We proposed an innovative phase interrogation method for localized surface plasmon resonance (LSPR) detection.
To our knowledge, this is the first demonstration of LSPR biosensor by phase interrogation. LSPR is realized as the
plasmonic resonance within confined metal nanoparticle. Nanoparticle couples the light by means of a non-radiative
inter-band absorption, and a scattering from surface plasmon oscillation, the total contribution is the optical
extinction of nanoparticles. Due to the variety of resonance types, LSPR is extensively studied in the field of
biological sensing, imaging, and medical therapeutics. Generally, LSPR is probed by optical intensity variation of
continuous wavelength, in other words, wavelength interrogation. LSPR sensitivity probed by this method is ranged
from several tens nm/RIU to less than 1000nm/RIU depending on the nanostructure and metal species, which at least
an order of magnitude less than conventional SPR biosensor in wavelength interrogation. In this work, an innovative
common-path phase interrogation system is applied for LSPR detection. Phase difference in our home-made system
is simply extracted through the correlation of optical intensity under different polarization without any heterodyne
optical modulator or piezoelectric transducer, and thus low down the cost and complexity in optical setup. In
addition, signal-to-noise ratio is substantially reduced since the signal wave and reference wave share the common
path. In our preliminary results, LSPR resolution of Au nanodisk array is 1.74 x 10<sup>-4</sup> RIU by wavelength
interrogation; on the other side, LSPR resolution of Au nanodisk array is 2.02x10<sup>-6</sup> RIU in phase interrogation.
LSPR sensitivity is around one order of magnitude enhanced. In conclusion, we demonstrated that LSPR sensitivity
can be further enhanced by phase interrogation.
Intensity interrogation of SPR biosensor owns high sensitivity, and is generally used
as SPR microscopy due to the optical intensity variation. Therefore, it is substantial to
improve its sensitivity to have a better sensing ability and image quality. In this paper,
we discussed numerically and experimentally the influence of sensitivity by metal
thickness, and provide a design rule of manifesting optimized thickness to maximize
sensitivity in intensity interrogation.