New laser nanoantenna shows unprecedented detail

In a stunning feat of nanotechnology engineering, researchers from Harvard University have demonstrated a laser with a wide-range of potential applications in chemistry, biology, and medicine. Called a quantum cascade (QC) laser antenna, the device is capable of resolving the chemical composition of samples, such as the interior of a cell, with unprecedented detail.

Spearheaded by graduate students Nanfang Yu and Ertugrul Cubukcu, and Robert L. Wallace Professor of Applied Physics Federico Capasso — all of Harvard’s School of Engineering and Applied Sciences (SEAS), the findings will be published as a cover feature of the Oct. 22 issue of Applied Physics Letters. The researchers have filed for U.S. patents covering this new class of photonic devices.

The laser’s design consists of two gold nanorods separated by a nanometer gap (a device known as an optical antenna) built on the facet of a QC 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 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. Likewise, the use of near-field infrared microscopes, which rely on an ultrasharp metallic tip scanned across the sample surface at nanometric distances, is limited due to their large size and limited tunability and wavelength coverage.

“By combining QC lasers with optical antennas, 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 chemical and biological specimens,” says Capasso. 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 ultrathin atomic 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.

The team’s co-authors are Kenneth Crozier, assistant professor of electrical engineering, and research associates Mikhail Belkin and Laurent Diehl, all of SEAS; 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.