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Often, in bio-sensing applications rely on fluorescence as the transduction mechanism. The emission from the fluorescently labeled molecules is detected and quantified to obtain the concentration. Most current techniques, such as ELISA, FISH, next generation DNA sequencing and others need one or several washing steps to remove the unreacted fluorescent molecules to reduce the background noise.
To address that problem, we propose an integrated nano-photonic solution for on-chip fluorescence detection. The proposed platform is tailored for bio-sensing applications and has the potential to analyze bio-molecular interactions down to the single molecule level. The technology is based on near-field excitation and collection using PECVD Silicon Nitride (SiN) nano-photonic rib waveguides. SiN provides the combination of high-index-contrast and compatibility with CMOS processing technology, and unlike silicon, Silicon Nitride does not absorb in the visible wavelength window.
The evanescent tail of the used SiN waveguide mode extends from 80 nm to 200 nm above the waveguide surface depending on the waveguide geometry, cladding material and excitation wavelength. Hereby, the evanescent field of the waveguide mode excites a very thin layer of molecules near to the surface. The subsequent emission from the excited fluorophore is collected in the near field by coupling to another waveguide. The coupling strength mainly depends on the distance between the waveguide and fluorophore. This way, both the excitation and collection efficiency have an exponential dependency on the distance between the molecule and waveguide surface. Therefore, exciting and collecting fluorescence using photonic waveguides improves the separation between the surface bound fluorescence signal from the bulk background noise, paving the way for wash free bio-sensing. Wash-free assays allow to examine the bio-molecular interactions in real time and simplify the sample/liquid handling system.
Next to bulk fluorescence, autofluorescence generated in the SiN waveguide is another large contributor to noise. In the proposed design, the two separate single mode waveguides we use to excite fluorophores and collect the emission, are placed orthogonally in a cross configuration. The orthogonal placement of two Transverse Electric (TE) mode waveguides makes sure that the auto-fluorescence generated in the excitation waveguide is not coupled to the emission waveguide. As a result, we observe an improved signal-to-noise-ratio (SNR) which is a very critical parameter towards single molecule detection.
In this talk, I will talk about the design, fabrication and characterization of the proposed cross-configured waveguide based fluorescence detection platform. Experimental results to quantify the excitation efficiency, collection efficiency and SNR will be discussed. A comparison will be shown between the Finite Difference Time Domain (FDTD) simulation and experimental results.
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