This work presents simulation and experimental results of ultra-thin optical ring resonators, having larger Evanescent Field (EF) penetration depths, and therefore larger sensitivities, as compared to conventional Silicon-on-Insulator (SOI)-based resonator sensors. Having higher sensitivities to the changes in the refractive indices of the cladding media is desirable for sensing applications, as the interactions of interest take place in this region. Using ultra-thin waveguides (<100 nm thick) shows promise to enhance sensitivity for both bulk and surface sensing, due to increased penetration of the EF into the cladding. In this work, the designs and characterization of ultra-thin resonator sensors, within the constraints of a multi-project wafer service that offers three waveguide thicknesses (90 nm, 150 nm, and 220 nm), are presented. These services typically allow efficient integration of biosensors with on-chip detectors, moving towards the implementation of lab-on-chip (LoC) systems. Also, higher temperature stability of ultra-thin resonator sensors were characterized and, in the presence of intentional environmental (temperature) fluctuations, were compared to standard transverse electric SOI-based resonator sensors.
© 2014 Optical Society of America
1. Introduction and background
Silicon nanophotonic optical resonators have proven to be an attractive research area for many applications such as modulators and filters [1, 2]. These sensors can be used for biosensing applications as well, ranging from basic medical research  to bioterror detection [4, 5] to smart home healthcare  diagnostics. These devices utilize the Evanescent Field (EF) of the light propagating in a waveguide to detect molecular interactions and have been the focus of biosensing research [7–10]. Their sensing mechanism relies upon the interaction of the evanescent tail of the guided mode in the waveguide with molecules in the cladding medium, outside the waveguide core. Changes in this region (induced by bulk solution changes or binding events on the surface of the waveguide’s core) change the effective refractive index of the propagating mode in the waveguide. This effective index change results in measurable changes in the optical characteristics of the system and can be easily quantified, thus these optical sensors show promise for label-free detection and real-time monitoring of molecular binding.
An additional advantage of these EF waveguide-based sensors is that they can be fabricated in CMOS-compatible SOI processes, taking advantage of existing foundry processes and the economies of scale provided by high-volume fabrication.
Among EF optical sensors, waveguide-based ring [11–13], disk , photonic crystal , and Bragg grating  resonators have shown excellent potential for sensing applications as well as for Lab-on-Chip (LOC) integration. In these types of sensors, changes in the cladding’s refractive index (resulting from molecular binding events or concentration changes) change the effective index of the propagating mode. The ratio of the change in effective index of the propagating mode to the changes in refractive index of the cladding medium, is defined as the mode sensitivity, Smode:7, 17, 18]: 7, 17]: 7, 17, 19]:
Our group has previously compared the ILODs of waveguide-based resonator sensors fabricated in silicon-on-insulator (SOI) operating near λ = 1550 nm. Ring , disk , and Bragg grating  resonators have been demonstrated with ILODs approaching the theoretical limit (due to the optical absorption of water) of 2.5 × 10−4 RIU .
One technique for improving the limit of detection of a waveguide-based resonator sensor is to increase its sensitivity, S. This can be done by increasing the interaction between the propagating optical mode and the cladding medium. When a larger portion of the optical field travels outside of the silicon waveguide core, both the mode sensitivity (Smode) and S increase, resulting in a larger shift in the resonant wavelength. This also results in higher loss due to the optical absorption of water, resulting in a lowered Q, and, thus, there is a trade-off that results in limited improvements in the ILOD.
Researchers have attempted to increase the interaction between the electric field of the propagating mode and the cladding through the use of slot waveguide resonators  and resonators employing the Transverse Magnetic (TM) modes of waveguides . Although these methods have yielded significant improvements in sensitivities, compared to Transverse Electric (TE) ring resonators with 220 nm thick waveguides, the modes propagating in both slot and TM waveguides tend to exhibit high bending losses, degrading their Qs and ILODs . Bragg grating resonators, on the other hand, achieved high Qs by avoiding the bends . However, these sensors are long and cascading them is challenging.
Another method of increasing the interaction of the EF of the optical mode with the cladding is the use of thinner waveguide cores. Theoretical analysis [23, 24] demonstrates that thinner SOI waveguide cores are more responsive (have higher sensitivities) both to bulk cladding concentrations and thin adsorbed biomolecule layers. Analysis to determine the optimum waveguide thicknesses for specific biological applications has also been performed [23, 24]. In addition, thin and ultra-thin waveguides have been fabricated in CMOS-compatible processes and exhibited low losses; for example a loss of about 2 dB/cm was reported for a 50 nm thick strip waveguide . Ultra-thin waveguide resonator sensors (based on using ultra-thin silicon cores) also offer the potential for improved thermal stability for biosensing applications. The refractive indices of the silicon core and silicon dioxide substrate materials increase with increasing temperature [26, 27], while the index of the water cladding decreases with increasing temperature [28, 29], thus moving more of the propagating field to the cladding decreases the overall effect of temperature on the modal effective index.
This paper presents the first experimental demonstration of ultra-thin waveguide resonator sensors fabricated in a CMOS-compatible SOI process. We have successfully simulated, fabricated, and tested ultra-thin waveguide resonators using waveguide core thicknesses (90 nm) available in commercial Multi Project Wafer (MPW) runs offered by Optoelectronic Systems in Silicon (OpSIS) and/or CMC Microsystems fabricated by the Institute of Microelectronics (IME) in Singapore. We focused on these thicknesses as they offered the best potential for integration with future CMOS-compatible processes. Using both simulated and experimental results, we have demonstrated that ultra-thin ring resonators have higher sensitivities to the changes in cladding medium and lower sensitivities to temperature. We also compared these ultra-thin resonators with standard 220 nm thick ring resonators and observed significantly more stable responses.
These sensors were tested with solutions of glucose, from which a prediction model was created to predict glucose concentrations. These sensors could also be used to study molecular bindings for chemical and biological research purposes .
2. Design Methods and Analysis
3D simulations using finite difference time domain (FDTD) for ring modulators have been demonstrated . Since 3D FDTD simulations do not take into account propagation loss (with surface roughness as a significant source) we found that our Matlab model, which consists of a directional coupler model (numerical analysis with eigen mode solver from Lumerical) and a waveguide model (with experimental propagation loss) is better suited to design ring resonators. Compact models for ring resonators have been shown to be efficient and computationally less expensive . For the purposes of this work, we employed the compact model method where the model for the directional coupler is simplified using κ and t.
We have used Lumerical MODE Solutions and analytical models in MATLAB to design optical resonators with various thicknesses. MODE is used to calculate the effective index for each waveguide thickness and, ultimately, the change in effective index as the cladding is altered. Figures 1(a–c) illustrate the mode profile of the propagating TE fundamental mode for waveguide thicknesses of 220, 150, and 90 nm. Racetrack resonators are usually optimized at critical coupling, resulting in maximum Extinction Ratio (ER). Propagation losses for various thicknesses are estimated to be between two to four dB/cm, according to experimental results performed in our group. These values are used to calculate the critical coupling length for racetrack resonators.
Considering estimated and simulated losses and simulated mode profiles for various sections of racetrack, analytical modelling in MATLAB is used to predict the response of the ring resonator, to achieve an optimal design.
The thinner the waveguides, the less confined are the propagating modes to their silicon cores and the higher are their penetration depths into the surrounding media. The evanescent field in the top cladding at the centre of the silicon slab can be approximated by:
The penetration depth (de) is defined as the distance from the silicon-cladding interface (at the centre of the silicon) at which the evanescent field decays to E0/e. This penetration depth (de) of the mode is calculated using MODE and Eq. (5) for the waveguide thicknesses under investigation, and is plotted in Fig 1(d).
Based on the higher simulated sensitivities for ultra-thin 90 nm resonator sensors, we decided to fabricate a few variations of these ultra-thin sensors as well as conventional 220 nm thick resonators for comparison purposes. In this research, resonators with radii of 10, 20, and 30 μm and waveguide widths of 800 nm to 950 nm were designed for critical coupling, in order to achieve the best ER and optimized response. The goal was to use 90 nm waveguide cores, the thinnest offered by the standard MPW foundries. When using ultra-thin waveguides, a wider silicon core is necessary to guide the mode and has the advantage of reduced scattering losses due to sidewall roughness . These reduced losses result in slightly higher Q values, but slightly lower sensitivities (as a small portion of the evanescent field is travelling outside of the core). This trade-off between Q and S leads to an optimum point for the ILOD. In addition, the single mode conditions for waveguides with 90 nm silicon cores are not disturbed until the waveguide widths reach 900 nm, after which the waveguides are multi-mode.
2.1. Sensitivities of TE resonator sensors to cladding refractive indices
An EF sensor operates by detecting changes in the effective index of the waveguide. The effective index of the waveguide can be affected by a change in the refractive index of the core and the refractive index of the substrate and/or the refractive index of cladding. We are most interested in the change in effective index of the waveguide as a function of the change in refractive index of the analyte in the cladding medium (Eqs. (1 and 2)).
If thinner cores are used in resonators, the mode is less confined to the core and a larger portion of the evanescent field travels outside the core, resulting in more interaction with the cladding. Therefore, the dependence of the effective index of the waveguide on the refractive index of the cladding medium is increased in thinner waveguides, resulting in higher sensitivity. MODE is used to calculate the effective index of strip waveguides with various widths, thicknesses, and claddings. For each case, the change in effective index as a function of the refractive index of the cladding is calculated. We then use the sensitivity relation for resonators (Eq. (2)) to find the estimated sensitivity for each case at λ0 = 1550 nm. Figure 2(a) shows the sensitivity that can be achieved in a strip waveguide resonator for three waveguide thicknesses: 90, 150, and 220 nm.
2.2. Sensitivities of TE resonator sensors to temperature variations
The sensitivity of a TE resonator sensor to temperature is defined as the wavelength shift in resonator’s response caused by a temperature change of the waveguide (core, substrate, and cladding). The change in refractive index as a function of a change in temperature ( ) for silicon can be approximated for T around 295 K, and λ0 around 1.5 μm, to be [26, 27]. Since the waveguide thickness is changing in our study, the contribution of the temperature variation of the silicon dioxide (SiO2) substrate would be significant and different for each case. Therefore, the sensitivity is estimated assuming that the temperature of the substrate is changing by the same amount as the temperature of the core. The sensitivity of the refractive index of silicon dioxide to temperature is about . Most solutions used for biological applications are aqueous. The dependence of the refractive index of water to temperature is [28, 29]. Figure 2(b) shows the sensitivity of strip waveguide resonators to temperature variations of waveguide (when the cladding is water).
3. Experimental Methods and Materials
The above mentioned designed ultra-thin racetrack resonator sensors were fabricated on a SOI chip using the E-Beam Lithography (EBL) System at the University of Washington - Washington Nanofabrication Facility (UW WNF). Figure 3 shows SEM images of two of these sensors. On-chip Grating Couplers (GCs) are used to couple light into and out of the SOI chip . These sensors were tested using techniques and reagents described in this section.
3.1. Experimental Setup
Our automated measurement setup consists of a tunable laser (Agilent 81682A, Agilent Technologies, Inc., USA) as the optical source that operates with an output range of 1460 nm to 1580 nm. An array of polarization maintaining (PM) optical fibers (PLC Connections, LLC., USA) is used to couple light from the tunable laser to the silicon waveguides on the chip through a GC. The guided mode in the waveguide will then travel through the biosensor and after interacting with the reagents will go to an output GC, which couples the light from the waveguide into another PM optical fiber. The intensity of the output light is then measured with an optical power sensor (Agilent 81635A, Agilent Technologies, Inc., USA). All of the above mentioned processes are controlled with MATLAB programs that control the tunable laser and optical detectors as well as a motorized stage holding the chip. The motorized stage is used to align the fiber array to the GCs on the chip; and move from one GC to another to interrogate various biosensors.
3.2. Reagents and Microfluidic Setup
Aqueous solutions of D-Glucose (D16-500, Fisher Chemicals, Fisher Scientific, Inc.) in distilled water with various concentrations were prepared using the multiple dilution method (0 to 2000 mg/dL). The refractive index of glucose solutions can be estimated according to the following Eq. :
4. Performance of the Ultra-Thin TE Resonator Sensors
In this section, we report on the performance of our fabricated devices.
4.1. Sensitivity Analysis Results
The optical responses of two ring resonators, one with the standard 220 nm thick silicon core and one with 90 nm ultra-thin silicon cores, in the presence of the various glucose concentrations were measured. These two devices were measured consecutively in the presence of the same solutions under the same experimental conditions. These experiments were repeated on three days, at three different temperatures between 298 and 299 K inclusive, therefore subjected to an intentional temperature variations of 1 K. From the data, the wavelength shifts as functions of the change in refractive index of the glucose concentrations were calculated and plotted for both sensors.
Figure 4 summarizes the wavelength shift responses of our two sensors, in the presence of the various concentrations of analyte, and plots these shifts as functions of refractive index change. The slopes of the best linear fits to these points represent the sensitivities in nm/RIU, which is the peak wavelength shift as a function of refractive index change. Figure 4(a) is the response of the traditional 220 nm thick resonator sensor, and Fig. 4(b) is the response of a 90 nm ultra-thin resonator sensor. The slope of the dashed lines in Figs. 4(a) and 4(b) is the sensitivity of these sensors that are 38.2 and 133 nm/RIU for the standard 220 nm thick and 90 nm ultra-thin resonator sensors, respectively. The wavelength shift in the 220 nm thick resonator shows strong perturbations (large errors) due to environmental variations such as temperature (average variations of 1 K were induced) or noise, whereas the wavelength shift in the 90 nm ultra-thin resonator sensor shows a strong linear relation with the changes in the refractive index of the analyte; i.e. lower sensitivity to temperature and noise. It is evident that the ultra-thin resonator sensor shows higher sensitivity as well as better stability (significantly smaller errors). Having higher sensitivity and better stability allows improved predictions. Note that the temperature was varied in these experiments to demonstrate and compare the performance of the sensors in the presence of intentional temperature variation; the errors are due to these variations. In typical experiments, where the temperature is not varied, the errors are significantly smaller.
To further demonstrate the lower sensitivity of ultra-thin resonators to temperature variations, and to characterize the temperature sensitivity of these sensors, the responses of the sensors at three different temperatures were measured and the corresponding resonant wavelength shifts were plotted as functions of temperature change. The shift in resonant wavelength as a function of temperature ( ) denotes the sensitivity of a sensor to temperature. Figure 5 shows the experimental results for the temperature sensitivity of a 220 nm thick and a 90 nm ultra-thin resonator sensor, to be 69 and 49 pm/K respectively. These values can be used to quantify the induced temperature variation on the sensitivity of the devices. For the conventional 220 nm thick resonator, the sensitivity is 38.2 nm/RIU +/− 69 pm/K, which means a 0.2% variations for 1 K temperature variation. Whereas for, Ultra-thin resonator sensors, the sensitivity is 133 nm/RIU +/− 49 pm/K, which corresponds to 0.04% variations due to 1 K temperature variations. Therefore, ultra-thin resonator sensors proved five fold improvement in terms of temperature stability.
One potential application of these sensors is the use of their bulk sensitivity in predicting concentrations of analyte in their cladding medium. Figure 6 shows the results of using linear models, based on their sensitivities and measured wavelength shifts, to predict the concentrations of the analyte in the cladding medium using our two sensors. The red (light) error bars show the prediction results using the conventional 220 nm thick resonator with the goodness of prediction (R2 value) of 0.85, and the thick black error bars are the result of predictions using the ultra-thin resonator sensor, with an R2 value of 0.993. The error bars are the result of intentional temperature variations of 1 K. Temperature controlling these sensors would significantly reduce the error bars, and the R2 value for ultra-thin sensors increasing to 0.998 (Fig. 7). Figure 6 demonstrates significant improvement in ultra-thin resonator’s ability to measure glucose concentrations in the presence of temperature variations, as compared to conventional 220 nm thick sensor.
4.2. Q Factor and Intrinsic Limit of Detection (ILOD)
The Q factor of an optical waveguide resonator, being a measure of number of optical oscillations until the resonating energy decays to 1/e of its max value, is defined in Eq. (3). This approximation is used to measure the Q values of our devices studied in this paper. The Q of a resonator is inversely proportional to the losses that are affecting the propagating mode (Eq. (3)). Various loss components contribute to the Q factor in ring resonators: scattering loss, bend loss, mode-mismatch loss, radiation loss in bends, and material absorption, including water absorption. The Q factor is lower in the thinner waveguide resonators, because of their higher losses due to their increased interaction with the biomolecules in the cladding medium. These increased interactions are desirable for sensing applications. The highest Q factor that was achieved by the ultra-thin resonators in water was around 24,000, and the maximum Extinction Ratio (ER) measured was 28 dB.
Figure 8 summarizes the modelling and experimental results of the EF sensors, presenting the intrinsic limit of detection of the EF sensors as functions of their Q factors and normalized sensitivities. The thin black dashed lines represent contours of constant ILOD. The thick light blue line is the theoretical limit of detection for (unloaded) resonant sensors due to water absorption at 1550 nm ; its locus represents different proportions of light travelling in the water versus in the waveguide core, namely a high Q / low sensitivity for highly confined modes (e.g., thick silicon waveguides  and disk resonators ), and a low Q / high sensitivity for weakly guided modes (e.g., thin silicon waveguides, TM polarized waveguides , and slot waveguides [20, 22]). The corresponding light blue markers are the modelling results for the specific unloaded resonators considered in this study, where only optical absorption due to water around the waveguide is considered. The thick black line is the theoretical limit for a critically coupled (CC) resonator. The corresponding black markers are the modelling results for critically coupled resonators, where the Q factor is determined from the simulated optical spectra; these also include additional losses (e.g. bend loss and mode-mismatch loss) except for the waveguide scattering loss. The experimental normalized sensitivities and Q values for each fabricated device are plotted (white markers), with the waveguide dimensions noted in the legend. The differences between the model (black markers) and the experimental results (white markers) are attributed to scattering losses and the excess losses of the directional couplers (the couplers were assumed to be ideal in the model, i.e., κ2 + t2 = 1). Note that the difference between the experimental and modelling results is much larger for the narrow waveguides (500 nm) as compared to the wide waveguides (800–950 nm). It is known that wide waveguides have much lower optical scattering loss [35–37]. Thus, we expect that the scattering losses of the thin and wide waveguides (800–950 nm) should be relatively small and, hence, we expect good agreement between the model and experiments. It is seen that the experimental results for the 90 nm ultra-thin sensors agree very well with the model, both in terms of sensitivity and quality factor. These sensors offer performance that matches the theoretical limit for a critically coupled resonator.
5. Conclusion and Discussion
We have investigated, both by simulations and experiments, ultra-thin TE resonator sensors within the constraint of available thicknesses in standard MPW foundries and services. We obtained sensitivities over 100 nm/RIU with the ultra-thin TE resonator sensors. We have demonstrated, by experiment and simulation, the increased stability of these ultra-thin resonators, as compared to the traditional 220 nm thick resonators, in the presence of temperature variations. We report Q factors on the order of 15,000 to 25,000, with the ILODs on the order of 5 × 10−4 RIU. In addition, good agreement between experimental results and simulations was demonstrated. The bulk sensitivity and capability of these sensors in predicting glucose concentrations, in the presence of intentional 1 K temperature fluctuations, was demonstrated and showed a manifold improvement in these predictions as compared to traditional 220 nm thick resonator sensors.
The ultra-thin resonator sensors developed here, using the smallest available thickness offered by MPW foundries, can be integrated with the on-chip detectors also offered by these standard foundries. Furthermore, given that they are fabricated using SOI technology, they are well positioned to be integrated with CMOS electronics to produce a lab-on-chip. Additionally, as compared to conventional 220 nm resonator sensors, our ultra-thin resonator sensors, with larger evanescent fields, have unique advantages for bio sensing, e.g., by sampling more of the measurand, these sensors provide the capability to sense larger particles.
The authors would like to thank CMC Microsystems, Lumerical Solutions, Inc. for providing the simulation software. We gratefully acknowledge NSERC CREATE Silicon Electronic Photonics Integrated Circuits (SiEPIC) training program. This work was made possible by a National Priorities Research Program grant from the Qatar National Research Fund. This work was supported by the University of Washington (UW) Royalty Research Fund, NSF CBET (Award numbers 1264174 and 0930411), and the Washington Research Foundation. We gratefully acknowledge Lumerical Solutions, Inc. for providing the simulation software. Part of this work was conducted at the University of Washington Nanofabrication Facility (WNF), a member of the NSF National Nanotechnology Infrastructure Network. The first author, Sahba Talebi Fard, wishes to thank Mr. Farshad Madhi, from UC Berkeley, for his contribution in editing the manuscript and for his insightful comments.
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