1. Introduction

Optical biosensors have been illustrated to be ideal candidates for the Lab-on-a-chip (LOC) applications, due to their advantages of small footprint, extremely high sensitivity, robustness, low cost, and potential for mass production and multiplexing [1,2]. Various micro-photonic devices have been used to achieve on-chip optical sensing, such as micro ring resonators (MRR) [3-7], Mach-Zehnder interferometers (MZI) [8,9], surface plasmon resonators [10,11], and photonic crystal (PhC) cavities [12-20]. In these resonator-based sensors, a tiny disturbance in the external cladding refractive index can cause a noticeable frequency shift of the resonant peaks, which can provide an easily measurable spectrum response. Due to the high sensitivity, high quality factor (Q) and low mode volume (${V_m}$) characteristics, PhC cavities provide unique advantages in optical sensing.

High sensitivity up to 1500 nm/RIU has been reported by using the 2D PhC cavities [20]. Compared to 2D PhC cavities, 1D PhC cavities possess a much smaller footprint and less fabrication difficulties, making it attractive to sensing in recent years. Conventional nanobeam cavity sensors are mostly designed to confine light within the high index region of the waveguide core. However, only the light energy distributed outside the core region can interact with the environmental analyte, which means a limitation of their sensitivity (∼200 nm/RIU) [21]. In order to achieve an enhanced light-matter interaction and sensitivity, nanoslot has been introduced into the 1D-PhC cavities. Yang et al. experimentally demonstrated a PhC cavity sensor with a sensitivity ∼ 451 nm/RIU by utilizing a well-designed slotted quadrabeam PhC cavity [16]. By introducing a stack shaped slotted PhC cavity, Xu et al. experimentally achieved the sensitivity around 410 nm/RIU [13].

In this paper, we demonstrate a suspended slotted PhC cavity (SSPhC) based on silicon-on-insulator (SOI) platform, which consists of series pairs of nano tentacles. Benefitting from the nano tentacles as well as the suspended architecture, the sensitivity of the designed air mode SSPhC cavity can be significantly enhanced. A recorded bulk sensitivity of 656 nm/RIU among the PhC nanobeam cavity based sensors has been experimentally demonstrated by exposing the SSPhC cavity to the NaCl solutions with different concentrations. Furthermore, the proposed cavity sensor with excellent sensing performance has an ultra-compact sensing region, making it favorable to the future large-scale on-chip sensing arrays.

2. Design and analysis

The basic structure is a suspended slotted air mode 1D PhC cavity based on the SOI platform, which includes a 220 nm thick silicon device layer and a 2 µm thick oxide buffer layer. The refractive indices of the silicon core and the silica buffer are taken as 3.46 and 1.45, respectively. Figure 1(a) shows the schematic of the proposed architecture, in the sensing region, the bottom oxide buffer layer is removed, thus the shape modulated suspended PhC cavity can be totally submerged into the liquid analyte with a refractive index ${n_{cl}}$ ∼ 1.32. As can be seen from Fig. 2(b), (a) series of etched holes with pairs of protruding nano tentacles were introduced to one normal solid waveguide to achieve extreme light limitations in a specific wavelength range. To minimize the radiation losses and increase the Q-factor, a systematical method is utilized to generate a Gaussian shaped field profile within the PhC cavity [22]. To achieve the preferred Gaussian field profile, Quan et al. [23,24] proposed an efficient method by introducing a linearly increased attenuation of the electromagnetic field from the center holes to the outside holes of the PhC cavity. In this work, the distances between the nano tentacles are parabolically modulated from $ga{p_{center}}$ to $ga{p_{end}}$. The gap widths are determined by the following formula: $ga{p_i} = ga{p_{center}} + {({i - 1} )^2}({ga{p_{end}} - ga{p_{center}}} )/{({N - 1} )^2}$, where i is from 1 to N, and N is the number of holes in each side of the cavity. The radii of the holes R and the width of the waveguide ${W_{wg}}$ are set to be constants.

 

Fig. 1. (a) Schematic of the proposed SSPhC cavity sensor and the zoomed view; (b) schematic of the cavity from top view and the zoomed view; (c) The electric field distribution from top view (xoy plane, z = 0) and (d) from side view (yoz plane, x = 0); (e) The calculated band diagram of the periodic SSPhC cavity unit cell with gap = 100 nm (red curve) and gap = 145 nm (blue curve), the black circle indicates the resonant frequency, and the blue region indicates the radiation modes.

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Fig. 2. Influence of (a) radius, (b) gap and (c) W_s, on the sensitivity and Q-factor of SSPhC cavity sensors; (d) Wavelength shift and variance of Q-factor over different background refractive indices; (e) Simulated transmission spectrum of the cavity.

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The three-dimensional finite-difference-time-domain (3D-FDTD) method is used to calculate the photonic band structure and field distribution. The resonant wavelength of the SSPhC cavity is designed around 1550 . Figures 1(c) and (d) shows the electric field distribution from top view (xoy-plane) of the SSPhC cavity and the cross-section view (zoy-plane). From Fig. 1(c) and Fig. 1(d), we can find that the electric field is strongly confined in the low index etched holes area, especially near the narrow slot gap area. Figure 1(e) shows the TE band diagrams of the suspended slotted PhC cavity, which are calculated from series band structure simulations with Bloch boundary conditions. The red curve in the Fig. 1(e) shows the band diagram of $ga{p_{center}}$=100 nm, while the blue one indicates the band diagram of $ga{p_{end}}$=145 nm. And the black circle indicates the resonant frequency of the SSPhC cavity. To keep the resonant wavelength near 1550 nm, the period is chosen as a = 520 nm and the waveguide width is chosen as ${W_{wg}}$= 640 nm. The period number on each side of the cavity is chosen to be N = 15 to obtain a high enough transmittance. The effective mode volume of the proposed cavity is calculated to be 1.86 ${({\lambda /{n_{si}}} )^3}$, where the mode volume is defined by ${V_m} = \smallint dV\varepsilon {|E |^2}/{[{\epsilon {{|E |}^2}} ]_{max}}^3$.

The geometry parameters for the SSPhC cavity (including the R, gap, W_s) are optimized in order to obtain extremely high sensitivity and high Q-factor. The detailed analysis results are illustrated in Figs. 2(a)–2(c). As can be seen from Fig. 2(a), the sensitivity will rise dramatically with the increase of the holes radius R, where the increasing ratio is ∼2.5 nm⁻¹, and the Q-factor reaches a maximum value at radius R ∼ 195 nm. In Fig. 2(b), the sensitivity rises much more slowly (∼0.94 nm⁻¹) with the increasing of the distances gap between the two nano tentacles, while the Q-factor will decrease slightly with the increasing distances gap. We finally chose a gap ∼ 100 nm, which is a tradeoff between the sensitivity and Q-factor. From Fig. 2(c), both the sensitivity and the Q-factor will increase with the decreasing widths of the nano tentacles (along x-axis) ${W_s}$. As the minimum width of the narrow silicon strip is limited by the fabrication process, we finally chose ${W_s}$ to be 100 nm to reduce the fabrication difficulty.

From the above analysis, the parameters of the SSPhC cavity are chosen to be: a = 520 nm, ${W_{wg}}$ ∼ 640 nm, R = 195 nm, ${W_s}$ = 100 nm, gap = 100 nm and N = 15 on each side. The transmission spectrum of the designed cavity was shown in Fig. 2(e), in which multiple slot modes will appear. We used the fundamental cavity mode for sensing since it provides a high Q factor and a high sensitivity. In the proposed SSPhC cavity, a large part of the light energy is confined in the liquid solutions region (∼57%), ensuring an enhanced light-matter interaction between the optical field and the environmental analytes as well as an ultra-high sensitivity. The resonant wavelength and Q-factor of the proposed SSPhC cavity with different cladding index (from 1.30-1.35) is given in Fig. 2(d), which shows a high sensitivity around ∼672 nm/RIU and a Q-factor larger than 1.2 ${\times} {10^4}$ over large range of background refractive indices.

3. Fabrication and measurement

The SSPhC cavities was fabricated on a commercial SOI wafer (SOITEC Inc.) with a silicon layer of 220 nm and a bottom oxide buffer layer of 2 µm. A positive tone E-beam resist (PMMA, 950 K) was spin-coated onto the SOI wafer at 4000 rpm, and the thickness of the PMMA film was 330 nm. Then the SOI wafer was baked on a hot plate at 180 °C for 15 minutes. The device pattern was firstly defined by the electron-beam lithography (Raith150 II) and then developed in a MIBK:IPA (1:3) mixture. In the next step, the defined pattern was transferred onto the silicon layer by inductively coupled plasma (ICP) etching using a gas mixture of SF_6­ and C₄F₈. Another overlay exposure followed by a shallow etching (70 nm) was performed to realize the TE mode grating coupler (TEGC) with a period of 630 nm and a duty cycle of 50:50. Finally, the SOI wafer was submerged in BOE (49%HF_(aq):40%NH₄F_(aq)=1:6) for 20 minutes to release the SiO₂ buffer layer under the SSPhC cavity.

Figure 3(a) shows the scanning electric microscopy (SEM) of the whole device, including the grating couplers, the supported brackets and the SSPhC cavity, while Figs. 3(b)–3(d) are the enlarged view of the device in different parts. In order to improve the mechanical stability of the suspended structure, pairs of sub-wavelength brackets were used to support the whole device, as can been seen from Fig. 3(c). Based on the measured results, all these 8 pairs of brackets introduced extra losses lower than 3 dB, where the period lengths of these brackets are 260 nm and the duty cycle are 50:50. The SSPhC cavity part is shown in Fig. 3(d), and the minimum feature size of the cavity is ∼100 nm. To characterize the fabricated SSPhC cavity sensors, a homemade testing system was used, including a broadband amplified spontaneous emission (ASE) source with a wavelength ranging from 1520 to 1610 nm and an optical spectrum analyzer (OSA, Yokogawa AQ6370D) to detect the output spectrum.

 

Fig. 3. The SEM pictures of the fabricated device. (a) The whole view of the device; (b) enlarged view of the TE mode grating coupler; (c) enlarged view of the supporting brackets and (d) enlarged view of the nanobeam cavity.

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The transmission spectra were measured by submerging the cavity into different mass concentrations NaCl solutions. Here a medical syringe was utilized to transfer the prepared NaCl solutions onto the SOI chip. We firstly pre-aligned the grating couplers on our chip with the coupling fiber and then add the droplet of solution onto the chip by using the medical syringe. A further alignment was then performed to ensure that the coupling gratings were well aligned to the optical fibers. The refractive index varies 0.0018 refractive index unit (RIU) for per 1% shift of the NaCl solution concentration at room temperature (20°C) [25]. In our experiment, the concentrations of NaCl solutions varied from 0% to 12% with a step of 3%; therefore, the refractive indices of the NaCl solutions were: 1.32, 1.3254, 1.3308, 1.3362 and 1.3416. After each measurement in NaCl solutions with different concentrations, the device was well cleaned by DI water in order to remove the residuals. Figure 4(b) shows the transmission spectrum of the SSPhC cavity sensor immerged into 9% NaCl solution, wherein the red circle indicates the experimental data and the black curve indicates the result of Lorentzian fit. The full width at half maximum (FWHM) of the cavity spectrum was ∼580 pm, which indicate a Q-factor ∼ 2719. The measured Q-factor was lower than the designed one, which is mainly caused by the light absorption from the surrounding water [26]. By measuring the resonant wavelength of the fundamental mode (Fig. 4(a)), the linear relationship between the resonances and the refractive indices can be obtained in Fig. 4(c). From the linear fit in Fig. 4(c), an experimental sensitivity of ∼ 656 nm/RIU can be obtained, which shows rather good agreement with the designed value (∼672 nm/RIU). The measured Q-factors are shown in Fig. 4(d), which were in the range of 2015 ∼ 2719. The measured sensitivity is the highest among all reported 1D PhC nanobeam cavity based sensors, which is 2.43 times to the dielectric PhC cavity (270 nm/RIU) [17] and 1.64 times to the slotted one (∼400 nm/RIU) [13,16].

 

Fig. 4. (a) Measured transmission response of the SSPhC cavity sensor immersed into NaCl solutions of different mass concentrations (from 0% to 12%); (b) The measured transmission of the SSPhC cavity sensor, and the red circle indicates the measured spectrum and the black curve indicates the Lorentzian fit result. (c) The resonant wavelengths of the cavity with different background indices, and the measured sensitivity is 656 nm/RIU by linear fit; (d) The measured Q-factor of the cavity sensor with different background indices.

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4. Summary

In conclusion, a highly sensitive silicon photonic biosensor was demonstrated by using a novel 1D-SSPhC cavity. From the simulated and experimental results, the cavity can be capable of achieving a largely enhanced light power distribution in the low index region of the etching circle hole, especially near the gap between the nano tentacles. Therefore, a greatly enlarged overlap between the optical field and the analyte has been achieved which indicates a significantly improved sensitivity of the SSPhC cavity. The measured sensitivity is about 656 nn/RIU, which agrees well with the designed value. Furthermore, the device possesses an extremely small size. The sensing area of the cavity sensor is only 15.6 ${\times} $ 0.64 µm², and the whole size of the device (including the grating couplers) is 320 × 40 µm². Therefore, such sensing device with characteristics of highly enlarged light-matter interaction, ultra-compact footprint and extremely high sensitivity will be conducive to the development of versatile Lab-on-a-chip applications.