The waveguide structure for the local evanescent array coupled (LEAC) biosensor is optimized theoretically
with Beam Propagation Method (BPM) simulations. The LEAC biosensor has successfully demonstrated
experimental results of a sensitivity of 16% /nm and a metrology limit of 14 pm. Considering the waveguide
thickness detector position used in previous experiments are far from optimized values, the detection performance of
the LEAC sensor can be significantly improved with the simulated optimal structure. With the optimized
parameters, when the upper cladding is air the estimated metrology limit is 0.8 pm; with water as the upper cladding
for real-time measurements in an intigrated microfluidic channel, the estimated metrology limit is 1.6 pm.
A Label-free optical waveguide immunosensor is investigated both theoretically and experimentally. The local
evanescent array coupled (LEAC) biosensor is based on a local evanescent field shift mechanism, which differs from
those of other evanescent waveguide sensors. Antigens specifically bound by immobilized antibodies on the waveguide
surface increase the refractive index of the upper cladding of the waveguide, and hence shift the evanescent field
distribution up. This local detection mechanism grants the LEAC sensor multi-analyte ability in a single optical path.
Compared to traditional biosensors, including surface plasmon resonance and ring resonance biosensors, the nonresonant
and temperature/wavelength insensitive properties of the LEAC biosensor relax its requirement on the optical
source. It requires no accessory off-chip instruments such as spectrometers, making it a chip-scale biosensing platform.
The on-chip detection is accomplished by integrating buried polysilicon detector arrays into silicon nitride waveguide in
a commercial complementary metal oxide semiconductor (CMOS) process. Protein antigens and IgG producing
biologically relevant antibody-antigen interactions were used to test the clinical utility of the LEAC biosensor platform.
Advanced analysis beam propagation method (BPM) simulations and chips with different geometric parameters were
used to study the relationship between the sensitivity and structure of LEAC biosensor.
Silicon photonic biosensors using SiNx/SiO2 waveguides fabricated in a commercial CMOS process are read out with an
integrated polysilicon photodetector array buried under the waveguide. Increased surface refractive index, such as that
generated by the specific binding of target molecules, locally decreases evanescent coupling to the detector array
resulting in reduced photocurrent from elements under the higher index region. The operating principle of this local
evanescent array coupled (LEAC) sensor differs from those of other waveguide sensors and has been explored using
numerical simulations. Experimental studies have been performed using adsorbed bovine serum albumin (BSA)
nanofilms as well as antigen-antibody pairs.
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