 26.9 ◦ and with the effect of ∆ps(λ) (b). Fig. 4. Resonance wavelength as a function of the RH of air with polynomial fits (a). Sensitivity as a function of the RH of air (b). To evaluate the response of the MDS under test, the sensitivity S RH to the relative humidity, defined as the change of the position of the dip δλr with respect to the change in the relative humidity δRH of the moist air, SRH = δλr δRH, (2) needs to be specified. For both responses (GM, GM-SR), the sensitivity S RH exhibits a linear dependence on the RH, as the resonance wavelength shift can be well fitted by a second-order polynomial, as shown in Fig. 4(a). A linear response is also possible when a greater angle of incidence θ (smaller angle α) is adjusted, but this is accompanied by a sensitivity decrease. The sensitivity S RH for the GM changes in a range of 0.031–0.114 nm/%RH, and for the GM-SR it is enhanced to 0.037–0.120 nm/%RH because the resonances are present at longer wavelengths. When we consider that the resonance wavelength is resolved with the precision ∆λ= 0.01 nm, the detection limit (DLRH), defined as DLRH =∆λ/SRH, reaches 0.08 %RH for the GM-SR. The above presented sensing principle for the relative humidity of air is due to sensitivity of the GMR-based sensor to RI changes. It is also desirable to evaluate the characteristic quantities for the RI. Thus, the sensitivity S n to the RI n, defined as the change of the position of the dip δλr Research Article Vol. 28, No. 20 / 28 September 2020 / Optics Express 28959 with respect to the change in the refractive index δnof the moist air Sn= δλr δn, (3) can be expressed using the relation between the RI of the moist air and its relative humidity. To measure the RI of the moist air in the sensing chamber, we utilized a simple method based on the knowledge of the dielectric function of the reference plasmonic structure of Au/Cr/SF10 [27]. By processing the positions of dips of the spectral reflectance ratios at a given angle of incidence, we obtained approximately a linear dependence of the RI of the moist air on its relative humidity with a slope of 1.5 × 10 −5 RIU/%RH. Thus, the sensitivity S n for the GM is linearly dependent on the RI and it changes in a range of 2067–7600 nm/RIU, and for the GM-SR it is enhanced to 2467–8000 nm/RIU. We can also evaluate the FOM, defined as the sensitivity S n divided by the FWHM of the dip FOM =Sn FWHM, (4) and for the GM-SR with the FWHM of 11.4 nm, the FOM is as high as 702 RIU −1 . The FOM obtained for the sensor is substantially higher than that for standard SPR-based sensors [28]. 5. Conclusions In this paper, a highly sensitive measurement of the relative humidity of air, based on the GMR of an MDS and the spectral interference of sand p-polarized waves reflected from the MDS, has been presented. The humidity measurement was performed for an MDS comprising four bilayers of TiO 2 /SiO 2 and a termination layer of TiO 2 . By employing a birefringent crystal, which affects the phase difference between polarized waves, the GMR is transformed into the resonance with a sharp dip, and the measured sensitivity reached 0.120 nm/%RH. We revealed that the highly sensitive measurement of the relative humidity of air is possible due to the sensitivity to the RI and FOM as high as 8000 nm/RIU and 702 RIU −1 , respectively. These values are substantially higher than those for standard SPR-based sensors [28]. In addition, due to mechanical and chemical stability of the MDS employed in the sensor, no hysteresis in the relative humidity measurement was revealed. Finally, owing to a quick response of the MDS to the relative humidity changes, the sensor with these advantages has potential to be applied in real-time measurements. 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