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Abstract
We present the design, fabrication, and characterization of cascaded side-coupled Silicon-on-Insulator (SOI) photonic crystal (PhC) nanobeam cavities for simultaneous measurement of refractive index (RI) and temperature. Due to the different mode distribution in air-mode cavity and dielectric-mode cavity, the two types of PhC nanobeam cavities have quite different sensitivities towards the changes of ambient RI and temperature. We demonstrated the feasibility to obtain RI and temperature simultaneously with a single measurement, obtaining a RI sensitivity of 254.6 nm/RIU (refractive index unit) and a temperature sensitivity of 30.1 pm/°C for air-mode cavity, while a RI sensitivity of 105.5 nm/RIU and a temperature sensitivity of 56.4 pm/°C for dielectric-mode cavity.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
During the past decade, the demands of the in situ monitoring for physical, chemical, and biological parameters have been increasing dramatically in prevention of global warming, manufacturing industries and protection of ecosystems. Simultaneous measurement of refractive index (RI) and temperature has become a critical issue for precise detection of concentrations of solutions because the RI of solution is strongly depended on ambient temperature [1–3]. In recent years, such dual parameter sensors have received great attention, various works based on optic fibers, such as tapered fiber Mach-Zehnder interferometers (MZI) [4], microfiber Fabry–Perot interferometer (MFPI) [5], dual-cavity Fabry–Perot fiber interferometer (FPFI) [6], and multimode microfiber (MMMF) based dual MZI [7] have been reported.
Benefited from the innovation of micro-nano manufacture technology, integrated optics devices have been extensively investigated to conform to the trend of miniaturization and intellectualization. To achieve the goal of “lab-on-chip” [8–10], significant efforts have been spent on developing integrated optical biosensors. Accordingly, various sensors utilizing different structures, e.g., microring resonators (MRR) [11–13], MZI [14], surface plasmons resonators [15], and photonic crystal (PhC) cavities [16–21], have been proposed and realized. Compared with the other approaches, PhC cavity is preferred due to its high Q factor, small footprints and no limitation of free spectral range (FSR). However, the resonance of PhC cavity typically shifts with the fluctuation of ambient temperature; therefore, it’s of great importance to distinguish them effectively during sensing measurement. One solution to avoid the influence of a temperature change is using additional devices like a thermo-electric cooler, which obviously makes the sensing system more complex and with high energy consumption.
In this Letter, we present the design, fabrication and characterization of cascaded side-coupled Silicon-on-Insulator (SOI) PhC nanobeam cavities [Fig. 1(a)] for simultaneous measurement of refractive index and temperature. By utilizing a buswaveguide with suitable width, the input TE-like optical mode is coupled into two PhC nanobeam cavities, which resonate respectively during the sensing measurement. Since the mode distribution of air-mode cavity and dielectric-mode cavity [22] are quite different, the responses to the external variation of RI and temperature exhibit large difference for the two cavities, which offers the possibility of dual-parameter measurement.
Fig. 1 (a) The schematic of the proposed cascaded side-coupled PhC nanobeam cavities. (b) The band diagram of the PhC air-mode nanobeam cavity with Wend1 = 500 nm (black line) and Wcenter1 = 700 nm (red line), the blue circle indicates the resonant frequency. (c) The band diagram of the PhC dielectric -mode nanobeam cavity with Rend2 = 80 nm (black line) and Rcenter2 = 125 nm (red line), the blue circle indicates the resonant frequency
2. Design principles
The optical mode of the air-mode cavity [cav1 in Fig. 1(a)] distributes more electric filed into surrounding cladding area, so it has a higher sensitivity towards the RI of up-cladding. While for the dielectric-mode cavity [cav2 in Fig. 1(a)], the majority of the electronic field is confined inside the silicon core, which has a relative high thermo-optic coefficient, so it has a higher sensitivity towards the variation of surrounding temperature. Meanwhile, the higher order modes of air-mode cavity locate at shorter wavelength region in the spectrum, however the higher order modes of dielectric-mode cavity locate at longer wavelength region in the spectrum. This characteristic naturally provides a wide sensing window without the influence of higher order modes for the aforementioned two PhC nanobeam cavities.
SOI platform with a 220 nm thick Si device layer and a 2 μm thick insulating layer is considered in our work. The refractive index of the silicon core and silica insulator layer are chosen to be 3.45 and 1.44, respectively. Since our main application is aimed for aqueous solution sensing, the refractive index of the up-cladding is then set to be that of the solution (n≈1.33) in our simulations. Firstly, the band diagrams are calculated using three-dimensional finite-difference time-domain (3D-FDTD) method (Lumerical solutions, Inc.) [23]. The resonant wavelengths of the air-mode cavity and dielectric-mode cavity are set be around 1530 nm and 1570 nm, so that the wavelength window of 40 nm would be wide enough for our sensing application. The parameters of air-mode cavity are chosen to be: a1 = 420 nm, r1 = 125 nm, Wend1 = 500 nm, Wcenter1 = 700 nm, while the parameters of dielectric-mode cavity are chosen to be: a2 = 340 nm, W2 = 700 nm, Rend2 = 80 nm, Rcenter2 = 125 nm. The band diagrams of the cavities are given in Fig. 1(b) and Fig. 1(c). To create a Gaussian-shape mode distribution to ensure high Q factor of the nanobeam cavities, the width of the air-mode cavity is quadratically tapered from Wcenter1 to Wend1, i.e., Wx = Wcenter1 + (Wend1-Wcenter1)∙x2/(N1∙a1)2, (N1 is the number of Gaussian mirrors on each side for air-mode cavity), while the filling factor of the dielectric-mode cavity is quadratically tuned from ffcenter2 (ffcenter2 = πRcenter22/(W2∙a2)) to ffend2 (ffend2 = πRend22/(W2∙a2)), i.e., ffi = ffcenter2 + (ffend2-ffcenter2)∙(i-1)2/(N2-1)2, (N2 is the number of Gaussian mirrors on each side for dielectric-mode cavity, i increases from 0 to N2). N1 and N2 are set to be 18 so that the Q factor of both unloaded cavity (without the side-coupled buswaveguide) could be above 30,000 (39,012 for air-mode cavity and 130,669 for dielectric-mode cavity), which would be large enough for our application. Besides, one more pair of additional mirror is placed on both ends of the aforesaid Gaussian mirrors for both cavities. A buswaveguide with width Wg1 and Wg2 is then placed adjacent to the cavity with a gap G1 and G2 to excite the cavity modes. To ensure an efficient coupling between the buswaveguide and the PhC cavity, the width of the buswaveguide session for cav1 (Wg1) and cav2 (Wg2) are chosen according to [(2π∙neffi)/λ = π/ai, i = 1,2] where neffi is the effective index of the buswaveguides. Thus, the parameters are chosen to be Wg1 = 330 nm and Wg2 = 420 nm. Compared with the unloaded cavity, there would be a trade-off between the Q factor and extinction ratio (ER) for the loaded cavity. Generally, a smaller gap would lead to a lower Q factor and a higher ER, while a larger gap would lead to a higher Q factor and a lower ER. For resonant cavity based refractive index sensors, loaded cavities with Q > 10,000 and ER > 10dB would be enough [24]. Based on the 3D-FDTD simulation, G1 = 600 nm results in Qcav1 = 12,136 and ERcav1 = 10.1 dB while G2 = 400 nm results in Qcav2 = 18,686 and ERcav2 = 16.2 dB. The parameters of the two cavities are given in Table1, and the power distributions of the fundamental air-mode and dielectric-mode, excited by the buswaveguide, are shown in Fig. 2(a) & (b).
Table 1. The geometric parameters of the PhC air-mode nanobeam cavity(cav1) and the PhC dielectric -mode nanobeam cavity(cav2)
Fig. 2 The electric field distribution (top view) taken at the center of the silicon core layer. (a) PhC air-mode nanobeam cavity (cav1); (b) PhC dielectric-mode nanobeam cavity (cav2). (c) & (d) The simulated resonant wavelength shifts and Q factors of the two PhC nanobeam cavities vary with different background RI (at constant room temperature T = 17°C). (e) & (f) The simulated resonant wavelength shifts and Q factors of the two PhC nanobeam cavities vary with different ambient temperature (in deionized water, n = 1.33).
The dual-parametric sensing performance of present cascaded side-coupled PhC nanobeam cavities can be described by defining a sensor matrix as following:
Sn,cav1 and ST,cav1 represent the sensitivity of RI and temperature for the air-mode cavity (cav1) while Sn,cav2 and ST,cav2 represent the sensitivity of RI and temperature for the dielectric-mode cavity (cav2).
From the electric field distribution shown in Fig. 2(a) and Fig. 2(b), one can see that the air-mode cavity distributes more electric field into the cladding area while the dielectric-mode cavity confines more electric field energy inside the silicon core. Taking the thermo-optic coefficient of silicon (∂nSi/∂T = 1.8 × 10−4 K−1) [25] and that of water-cladding (∂nwater/∂T = −0.8 × 10−4 K−1) [26] into consideration, the sensitivities and Q factors of the two cavities can be obtained from the simulations. The simulated resonant wavelength-shifts and Q factors of the two PhC nanobeam cavities with different background RI and ambient temperature are obtained by a series of individual simulations with a systematically increased RI and temperature, respectively. The theoretical sensitivities and Q factors are given in Fig. 2(c)&(d) and Fig. 2(e)&(f), one should note that the sensitivity ratio of RI sensing ration = Sn,cav1/Sn,cav2 = 2.4 and the sensitivity ratio of temperature sensing ratioT = ST,cav1/ST,cav2 = 0.5 are quite different, which ensures the feasibility of simultaneous measurement of RI and temperature with the proposed structure.
3. Fabrication and characterization
The fabrication of the present dual parameter sensor is processed on a 220nm-thick SOI wafer with a 2 μm silica buffer layer (SOITEC Inc.). A positive tone e-beam resist (PMMA 950K) with a thickness around 300 nm is firstly spin coated onto the SOI wafer. The electron-beam lithography (Raith 150-II) at 20KV acceleration voltage with the exposure dose of 200 μC/cm2 is used to define the device patterns, which was then transferred to the silicon layer with an inductively coupled plasma reactive-ion-etching (ICP-RIE) process with a gas mixture of C4F8 and SF6. The residual resist is removed by acetone in ultrasonic cleaner for 30 minutes, then the device wafer is rinsed into de-ionized (DI) water. In order to achieve efficient coupling between the optical fiber and the chip, grating couplers with the period of 630 nm and the duty cycle of 50:50 are fabricated at the ends of the input/output waveguides through another step of overlay electron-beam lithography exposure followed by a shallow etching (70 nm). Figure 3(a) shows the microscope image of the fabricated dual parameter sensor. The scanning electron microscope (SEM) pictures of the two PhC nanobeam cavities are shown in Fig. 3(b) & (c), respectively.
Fig. 3 (a) Optical microscopy image of the dual parameter sensor. (b) The zoomed in scanning electron microscope (SEM) image of the PhC air-mode nanobeam cavity. (c) The zoomed in scanning electron microscope (SEM) image of the PhC dielectric-mode nanobeam cavity.
To characterize the fabricated dual parameter sensor, a continuous wave (CW) broad-band amplified spontaneous emission (ASE) light source (B&A technology OS8143) is used in our measuring. The TE-polarized incident light is guaranteed by the polarization sensitive grating coupler, which typically has a polarization extinction ratio of more than 20dB [27].The output optical signal from the chip is received by an optical spectrum analyzer (OSA) (Yokogawa AQ6370D). Aqueous solutions of glucose are utilized to characterize our device as a proof-of-principle.
The external RI response is examined by immerged the device into aqueous solutions of glucose with different mass concentrations, the ratio of the RI change to the mass concentration change for glucose solution is around 0.0016 RIU/1% at 17 °C [28]. In our measurement, the concentration of glucose solution varies from 0% to 16% with a step of 4%, the corresponding RI of the glucose solutions are 1.33, 1.3356, 1.3412, 1.3468 and 1.3524, respectively. The temperature remains at 17 °C during the measurement. The measured transmission spectrum when the dual parameter sensor immersed into DI water is shown in Fig. 4(a), where the two resonant dips locate at 1522.65nm and 1575.62nm, respectively. The Lorentz fitted full width at half maximum (FWHM) for the air-mode and dielectric-mode are 838 pm [Fig. 4(b)] and 174 pm [Fig. 4(c)], corresponding to Q factors of 1817 and 9055, respectively. The degradation of measured Q-factor compared to the simulation value may come from fabrication errors, absorption loss and also scatterings due to impurities in the analytes. Fig. 5(a) & (b) shows the measured transmission spectra when the sensor is immerged into glucose solutions with different mass concentrations. One can find that both resonance modes shift towards longer wavelengths as the ambient RI increases, while the shift of air-mode is much larger than that of dielectric-mode, which agrees well with the above analysis. The fitting lines obtained from the measured data of wavelength shifts extracted from Fig. 5(a) & (b) are given in Fig. 5(c). The RI sensitivities for the two cavities are Sn,cav1 = 254.6 ± 9.2 nm/RIU and Sn,cav2 = 105.5 ± 3.1 nm/RIU, respectively. Since the resolution of the two cavities are Rcav1 = 0.073 nm and Rcav2 = 0.031 nm [24], the corresponding detection limits of RI sensing DLn,cav1 = Rcav1/Sn,cav1 = 2.8 × 10−4 RIU and DLn,cav2 = Rcav2/Sn,cav2 = 2.9 × 10−4 RIU are also obtained.
Fig. 4 (a) Measured normalized spectrum of the dual parameter sensor when immerged into DI water. The zoomed in view are measured transmission of the resonance of air-mode (b) and dielectric-mode (c) The black dots are the experimental data and the red lines show the Lorentz fit.
Fig. 5 (a) Normalized transmission spectrum of the PhC air-mode nanobeam cavity immersed into glucose solutions of different mass concentrations (ranging from 0% to 16%, at the temperature of 17°C). (b) Normalized transmission spectrum of the PhC dielectric-mode nanobeam cavity immersed into glucose solutions of different mass concentrations (ranging from 0% to 16%, at the temperature of 17°C). (c). The extracted resonant wavelength shifts of the dual parameter sensor with different ambient refractive indices.
The temperature response of the device is investigated by placing the device on a thermo-electric cooler (TEC) with a temperature resolution of 0.1 °C. The device was heating from 17 to 39 °C while immerged into de-ionized (DI) water during the measurement. The measured transmission spectra given in Fig. 6(a) & (b) show that both resonant modes shift toward longer wavelengths with the increasing of the temperature. The resonant shifts for the dielectric-mode is almost twice to that of the air-mode as we expected. By fitting the measurement data [obtained from Fig. 6(a) & (b)], we obtain fitted temperature sensitivities [Fig. 6(c)] ST,cav1 = 30.1 ± 0.4 pm/°C and ST,cav2 = 56.4 ± 0.5 pm/°C. The detection limits of temperature sensing DLT,cav1 = Rcav1/ST,cav1 = 2.4 °C and DLT,cav2 = Rcav2/ST,cav2 = 0.5 °C are also obtained.
Fig. 6 (a) Normalized transmission spectrum of the PhC air-mode nanobeam cavity (cav1) immersed into DI water at different temperature (17°C, 21°C, 27°C, 33°C and 39°C). (b) Normalized transmission spectrum of the PhC dielectric-mode nanobeam cavity (cav2) immersed into DI water at different temperature (17°C, 21°C, 27°C, 33°C and 39°C). (c). The extracted resonant wavelength shifts of the dual parameter sensor with different temperature.
With the sensitivities of RI and temperature obtained in the aforementioned separate measurements, once the wavelength shift of air-mode mode (∆λcav1) and that of dielectric-mode (∆λcav2) are measured, the variation of RI (∆RI) and temperature (∆T) can be mathematically solved by calculating the following matrix:
Furthermore, we operate the following measurement to experimentally demonstrate the feasibility of achieving simultaneous measurement of RI and temperature with our device. The base RI RIbase and base temperature Tbase are set to be 1.33 (the RI of deionized water) and 17°C, respectively. The corresponding base resonant wavelength of the air-mode (λbase,cav1) and that of the dielectric-mode (λbase,cav2) are measured to be 1522.65nm and 1575.62nm. Five groups of measurement are carried out to get the ∆λcav1 and ∆λcav2 relative to the base condition. After that, we use Eq. (2) to obtain the variation of RI (∆RImea) and temperature (∆Tmea). The results are shown in Table 2. The achieved detection accuracy for the RI and temperature are within 5 × 10−4 RIU and 1.3 °C, which are close to the detection limits from our analysis above. The sensitivities of state-of-the-art photonics base RI and temperature sensors are given in Table 3 for comparison. By undercutting the PhC cavity (especially for the air-mode cavity [21]), one may further improve the sensitivity to RI at a cost of higher fabrication complexity.
Table 2. Simultaneous Measurement of Refractive Index and Temperature Using the cascaded side-coupled Phc nanobeam cavities
Table 3. The sensitivities of state-of-the-art photonics base RI and temperature sensors
4. Conclusion
In summary, we have proposed and demonstrated an approach to achieve simultaneous measurement of RI and temperature by utilizing a dual cascaded side-coupled SOI Phc nanobeam cavities. Since the two types of cavity modes have strong birefringence to ambient temperature and RI change, the resonant spectra for two cavity modes have different temperature and RI sensitivities. Therefore, it is possible to discriminate coexisted sensitivities. By analyzing the wavelength shifts of certain resonant modes for glucose solution of different concentrations at various temperatures, RI sensitivities of 254.6 and 105.5 nm/RIU, as well as temperature sensitivities of 30.1 and 56.4 pm/°C are obtained experimentally. Thus, we can obtain RI and temperature simultaneously with a single measurement. With the advantages of simple configuration, convenient integration and easy fabrication, the structure has great potential in multiparametric chemical and biological sensing applications.
Funding
National Natural Science Foundation of China (NSFC) (Grant No. 61675178 & 61377023); National Key Research and Development Program (2016YFB0402502).
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