Unique surface sensing property and enhanced sensitivity in microring resonator biosensors based on subwavelength grating waveguides

In this paper, unique surface sensing property and enhanced sensitivity in microring resonator biosensors based on subwavelength grating (SWG) waveguides are studied and demonstrated. The SWG structure consists of periodic silicon pillars in the propagation direction with a subwavelength period. Effective sensing region in the SWG microring resonator includes not only the top and side of the waveguide, but also the space between the silicon pillars on the light propagation path. It leads to greatly increased sensitivity and a unique surface sensing property in contrast to common evanescent wave sensors: the surface sensitivity remains constantly high as the surface layer thickness grows. Microring resonator biosensors based on both SWG waveguides and conventional strip waveguides were compared side by side in surface sensing experiment and the enhanced surface sensing capability in SWG based microring resonator biosensors was demonstrated. © 2016 Optical Society of America OCIS codes: (280.4788) Optical sensing and sensors; (130.3120) Integrated optics devices; (050.6624) Subwavelength structures References and links 1. M. C. C. Estevez, M. Alvarez, and L. M. M. Lechuga, "Integrated optical devices for lab-on-a-chip biosensing applications," Laser Photon. Rev. 6, 463–487 (2012). 2. V. Passaro, C. Tullio, B. Troia, M. Notte, G. Giannoccaro, and F. Leonardis, "Recent Advances in Integrated Photonic Sensors," Sensors 12, 15558–15598 (2012). 3. F. Vollmer, L. Yang, and S. Fainman, "Label-free detection with high-Q microcavities: A review of biosensing mechanisms for integrated devices," Nanophotonics 1, 267–291 (2012). 4. B. Zhang, A. W. Morales, R. Peterson, L. 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Introduction
Micro-and nano-scale photonic biosensors have become a fast growing research topic driven by the need of portable bio-detection systems with high sensitivity, high throughput, real-time and label-free detection [1][2][3].Various devices, including surface plasmon devices [4][5][6], microring resonators [7][8][9], silicon nanowires [10], nanoporous silicon waveguides [11], onedimensioanl (1D) and two-dimensional (2D) photonic crystal (PC) microcavities [12][13][14][15], have been proposed and demonstrated.Most of the proposed structures are based on the interaction between the evanescent wave and the biomolecules that are absorbed or immobilized on the sensor surface.For example, microring resonators built on silicon-oninsulator (SOI) substrate can detect biomolecule layers immobilized on the surface of the microring through the induced resonance shift in the transmission spectrum [7].Extensive efforts have been made on this type of sensors focusing on increasing the sensitivity and lowering the detection limit [16][17][18][19][20].However, this type of evanescent wave sensing mechanism faces limitation in surface sensing: the sensitivity drops inevitably with increasing thickness of the surface layer accumulated on the sensor surface.In real applications, this layer includes necessary oxide and chemical layers generated by surface treatment, probe proteins, target proteins and any other reagents that are used to enhance the signal.These can amount to a total layer thickness ranging from several nanometers to a few tens of nanometers, within which the sensitivity of the evanescent wave could drop considerably before it reaches the final target to be detected [21][22][23][24][25].
Recently, novel subwavelength grating (SWG) based waveguides and photonic devices were proposed and demonstrated [26][27][28][29].The SWG waveguide consists of periodic silicon pillars in the propagation direction with a period much smaller than the operating wavelength.Within such a structure, the wave propagates in a similar way to conventional strip waveguides, but the interaction region between light and the cladding materials is greatly extended compared to the aforementioned evanescent wave based biosensors.Therefore, SWG structure shows great promise in integrated optical biosensors.In [24,30], microring resonators based on SWG waveguides were first demonstrated with bulk sensitivity (the ratio of resonance shift to the change of surrounding refractive index) greater than 400 nm/RIU, which is several times higher than conventional microring resonators based on strip waveguides [7].However, the enhanced surface sensing capability in SWG waveguide, which is a unique advantage in this structure, has never been revealed and carefully studied.
In this paper, we analyzed and demonstrated enhanced surface sensing capability in microring resonator biosensor based on SWG waveguide structures.In the SWG waveguides, effective sensing region includes not only the top and side of the waveguide, where evanescent wave exists, but also the space between silicon pillars on the light propagation path.This leads to greatly increased sensitivity as well as a unique property of thicknessindependent surface sensitivity in comparison to conventional microring resonator biosensors.The surface sensitivity (the ratio of resonance shift to the change of surface layer thickness) remains constantly high in SWG microring resonator even when surface layer thickness grows.Simulation shows that the surface sensitivity remains around 1.0 nm/nm in the first 25-nm thick layer upon the surface in the studied case.In the experimental demonstration, microring resonator biosensors based on both SWG waveguides and conventional strip waveguide were fabricated and compared side by side in a biosensing experiment.Special tuning of the pillar shape in the SWG was utilized to minimize the bending loss in the SWG microring and a high quality factor of 9100 in water environment was achieved.A comparison between the two types of sensors in the biosensing test verified the superior surface sensing capability in the sensors constructed by SWG waveguides.The SWG microring sensor was also used to detect microRNA with concentration as low as 1 nM.

Simulation and analysis
The structure of the SWG microring resonator is shown in Fig. 1(a).It is constructed by replacing the strip waveguides in a regular microring resonator with SWG waveguides.The silicon SWG microring resonator sits on top of the buried oxide layer and is covered by sensing medium (water or other biological buffers, assuming refractive index n = 1.32).strip medium is pumped and flowed onto the sensor at a controlled flow rate.The stage is also thermally controlled with Newport 3040 Temperature Controller to avoid temperatureinduced resonance shift in the biosensors.The whole system is connected to a computer and the transmission spectra of the device is monitored automatically.

Sensing experiment methods
To characterize the bulk refractive index sensitivity of the fabricated SWG microring resonator, different concentrations of glycerol in water solution (0%, 5%, 10%, 20%, v/v) were prepared and flowed onto the chip through microfluidic channels.The resonance wavelengths were recorded and the sensitivity was calculated by / S n λ = Δ Δ , the resonance shift versus refractive index change.Refractive index data was obtained based on [34].
To demonstrate the enhanced surface sensitivity, both SWG microring resonator and conventional microring resonator were fabricated on the same chip and characterized with biosensing test.Before the test, the chip was first silanized by 2% (v/v) APTES in toluene.Then the chip was further treated with 2.5% (v/v) glutaraldehyde in phosphate buffer saline (PBS) to provide aldehyde group linker that is able to immobilize protein covalently [21,25].Next, anti-streptavidin antibody (50 μg/mL, from Abcam), bovine serum albumin (BSA, 1 mg/mL), streptavidin (100 μg/mL, from Sigma-Aldrich), and biotinylated BSA (1 mg/mL, from Thermo Fisher Scientific) were flowed in sequence into the microfluidic channel containing both microring sensors.Anti-streptavidin antibody was immobilized on the sensor surface as probe protein.BSA was used as blocking buffer to block any vacant sites.Streptavidin binds to probe protein and later capture biotinylated BSA through biochemical interactions.Before switching reagent at each of the above steps, PBS buffer was flowed to remove any unbound biomolecules.Resonance wavelengths of both the SWG and conventional microring resonator were recorded and resonance shifts were compared.
The SWG microring resonator was also used to detect low concentrations of microRNA (miRNA).The chip containing SWG microring biosensors was chemically modified with APTES and glutaraldehyde as described above.Then capture DNA (1 mM) was flowed to cover the sensor surface followed by applying blocking buffer.The conjugate miRNA (1 nM and 100 nM) was then flowed in the microfluidic channels to conjugate with the capture DNA.Anti-DNA:RNA antibody was flowed last to amplify the signal.

Results and discussion
Scanning electron microscope (SEM) images of the fabricated SWG microring resonator are shown in Fig. 5(a) with the coupling region enlarged to show the trapezoidal pillars in the microring and the rectangular pillars in the bus waveguide.A transmission spectrum of the SWG microring is shown in Fig. 5(b), from which the free spectral range is measured to be 12.5 nm, corresponding to group index 2 / (2 ) 3.0 . The estimated quality factor ( ~/ Q λ δλ ) is as high as 9100 due to the use of trapezoidal pillars in the SWG microring.The trapezoidal shape induces pre-distorted refractive index profile to significantly reduce mode mismatch and radiation loss in the SWG bend [31].It is also worth noting that the absorption loss in water contributes to the total loss in the SWG microring.Moving to a shorter wavelength such as 1310 nm for operation and adjust the design slightly to compensate for the wavelength change would potentially further improve the quality factor.The device would suffer about an order of magnitude less absorption loss [30].
Next, bulk refractive index sensitivity of the SWG microring biosensor was characterized.The resonance peak wavelength was monitored during the experiment and plotted in Fig. 5(c).Thus, the bulk sensitivity can be estimated by a linear fit on the resonance shift versus refractive index change plot, as shown in Fig. 5(d).The fit curve shows a bulk sensitivity of S b = 440.5 ± 4.2 nm/RIU, which is a typical value for SWG microring resonators [24], and is  The fabricated SWG microring biosensor was also tested with low concentration biosamples to further demonstrate the enhanced sensitivity for biosensing applications.The experiment is performed as described in the Methods section.The test result is shown in Fig. 7.A net resonance wavelength shift of 0.11 nm was observed for 1nM miRNA with anti-DNA:RNA antibody amplification.A net resonance wavelength shift of 0.19 nm was observed for 100nM miRNA with antibody amplification.It shows that the SWG microring biosensor is promising in detecting low concentration of biomolecules in real applications.

Conclusion
In conclusion, we have shown that microring resonator biosensors based on SWG waveguides possess unique property of thickness-independent surface sensitivity and enhanced sensitivity compared to conventional microring resonators.Numerical simulation reveals that due to periodic pillar structure in the propagation direction, the effective sensing region includes not only top surface and sidewall of the waveguide, but also the space on the propagation path between the periodic pillars.It is the strong optical field between the periodic pillars that leads to significantly enhanced interaction with the sensing medium.Biosensing experiment on both SWG microring and conventional microring demonstated the superior surface sensing capability of the SWG waveguide.Along with the demonstration of miRNA detection at 1 nM concentration, SWG microring resonator is shown to be promising in real biosensing applications.

Figure 1
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Fig. 6 .
Fig. 6.(a) Real time monitoring of the resonance shift in SWG microring biosensor during the biosensing experiment; Blue region indicate buffer washing steps and other steps are marked with the corresponding reagents used.Anti-SA: anti-streptavidin antibody, SA: streptavidin, bio-BSA: biotinylated BSA.(b) Resonance shift in both SWG microring and regular microring; insets show SEM images of both microrings; GLU: glutaraldehyde (c) Surface sensitivity with respect to estimated thickness in both SWG microring and regular microring.
Department of Energy (DOE) (Contract #: DE SC-0013178); National Cancer Institute/ National Institutes of Health (NCI/NIH) (Contract #: HHSN261201500039C); National Natural Science Foundation of China (No. 61372038); Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, IPOC2015ZC02); China and Postgraduate Innovation Fund of SICE, BUPT, 2015.