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High performance SOI microring resonator for biochemical sensing
Optics & Laser Technology 59 (2014) 60–67
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
High performance SOI microring resonator for biochemical sensing C. Ciminelli n, F. Dell’Olio, D. Conteduca, C.M. Campanella, M.N. Armenise Optoelectronics Laboratory, Politecnico di Bari, Via Re David 200, 70125 Bari, Italy
art ic l e i nf o
a b s t r a c t
Article history: Received 4 April 2013 Received in revised form 12 December 2013 Accepted 14 December 2013 Available online 8 January 2014
In this work we have investigated different silicon-on-insulator (SOI) microcavities based on a planar geometry having a footprint on chip as small as 100 μm2 with a ring, disk and hybrid configurations with the aim of being poorly intrusive for both in-body and out-of-body biosensing purposes. Accurate numerical results have been achieved by using the 3D finite element method and compared to 3D finite discrete time domain ones with a good agreement for both methods. The most promising resonator among the devices we have analyzed shows a Q-factor of the order of 105, that allows a limit of detection for the sensor equal to 10 6 RIU and a sensor sensitivity of 120 nm/RIU. The resonator has been designed for glucose biosensing, considering both the homogeneous sensing and the surface one, that enhances the sensor selectivity by the device functionalization with a glucose-oxidase (GOD) layer. The glucose concentration has been evaluated both with the microcavity surrounded by a water solution and with water only in the inner part of the cavity. The achieved performance is really attractive not only for the reduced size of the cavity, but also for the planar coupling configuration of the annulus and the waveguides composing the cavity since it appears to be a very promising configuration for the practical packaging of micro systems containing whispering gallery mode resonators. In this paper the concept of an on-chip platform for a high throughput and multichannel detection relying on an array of resonant cavities interacting with a single nanofluidic channel, is also discussed. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Biochemical sensors Optical ring resonator Glucose sensor
1. Introduction Since the last decades, miniaturized sensors have been used for clinical analysis, healthcare, and environmental monitoring [1]. The minimal intrusiveness and the capability to be easily implanted into the human body or placed in a harsh environment without modifying the surrounding conditions, make them really attractive on the market. Another interesting aspect is related to the capability of realizing a very large scale integration (VLSI) platform for biological sensing in a multiplexed configuration [2]. In addition to these characteristics, miniaturized sensors inherit all the advantages coming from the micro-fabricated devices such as low power consumption, quick response time, possibility to be integrated in a closed loop system including sensors and actuators, accurate control of infusion drug in case of an implantable or external drug delivery system, possibility for chemical functionalization and interaction with microfluidic channels. The use of integrated photonic devices for biosensing application, ranging from the detection of a certain amount of analyte in solution to the single nanoparticle or virus detection, offers different
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additional benefits with respect to both mechanical and electrical biosensors, such as the immunity to any electromagnetic interference and the higher sensitivity [3]. An overview on biological sensors relying on optical devices can be found in Ref. [4]. Whispering gallery modes (WGMs) optical microresonators have been proposed as a competitive platform for biological sensing, showing a resolution or detection limit up to a single nanoparticle or virus [5,6]. WGM resonators are optical cavities having circular shape where the light optical path is confined along the outer periphery, i.e. it propagates along the interface resonator-surrounding medium suiting the total internal reflection phenomenon [7]. Different geometries have been proposed for realizing WGM resonant microcavities, including planar configurations (i.e. rings, racetracks, spirals, disks), liquid core optical ring resonators, LCORR, (i.e. micro-tubes, micro-bottles and micro-bubbles), microspheres and microtoroids. Despite their sensitivity and detection limit up to a single molecule [5,6], the latter suffer from issues related to reproducibility and integration on chip in case of implantable or anyway compact devices. These issues can be overcome by using planar structures which allow for a monolithic integration onto a chip, a good reproducibility and controllability of the fabrication process and thus for the possibility of creating an array of resonators to be used for multiplexing [2].
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On the basis of these considerations, here we propose a planar microresonator for biosensing applications having a radius as small as 5 mm. The main idea this paper relies on is to improve the resonator Q factor and thus the resolution without affecting the cavity length in order to develop an integrated optics platform for biochemical sensing purposes, i.e. a lab on chip (LOC), with the characteristics of being potentially implantable in human body or anyway portable.
2. The designed cavity Whispering gallery modes based microresonator are really attractive due to the very sharp resonances characterizing their transmission spectrum. As already both theoretically and experimentally demonstrated, the detection limit, LOD, of a sensor based on a resonant cavity is dependent on the full width at half maximum (FWHM) of resonances, i.e. LOD p(FWHM). In Ref. [4] a more detailed description of the WGM resonator parameters is reported. One of the main resonator parameters to be explored for sensing application is the quality factor Q, accounting for the temporal confinement of the light within a cavity. A high Q-factor means that light is trapped in a cavity for a long time and so interacts longer with the analyte. The Q value is affected by the cavity length, the coupling gap between the straight and the curved guide, and the cavity loss. An additional figure to be taken into account for the design of an optical biosensor is the mode extinction ratio ER, which is strictly related to the coupling efficiency since it is associated to the resonant dips depth (usually expressed in dB) in the resonator transmission spectrum. Deeper is the dip more energy is coupled from the bus waveguide to the resonator, higher is the ER value and higher is the coupling efficiency, but lower is the Q-factor. An extinction ratio ER of 12 dB can be considered an acceptable compromise, because this condition provides an evident dip at the resonance with high values of both the coupling efficiency and the Q-factor at the same time. According to theoretical studies [8], an improvement of Q factor value can be obtained if a planar resonator is coupled to a single waveguide (all-pass configuration) rather than to two distinct waveguides (add-drop configuration) since coupling loss are reduced and light circulates inside the cavity a huge number of times giving rise to a long photons life-time within cavity. However, the add-drop configuration allows high values of ER, then a trade-off between the Q-factor and the ER has to be identified based on the specific application. Besides the coupling loss reduction, an additional possible way of increasing a resonator Q-factor relies on employing a disk configuration since the WGM mode propagating along the disk periphery is not affected by a double side-walls confinement as for a ring resonator and thus it experiences reduced propagation losses. A disk resonator, anyway, supports more than a single propagating mode and this multi-mode condition should be avoided in sensing applications, especially if modes are not widely spaced each other and their suppression ratio is not so high. Alternatively, the Q-factor could be increased by increasing the cavity length, but the integration of the device is more difficult with a long cavity length, for systems with an array of resonant cavities. A possible way to increase the device Q-factor without modifying the cavity size, is by enlarging the annulus composing the ring [9,10], so the fabrication steps are less critical and sidewall losses are reduced. If the ring waveguide width is bigger than the one of the bus waveguide, the mode propagating inside the resonator will be confined only along a sidewall, experiencing a
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loss reduction. In this way, a higher tolerance during the etching phase of fabrication process is allowed. Different sources of loss, indeed, affect the resonator Q-factor and extinction ratio, including substrate leakage, bending loss in the waveguide, coupling loss between the waveguide and the ring resonator and radiation loss at the etched waveguide sidewalls. Although a strong confinement of the mode in a Si wire can be achieved, the exponential tail of the mode decaying into the substrate causes leakage [11]. Due to the exponential dependence of the bending losses on the bend radius, a way to reduce these losses is to design a resonator having a large bending radius, that we assume R¼5 mm. Coupling loss and coupling efficiency, as stated before, are related to the gap between the straight and the circular waveguide. Scattering or radiation loss due to the etching process at resonator sidewalls, instead, can be reduced by using etchless silicon photonic waveguides [12] which are characterized by ultra-smooth sidewalls induced by selective oxidation. Alternatively, a proper design of the device can also lead to a reduced interaction between the inner sidewall and the mode energy. In this way, only the outer sidewall loss will affect light propagation. The proposed resonator is named “hybrid resonator” [13], because it shows a microdisk optical behavior in terms of number of guided modes and energy stored (i.e. Q-factor) within the resonator and, at the same time, a microring shape with an annulus width ΔR, as shown in Fig. 1. Here we assume the resonator made in SOI technology due to its consolidated fabrication process and the high index contrast between the guiding (Si) and the substrate (SiO2) layer which enables a strong light confinement. Several works on biochemical platforms in SOI technology have been investigated so far, as reported in Refs. [4,14,15]. A silicon wire having a cross-section of 500 nm 220 nm has been used as guiding structure. The core thickness has been chosen according to commercially available SOI wafers and referring to the propagation losses experimentally demonstrated as low as 0.92 70.12 dB/cm [16]. Coupling efficiency as a function of the gap is shown in Fig. 2. The reported data have been obtained by the 3D finite element method (FEM) and an exponential fitting algorithm has been used to interpolate them. Since the input waveguide and the mode propagating within that guiding structure is unchanged in our study as well as the mode evanescent tail penetration depth, we expect a trend of the coupling efficiency k similar for all the investigated configurations, as it has also been confirmed by the 3D FEM simulations (see Fig. 2), in which k has been evaluated for both a 500 nm wide ring and a 1000 nm wide annulus, for different gap values. Only a slight change in the k value, of the order of a fraction of few % points has been observed when the cavity cross-section at the coupling section is asymmetric, i.e. the input waveguide is coupled to a “hybrid” resonator up to the limit of a disk configuration rather than to a ring resonator. As an example, for a 200 nm gap, the coupling efficiency value changes for only 0.4% when ΔR varies from 500 nm to 1000 nm.
Fig. 1. Hybrid configuration with an annulus width of ΔR in the add/drop configuration.
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Fig. 2. Coupling efficiency of (solid black line) a 500 nm wide ring and (dashed red line) an annulus 1000 nm wide as a function of the resonator-waveguide gap.
A low coupling efficiency corresponds to a high Q factor value as previously stated. This condition corresponds also to a significant reduction of the resonator extinction ratio ER which is undesirable for sensing applications. To keep ER Z12 dB, the coupling gap should be far from the case of weak coupling condition, which occurs for a gap 4250 nm [17,18]. For this reason, we have investigated resonators having a gap r220 nm. Therefore the coupling efficiency is 42% in all the considered devices. The resonator outer radius Rout has been fixed to the value of 5 μm in order to realize a really small footprint size cavity, while the inner radius Rin value has been changed in order to switch from a classical ring configuration to a disk resonator configuration. We remind that classical ring means that the width of ring annulus is equal to the width of waveguide cross-section, i.e. w¼500 nm. A disk resonator is instead obtained by setting Rin to zero. We have used the 3D FEM to simulate the resonator electromagnetic behavior for all devices. The cavity performance obtained by 3D FEM has been also compared to 3D finite difference time domain (FDTD) results, confirming a good agreement for both methods. We have chosen 3D FEM to 3D FDTD for a higher accuracy and a shorter solution time.
3. Comparison of ring, hybrid and disk resonator configurations We have compared the ring, the disk and the hybrid resonator aiming at the identification of the best trade-off between Q-factor, number of resonant modes and ER. The modes supported by the analyzed cavities are depicted in Fig. 3 where the energy density is plotted in the cross sections for the fundamental TE mode of the microring, as well as for the first two TE modes of the hybrid resonator and, finally, for the first three radial TE modes of the disk resonator. The transmission spectrum of the ring (ΔR¼500 nm), hybrid (ΔR¼1000 nm) and disk (ΔR¼ 5000 nm) resonators, with a coupling gap of 100 nm between the bus waveguides and the annulus, is shown in Fig. 4a. The Q-factor and the ER dependence on ΔR is reported in Fig. 4b and c, respectively. We have found that the hybrid resonator acts as a disk, i.e. it supports a maximum mode radial order equal to 3, in case of ΔR 41000 nm, and equal to 2 if 750 nm r ΔR r1000 nm. The hybrid resonator with ΔR ¼1000 nm can be considered the optimized structure because the Q-factor is 7.8 104, close to Q¼8 104 obtained with the disk configuration (ΔR¼5000 nm),
as shown in Fig. 4b, but with two resonance modes unlikely the disk with a higher mode radial order. We have simulated both the ring resonator and the hybrid one (ΔR¼ 1000 nm) to evaluate the resonant transmission and the Qfactor with a single bus waveguide and an add-drop configuration. Even if we have demonstrated higher values of the Q-factor and the ER with the hybrid resonator, we have also considered the ring configuration to compare their performance and to quantify the improved behavior of the hybrid cavity, also when used as a biosensor. The hybrid configuration with ΔR¼1000 nm and the ring (ΔR¼ 500 nm) have been simulated for different values of the gap G between the cavity and the bus waveguide to estimate the Q-factor and the ER for a single bus waveguide and an add-drop configuration. We have calculated the Q-factor and the ER for G in a range from 100 nm to 220 nm. In Fig. 5, the Q-factor vs. the ER has been plotted. Fig. 5 shows that the Q-factor decreases as the ER increases. Therefore, as already mentioned, high Q and high ER are conflicting requirements and a trade-off between the two performance parameters should be identified. The specific application demands an accurate estimation of the resonance dip whose shift allows the concentration measure. On the basis of our simulations we have concluded that a value of ER Z12 dB is appropriate to achieve a good accuracy in the measurement of the resonance shift. In our design we impose ER ¼12 dB and optimize the resonator Q-factor. For ER ¼ 12 dB, the add-drop configurations provide a higher Q-factor than cavities with a single bus waveguide layout, as shown in Fig. 5. The gap value of the hybrid resonator in an add-drop configuration that provides ER ¼ 12 dB is G ¼ 215 nm. For that value of the gap Q¼4.5 105. For the ring with an adddrop layout the same layout, ER ¼ 12 dB implies G ¼205 nm, with Q¼1.7 105. By comparing the cavities with an add-drop layout, we have observed that the hybrid resonator exhibits a higher Q-factor than the ring resonator. We have investigated the optimized configurations of the ring and the hybrid resonator as a glucose sensor to define the improvement of the sensitivity and the detection limit obtained with the hybrid resonator in comparison to the performance of the microring. The glucose concentration has been evaluated in a water solution (n¼ 1.31), as shown in Fig. 6. The presence of a water solution on the ring rather than an air cladding provides a red-shift around 10 nm of the resonant wavelength for both configurations (λ 1560 nm) due to an increase of the waveguide effective index. We have observed a decrease of the Q-factor in a water solution and an increase of ER of 2–3 dB for both configurations. Thus when the resonant cavity is surrounded by a water solution its ER is around 15 dB, which is considered an optimum value for sensing applications [17]. 3.1. Homogeneous sensing for glucose concentration biosensor We have studied the configurations of the optimized resonators for homogenous sensing, based on refractive index change, detect glucose concentration in a water solution. The presence of glucose provides a change of the effective index and a linear red-shift of the resonance wavelength. The following dependence between the average refractive index of the solution ng/l and the glucose concentration C has been assumed [19] ng=l ¼ aðλÞC þ bðλÞ
ð1Þ
The water refractive index at λ ¼1550 nm, which is the wavelength of interest, is equal to 1.3101 RIU [17] and coincides with the b(λ) value, while the a(λ) value is assumed to be 1.189 10 4 according to Ref. [19].
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Fig. 3. Microresonators geometrical cross-section in the coupler section. The electric energy density within each cavity is shown for all the propagating modes in the section far from the coupler. Figures are not in scale.
Fig. 6. (a) Q-factor and (b) resonance transmission of the ring with ΔR ¼500 nm and hybrid resonator with ΔR ¼1000 nm in air and water solution. Fig. 4. (a) Transmission spectrum of a ring, a hybrid and a disk resonator and (b) Qfactor and (c) ER for different values of ΔR with a coupling gap of 100 nm.
Fig. 5. Q-factor vs. ER for a hybrid device with ΔR ¼1000 nm and a ring with a single bus waveguide and an add-drop configuration.
The resulting refractive index of samples containing a glucose concentration ranging from 0 to 180 g/l is reported in Table 1. We have evaluated the sensitivity and the detection limit for homogeneous sensing with the optimized structures of the ring (G ¼205 nm) and the hybrid resonator (G ¼215 nm) with glucose in the water solution, as shown in Fig. 7. The maximum refractive index change considered to define the sensor performance is 5 10 3 RIU, at which corresponds a glucose concentration of 46 g/l, as shown in Fig. 8. The hybrid resonator provides a higher sensitivity (S¼ 120 nm/ RIU) than the ring resonator (S ¼65 nm/RIU), confirming a better performance of this novel configuration as biosensor. We have also evaluated the detection limit for both configurations, assuming the use of a thermal electric cooler (TEC) for the thermal stabilization
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Table 1 Glucose concentration [g/l] in solution and solution refractive index at λ ¼ 1550 nm. Absolute uncertainty of the solution refractive index is about 10 4. C [g/l]
ng/l
0 20 40 60 80 100 120 140 160 180
1.3101 1.3125 1.3149 1.3172 1.3196 1.3220 1.3244 1.3267 1.3291 1.3315
Fig. 7. Hybrid resonator with ΔR ¼1000 nm in a water solution with glucose.
of the sensor. The LOD can be expressed as: LOD ¼ 3s=S
ð2Þ
where s is the standard deviation of the total noise of the system and is evaluated by summing all noise contributions limiting the system resolution, i.e. thermal and shot noise in the photodetector, laser relative intensity noise, and quantization error [20].We have also assumed a typical value of SNR of the system of about 60 dB and an instruments resolution of 1 pm (i.e. for the laser source and the optical spectrum analyzer). Since the thermal noise is negligible due to the use of the TEC, the most relevant noise source is related to the measuring instruments. We have obtained for the optimized ring and hybrid resonator LOD ¼1.7 10 5 RIU and LOD ¼2.3 10 6 RIU, respectively. The hybrid resonator allows to detect a lower detection limit and a lower glucose concentration than the ring configuration. The minimum detectable glucose concentration with the hybrid resonator is 13 mg/l, while for the ring it is equal to 95 mg/l. Of course, the LOD obtained by the hybrid resonator significantly gets worse without the TEC in the system, because the thermal noise can be considered the most significant noise source in this case. We have simulated the hybrid resonator with ΔR¼1000 nm and G¼215 nm considering the influence of the temperature changes on the resonance wavelength, obtaining Δλ/ΔT¼70 pm/K. It means that for an increment of 1 K, a red-shift of the resonance wavelength of 70 pm is expected. A degradation of about three order of magnitude in the LOD has been evaluated without the TEC, confirming the great influence of the temperature changes on the biosensor performance. 3.2. Homogeneous sensing for glucose concentration biosensor with a water solution in the inner part of the cavity We have simulated the optimized hybrid and ring resonators as glucose sensor with a different configuration, in which the water is only in the inner part of the cavity, as in Fig. 9. Due to this
Fig. 8. Sensitivity and resonance shift of (a) a ring with G ¼ 205 nm and (b) a hybrid resonator with G ¼ 215 nm, as a function of ng/l.
Fig. 9. Hybrid resonator filled in the inner part with water solution for glucose homogeneous sensing.
configuration, the device fabrication can be simplify, avoiding the manufacturing of a microfluidic channel above the cavity. As shown in Fig. 3, the resonator fundamental mode of the WGM cavity is in the outer part of the annulus and it does not directly interact with water. We have obtained an improvement of the sensitivity and a worsening of the detection limit for both configurations, up to S ¼15 nm/RIU and LOD ¼2 10 5 RIU with a minimum detectable glucose concentration of 110 mg/l for the hybrid resonator, and S¼ 13.5 nm/RIU, LOD¼ 2.1 10 5 and a minimum detectable glucose concentration of 120 mg/l for the ring cavity, confirming a degradation of the performance. In Table 2 the comparison (i.e. Q-factor, LOD and sensitivity) for ring and hybrid biosensors with the water in the inner part of the device and on the entire cavity have been reported.
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Table 2 Comparison between the performance of the ring and the hybrid resonator.
Q ER (dB) S (nm/RIU) LOD (RIU) Minimum detectable glucose concentration (mg/l)
Ring resonator (G ¼205 nm)
Hybrid resonator (G ¼215 nm)
Water in the inner part
Water on the entire cavity
Water in the inner part
Water on the entire cavity
8.6 104 13.1 13.6 2.1 10 5 120
4.4 104 15.3 65 1.7 10 5 95
3.0 105 12.5 15 2 10 5 110
1.73 105 14.1 120 2.3 10 6 13
Fig. 10. Hybrid resonator with a functionalized surface with GOD for surface sensing of glucose.
Fig. 11. Cavity effective index change of the hybrid resonator (ΔR¼ 1000 nm) as a function of the thickness of the immobilized receptors layer.
3.3. Surface sensing for glucose concentration biosensor We have also considered another detection scheme based on the use of a functionalized layer (Fig. 10), to improve the biosensor selectivity [4]. It is an affinity-based biosensor, in which a receptor or ligand is attached to the surface of the sensor and provides an interaction with a particular analyte. The link between the receptor and the analyte increases the thickness of the functionalized surface and the device sensitivity is proportional to the growth of the thickness of the functionalized layer. Many approaches have been proposed to functionalize biosensors surface, aiming at reaching a high affinity constant and a very stable ligand/receptor interaction [21–23]. For glucose biosensor it has been assumed the whole cavity surface to be activated and functionalized via the gluco-oxidase (GOD), enzymes in order to potentially detect only glucose (β-Dglucose) as analyte. GOD immobilization on commercial Si wafers has been demonstrated through conventional methods [19]. In 3D FEM simulations we have assumed the bonding of the analyte at the cavity surface as an add uniform layer. GOD refractive index has been assumed to be 1.45 RIU which is the average RI value of proteins at λ ¼1550 nm. The sensor sensitivity, for surface detection scheme is given by S¼
Δλ Δt
ð3Þ
where t is the thickness of the functionalized layer [17]. Effective index change as a function of the immobilized layer thickness on the functionalized surface of the hybrid resonator has been investigated, as depicted in Fig. 11. A linear red shift of the resonant wavelength has been observed. For the ring resonator we have obtained a sensitivity of S¼ 0.09 nm/ nm. The hybrid configuration is more suitable for surface sensing, thanks to a larger functionalized area for the bonding of glucose and provides an improvement of sensitivity up to S¼ 0.145 nm/nm. The state-of-the-art of sensitivity, detection limit, and footprint of biosensors for homogeneous and surface sensing are shown in Table 3. Despite other resonant cavities having a higher sensitivity and a lower detection limit [14,24,25], the proposed hybrid resonator has a smaller footprint more than two order of magnitude. In Ref.
Table 3 State of the art of biosensors for homogeneous and surface sensing. Type of sensor
Detection scheme
Detection limit (RIU)
Footprint (lm2)
Interferometer [24] Microring with cernier effect [25] Plasmonic ring [26] Hybrid resonator (this paper)
Surface Homogeneous
9 10 9 RIU 4.8 10 6
106 1.4 104
Homogeneous Homogeneous
1.5 10 4 2.3 10 6
5 100
[14] a plasmonic ring with a smaller footprint and a higher sensitivity then the hybrid resonator has been discussed, but with a higher detection limit. We have also obtained a device sensitivity with the surface functionalization close to the value demonstrated in Ref. [27] with a footprint of our device about 20% less than the racetrack configuration, demonstrating the usefulness of a larger annulus to increase the functionalized surface area and the interaction with the analyte to detect.
4. Discussion and conclusions The choice of a cavity footprint on a chip as small as 100 mm2 could lead to the development of an implantable or portable bio-assay platform allowing for the refractometric detection of the concentration of the interest analyte in a complex solution. A possible on chip configuration for a high throughput and multianalyte detection relies on an array of resonant cavities interacting with a single nano-fluidic channel, as reported in Fig. 12. The surface of each resonator should be chemically activated and functionalized by depositing a layer of receptors having a high binding affinity with the analytes. In Fig. 12 a pumping technique based on pressure driven flow has been assumed. It relies on creating a pressure gradient by modifying the channel section according to Bernoulli0 s principle. The outlet channel section should thus be chosen in order to slow the solution flow and so to increase the time the analyte interacts with the functionalized resonator surface since a real time analysis
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promising configuration in terms of trade-off between sensitivity, detection limit and extinction ratio for biosensing application is the one composed by a 1 mm wide annulus coupled to two distinct straight waveguides. Benefits coming from this cavity are related to a LOD value as low as 2.3 10 6 RIU, a sensitivity of the order of 120 nm/RIU and a Q-factor as high as 1.73 105 which makes the cavity really competitive in the field of biosensing, thanks to its small volume. We have also simulated the hybrid resonator for surface sensing with a deposition of a GOD layer on the cavity to increase the selectivity and performance of the biosensor, obtaining a sensitivity of 0.145 nm/nm, close to biosensor performance that have already discussed, but with a smaller footprint.
References
Fig. 12. Perspective (a) and top-side (b) view of a microcavities array.
of molecular interactions takes several seconds as from experimental works [28]. In the last decade a lot of systems for multianalyte detection have been investigated, thanks to the possibility of their use in many areas, i.e. clinical analysis and healthcare. A surface functionalization with different biochemical receptors of an array of resonant cavities (i.e. microrings) provides the detection of many analytes at the same time. It is very useful for areas in which a single analyte detection does not provide enough information to define a complete clinical analysis, e.g. in Ref. [29] a microring biosensor has been realized to detect T-cells, that have a fundamental rule in the recognition of the cancer pathology and definition of the disease progression. Recently the interaction between the glucose concentration and cancer cells production has been verified [30], therefore a system that provides the detection at the same time of the glucose concentration, trapping cells selectively thanks to the high energy stored in the ring resonators at the resonance and monitoring the presence of proteins and blood cells (i.e. T-cell) can improve the biosensor performance, reducing false negative cases in real time for a clinical analysis, making the detection system more efficient [31]. The hybrid resonator provides higher Q-factors and an enlargement of surface area for the functionalization, making the device more suitable for biosensing than a ring configuration, ensuring at the same time a small footprint, a planar geometry and consolidated fabrication processes. Different SOI microcavities having a footprint on chip as small as 100 mm2 have been investigated in this work. The most
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Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
High performance SOI microring resonator for biochemical sensing C. Ciminelli n, F. Dell’Olio, D. Conteduca, C.M. Campanella, M.N. Armenise Optoelectronics Laboratory, Politecnico di Bari, Via Re David 200, 70125 Bari, Italy
art ic l e i nf o
a b s t r a c t
Article history: Received 4 April 2013 Received in revised form 12 December 2013 Accepted 14 December 2013 Available online 8 January 2014
In this work we have investigated different silicon-on-insulator (SOI) microcavities based on a planar geometry having a footprint on chip as small as 100 μm2 with a ring, disk and hybrid configurations with the aim of being poorly intrusive for both in-body and out-of-body biosensing purposes. Accurate numerical results have been achieved by using the 3D finite element method and compared to 3D finite discrete time domain ones with a good agreement for both methods. The most promising resonator among the devices we have analyzed shows a Q-factor of the order of 105, that allows a limit of detection for the sensor equal to 10 6 RIU and a sensor sensitivity of 120 nm/RIU. The resonator has been designed for glucose biosensing, considering both the homogeneous sensing and the surface one, that enhances the sensor selectivity by the device functionalization with a glucose-oxidase (GOD) layer. The glucose concentration has been evaluated both with the microcavity surrounded by a water solution and with water only in the inner part of the cavity. The achieved performance is really attractive not only for the reduced size of the cavity, but also for the planar coupling configuration of the annulus and the waveguides composing the cavity since it appears to be a very promising configuration for the practical packaging of micro systems containing whispering gallery mode resonators. In this paper the concept of an on-chip platform for a high throughput and multichannel detection relying on an array of resonant cavities interacting with a single nanofluidic channel, is also discussed. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Biochemical sensors Optical ring resonator Glucose sensor
1. Introduction Since the last decades, miniaturized sensors have been used for clinical analysis, healthcare, and environmental monitoring [1]. The minimal intrusiveness and the capability to be easily implanted into the human body or placed in a harsh environment without modifying the surrounding conditions, make them really attractive on the market. Another interesting aspect is related to the capability of realizing a very large scale integration (VLSI) platform for biological sensing in a multiplexed configuration [2]. In addition to these characteristics, miniaturized sensors inherit all the advantages coming from the micro-fabricated devices such as low power consumption, quick response time, possibility to be integrated in a closed loop system including sensors and actuators, accurate control of infusion drug in case of an implantable or external drug delivery system, possibility for chemical functionalization and interaction with microfluidic channels. The use of integrated photonic devices for biosensing application, ranging from the detection of a certain amount of analyte in solution to the single nanoparticle or virus detection, offers different
n
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additional benefits with respect to both mechanical and electrical biosensors, such as the immunity to any electromagnetic interference and the higher sensitivity [3]. An overview on biological sensors relying on optical devices can be found in Ref. [4]. Whispering gallery modes (WGMs) optical microresonators have been proposed as a competitive platform for biological sensing, showing a resolution or detection limit up to a single nanoparticle or virus [5,6]. WGM resonators are optical cavities having circular shape where the light optical path is confined along the outer periphery, i.e. it propagates along the interface resonator-surrounding medium suiting the total internal reflection phenomenon [7]. Different geometries have been proposed for realizing WGM resonant microcavities, including planar configurations (i.e. rings, racetracks, spirals, disks), liquid core optical ring resonators, LCORR, (i.e. micro-tubes, micro-bottles and micro-bubbles), microspheres and microtoroids. Despite their sensitivity and detection limit up to a single molecule [5,6], the latter suffer from issues related to reproducibility and integration on chip in case of implantable or anyway compact devices. These issues can be overcome by using planar structures which allow for a monolithic integration onto a chip, a good reproducibility and controllability of the fabrication process and thus for the possibility of creating an array of resonators to be used for multiplexing [2].
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On the basis of these considerations, here we propose a planar microresonator for biosensing applications having a radius as small as 5 mm. The main idea this paper relies on is to improve the resonator Q factor and thus the resolution without affecting the cavity length in order to develop an integrated optics platform for biochemical sensing purposes, i.e. a lab on chip (LOC), with the characteristics of being potentially implantable in human body or anyway portable.
2. The designed cavity Whispering gallery modes based microresonator are really attractive due to the very sharp resonances characterizing their transmission spectrum. As already both theoretically and experimentally demonstrated, the detection limit, LOD, of a sensor based on a resonant cavity is dependent on the full width at half maximum (FWHM) of resonances, i.e. LOD p(FWHM). In Ref. [4] a more detailed description of the WGM resonator parameters is reported. One of the main resonator parameters to be explored for sensing application is the quality factor Q, accounting for the temporal confinement of the light within a cavity. A high Q-factor means that light is trapped in a cavity for a long time and so interacts longer with the analyte. The Q value is affected by the cavity length, the coupling gap between the straight and the curved guide, and the cavity loss. An additional figure to be taken into account for the design of an optical biosensor is the mode extinction ratio ER, which is strictly related to the coupling efficiency since it is associated to the resonant dips depth (usually expressed in dB) in the resonator transmission spectrum. Deeper is the dip more energy is coupled from the bus waveguide to the resonator, higher is the ER value and higher is the coupling efficiency, but lower is the Q-factor. An extinction ratio ER of 12 dB can be considered an acceptable compromise, because this condition provides an evident dip at the resonance with high values of both the coupling efficiency and the Q-factor at the same time. According to theoretical studies [8], an improvement of Q factor value can be obtained if a planar resonator is coupled to a single waveguide (all-pass configuration) rather than to two distinct waveguides (add-drop configuration) since coupling loss are reduced and light circulates inside the cavity a huge number of times giving rise to a long photons life-time within cavity. However, the add-drop configuration allows high values of ER, then a trade-off between the Q-factor and the ER has to be identified based on the specific application. Besides the coupling loss reduction, an additional possible way of increasing a resonator Q-factor relies on employing a disk configuration since the WGM mode propagating along the disk periphery is not affected by a double side-walls confinement as for a ring resonator and thus it experiences reduced propagation losses. A disk resonator, anyway, supports more than a single propagating mode and this multi-mode condition should be avoided in sensing applications, especially if modes are not widely spaced each other and their suppression ratio is not so high. Alternatively, the Q-factor could be increased by increasing the cavity length, but the integration of the device is more difficult with a long cavity length, for systems with an array of resonant cavities. A possible way to increase the device Q-factor without modifying the cavity size, is by enlarging the annulus composing the ring [9,10], so the fabrication steps are less critical and sidewall losses are reduced. If the ring waveguide width is bigger than the one of the bus waveguide, the mode propagating inside the resonator will be confined only along a sidewall, experiencing a
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loss reduction. In this way, a higher tolerance during the etching phase of fabrication process is allowed. Different sources of loss, indeed, affect the resonator Q-factor and extinction ratio, including substrate leakage, bending loss in the waveguide, coupling loss between the waveguide and the ring resonator and radiation loss at the etched waveguide sidewalls. Although a strong confinement of the mode in a Si wire can be achieved, the exponential tail of the mode decaying into the substrate causes leakage [11]. Due to the exponential dependence of the bending losses on the bend radius, a way to reduce these losses is to design a resonator having a large bending radius, that we assume R¼5 mm. Coupling loss and coupling efficiency, as stated before, are related to the gap between the straight and the circular waveguide. Scattering or radiation loss due to the etching process at resonator sidewalls, instead, can be reduced by using etchless silicon photonic waveguides [12] which are characterized by ultra-smooth sidewalls induced by selective oxidation. Alternatively, a proper design of the device can also lead to a reduced interaction between the inner sidewall and the mode energy. In this way, only the outer sidewall loss will affect light propagation. The proposed resonator is named “hybrid resonator” [13], because it shows a microdisk optical behavior in terms of number of guided modes and energy stored (i.e. Q-factor) within the resonator and, at the same time, a microring shape with an annulus width ΔR, as shown in Fig. 1. Here we assume the resonator made in SOI technology due to its consolidated fabrication process and the high index contrast between the guiding (Si) and the substrate (SiO2) layer which enables a strong light confinement. Several works on biochemical platforms in SOI technology have been investigated so far, as reported in Refs. [4,14,15]. A silicon wire having a cross-section of 500 nm 220 nm has been used as guiding structure. The core thickness has been chosen according to commercially available SOI wafers and referring to the propagation losses experimentally demonstrated as low as 0.92 70.12 dB/cm [16]. Coupling efficiency as a function of the gap is shown in Fig. 2. The reported data have been obtained by the 3D finite element method (FEM) and an exponential fitting algorithm has been used to interpolate them. Since the input waveguide and the mode propagating within that guiding structure is unchanged in our study as well as the mode evanescent tail penetration depth, we expect a trend of the coupling efficiency k similar for all the investigated configurations, as it has also been confirmed by the 3D FEM simulations (see Fig. 2), in which k has been evaluated for both a 500 nm wide ring and a 1000 nm wide annulus, for different gap values. Only a slight change in the k value, of the order of a fraction of few % points has been observed when the cavity cross-section at the coupling section is asymmetric, i.e. the input waveguide is coupled to a “hybrid” resonator up to the limit of a disk configuration rather than to a ring resonator. As an example, for a 200 nm gap, the coupling efficiency value changes for only 0.4% when ΔR varies from 500 nm to 1000 nm.
Fig. 1. Hybrid configuration with an annulus width of ΔR in the add/drop configuration.
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Fig. 2. Coupling efficiency of (solid black line) a 500 nm wide ring and (dashed red line) an annulus 1000 nm wide as a function of the resonator-waveguide gap.
A low coupling efficiency corresponds to a high Q factor value as previously stated. This condition corresponds also to a significant reduction of the resonator extinction ratio ER which is undesirable for sensing applications. To keep ER Z12 dB, the coupling gap should be far from the case of weak coupling condition, which occurs for a gap 4250 nm [17,18]. For this reason, we have investigated resonators having a gap r220 nm. Therefore the coupling efficiency is 42% in all the considered devices. The resonator outer radius Rout has been fixed to the value of 5 μm in order to realize a really small footprint size cavity, while the inner radius Rin value has been changed in order to switch from a classical ring configuration to a disk resonator configuration. We remind that classical ring means that the width of ring annulus is equal to the width of waveguide cross-section, i.e. w¼500 nm. A disk resonator is instead obtained by setting Rin to zero. We have used the 3D FEM to simulate the resonator electromagnetic behavior for all devices. The cavity performance obtained by 3D FEM has been also compared to 3D finite difference time domain (FDTD) results, confirming a good agreement for both methods. We have chosen 3D FEM to 3D FDTD for a higher accuracy and a shorter solution time.
3. Comparison of ring, hybrid and disk resonator configurations We have compared the ring, the disk and the hybrid resonator aiming at the identification of the best trade-off between Q-factor, number of resonant modes and ER. The modes supported by the analyzed cavities are depicted in Fig. 3 where the energy density is plotted in the cross sections for the fundamental TE mode of the microring, as well as for the first two TE modes of the hybrid resonator and, finally, for the first three radial TE modes of the disk resonator. The transmission spectrum of the ring (ΔR¼500 nm), hybrid (ΔR¼1000 nm) and disk (ΔR¼ 5000 nm) resonators, with a coupling gap of 100 nm between the bus waveguides and the annulus, is shown in Fig. 4a. The Q-factor and the ER dependence on ΔR is reported in Fig. 4b and c, respectively. We have found that the hybrid resonator acts as a disk, i.e. it supports a maximum mode radial order equal to 3, in case of ΔR 41000 nm, and equal to 2 if 750 nm r ΔR r1000 nm. The hybrid resonator with ΔR ¼1000 nm can be considered the optimized structure because the Q-factor is 7.8 104, close to Q¼8 104 obtained with the disk configuration (ΔR¼5000 nm),
as shown in Fig. 4b, but with two resonance modes unlikely the disk with a higher mode radial order. We have simulated both the ring resonator and the hybrid one (ΔR¼ 1000 nm) to evaluate the resonant transmission and the Qfactor with a single bus waveguide and an add-drop configuration. Even if we have demonstrated higher values of the Q-factor and the ER with the hybrid resonator, we have also considered the ring configuration to compare their performance and to quantify the improved behavior of the hybrid cavity, also when used as a biosensor. The hybrid configuration with ΔR¼1000 nm and the ring (ΔR¼ 500 nm) have been simulated for different values of the gap G between the cavity and the bus waveguide to estimate the Q-factor and the ER for a single bus waveguide and an add-drop configuration. We have calculated the Q-factor and the ER for G in a range from 100 nm to 220 nm. In Fig. 5, the Q-factor vs. the ER has been plotted. Fig. 5 shows that the Q-factor decreases as the ER increases. Therefore, as already mentioned, high Q and high ER are conflicting requirements and a trade-off between the two performance parameters should be identified. The specific application demands an accurate estimation of the resonance dip whose shift allows the concentration measure. On the basis of our simulations we have concluded that a value of ER Z12 dB is appropriate to achieve a good accuracy in the measurement of the resonance shift. In our design we impose ER ¼12 dB and optimize the resonator Q-factor. For ER ¼ 12 dB, the add-drop configurations provide a higher Q-factor than cavities with a single bus waveguide layout, as shown in Fig. 5. The gap value of the hybrid resonator in an add-drop configuration that provides ER ¼ 12 dB is G ¼ 215 nm. For that value of the gap Q¼4.5 105. For the ring with an adddrop layout the same layout, ER ¼ 12 dB implies G ¼205 nm, with Q¼1.7 105. By comparing the cavities with an add-drop layout, we have observed that the hybrid resonator exhibits a higher Q-factor than the ring resonator. We have investigated the optimized configurations of the ring and the hybrid resonator as a glucose sensor to define the improvement of the sensitivity and the detection limit obtained with the hybrid resonator in comparison to the performance of the microring. The glucose concentration has been evaluated in a water solution (n¼ 1.31), as shown in Fig. 6. The presence of a water solution on the ring rather than an air cladding provides a red-shift around 10 nm of the resonant wavelength for both configurations (λ 1560 nm) due to an increase of the waveguide effective index. We have observed a decrease of the Q-factor in a water solution and an increase of ER of 2–3 dB for both configurations. Thus when the resonant cavity is surrounded by a water solution its ER is around 15 dB, which is considered an optimum value for sensing applications [17]. 3.1. Homogeneous sensing for glucose concentration biosensor We have studied the configurations of the optimized resonators for homogenous sensing, based on refractive index change, detect glucose concentration in a water solution. The presence of glucose provides a change of the effective index and a linear red-shift of the resonance wavelength. The following dependence between the average refractive index of the solution ng/l and the glucose concentration C has been assumed [19] ng=l ¼ aðλÞC þ bðλÞ
ð1Þ
The water refractive index at λ ¼1550 nm, which is the wavelength of interest, is equal to 1.3101 RIU [17] and coincides with the b(λ) value, while the a(λ) value is assumed to be 1.189 10 4 according to Ref. [19].
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Fig. 3. Microresonators geometrical cross-section in the coupler section. The electric energy density within each cavity is shown for all the propagating modes in the section far from the coupler. Figures are not in scale.
Fig. 6. (a) Q-factor and (b) resonance transmission of the ring with ΔR ¼500 nm and hybrid resonator with ΔR ¼1000 nm in air and water solution. Fig. 4. (a) Transmission spectrum of a ring, a hybrid and a disk resonator and (b) Qfactor and (c) ER for different values of ΔR with a coupling gap of 100 nm.
Fig. 5. Q-factor vs. ER for a hybrid device with ΔR ¼1000 nm and a ring with a single bus waveguide and an add-drop configuration.
The resulting refractive index of samples containing a glucose concentration ranging from 0 to 180 g/l is reported in Table 1. We have evaluated the sensitivity and the detection limit for homogeneous sensing with the optimized structures of the ring (G ¼205 nm) and the hybrid resonator (G ¼215 nm) with glucose in the water solution, as shown in Fig. 7. The maximum refractive index change considered to define the sensor performance is 5 10 3 RIU, at which corresponds a glucose concentration of 46 g/l, as shown in Fig. 8. The hybrid resonator provides a higher sensitivity (S¼ 120 nm/ RIU) than the ring resonator (S ¼65 nm/RIU), confirming a better performance of this novel configuration as biosensor. We have also evaluated the detection limit for both configurations, assuming the use of a thermal electric cooler (TEC) for the thermal stabilization
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Table 1 Glucose concentration [g/l] in solution and solution refractive index at λ ¼ 1550 nm. Absolute uncertainty of the solution refractive index is about 10 4. C [g/l]
ng/l
0 20 40 60 80 100 120 140 160 180
1.3101 1.3125 1.3149 1.3172 1.3196 1.3220 1.3244 1.3267 1.3291 1.3315
Fig. 7. Hybrid resonator with ΔR ¼1000 nm in a water solution with glucose.
of the sensor. The LOD can be expressed as: LOD ¼ 3s=S
ð2Þ
where s is the standard deviation of the total noise of the system and is evaluated by summing all noise contributions limiting the system resolution, i.e. thermal and shot noise in the photodetector, laser relative intensity noise, and quantization error [20].We have also assumed a typical value of SNR of the system of about 60 dB and an instruments resolution of 1 pm (i.e. for the laser source and the optical spectrum analyzer). Since the thermal noise is negligible due to the use of the TEC, the most relevant noise source is related to the measuring instruments. We have obtained for the optimized ring and hybrid resonator LOD ¼1.7 10 5 RIU and LOD ¼2.3 10 6 RIU, respectively. The hybrid resonator allows to detect a lower detection limit and a lower glucose concentration than the ring configuration. The minimum detectable glucose concentration with the hybrid resonator is 13 mg/l, while for the ring it is equal to 95 mg/l. Of course, the LOD obtained by the hybrid resonator significantly gets worse without the TEC in the system, because the thermal noise can be considered the most significant noise source in this case. We have simulated the hybrid resonator with ΔR¼1000 nm and G¼215 nm considering the influence of the temperature changes on the resonance wavelength, obtaining Δλ/ΔT¼70 pm/K. It means that for an increment of 1 K, a red-shift of the resonance wavelength of 70 pm is expected. A degradation of about three order of magnitude in the LOD has been evaluated without the TEC, confirming the great influence of the temperature changes on the biosensor performance. 3.2. Homogeneous sensing for glucose concentration biosensor with a water solution in the inner part of the cavity We have simulated the optimized hybrid and ring resonators as glucose sensor with a different configuration, in which the water is only in the inner part of the cavity, as in Fig. 9. Due to this
Fig. 8. Sensitivity and resonance shift of (a) a ring with G ¼ 205 nm and (b) a hybrid resonator with G ¼ 215 nm, as a function of ng/l.
Fig. 9. Hybrid resonator filled in the inner part with water solution for glucose homogeneous sensing.
configuration, the device fabrication can be simplify, avoiding the manufacturing of a microfluidic channel above the cavity. As shown in Fig. 3, the resonator fundamental mode of the WGM cavity is in the outer part of the annulus and it does not directly interact with water. We have obtained an improvement of the sensitivity and a worsening of the detection limit for both configurations, up to S ¼15 nm/RIU and LOD ¼2 10 5 RIU with a minimum detectable glucose concentration of 110 mg/l for the hybrid resonator, and S¼ 13.5 nm/RIU, LOD¼ 2.1 10 5 and a minimum detectable glucose concentration of 120 mg/l for the ring cavity, confirming a degradation of the performance. In Table 2 the comparison (i.e. Q-factor, LOD and sensitivity) for ring and hybrid biosensors with the water in the inner part of the device and on the entire cavity have been reported.
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Table 2 Comparison between the performance of the ring and the hybrid resonator.
Q ER (dB) S (nm/RIU) LOD (RIU) Minimum detectable glucose concentration (mg/l)
Ring resonator (G ¼205 nm)
Hybrid resonator (G ¼215 nm)
Water in the inner part
Water on the entire cavity
Water in the inner part
Water on the entire cavity
8.6 104 13.1 13.6 2.1 10 5 120
4.4 104 15.3 65 1.7 10 5 95
3.0 105 12.5 15 2 10 5 110
1.73 105 14.1 120 2.3 10 6 13
Fig. 10. Hybrid resonator with a functionalized surface with GOD for surface sensing of glucose.
Fig. 11. Cavity effective index change of the hybrid resonator (ΔR¼ 1000 nm) as a function of the thickness of the immobilized receptors layer.
3.3. Surface sensing for glucose concentration biosensor We have also considered another detection scheme based on the use of a functionalized layer (Fig. 10), to improve the biosensor selectivity [4]. It is an affinity-based biosensor, in which a receptor or ligand is attached to the surface of the sensor and provides an interaction with a particular analyte. The link between the receptor and the analyte increases the thickness of the functionalized surface and the device sensitivity is proportional to the growth of the thickness of the functionalized layer. Many approaches have been proposed to functionalize biosensors surface, aiming at reaching a high affinity constant and a very stable ligand/receptor interaction [21–23]. For glucose biosensor it has been assumed the whole cavity surface to be activated and functionalized via the gluco-oxidase (GOD), enzymes in order to potentially detect only glucose (β-Dglucose) as analyte. GOD immobilization on commercial Si wafers has been demonstrated through conventional methods [19]. In 3D FEM simulations we have assumed the bonding of the analyte at the cavity surface as an add uniform layer. GOD refractive index has been assumed to be 1.45 RIU which is the average RI value of proteins at λ ¼1550 nm. The sensor sensitivity, for surface detection scheme is given by S¼
Δλ Δt
ð3Þ
where t is the thickness of the functionalized layer [17]. Effective index change as a function of the immobilized layer thickness on the functionalized surface of the hybrid resonator has been investigated, as depicted in Fig. 11. A linear red shift of the resonant wavelength has been observed. For the ring resonator we have obtained a sensitivity of S¼ 0.09 nm/ nm. The hybrid configuration is more suitable for surface sensing, thanks to a larger functionalized area for the bonding of glucose and provides an improvement of sensitivity up to S¼ 0.145 nm/nm. The state-of-the-art of sensitivity, detection limit, and footprint of biosensors for homogeneous and surface sensing are shown in Table 3. Despite other resonant cavities having a higher sensitivity and a lower detection limit [14,24,25], the proposed hybrid resonator has a smaller footprint more than two order of magnitude. In Ref.
Table 3 State of the art of biosensors for homogeneous and surface sensing. Type of sensor
Detection scheme
Detection limit (RIU)
Footprint (lm2)
Interferometer [24] Microring with cernier effect [25] Plasmonic ring [26] Hybrid resonator (this paper)
Surface Homogeneous
9 10 9 RIU 4.8 10 6
106 1.4 104
Homogeneous Homogeneous
1.5 10 4 2.3 10 6
5 100
[14] a plasmonic ring with a smaller footprint and a higher sensitivity then the hybrid resonator has been discussed, but with a higher detection limit. We have also obtained a device sensitivity with the surface functionalization close to the value demonstrated in Ref. [27] with a footprint of our device about 20% less than the racetrack configuration, demonstrating the usefulness of a larger annulus to increase the functionalized surface area and the interaction with the analyte to detect.
4. Discussion and conclusions The choice of a cavity footprint on a chip as small as 100 mm2 could lead to the development of an implantable or portable bio-assay platform allowing for the refractometric detection of the concentration of the interest analyte in a complex solution. A possible on chip configuration for a high throughput and multianalyte detection relies on an array of resonant cavities interacting with a single nano-fluidic channel, as reported in Fig. 12. The surface of each resonator should be chemically activated and functionalized by depositing a layer of receptors having a high binding affinity with the analytes. In Fig. 12 a pumping technique based on pressure driven flow has been assumed. It relies on creating a pressure gradient by modifying the channel section according to Bernoulli0 s principle. The outlet channel section should thus be chosen in order to slow the solution flow and so to increase the time the analyte interacts with the functionalized resonator surface since a real time analysis
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promising configuration in terms of trade-off between sensitivity, detection limit and extinction ratio for biosensing application is the one composed by a 1 mm wide annulus coupled to two distinct straight waveguides. Benefits coming from this cavity are related to a LOD value as low as 2.3 10 6 RIU, a sensitivity of the order of 120 nm/RIU and a Q-factor as high as 1.73 105 which makes the cavity really competitive in the field of biosensing, thanks to its small volume. We have also simulated the hybrid resonator for surface sensing with a deposition of a GOD layer on the cavity to increase the selectivity and performance of the biosensor, obtaining a sensitivity of 0.145 nm/nm, close to biosensor performance that have already discussed, but with a smaller footprint.
References
Fig. 12. Perspective (a) and top-side (b) view of a microcavities array.
of molecular interactions takes several seconds as from experimental works [28]. In the last decade a lot of systems for multianalyte detection have been investigated, thanks to the possibility of their use in many areas, i.e. clinical analysis and healthcare. A surface functionalization with different biochemical receptors of an array of resonant cavities (i.e. microrings) provides the detection of many analytes at the same time. It is very useful for areas in which a single analyte detection does not provide enough information to define a complete clinical analysis, e.g. in Ref. [29] a microring biosensor has been realized to detect T-cells, that have a fundamental rule in the recognition of the cancer pathology and definition of the disease progression. Recently the interaction between the glucose concentration and cancer cells production has been verified [30], therefore a system that provides the detection at the same time of the glucose concentration, trapping cells selectively thanks to the high energy stored in the ring resonators at the resonance and monitoring the presence of proteins and blood cells (i.e. T-cell) can improve the biosensor performance, reducing false negative cases in real time for a clinical analysis, making the detection system more efficient [31]. The hybrid resonator provides higher Q-factors and an enlargement of surface area for the functionalization, making the device more suitable for biosensing than a ring configuration, ensuring at the same time a small footprint, a planar geometry and consolidated fabrication processes. Different SOI microcavities having a footprint on chip as small as 100 mm2 have been investigated in this work. The most
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