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 compact device is capable of resolving the chemical composition of surfaces and tissues with unprecidented detail, and could reduce the overall size of an infrared microscope dramatically by replacing bulky laser sources.
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, that is 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.
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, that is 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.
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