Near-Field Microscopy Catches Smallest Laser in Action
Using a near-field scanning optical microscope, researchers at the University of California in Berkeley have captured images of tiny nanowire lasers in action. Led by Richard Saykally, they have applied a combination of femtosecond spectroscopy and near-field optics to measuring the new lasers.
Discovered last year by university researchers led by Peidong Yang, the lasers are created by a high-temperature process that grows vertical arrays of nanowires on a sapphire substrate coated with gold. They reported gross measurements of the lasing properties of an entire array. However, they have not measured lasing characteristics of individual nanowires apart from the array.
"We have been developing a new type of microscopy for measuring the optical and chemical properties of nanoscopic systems, based on combining femtosecond laser spectroscopy with near-field microscopes," explained Justin Johnson, a member of Saykally's group. "Our system is perfect for imaging the lasing action of single wires and for measuring the nonlinear properties of these wires as well."
The wires, which are between 40 and 150 nm in diameter, grow vertically on the substrate. In an electron microscope image, they look like a neat arrangement of tiny needles stood on end. To image an individual wire, the researchers used sound waves to remove it from the substrate. They pumped the nanowire with 285-nm radiation made by doubling the sum frequency of a tunable near-IR pulse and an amplified 800-nm pulse from a mode-locked Ti:sapphire oscillator. Johnson said that these lasers have very high peak powers that can excite the lasing response from the nanowire.
Then the scientists used a near-field optical microscope operating with a shear-force feedback system to collect light emitted from the wire. The microscope positioned the tip of an etched optical fiber within 10 nm of the surface of the nanowire surface. In conventional optics, resolution is limited to roughly one-half the wavelength of light used in the imaging. Near-field optics overcome this by keeping the aperture of the microscope -- the optical fiber tip -- and the distance to the subject smaller than the wavelength of the imaging light.
Johnson said that the group characterized the output of the nanowire lasers and that the gain curve of a single nanolaser lying on the substrate is quite different from that of the entire array. "We found that only a small fraction of the nanowires actually exhibited lasing, and we concluded that this implied that the condition of the ends of the wires and the amount of substrate contact was crucial to this behavior." In other words, he explained, the ends of the wire have to act as reasonably good mirrors to achieve lasing.
In the long run, Johnson said, the wires may be useful in communications systems as well as for biological applications. The group is continuing its work by studying the wires' second- and third-harmonic responses to femtosecond excitation, which will help determine how to create nanowire light sources in other colors. "We also want to study the temporal properties of the lasing action and to study the effects of the environment on lasing properties."
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