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Optical antenna allows submicron laser scans

Posted: 17 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:spectral scanners? quantum-cascade lasers? optical antenna?

Spectral scanners using tiny semiconductor quantum-cascade (QC) lasers hold promise for handheld devices that can read out the chemical composition of nearly any sample. Unfortunately, QC laser's wavelengths are measured in microns, limiting scanners to micron-scale resolution, since a focused laser's spot cannot ordinarily be smaller than its wavelength.

Now, co-inventor of the QC laser, Harvard professor Federico Capasso, has devised an optical antenna that enables QC lasers to perform submicron scans by focusing the laser's spot with nanoscale accuracy.

Mounting an optical antenna atop a QC laser can sharpen its resolution up to 100x smaller than the wavelength of its light. Spectral-photonic scanners using this more focused QC laser could image the submicron chemical composition of surfaces in realtime. Capasso has applied for a U.S. patent on what he claims is a new class of photonic-scanning device, which he invented in collaboration with doctoral candidates Nanfang Yu and Ertugrul Cubukcu.

Capasso's optical antenna consists of two gold rods, 1.2?m long, with a 100nm gap between them integrated above the QC laser. When set to 7?m, the laser can power a spectral scanner that reveals chemical composition. Ordinarily, a 7?m laser could only be focused to a spot no smaller than 7?m. By focusing the laser with the optical antenna, a spot less than 100nm across can be produced, enabling sub-micron resolution in measurements of chemical composition by merely scanning the laser across a sample.

Capasso and his group at Bell Labs invented the QC laser in 1994a semiconductor laser smaller than bulk-crystal laser. The QC laser is created by stacking alternating layers of semiconductor materials on top of each othervarying their thickness to tune for specific wavelengths.

In conventional lasers, an electron emits a single photon by jumping from the semiconductor's conduction band to its valence band. But a QC laser arranges its layers to realize from 25 up to 75 quantum wellseach at a progressively lower energy level. When an electric current flows through the QC laser, electrons cascade down the energy staircase, emitting a photon at each step.

QC lasers are being customized for various apps, including semiconductor metrology and pollution monitoring.

- R. Colin Johnson
EE Times

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