We present a new optical biosensing integration approach with multifunctional capabilities using plasmonic and photonic components on the same chip and a new methodology to design interferometric biosensors exhibiting record high sensitivity and enhanced resolution relying on a planar surface plasmon polariton (SPP) waveguide. First, we use this approach to demonstrate a proof of concept integrated plasmo-photonic liquid refractive index sensor based on a silicon nitride (Si 3 N 4 ) Mach− Zehnder Interferometer (MZI). A 70 μm long, gold metal stripe is incorporated in the sensing arm serving as the transducer element. A variable optical attenuator and a thermo-optic phase shifter are deployed in the Si 3 N 4 reference arm for performance optimization. The variable optical attenuator stage targets high extinction ratio of the resonance at the interferometer output by balancing the power between the two arms whereas the phase shifter is used to tune the MZI at the desired spectral window. Experimental results matched well with numerical simulations showing bulk sensitivity up to 1930 nm/RIU and a resonance extinction ratio of 37 dB. We also provide a theoretical analysis for correlating the sensitivity performance of the sensor with its free spectral range (FSR). Based on this analysis, we propose optimized sensor designs and show that, by engineering the free spectral range of the sensor in the range of 600 nm, sensitivity may be boosted up to 60000 nm/RIU.
Co-integrating CMOS plasmonics and photonics became the “sweet spot” to hit in order to combine their benefits and allow for volume manufacturing of plasmo-photonic integrated circuits. Plasmonics can naturally interface photonics with electronics while offering strong mode confinement, enabling in this way on-chip data interconnects when tailored to single-mode waveguides, as well as high-sensitivity biosensors when exposing Surface-Plasmon-Polariton (SPP) modes in aqueous environment. Their synergy with low-loss photonics can tolerate the high plasmonic propagation losses in interconnect applications, offering at the same time a powerful portfolio of passive photonic functions towards avoiding the use of bulk optics for SPP excitation and facilitating compact biosensor setups. The co-integration roadmap has to proceed, however, over the utilization of fully CMOS compatible material platforms and manufacturing processes in order to allow for a practical deployment route. Herein, we demonstrate for the first time Aluminum plasmonic waveguides co-integrated with Si3N4 photonics using CMOS manufacturing processes. We validate the data carrying credentials of CMOS plasmonics with 25 Gb/s data traffic and we confirm successful plasmonic propagation in both air and water-cladded waveguide configurations. This platform can potentially fuel the deployment of co-integrated plasmonic and photonic structures using CMOS processes for biosensing and on-chip interconnect applications.
We demonstrate a water cladded plasmo-photonic waveguide, by exploiting the directional coupling scheme to vertically divert light from a 360 × 800 nm (height × width) Si 3 N 4 waveguide to a plasmonic slot waveguide, enabling the excitation of a pure plasmonic mode within a 210-nm-wide slot at 1550 nm. The 150-nm-thick plasmonic slot waveguide was deposited on the top of an oxide cladded Si 3 N 4 waveguide exhibiting an experimental plasmonic-to-photonic insertion loss of 2.24 ± 0.3 dB and a plasmonic propagation length (L spp ) of 10.8 μm at 1550 nm. The proposed plasmo-photonic waveguide holds a promise as an optical transducer element for highly sensitive and low-cost interferometric biosensors due to the significant phase change achieved per unit propagation length.
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