Hank Hogan, email@example.com
EVANSTON, Ill. – A new class of biosensors could be at hand, according to Northwestern
University researchers. They constructed composite metal-dielectric-metal nanoantennas
on the facet of a quantum cascade laser, thereby boosting the incident optical field
intensity in the mid-IR as much as 4000-fold. Their device had a lasing mode spot
size more than 10 times smaller than the laser wavelength.
This light squeeze solves a fundamental problem, said team leader
Hooman Mohseni. The wavelength of a mid-IR laser is thousands of times larger than
the size of biomolecules, making interaction with the light poor, signals weak and
Metal-dielectric-metal nanoantennas (scanning electron microscope
image on right) fabricated on and integrated with the facet of a quantum cascade
laser (center) concentrate infrared emission (top left inset). The laser’s
current voltage characteristics are shown. Reprinted from Optics Letters. Images
courtesy of Dibyendu Dey and Hooman Mohseni, Northwestern University.
This obstacle can be overcome if the researchers can reduce the
detection volume by up to an order of magnitude, according to Mohseni, an associate
professor of electrical engineering and computer science.
He added that quantum cascade lasers are a natural choice for
a source in the range of 2 to 30 μm, a region where many important biomolecules
have an identifying spectral signature. Metal nanoantennas have been used in the
past to concentrate laser light in this range. These antennas interact with the
light and compress it into a small volume.
Side view of simulated intensity profile for metal dielectric metal
nanoantenna (Au/SiO2/Au, 70/30/70 nm) at a resonance antenna length of 2 μm.
Reprinted from Optics Letters.
The Northwestern group used a different type of nanoantenna, fabricating
the devices out of multiple layers of metal and dielectric. They chose this approach,
said graduate student Dibyendu Dey, because these composite-material nanoantennas
increase near-field enhancement. Indeed, the group achieved four times the enhancement
previously seen, according to an Aug. 15, 2010, Optics Letters paper, of which Dey
was the lead author.
The scientists simulated the effect of the thickness of a dielectric
layer of silicon dioxide from 0 to 100 nm. In these calculations, they kept the
total thickness of the nanoantennas fixed at 170 nm. Thus, those devices with thicker
oxide had thinner layers of gold sandwiching the dielectric. The metal thickness
ranged from a high of 85 to a low of 35 nm for each layer. The researchers found
that a 30-nm oxide thickness delivered the best results.
They then constructed the optimum composite nanoantennas on the
facet of a quantum cascade laser by first depositing the alternating gold and oxide
layers on top of a 100-nm buffer oxide. Using a focused ion beam, they then carved
coupled nanorods out of the deposited layers, with each rod about 500 nm wide and
about 2 μm long. They fabricated the two so that their short ends faced each
other with a separation in the tens of nanometers. The antenna had a resonant wavelength
of about 2 μm.
To evaluate the results, they turned to a custom-built near-field
scanning microscope. With this, they measured the optical hot spots, finding that
these results agreed well with what simulations had predicted. The lasing spot size
was about 450 nm, an order of magnitude smaller than the laser wavelength.
As for the future, quantum cascade lasers are already being used
for gas sensors. Because it concentrates light, the nano-antenna approach promises
to enable quantum cascade laser-based biosensors. Since the nanoantennas are integrated
onto the laser itself, the resulting device will be very compact. As an added bonus,
device operation can be tuned by varying laser power or by employing different layer
combinations. Once the remaining problems are solved, the researchers are certain
that there will be considerable commercial interest in the technique.
One challenge is how to position the biomolecules in the optical
hot spot, which is at specific locations with respect to the nanorods. The researchers
are evaluating how best to achieve this, Dey said. “We are working on basically
two schemes: to use dip-pen nanolithography to specifically position the molecule
at a particular location and a microfludic approach.”