Single nano-particle sensing exploiting crossed polarizers to improve the
signal-to-noise ratio
Jon O. Grepstad1,*, Peter Kaspar2, Olav Solgaard3, Ib-Rune Johansen4, Aasmund Sudbø5
1
Norwegian University of Science and Technology, NO-7491 Trondheim, Norway, 2Electronics Laboratory, ETH
Zurich, CH-8092 Zurich, Switzerland, 3E.L. Ginzton Laboratory, Stanford University, CA-94305, USA, 4SINTEF ICT,
Microsystems and Nanotechnology, NO-0373, Norway, 5Department of Physics, University of Oslo, NO-0316, Norway
*jonolav.grepstad@sintef.no
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
[1] 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 [2].
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.
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Figure 1 (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 [2].
1.
2.
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)