Semiconductor lasers now permeate
our society, as key components in widespread commercial technologies such as
optical fiber communications and CD
and DVD players. However, their beams
usually have a large divergence angle (of
around tens of degrees) due to the significant diffraction caused by the small
emission aperture of the devices. In
addition, light output is mostly linearly
polarized along a single direction, which
is determined by the optical selection
rules of the gain medium.
Clearly new beam-shaping schemes
could improve myriad technologies by
greatly reducing beam divergence and by
making available a wide range of polarization states, such as circular polarization and linear polarization along
different directions. Such a feat would
be technologically important. Specifically, we can envision collimators and
polarizers consisting of metallic gratings,
apertures, antennas or other configurations. Another challenge is to concentrate laser light in regions much smaller
than the wavelength—which would
dramatically improve the information
capacity of optical disks. In this article,
we address some of these questions in
detail and demonstrate specific solutions.
Plasmonic laser antennas
A variety of strategies have emerged
that take advantage of localized near-fields that are generated by suitably
shaped metallic nanoparticles known as
optical antennas. Optical antennas can
produce very high near-field intensities
when their size is properly matched to
the wavelength of the incident light.
This resonance condition is achieved
when the length of the antenna segments is chosen to be approximately
half the wavelength of surface plasmons
excited in the metal by the incident
radiation and the latter is polarized
along the antenna length.
Recently, there has been a great
deal of scientific interest in the use of
active optical devices that can generate sub-wavelength optical spots for
various applications. Researchers have
demonstrated very-small-aperture lasers
[ Resonant optical antenna designs ]
The antenna resonance condition leads
to a strong near-field enhancement at
the antenna ends with respect to the
incident field (bottom). The coupled-rods design (middle) is used to create
highly localized, well-defined light spots
at the central antenna gap. The bow-tie
design (top) maximizes field concentration in the nanogap compared to the
outer ends of the antenna segments.
Pink and bright cyan stand for positive
and negative charge distributions in the
antennas, respectively. Resonant optical antennas can capture a light beam
with a cross-section much larger than
their geometrical size.
A semiconductor laser with a resonant
optical antenna integrated on the facet.
that consist of a laser diode with its
facet coated by a metal film on which a
sub-wavelength hole is etched by focused
ion beam milling. However, these and
related devices suffer from limited
throughput, since the transmitted power
scales strongly with the hole diameter
(with the fourth power through a sub-wavelength hole in a perfect metal).
Plasmonic laser antennas, which
consist of a resonant optical antenna
integrated on the facet of a semiconductor laser, don’t suffer from this problem.
Resonant antennas indeed can capture
a substantial part of the power of the
incident laser beam and concentrate it
into a spot the size of the nanometric
antenna gap, leading to a light intensity hundreds or even thousands of
times stronger than that of the incident
beam. Such a compact laser source with
sub-wavelength spatial resolution could
provide distinct advantages in a number
of applications, including microscopy,
spectroscopy, terabyte-level optical data
storage, lithography and laser processing.
We fabricated an optical antenna that
comprises a pair of gold nanoparticles on
the facet of a commercial near-infrared
edge-emitting laser diode (Sanyo, Inc.).
A thin silicon dioxide insulating layer
separated the metal from the facet to
avoid electrical shorting. We used an
aperture-less near-field scanning optical
microscope to map the optical near-field
distribution in the fabricated devices.
In this technique, a nanoscale sharp
gold-coated atomic force microscope tip,
driven at its resonant frequency, scatters
the light from the near-field as it scans
over the sample. The weak light backscattered into the laser is collected by
the back-facet photodiode incorporated
in the laser package, thus providing a
two-dimensional map of the near-field,
as shown in the top figure on the facing
page. This device is capable of generating
an intense optical spot (~100 MW/cm
at a distance of 10 nm above the center
of the antenna) that is localized within
an area 50 times smaller than what
one would obtain with conventional
diffraction-limited optics such as lenses.
We have also demonstrated plasmonic laser antennas with 100-nm size
spots at mid-infrared wavelengths using
quantum cascade lasers. This work opens
up intriguing opportunities for sub-wavelength chemical-biological imaging—of a cell’s interior, for example.
Optical antenna arrays fabricated at
the end of an optical fiber can be used
for detecting molecules with surface-enhanced Raman scattering. Here, the
strong near-fields of the coupled antennas
lead to an enhancement of the Raman
signal by many orders of magnitude,
since both the incident laser field and the
Stokes field are resonantly enhanced.
This device could have important
applications for in situ chemical sensing