The divergence in the vertical direction
is reduced by more than a factor of 20
with respect to the same device before
patterning—in very good agreement
with the simulations, as shown in the
panel “Simulated and measured far-fields
of 1D collimator.”
Since mid-infrared surface plasmons
are capable of propagating over long
distances of a few millimeters along
unpatterned gold-air interfaces, hundreds of grating grooves can be used to
couple most of the energy of the plasmons back into free space. The power
throughput of the device can therefore
be preserved while the divergence angle
is simultaneously reduced. Collimators
and other devices for beam engineering
based on surface plasmons can be used
down to near-infrared wavelengths. At
visible wavelengths, on the other hand,
the shortcomings of increased optical
losses will limit the collimation effect
and power throughput. For example, at
visible wavelengths, surface plasmons
can only propagate a distance comparable to 100 wavelengths, which is one
order of magnitude smaller than their
mid-infrared counterparts.
The grating discussed so far extends
only along the laser polarization direction. The lateral dimension of the slit
aperture is several times larger than the
free space wavelength l , so that surface
o
plasmons propagate mainly in the direction perpendicular to the aperture slit;
as a result, beam collimation is only
realized in one direction.
To achieve collimation in the entire
beam cross-section, we designed an
Intensity [a.u.]
1.0 1.0
1.0
0.8
Intensity [a.u.]
0.8 0.8
0.6
2. 2°
Intensity [a.u.]
0.6 0.6
0.4
0.4 0.4
0.2
x3
0.0
0.2 0.2
– 5 0 5 10 15
Angle [degree]
0.0 0.0
–80 –60– 40 – 20 0 20 40 60 80 – 20 – 10 0 10 20 30
Angle [degree] Angle [degree]
(Left) Simulated far-field in the vertical direction, including zoom-in view of the dominant
lobe for a device with 24 grooves emitting at l = 10 mm. (Right) Measured far-field
o
of the patterned laser (right side of bottom figure on p. 25) in the vertical direction
(red curve) and of the unpatterned laser (black curve) showing a reduction of the
vertical divergence angle from about 60° to 2. 5°. The central lobe of the far-field of the
patterned devices contains about 70% of the total output power, which is comparable
to that of the unpatterned device (peak value ~120 m W).
aperture that can launch surface plasmons in two dimensions and created a
two-dimensional plasmonic collimator
that can coherently scatter the surface
plasmon radiation into the far-field. This
leads to constructive interference in both
the vertical and lateral directions, thus
achieving excellent collimation in the
plane perpendicular to the laser beam.
Several plasmonic structures can
satisfy these requirements; we chose a
relatively simple design consisting of
a rectangular aperture and a concentric half-ring grating. We performed a
detailed study for patterned quantum
cascade lasers with l = 8.06 µm. A sub-
o
wavelength rectangular aperture ( 10 µm
2
area) is positioned in the center of the
active region. The grating period is
comparable to the wavelength, while
the grooves’ widths and depths are sub-wavelength, as shown in the left side of
the panel “2D collimation.” There was a
dramatic improvement in collimation,
as demonstrated by the large reduction in beam divergence shown in the
right side of the panel “2D collimation.”
The beam quality factor (M factor) of
2
a device with 20 rings is about 2.0 in
both the vertical and lateral directions.
10
deg 0.9
0.8
5 0.7
0.6
0 0.5
0.4
– 5 0.3
0.2
0.1
Intensity
[normalized]
– 10 – 5 0 5 10deg
(Left) Electron micrograph of the facet of a l = 8.0-mm wavelength quantum cascade
o
laser patterned with a 2D plasmonic collimator. (Right) Measured 2D far-field intensity
profile for the device demonstrating a major reduction in beam divergence by a factor
of 30 and 10 in the vertical and lateral directions, respectively, compared to the
original unpatterned laser.
Plasmonic control of polarization
Plasmons can be used to control semiconductor laser polarization by means
of metallic gratings and sub-wavelength
apertures patterned on the laser emission facet. An integrated plasmonic
polarizer can project the polarization
of a semiconductor laser onto other
directions. Alternatively, a circularly
polarized laser beam can be achieved by
designing a facet with two orthogonal
grating-aperture structures.
To project the laser polarization onto
a direction defined by u, we used an
aperture-grating plasmonic structure
similar to that of the one-dimensional
collimator, except that here the normal to the slit aperture and the grating
grooves is at an angle u from the vertical
direction. Only the component of the
laser polarization perpendicular to the
aperture slit couples to surface plasmons
propagating along the grating. The
