research activities in 2007 - CSEM

Towards Plasmon Enhanced Detectors
L. A. Dunbar, M. Guillaumée, R. Eckert, E. Franzi, R. P. Stanley
Surface plasmons play a key role in the recent experiments on extra-ordinary optical transmission through nano-structured metals. By tailoring
these nanostructures they can be used to enhance optical transmission, with polarisation and spectral selection. Incorporating these nanostructures
directly onto photon detectors can improve their performance.
To enhance the signal to noise ratio of detectors one can
either enhance the signal or reduce the noise. Recently
Ebbesen et al [1] measured extraordinary optical transmission
through sub-wavelength holes in an otherwise opaque metal.
By exploiting this effect, light can be harvested on
photodetectors and the signal to noise ratio can be increased.
Currently CSEM is fabricating metallic nanostructures on
image sensors to do just this. Additionally these
nanostructured metallic films can act as spectral and
polarization filters or a combination of the two.
Initial trials have been made to investigate the enhanced
transmission through a single slit surrounded by a series of
grooves. Incident light is converted into a surface wave
(plasmon) by the periodic array of grooves. The light is
transported along the surface towards the central slit. This
allows light which does not fall on the central slit, to pass
through the slit and increase the signal that can be detected
with the detector under the slit. As the shot noise depends on
the size detector, increasing the signal whilst maintaining the
same detector size increases the signal to noise ratio. Initial
results give a factor of 5 enhancement per surface area over a
spectral bandwidth of 25 nm.
Figure 1: Scanning electron micrograph showing a slit delimited by 5
groove structures
A typical structure is shown in Figure 1. Here 140 nm of gold
has been deposited onto a CSEM made Vision Sensor. The
metal has a very low surface roughness.
The structures can be optimised for improved signal to noise
ratio, for a specific spectral width or for polarisation sensitivity.
In order to do this the metal thickness, the width and depth of
the groove, the period of the grooves and the initial distance
from the centre of the slit to the first groove need to be
optimised. Slit structures are intrinsically polarisation selective
and by varying these parameters spectral selectivity can also
be modified.
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Figure 2 shows the transmission through a structured film for
different film thicknesses and shows that the spectral features
can be tuned by varying the metal thickness.
Figure 2: Transmission spectra calculated using 2D-Finite Difference
Time Domain (FDTD) for different metal thicknesses. The peak
transmission shifts to longer wavelengths with increased metal
thickness.
To obtain the desired optical properties is not straight forward
as there is a complex interplay between the various
parameters. Moreover the metal smoothness is crucial both
for losses and to obtain the necessary fabrication tolerances.
The fabrication of tailored nanostructures is currently being
undertaken by focused ion beam which gives both the
flexibility and the precision required.
This work was partly funded by the PLEAS as part of a EU
Project in framework 6.
[1] T. W. Ebbesen, H. J. Lezec, H. Ghaemi, T. Thio, P. A. Wolf,
"Extraordinary optical transmission through sub-wavelength hole
arrays," Nature 391, (1998), 667-669