research activities in 2007 - CSEM

Towards an Optical Switch with J-aggregates Monolayers
R. Eckert, J. Dintinger • , T. Ebbesen • , R. P. Stanley
J-aggregates are an efficient non-linear optical material. They have been successfully used in ultrafast all optical switches, which exploit plasmonic
effects of nano-structured metal surfaces. This work aims to replace 100 nm thick J-aggregate layers in such switches by a monolayer of
J-aggregates using a unique deposition technique.
Extraordinary optical transmission (EOT) through an array of
subwavelength holes drilled in a metal film relies on the
interplay of surface plasmons (SP) with the periodicity of the
nanostructure to make the otherwise opaque film transparent.
EOT is not only a very active field of basic research but is
currently finding its way into real world applications and
devices.
The concept of an ultrafast all–optical switch, which exploits
EOT, has been demonstrated recently [1] . Its basic component
is a gold film with an array of holes covered with J-aggregates
(Figure 1). J-aggregates (JA) are formed from cyanine
molecules and have well-defined absorption bands. The
difference between their ground and excited states is sufficient
to change the transmission properties of the array.
Consequently, one beam of light (the probe beam) can be
switched on and off by exciting the JA with a second beam of
light (pump beam).
Figure 1: Concept of an all optical switch based on EOT
In the original work the JA were randomly dispersed in a
200 nm thick polymer film coated on one side of the hole
array. The present work aims at achieving the same effect by
using only a monolayer of highly ordered JA formed by a
process developed at CSEM using a template of selfassembled
dendrimers. Both, the remarkable absorption
efficiency of these monolayers and the possibility of coating
both sides of the hole array instead of only one offer the
potential to achieve bigger switching amplitudes between on
and off states of the device.
A typical hexagonal hole array with a JA monolayer on one
side and its transmission spectrum are shown Figure 2. The
holes are milled by focused ion beam (FIB) into a 200 nm
thick gold film deposited on a glass substrate. The array
period is 525 nm and the hole diameter is 175 nm. The
transmission spectrum exhibits two main peaks, which
correspond to the surface plasmon resonances at the
gold/JA/air interface (left) and the glass/gold interface (right).
The dip in the transmission peak at 550 nm is due to the
absorption of the JA monolayer which is less than 10 nm thick.
48
In this experiment, the pump power provided by a cw laser
was not sufficient to excite all JA in the excitation volume and
thereby to induce a sufficiently big change of the index of
refraction necessary to shift the transmission peaks of the
hole array.
Figure 2: a) Electron micrograph of a hole array b) Transmission
spectrum of the hole array
In the next step of experiments, the JA will be pumped by a
pulsed laser, powerful enough to saturate the JA absorption,
which ultimately should cause a shift of the transmission
spectrum to the blue and thus a real switching of the device.
This work was partly funded by the Network of excellence
Plasmon-Nano-Devices within the European Framework 6.
CSEM thanks them for their support.
•
ISIS, Université Louis Pasteur, Strasbourg, France
[1] J. Dintinger, I. Robel, P. V. Kamat, C. Genet, T. W. Ebbesen,
“Terahertz all-optical molecule-plasmon modulation”, Adv. Mat.
18 (2006) 1645