Label-Free Plasmonic Biosensor for Cancer Marker Detection Based on Phase Singularity-Induced Lateral Position Shift

Optical biosensors have been used powerful tools for disease diagnosis and molecular biology research. However, current plasmonic sensors based on intensity and wavelength scanning are limited to obtain ultrahigh sensitivity without further signal amplification of secondary labels (e.g., fluorescent molecules and plasmonic nanotags). To address this limitation, we developed a plasmonic biosensor based on enhanced phase singularity. This singularity is characterized by a sharp phase retardation at the dark point of reflection from the plasmonic substrate, leading to a giant position shift in the reflected beam, which is known as the Goos-Hänchen (GH) shift [1]. According to stationary phase theory, the GH shift is defined as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta \text{GH}=-(\lambda/2\pi n_{0})\bullet(\Delta\phi/\Delta\theta)$</tex> in Kretschmann structure, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda$</tex> is the wavelength of incident light, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n_{o}$</tex> is the refractive index of the prism, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\phi$</tex> is the optical phase, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\theta$</tex> is incident angle.