Polarization-sensitive surface plasmon enhanced ellipsometry biosensor using the photoelastic modulation technique

A surface plasmon enhanced ellipsometry (SPEE) biosensor scheme based on the use of a photoelastic modulator (PEM) is reported. We show that the polarization parameters of a laser beam, tan ψ, cos Δ and ellipse orientation angle ϕ, can be directly measured by detecting the modulation signals at the first and second harmonics of the modulated frequency under a certain birefringence geometry. This leads to accurate measurement of refractive index variations within the evanescent field region close to the gold sensor surface, thereby enabling biosensing applications. Our experimental results confirm that the new scheme offers a respectable detection limit of 6 × 10−7 refractive index unit (RIU) or 15 ng/ml of biomolecule solute concentration without any compromise in dynamic range. In addition, PEM offers the possibility of single-beam phase-sensitive SPR detection that drastically reduces the complexity of the optical system, thus readily making it possible for SPR to be adopted in label-free micro-array biochips.