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Laser antenna holds promise for nanoscale imaging

26 Oct 2007

A laser that focuses infrared light to a spot size of 100 nm or less could enable high-resolution imaging and spectroscopy.

A team led by Harvard professor Federico Capasso has combined a nanoscale optical antenna with a quantum cascade laser (QCL) to confine mid-infrared light to a resolution 100 times smaller than its wavelength. According to Capasso, the small spot size could make it possible to image the sub-micron chemical composition of surfaces and tissues in real time, and avoid the need for the cumbersome CO2 laser sources previously needed for high-resolution imaging in the mid-IR. (Applied Physics Letters, 91, 173113.)

The laser's design, claimed to be a new class of photonic device, consists of an optical antenna built on the facet of the QCL. The antenna is made from two gold rods 1.2 µm long, separated by a nanometer-scale gap.

"The two rods are oriented along the polarization of the quantum cascade laser, ie. along the direction of the oscillating electric field," Capasso explained to optics.org. "This sets the electrons in the antenna into oscillation. The length of each rod is designed to be half the incident wavelength, and the result is a strong accumulation of charge at the ends of the rods and a very strong electric field across the gap." This produces an intense laser spot, localized in the gap between the rods.

"Effectively the antenna behaves like an optical funnel," continued graduate student Nanfang Yu. "It efficiently captures the energy of the laser output and transfers it into the intense, subwavelength optical spot in the antenna gap."

The team demonstrated that using a QCL source at 7 µm and an antenna gap of 100 nm produced a spot size comparable to the gap dimension, considerably below the laser wavelength. Using a 5 µm source with a 75 nm antenna gap produced field confinement of about 70 nm.

"In imaging applications, scanning such a highly localized light spot across a material allows details to be resolved at sizes much smaller than the source wavelength," said Capasso. The mid-IR is of particular interest for bioimaging applications because it is the so-called "fingerprint" region, from about 3 to 20 µm, where many molecules have characteristic absorption peaks.

"Scanning a normal mid-IR beam across a specimen can only lead to a spatial resolution larger than a few microns, which can sometimes barely resolve a single cell," commented Fu. "On the other hand, the mid-IR laser antenna can produce an intense optical spot of subwavelength size, which could spatially resolve even sub-cellular features."

Capasso told optics.org that one of the biggest hurdles was to fabricate the nanoantenna. However, making an antenna for use in the mid-infrared proved easier than previous work on near-infrared antennas for high-density data storage. Since the size of antenna rod is designed to match half the incident wavelength, a near-IR antenna must be less than 1 µm long. The mid-IR antennas, with lengths of a few microns, were more straightforward to produce using focus ion-beam milling techniques.

"The biggest challenges were to coat a QCL with a film of metal without electrically shorting the device, and to open the antenna gap as small as possible," explained Fu. "A third challenge was more intrinsic: to design an antenna structure to suppress as much as possible the two side optical spots at the outer ends of the antenna, since ideally only the center optical spot is needed for applications." The team has also reported work on a "bowtie" antenna design that can suppress the side spots more effectively than the double-rod system.

According to Capasso, the next stage will be to build a microscope from the modified QCL. The localization of light in the gap is a "near field" effect which means that samples would need to be brought very close to the gap to use the light for imaging or spectroscopy.

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