Microsystems and Nanotechnology, NO-0373, Norway, 5Department of Physics, University of Oslo, NO-0316, Norway
Crossed polarized excitation and detection has been used to improve signal-to-noise ratio in an optical nano-particle sensor exploiting guided-resonance-modes in photonic crystal membranes. The sensor is illustrated in Figure 1B. It is comprised of a diode laser that serves as backside illumination in a standard optical microscope, where the transmission through a photonic crystal membrane is recorded with spacial resolution using a CCD camera. Two polarizers rotated 90 degree relative to each other are placed before and after the photonic crystal. In this way, the illumination and detected light have orthogonal polarizations.
The sensor has been tested by imaging the 75nm radius nano-particle trapped in the photonic crystal, shown in the SEM-image in Figure 1A. Figure 1C and Figure 1D are taken using a system having parallel  and crossed polarizers. The signal-to-noise ratio improves from 17 to 59, respectively. We estimate that single nano-particles with a diameter less than 40 nm can be detected using crossed polarizers .
Optical sensing enabling detection of nano-particles of this size is highly applicable. Provided means for specific molecule capture, the sensor should be able to detect single viruses. Viruses typically have a diameter of 20-300nm. Moreover, the simplicity of the sensor enables it to be made compact and inexpensive.
Figure (A) SEM image of fabricated photonic crystal and (B) the optical setup used for characterization. The upper right figures show how the polarizers in the setup are oriented with respect to the hole pattern in the photonic crystal. The lower left (C) and right (D) figures display images recorded with parallel and crossed polarizers, respectively. Line plots are included for a row of pixels crossing the particle location in the vertical direction .
J. O. Grepstad, P. Kaspar, O. Solgaard, I.-R. Johansen, and A. S. Sudbø, “Photonic-crystal membranes for optical detection of single nano-particles, designed for biosensor application,” Opt. Express 20, 7954 – 7965 (2012).
J. O. Grepstad, P. Kaspar, O. Solgaard, I.-R. Johansen, and A. Sudbø, “Single nano-particle sensing exploiting crossed polarizers to improved the signal-to-noise ratio”, Oral presentation (To be presented at CLEO 2013, San Jose)