The demonstration of a quantum cascade (QC) laser nanoantenna, a device capable of resolving the chemical composition of samples such as the interior of a cell with unprecedented detail, is being described as a major feat of nanotechnology by the team that spearheaded the project.
Harvard graduate students Nanfang Yu and Ertugrul Cubukcu and physics professor Federico Capasso, all of Harvard’s School of Engineering and Applied Sciences, have filed for US patents covering this new class of photonic devices and are publishing their findings in Applied Physics Letters.
Capasso said, “There’s currently a major push to develop powerful tabletop microscopes with spatial resolution much smaller than the wavelength that can provide images of materials, and in particular biological specimens, with chemical information on a nanometric scale.”
The laser is comprised of two gold rods separated by a nanometer gap (a device known as an optical antenna) built on the facet of a quantum cascade laser, which emits invisible light in the region of the spectrum where most molecules have their telltale absorption fingerprints. The nanoantenna creates a light spot of nanometric size about 50 to 100 times smaller than the laser wavelength; the spot can be scanned across a specimen to provide chemical images of the surface with superior spatial resolution.
While infrared microscopes — based on the detection of molecular absorption fingerprints — are commercially available and widely used to map the chemical composition of materials, their spatial resolution is limited by the range of available light sources and optics to well above the wavelength, the university said in a statement. Likewise, the so-called near field infrared microscopes — which rely on an ultrasharp metallic tip scanned across the sample surface at nanometric distances — can provide ultrahigh spatial resolution, but applications are so far strongly limited by the use of bulky lasers with very limited tunability and wavelength coverage.
"By combining quantum cascade lasers with optical antenna nanotechnology, we have created for the first time an extremely compact device that will enable the realization of new ultrahigh spatial resolution microscopes for chemical imaging on a nanometric scale of a wide range of materials and biological specimens," Capasso said.
QC lasers were invented and first demonstrated by Capasso and his group at Bell Labs in 1994. These compact, millimeter-length semiconductor lasers (which are now commercially available) are made by stacking nanometer-thick layers of semiconductor materials on top of each other. By varying the thickness of the layers, one can select the wavelength of the QC laser across essentially the entire infrared spectrum where molecules absorb, thus custom designing it for a specific application. In addition, by suitable design, the wavelength of a particular QCL can be made widely tunable. Potential applications of QC laser-based chemical sensors include pollution monitoring, chemical sensing, medical diagnostics such as breath analysis, and homeland security.
The team’s co-authors are Kenneth Crozier, assistant professor of electrical engineering, and research associates Mikhail Belkin and Laurent Diehl, all of Harvard’s School of Engineering and Applied Sciences; and David Bour, Scott Corzine and Gloria Höfler, all formerly with Agilent Technologies. The research was supported by the Air Force Office of Scientific Research and the National Science Foundation. The authors also acknowledge the support of two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.
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