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Abstract
We demonstrate high-Q Si3N4 racetrack-spiral and spiral-spiral coupled-resonator devices for sensing applications. The coupled-resonator architecture resolves the confusion caused by small free spectral range (FSR) in conventional long resonators. The sensitivity of the racetrack-spiral coupled-resonator device for sodium chloride (NaCl) and biomolecule detection is numerically achieved at around 199 nm/RIU (bulk sensitivity) and 183 pm/nm (surface sensitivity), respectively. We fabricate a racetrack-spiral coupled-resonator device with intrinsic Q of 560,000 (263,000) at near-infrared wavelengths, at around 1300 nm, for air (water) cladding. We extract an experimental bulk sensitivity of around 131 nm/RIU for this device from the wavelength shift of the device with different concentrations of the NaCl solution. The performance of the spiral-spiral coupled-resonator device for NaCl and biomolecule detection is theoretically calculated. For biomolecular detection, this device is self-referenced, and its FSR increases almost linearly with the refractive index of biomolecular layer in a certain range. A theoretically defined FSR sensitivity of 74 nm/RIU is extracted from the experimental data. We fabricate a spiral-spiral coupled-resonator device with intrinsic Q of 276,000 (112,000) at around 1300 nm for air (water) cladding, and a bulk sensitivity of 167 nm/RIU for NaCl detection.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Integrated photonic microresonators are promising for sensing applications as they enable chip-scale, multiplexed, label-free detection of biomolecules, and provide the opportunity of miniaturizing and integration of both passive and active optical components, such as lasers and photodetectors, onto the same platform. Various integrated optical sensors have been developed based on optical resonators [1–3], plasmonic resonators [4], nanowire [5], and nanomechanical resonators [6], to name a few. Optical resonance is a major sensing method for detecting biomolecules, i.e., biomarkers, which are essential for medical diagnostics, e.g., cancer detection at early stages or heart-failure detection [7]. High-quality-factor (high-Q) optical resonators with high field enhancement and sharp resonance profiles provide sensitive and fast detection features with miniaturized footprints. Spiral resonators with various cavity lengths have been demonstrated on different platforms to achieve the high Q, such as Q = 35,000 for a silicon-on-insulator (SOI) spiral resonator with a cavity length of 2400 µm [8] and Q = 140,000,000 for a silica-on-silicon spiral resonator with a cavity length of 1.2 m [9]. While the long spiral resonator enables a high Q, it also complicates the determination of the actual wavelength shift when the shift is larger than one free spectral range (FSR), which is very small due to the large spiral length. Coupled optical resonators [10] could thus be used to solve this shortcoming by extending the FSR extensively [11,12].
Silicon Nitride (Si3N4) is a promising material platform for integrated photonic sensing as it provides various advantages over other options, such as SOI and silica-on-silicon. Si3N4 offers a very low material loss over a wide transparency window (from visible to infrared) [14–16] while silicon (Si) is opaque in the visible wavelengths. Besides, Si3N4 has a small thermo-optic coefficient ($TOC = 2.45 \times {10^{ - 5}}/K$), which leads to the suppression of the thermal noise by an order of magnitude compared with Si ($TOC = 1.8 \times {10^{ - 4}}/K$) [17,18]. Si3N4 also provides a medium refractive index with a value of approximately 2 and it is fully compatible with standard complementary metal-oxide-semiconductor (CMOS) fabrication processes with no undercutting while silicon-oxide (SiO2) devices on Si usually require undercutting for light confinement in the SiO2 layer [9,13], due to the small refractive index of SiO2. Several Si3N4 microresonator-based sensors working at various wavelengths have been developed to achieve high bulk sensitivity, but the Q of the reported devices are relatively low, e.g., Q = 75,000 for microring resonator at around 970 nm [19], Q = 1,300, Q = 1,800 and Q = 3,000 for slot-rings at around 1300 nm [20–22], Q = 15,000 and Q = 10,000 for suspended microdisk resonators at around 780 nm and around 1550 nm [23,24], which degrades the value of the high sensitivity.
In this work, we report two high-Q ultrasensitive Si3N4 coupled-resonator architectures that are promising for sensing applications at around 1300 nm wavelengths, for which the light absorption of water is low [25]. The first device uses a racetrack-spiral coupled-resonator (Fig. 1, top), where the racetrack resonator works as a filter with a relatively large FSR to resolve the confusion caused by the small FSR of the spiral resonator while enabling the detection of very small spectral shifts through the high resonator Q. The second device uses a spiral-spiral coupled-resonator (Fig. 1, bottom), where the FSR of the coupled-resonator device (FSRD) is extended through the Vernier operation (resonators of different lengths, i.e., 3600 µm and 3960 µm for the device shown in Fig. 1, bottom). This device also resolves the confusion caused by small FSRs in conventional resonators while maintaining a high Q. If we coat only one of the spiral resonators with target biomolecules while keeping the other spiral resonator unchanged, the FSR of the sensing spiral resonator will change, leading to the variation of FSRD. In this way, we could use one spiral resonator as a sensor while the other one as a reference resonator to compensate the unwanted changes, leading to a self-referenced device.
Fig. 1. The SEM images of (top) a racetrack-spiral coupled-resonator device (racetrack total length: 317 µm, spiral total length: 3800 µm, the gap between the waveguide and the racetrack resonator: 0.2 µm with a coupling length of 8.5 µm, the gap between the racetrack and the spiral resonator: 0.55 µm with a coupling length of 1.5 µm) and (bottom) a spiral-spiral coupled-resonator device (two spiral total lengths: 3600 µm and 3960 µm, the gap between the waveguide and the spiral resonator: 0.2 µm with a coupling length of 5.7 µm, the gap between the two spiral resonators: 0.5 µm with a coupling length of 1 µm). Insets are the zoomed-in SEM images of the racetrack resonator and the spiral resonator.
2. Simulation and fabrication
The resonators in this work are designed for the fundamental quasi-transverse-magnetic (TM)-polarized mode (electric field out of the plane of the resonator) to achieve a higher sensitivity. An in-house code based on the coupling transfer matrix approach, developed in MATLAB, and the finite-element method (2D FEM) to obtain the effective refractive index (neff) and group index (ng), implemented using COMSOL, are utilized to model the transmission spectra of racetrack-spiral and spiral-spiral coupled-resonator devices in any sensing scenario. The waveguide widths of both devices are 1 µm for single-mode operation. The total lengths of the racetrack and the spiral resonators in the racetrack-spiral coupled-resonator device are 317 µm and 3800 µm, respectively. The total lengths of the two spiral resonators in the spiral-spiral coupled-resonator device are 3600 µm and 3960 µm. The grating couplers are designed for achieving the maximum coupling efficiency to the fundamental TM-polarized mode of the input waveguide at around 1300 nm wavelength. The finite-difference time-domain (FDTD) method, implemented in Lumerical, is utilized to numerically calculate the coupling efficiency of the grating coupler. The perfect matched layer (PML) boundary condition is applied in all directions. Our grating, with a pitch of 1130 nm, a duty cycle of 0.6089, and an etch depth of 224 nm, shows a maximum coupling efficiency of -7.96 dB (or 16%) at around 1300 nm.
The designed devices are fabricated on a 4-µm-thick SiO2 layer on top of a Si substrate through several processing steps with the structural parameters given in Tables 1 and 2. First, a 300-nm-thick Si3N4 layer of high quality is deposited as the device layer using low-pressure chemical vapor deposition (LPCVD) on top of the SiO2 layer. Then, electron-beam (e-beam) lithography (EBL, Elionix ELS-G100) is used to pattern Si3N4 with the spin-coated e-beam resist being flowable oxide (FOx, Dow Corning), which provides high resolution and good etching selectivity. A two-step etching process is implemented to ensure the grating coupling efficiency to the TM mode of the waveguide (shallow etching in the grating coupler area) while maintaining the high Q of the resonators (full etching in all other areas). Inductively coupled plasma etching (ICP, Plasma-Therm) using carbon tetrafluoride (CF4) is first utilized to etch the Si3N4 layer shallowly (etch depth: 224 nm) with FOx as the hard mask. After that, we pattern the grating coupler area to protect it from full ICP etching with ma-N being the e-beam resist. The remaining Si3N4 layer is then etched using the ICP system with ma-N as the hard mask for the grating coupler area. The ma-N and FOx resists are consequently removed using Microposit Remover 1165 and buffered oxide etch (BOE), respectively. Finally, an annealing process is implemented in a furnace (Mini Tystar) with an optimized recipe of 8 hours in an O2 ambient at 1100 °C and then 4 hours in an N2 ambient at 1100 °C. The annealing process not only minimizes the material absorption loss by reducing the hydrogen content but also make the sidewall of the waveguide smooth, which are essential factors to achieve the high Q.
Table 1. Structural Parameters of the Racetrack-Spiral Coupled-Resonator Device in Fig. 1, Top
Table 2. Structural Parameters of the Spiral-Spiral Coupled-Resonator Device in Fig. 1, Bottom
Figure 1 shows the scanning electron microscopy (SEM) images of a fabricated racetrack-spiral coupled-resonator device (upper part) and a fabricated spiral-spiral coupled-resonator device (lower part). The structural parameters of the two devices in Fig. 1 are listed in Table 1 and Table 2, respectively. The transmission spectra of the racetrack-spiral coupled-resonator device (Fig. 2(a)) exhibit the highest intrinsic Q of 560,000 and 263,000 with air and water cladding, respectively. These numbers for the spiral-spiral coupled-resonator device are 276,000 and 112,000, respectively, as shown in Fig. 2(b). To the best of our knowledge, these values are the highest reported Q for Si3N4 microresonator-based sensors.
Fig. 2. The transmission spectra of (a) the racetrack-spiral resonator (the gap between the waveguide and the racetrack resonator is 0.3 µm, and all other parameters are the same as those in Table 1) and (b) the spiral-spiral coupled-resonator device (all structural parameters are the same as those in Table 2).
3. Theoretical results for sensing
The resonance wavelength of a resonator is related to the refractive index of the cladding material of the resonator through the equation [26]
where ${\lambda _r}$ is the resonance wavelength, m is the azimuthal mode order, and L is the length of the resonator. The neff of the waveguide that forms the resonator is dependent on the refractive index of the cladding material. Thus, the refractive index change of the cladding material within the evanescent tail of the optical mode leads to a resonance wavelength shift. The fraction of power in the evanescent field at the top cladding of the waveguide for the devices in Fig. 1 is approximately 20% at around 1300 nm with water cladding. Both chemicals (e.g., NaCl) and biomolecules covered the evanescent tail of the optical mode can be detected by monitoring the resonance wavelength shift.The theoretical results for the transmission spectra of the two devices with different concentrations of NaCl solution are shown in Figs. 3(a) and 3(b). The numerical bulk sensitivities (${S_{RIU}}$) are 199 nm/RIU and 200 nm/RIU, respectively, which is calculated based on the refractive index variation for different concentrations of NaCl solutions [27] using the equation [28]
Fig. 3. Theoretical transmission spectra of (a) racetrack-spiral and (b) spiral-spiral coupled-resonator devices in Fig. 1 with different concentrations of NaCl solution. The top figures in (a) and (b) are the corresponding transmission spectra of the two devices with DI water as the cladding.
In order to investigate the capability of the two coupled-resonator devices for biomolecular layer sensing (BMLS), the sensitivity for the detection of biomolecules is theoretically calculated for the racetrack-spiral coupled-resonator device, and the performance of the spiral-spiral coupled-resonator device is simulated and optimized by optimizing the total lengths of the two spirals. The biomolecular layer (BML) is modeled with a thickness of 10 nm, a density of $1.33\; \textrm{g}\cdot\textrm{c}{\textrm{m}^{ - 3}}$ and a refractive index of 1.46. Deionized (DI) water cladding with a refractive index of 1.32 is implemented on top of both coupled-resonator devices before and after applying the BML. Figure 4(a) exhibits the theoretical results of the transmission spectra of the racetrack-spiral coupled-resonator device before and after a thin BML is applied. The resonance wavelength undergoes a red shift of 1.83 nm due to the enhancement of the refractive index. The theoretical surface sensitivity of the racetrack-spiral coupled-resonator device for biomolecule detection is achieved at around 183 pm/nm, which can be calculated using the equation [28]
Fig. 4. Theoretical transmission spectra of (a) racetrack-spiral and (b) spiral-spiral coupled-resonator device before and after a 10-nm-thick BML, with refractive index of 1.46, applied on top of the coupled-resonator device (for (a)) and the spiral with total length of 3600 µm (for (b)). The total length of the second spiral in (b) is 3720 µm. The cladding for both spirals is water. (c) The theoretically calculated FSRD as a function of the refractive index of BML.
4. Experimental results and discussions
To test these conclusions experimentally, the transmission spectra of the fabricated devices in Fig. 1 are measured for different concentrations of an NaCl (Sigma Aldrich) solution that covers both resonators in the racetrack-spiral and spiral-spiral coupled-resonator architectures. A tunable laser (TSL-550, Santec) is used as the light source. A pair of cleaved single-mode optical fiber is utilized to couple light into and out of the device. The output signal is detected by a photodetector (PDB 150C, Thorlabs). A thermal control stage is used to keep the temperature fixed at the room temperature during characterization. The data is finally collected for saving and post-processing to a computer through a data acquisition (DAQ) card (PCI-6251, National Instruments). Figure 5(a) shows the variation of the spectrum of a single azimuthal mode of the Si3N4 racetrack-spiral coupled-resonator device. Note that the resonator has several azimuthal modes separated by an FSR of 2.65 nm, and the results shown in Fig. 5(a) are for a single azimuthal mode. It is seen that the resonance wavelength shifts to longer wavelengths as the concentration of the NaCl solution increases (corresponding to the enhancement of the refractive index of the NaCl solution, which is extracted from the data in Ref. [27]). By using the values of the refractive index of different NaCl solutions from Ref. [27] and linear fitting of the experimental data, a measured bulk sensitivity of around 131 nm/RIU is obtained for this device. Figure 5(b) shows the experimentally measured transmission spectra of the spiral-spiral coupled-resonator device with different concentrations of NaCl solution, showing a measured bulk sensitivity of 167 nm/RIU and FSRD being almost unchanged under different solutions (applied to both resonators). The measured bulk sensitivities are lower than the theoretical values due to fabrication imperfections and the variation between the refractive indices of different NaCl solutions in [27] and those in our work.
Fig. 5. The experimentally measured transmission spectra of (a) racetrack-spiral and (b) spiral-spiral coupled-resonator device shown in Fig. 1 with different concentrations of NaCl solution.
The comparison with other Si3N4 sensing works is summarized in Table 3. The small full-width-at-half-maximum (FWHM) represents the narrow resonance linewidth, which enables the detection of very small spectral shifts. It is extracted from the intrinsic Q (Q0) or the loaded Q (QL) and the resonance wavelength. Without considering the fragile suspended devices [24], the measured bulk sensitivities here are in the same range as those in previous reports [19–22]. Nevertheless, the Q of our devices is around 1-2 orders of magnitude higher than those of these reports, and it is achieved by the optimization of the design of the spiral resonators and the fabrication processes. Here, we define a parameter of sensitivity/FWHM to represent the detection capability of the device. It is seen that this parameter of our device is orders of magnitude higher than those of other works, indicating higher detection capability. The coupled-resonator architectures in our work resolve the resonance shift confusion caused by small FSRs in conventional long resonators. The larger size of our devices increases the interaction area with analytes, and thus, for larger molecule sensing, the noise due to the nonspecific binding is smaller than that of the miniaturized devices.
Table 3. Comparison with other Si3N4 sensing works
5. Conclusion
In summary, we showed here a racetrack-spiral coupled-resonator device with resonance wavelength around 1300 nm, numerically calculated bulk and surface sensitivities for NaCl and biomolecule detection of around 199 nm/RIU and 183 pm/nm, intrinsic Q of 560,000 (263,000) for air (water) cladding, and an experimentally measured bulk sensitivity of 131 nm/RIU. We theoretically calculated the performance of a spiral-spiral coupled-resonator device for bulk NaCl sensing with a sensitivity of 200 nm/RIU, and BML sensing with an FSR sensitivity of 74 nm/RIU, which also shows that it is a self-referenced device. We experimentally demonstrated a spiral-spiral coupled-resonator device with resonance wavelength around 1300 nm, intrinsic Q of 276,000 (112,000) for air (water) cladding, and an experimentally measured bulk sensitivity of 167 nm/RIU. These coupled-resonator structures avoid the confusion caused by small FSRs of the spiral resonators. With these properties, the two structures shown in this work can be of great interest for various sensing applications.
Funding
Qatar National Research Fund (NPRP grant (10-0101-170076)).
Acknowledgment
This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542174).
Disclosures
The authors declare no conflicts of interest.
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