Vertical silicon nanowire-based racetrack resonator optical sensor
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We propose a highly sensitive optical sensor which is built from silicon nanowires. The silicon nanowires are arranged to form a ring resonator. The silicon nanowires cladding and voids are filled with the analyte. The sensor has a small footprint of 16 μm × 16.5 μm. The insertion loss of the sensor is only 0.4 dB, while it is characterized by its high sensitivity of 430 nm/RIU. As a biosensor, our device showed a 100 nm/RIU sensitivity when a thin biolayer of 10 nm thickness is attached to the silicon nanowire structures.
Silicon on insulator “SOI” waveguides are the most commonly used type of waveguides in photonic components and devices because of their low losses and complementary metal oxide semiconductor “CMOS” fabrication compatibility [1, 2]. In SOI waveguides, the majority of the photonic mode is confined in the silicon core, while an exponentially decaying evanescent tail extends through the cladding, quantified by the confinement factor . Thus, the effective refractive index of the guided mode is defined by the silicon core index with minimum contribution from the surrounding medium. However, in applications such as optical modulators and sensors, it is usually needed to increase the overlap between the photonic mode and the surrounding medium “cladding”. In this context, it is reasonable to discuss the plasmonic [4, 5] and silicon nanowires “SiNWs” platforms [6, 7].
The plasmonic platform enables the confinement and enhancement of fields at metal–dielectric interfaces, and in slot waveguides . Thus, plasmonic sensors benefit from the strong interaction between the effective mode and the analyte in contact with the metal surface . The SiNWs platform, which is built of arrays of SiNWs on an insulator substrate, is characterized by having voids between the SiNWs . So, SiNWs sensors benefit from the diffusion of the analyte through these voids, as well as the cladding.
However, the SiNWs platform has two main advantages when compared with the plasmonic platform. First, plasmonics are usually associated with high losses due to the large free electron density in metals and doped semiconductors , such high losses are not observed in SiNWs. Second, in contrast to plasmonic materials, SiNWs are CMOS compatible. This enables the integration of the photonic and electronic components on the same chip, since they are both fabricated by the CMOS fabrication technology .
Moreover, various applications have been developed using the SiNWs platform such as routers , and sensors . The propagation loss of SiNWs waveguides is negligible when compared with the bend loss. Since at sharp bends, the electromagnetic energy may escape through the poorly guiding voids, different parameters of SiNWs should be carefully studied such as the nanowire diameter, pitch, height, and bend radius .
Hereby, we demonstrate a fully compatible CMOS optical sensor. The sensor is based on the ring resonator mechanism. The sensor is built of SiNW arrays on SiO2 substrate, the SiNWs are arranged to form a ring structure. A commercial-grade simulator eigenmode solver and propagator  was used to analyze the modes and effective index. A 3D simulator based on the finite difference time domain method  was used to study the sensor performance in terms of its spectrum, extinction ratio, insertion loss, resonator Q-factor, and sensitivity.
2 SiNWs and SOI waveguides
In general, we may need to increase the Si existence and reduce free space (voids), to reduce the radiation of optical power at the bends, i.e. decreases the bend loss. In particular, we may need to increase the number, diameter of the SiNWs, and/or decrease the pitch. So, we need a compromise between the bend loss, and the sensitivity of the sensor. On one hand, increasing the voids and reducing Si results in higher interaction of the photonic mode with the analyte which leads to higher sensitivity, but higher bend loss. On the other hand, decreasing the voids and increasing the Si results in lower bend loss, but it decreases the sensitivity of the sensor.
The height of 600 nm remained constant between design A and design B. Shorter nanowires are more difficult to fabricate using the suggested fabrication methods (see Sect. 3.2). Longer nanowires would require the use of a taller input waveguide, which would likely support more than one mode, which is undesirable. We start the optimization of the sensor by considering design A, however, the large voids of 100 nm results in high radiation at the bends, with minimal optical power detected at the output such that the insertion loss reaches 5 dB, such design is inefficient for its huge power loss Thus, it is obvious that we need to increase the Si content on the expense of the voids between the SiNWs. Therefore we decreased the pitch from 200 to 150 nm, such that the distance between two adjacent nanowires is 50 nm, and we increased the number of SiNWs from 3 to 4 as shown in Fig. 1c.
We further investigate one more design (design C) of four SiNWs and 25 nm spacing between two adjacent nanowires, this design results in a small insertion loss of 0.4 dB. However, design C is challenging in terms of fabrication feasibility because of its small spacing of 25 nm, and also its measured sensitivity is limited to 400 nm/RIU which is less than that of design B (430 nm/RIU) as will be discussed later.
3 SiNW ring resonator
3.1 Device structure
3.2 Fabrication procedure
In Fig. 6b, a regular n-type silicon substrate was used.
4 Results and discussion
4.1 Device optimization
Also, the Q-factor of the resonator was calculated for different gaps, \(Q = \lambda /\Delta \lambda\), where ∆λ is the full width at half maximum of the 1.55 μm resonance. For gaps larger than 200 nm, the Q-factor increases as shown in Fig. 7b, but their corresponding ER increases. In other words, as the gap increases, the resonances become sharper, while less power is coupled to the resonator. Thus, the gap was set to 200 nm to achieve critical coupling, resulting in a Q-factor of 310.
4.2 Sensitivity measurements
Previously reported SiNWs sensors  used a ring resonator design that resulted on 243 nm/RIU sensitivity, where the number, diameter, pitch, height of the SiNWs were 9, 50 nm, 75 nm, and 700 nm, respectively. However, our sensor achieved a higher sensitivity of 430 nm/RIU.
4.3 Biosensing applications
Biosensing applications are vital in the photonics research community, where a vast number of biosensors were developed using different platforms such as the silicon nanowire biosensors , plasmonic biosensors , and photonic crystal biosensors .
The slope of the curve of Fig. 11 reveals that our sensor has a sensitivity of 100 nm/RIU for a 10 nm thick biolayer.
Silicon nanowire-based optical sensor has been developed. The sensor utilizes the ring resonator configuration. Critical coupling to the resonator as well the Q-factor of the resonator were studied. The spectrum of the resonator showed red-shift with the increased analyte index. The sensor was characterized by its low insertion loss of 0.4 dB, and high sensitivity of 430 nm/RIU. Furthermore, the functionality of the sensor as a biosensor was investigated by attaching a biolayer of a small thickness of 10 nm to the silicon nanowires, the sensitivity measured was 100 nm/RIU for such a small thickness.
Open Access funding provided by the Qatar National Library. This work was made possible by a NPRP award [NPRP7456-1-085] from the Qatar National Research Fund (member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors.
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