Optical antenna helps QC laser hit the spot

PORTLAND, Ore. -- 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 quantum-cascade laser, Professor Federico Capasso, of Harvard, 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 100-times smaller than the wavelength of its light--that is, 70 nanometers for a seven micron laser. Spectral-photonic scanners utilizing this more focused quantum-cascade laser could image the submicron chemical composition of surfaces in real-time--from semiconductors to medical samples. Capasso has applied for U.S. patents 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-microns long, with a 100-nanometer gap between them integrated above the QC laser. When set to seven microns, the laser can power a spectral scanner that reveals chemical composition. Ordinarily, a seven-micron laser could only be focused to a spot no smaller than seven microns. But by focusing the laser with the optical antenna, a spot less than 100 nanometers 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 quantum cascade laser in 1994--a semiconductor laser that's smaller than bulk-crystal or gas lasers. The QC laser is created by stacking alternating layers of semiconductor materials on top of each other--varying 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 wells--each 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 a variety of applications, from spectral scanners for semiconductor metrology, pollution monitoring, chemical sensing and medical diagnostics, to homeland security.