output of the device is thus an interference pattern produced by direct emission
from the aperture and coherent scattering from the grating grooves due to
Bragg diffraction of surface plasmons by
the second-order grating. The polarization of the reemitted light is along the u
direction, determined by the orientation
of the grooves.
The central figure of the panel “
Plasmonic polarizer” shows that the grating
greatly reduces the beam divergence angle
along u= 45° direction due to the interference effect. By detecting the power
transmission of the device output through
a conventional wire-grid polarizer, we
found that the polarization of the main
lobe of the far-field is essentially linearly
polarized along the 45° direction.
A circularly polarized laser could
find many applications, ranging from
the spectroscopy of chiral molecules to
satellite communications and quantum
information processing. A circularly
polarized laser beam can be constructed
by coherently combining two linearly
polarized beams satisfying three conditions: The two beams should have
perpendicular polarizations, a 90° phase
difference, and the same amplitude.
These conditions can be met by defining
two orthogonal aperture-grating structures on the device facet. By tailoring
the separations between the aperture and
the nearest groove in the two aperture-grating structures, we can control the
respective phase and amplitude of the
scattered light from the left grating and
from the right. Note that direct emission
from the apertures into the far-field must
be blocked through suitable design.
Intensity
de g
30 0.9
20 0.8
0.7
10
0.6
0 0.5
– 10 0.4
0.3
– 20 0.2
– 30 0.1 90
120 60
150 30
180 0
210 330
240 300
270
[normalized]
– 20 0 20deg
(Left) Electron micrograph of the facet of a l =10-mm quantum cascade laser
o
patterned with a plasmonic polarizer that can project the vertical polarization of the
original laser onto the 45° direction. (Center) Measured 2D far-field intensity profile
for the device. Beam divergence is reduced along the 45° direction. (Right) A wire-grid polarizer was used to determine the beam’s polarization state. As its axis is
rotated, the maximum optical power was detected for orientation normal to the grating
grooves, with a high polarization suppression ratio. Red circles are experimental data.
What about beams that are radially or
azimuthally polarized, or beams with
orbital angular momentum, which can
be used to rotate small particles?
Another issue of substantial conceptual and practical interest is whether
researchers can determine, from a
prescribed “designer wavefront,” the sub-wavelength structure that is required to
generate it.
Semiconductor lasers and single-mode optical fibers can be viewed as the
“optical benches” on which an entire
plasmonic network—consisting of
surface plasmon waveguides and active/
passive nodes (metallic and core-shell
nanoparticles, nanocrystals, quantum
dots, etc.)—can be built to perform local
optical processing. In addition, plasmonic beam shaping can be applied to the
optical trapping of nanoscale particles.
(Ken Crozier at Harvard is pursuing this
line of research.) Finally, using materials
with a negative-refractive index fabricated on the facet of laser diodes and fibers,
researchers and engineers will be able to
create light sources with sub-wavelength
focusing in the far-field as well as other
interesting functionalities. t
and H. Kan. The authors would like also to
acknowledge funding and support from NSF,
AFOSR MURI, DRAPA, NIH, Center for
Nanoscale Systems (CNS) at Harvard University, and Hamamatsu Photonics.
Federico Capasso ( capasso@seas.harvard.edu),
Nanfang Yu and Elizabeth Smythe
Member are with the School of Engineering
and Applied Sciences at Harvard University,
Cambridge, Mass., U.S.A. Ertugrul Cubukcu is
with the physics department at the University of
California, Berkeley, Calif., U.S.A.
Looking forward
There are rich opportunities in far- and
near-field engineering with plasmons,
from both scientific and technological
points of view. Some of the challenging
questions that scientists face moving forward include: Is it possible to
integrate plasmonic structures into lasers
to achieve sub-wavelength focusing in
the far-field? Can we use plasmonics
to create special beams such as Bessel
beams, which are diffraction-free beams?
The authors acknowledge the following people
who also contributed to the work reported
in this article: E.A. Kort, K.B. Crozier,
M.D. Dickey, J. Bao, G.M. Whitesides, R.
Blanchard, J. Fan, Q.J. Wang, C. Pflügl, L.
Diehl, T. Edamura, S. Furuta, M. Yamanishi,
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