CSEM Scientific and Technical Report 2008

Polarization Imaging using Nanostructured Metal Grids
M. Guillaumée, L. A. Dunbar, C. Santschi, E. Grenet, R. Eckert, R. P. Stanley
Efficient wire grid polarizers have been designed, fabricated and characterized on CMOS photodetectors. Placing such polarizers on each pixel of a
CMOS sensor allows the reconstruction of the polarization state, providing additional information on object properties being imaged.
CMOS image sensors are found in many devices, from mobile
phone cameras to high performance industrial sensors. They
are sensitive across the visible, and color information can be
extracted through filters on individual pixels. The integration of
micro-polarizers on each individual pixel, in a configuration
similar to the Bayer one (cf. Figure 1), allows the
reconstruction of the polarization state [1] . Polarization imaging
provides information such as shape and material properties.
For example, polarization imaging can improve biomedical
imaging contrast due to the birefringence of biological tissue [2] .
In the present work, the integration of wire grid polarizers on
pixels of CMOS photodetectors is achieved.
Figure 1: The Bayer-like arrangement of micro-polarizers on the pixel
array of an image sensor
Metallic wire grids (see Figure 2) are well established as
efficient polarizers (high transmission and high extinction ratio),
providing high acceptance angle and broadband polarization
extinction ratio (ER). Wire grids are commonly used from
radio-waves to the infrared. Their extension to the visible is
challenging: to obtain both high transmission and ER in a wire
grid polarizer, the pitch p of the grid (Figure 2) has to be much
smaller than the incident wavelength λ (p < λ/10), i.e.
p < 100 nm.
Figure 2: A schematic of a wire grid polarizer on a glass substrate
and definitions used for the grid dimensions
It would be commercially interesting to fabricate these grids
directly in the CMOS process. The period and slit width need
to be compatible with typical CMOS dimensions while
preserving efficient polarization properties. Suitable designs
can be found by optimizing both the aperture width w and the
film thickness h. An analytical model was used to understand
the physics behind the transmission process of the polarizer.
This model was used to optimize the design of the polarizer,
which CSEM was able to demonstrate.
As a first step, metallic gratings are on glass substrates. Good
agreement with theory is observed [3] . A polarization extinction
ratio of around 100 is measured from 600 nm to 900 nm with
p = 500 nm, w = 150 nm and h = 285 nm. The gratings also
show high transmission (>60%). Gold gratings with similar
dimensions were fabricated and characterized on
photodetectors. An extinction ratio higher than 200 is
measured (cf. Figure 3), demonstrating the possibility to
integrate efficient metallic gratings on CMOS detectors. The
fringes observed in Figure 3 are due to a Fabry-Pérot effect
due to the silicon oxide layer of several microns above the
active region inherent to the CMOS fabrication process used
for this device.
Figure 3: Extinction ratio measured for a gold grating fabricated on a
CMOS photodetector pixel (p = 500, h = 340 nm and w = 150 nm)
This work demonstrates both theoretically and experimentally
that it is possible to design efficient metallic grid polarizers
within CMOS process constraints. An array of such polarizers
placed above the pixel array of a CMOS camera would allow
real time polarization imaging. These structures could also be
used for other optical devices such as projection displays.
This work was funded by the European Community, project no.
IST-FP6- 034506 'PLEAS'.
[1] G. P. Nordin, J. T. Meier, P. C. Deguzman, M. W. Jones,
“Micropolarizer array for infrared imaging polarimetry”, J. Opt.
Soc. Am. A, 16 (1999) 1168
[2] S. Makita, Y. Yasuno, T. Endo, M. Itoh, T. Yatagai, “Polarization
contrast imaging of biological tissues by polarization-sensitive
Fourier-domain optical coherence tomography”, Appl. Opt., 45
(2006) 1142
[3] M. Guillaumée, L. A. Dunbar, C. Santschi, E. Grenet, R. Eckert,
R. P. Stanley, “Polarization imaging on CMOS photodetector
using nanostructured metallic grids”, Appl. Phys. Lett., submitted
(2009)
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