in remote locations and microfluidic
environments. Also, the fabrication of
the array required us to develop a powerful new soft-lithography technique to
transfer nanoscale patterns that had been
defined by e-beam lithography. Soft
lithography has a unique potential to be
used for nanoscale patterning of unconventional and non-planar substrates.
Plasmonic collimators:
small divergence lasers
Thomas Ebbesen of the University
of Strasbourg and coworkers have systematically studied experimentally
and theoretically the high directional
emission from a sub-wavelength aperture
surrounded by surface corrugations.
Our plasmonic collimators are based on
these findings. They work by coupling
laser emission from a sub-wavelength
aperture into surface plasmons propagating along the laser facet patterned
with a second-order grating. Radiation
from the grating groves and the aperture
interfere constructively in the far-field,
leading to greatly reduced divergence
compared to the unpatterned laser.
The 1D plasmonic collimator consists
of a slit aperture and a grating patterned
on the metal-coated laser facet. The slit
aperture and the grating grooves are
oriented to be perpendicular to the laser
polarization. The aperture slit couples
part of the laser output into surface
plasmons, which propagate through
the grating along the vertical direction
and are scattered by the grating grooves
as outgoing radiation. The separation
between the aperture slit and the grating
and between the grating grooves are
chosen to ensure maximum constructive
interference in the far-field in the direction normal to the facet, leading to a
strong reduction in the beam divergence
along the vertical direction. The grating
period is comparable to the wavelength,
while the grooves’ widths and depths are
sub-wavelength.
Effectively, the aperture and the
grooves in the plasmonic collimator act
as an array of coherent light sources, in
analogy with phased array antennas.
150 9
y [nm]
100 8
7 50 6
05
– 50 4
3
–100 2
–150 1
–300 –200 –100 0 100 200 300
x [nm]
Atomic force microscope topography (left) and near-field scanning optical microscope
image (right) of a resonant optical antenna fabricated on the facet of a l =0.83-µm
o
diode laser. Color coding is arbitrary.
300
50
800
y [nm]
600
0
400
200
– 50
0
–200 –150 –100 – 50 0 50 100 150 200
x [nm]
200
100
0
(Left) Numerical simulation of the optical intensity enhancement with respect to the
incident intensity at l =0.83 µm. (Right) Map of optical intensity enhancement for a
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bow-tie optical antenna designed for a vertical-cavity surface-emitting laser emitting
at l =0.85 µm.
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Transferred optical
antenna arrays
Laser
Spectrometer
Laser line
filter
100 nm
2,500
Intensity [Counts]
2,000
1,500
1,000
500
0
SERS fiber probe
notch filter lens
Laser Objective
1,000 1,200 1,400 1,600
Wavelength [nm]
Beamsplitter
(Left) Configuration used to collect signal from the surface-enhanced Raman scattering
(SERS) fiber probe. The spectrometer measures the SERS signal from analytes
interacting with optical antennas transferred to a fiber facet (inset). (Right) Benzenethiol
remotely detected through its SERS spectrum measured with the fiber sensor.
(Left panel) Cross-section of 1D plasmonic collimator fabricated on a semiconductor laser facet. A thin dielectric
electrically isolates the metal film from
the facet. Light waves diffracted by the
sub-wavelength aperture and the grating
grooves constructively interfere in the
far-field to produce a low divergence
beam in the vertical direction. (Right
panel) Electron micrograph of the facet
of a patterned mid-infrared quantum
cascade laser emitting at l = 10 mm.
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