August 18, 2009--Researchers have modified nanoparticles known as "Cornell dots" to make the world's tiniest laser (a nanolaser or technically a spaser)--so small it could be incorporated into microchips to serve as a light source for photonic circuits. The device may also have applications for sensors, solar collectors, and in biomedicine. The research is reported in the Aug. 16 online issue of the journal Nature and will appear in a coming print issue (see also "Nanolaser becomes a refractive-index sensor").
The original Cornell dots, created by Ulrich Wiesner, the Spencer T. Olin Professor of Engineering at Cornell, consist of a core of dye molecules enclosed in a silica shell to create an unusually luminous particle. The new work by researchers at Norfolk (Virginia) State University (NSU), Purdue University (West Lafayette, IN), and Cornell University (Ithaca, NY) uses what Wiesner calls "hybrid Cornell dots" which have a gold core surrounded by a silica shell in which dye molecules are embedded. Using nanoparticles 44 nm wide, the device is the smallest nanolaser reported to date, and the first operating in visible light wavelengths, the researchers said.
An optical laser this small is normally impossible because a laser develops its power by bouncing light back and forth in a tuned cavity whose length must be at least half the wavelength of the light to be emitted. In the first tests of the new device, the light emitted had a wavelength of 531 nm, in the green portion of the visible spectrum.
In a conventional laser, molecules are excited by an outside source of energy, which may be light, electricity or a chemical reaction. Some molecules spontaneously release their energy as photons of light, which bounce back and forth between two reflectors, in turn triggering more molecules to emit photons.
In the new device, dye molecules in the nanoparticle are excited by a pumping laser. A few molecules spontaneously release their added energy to generate a plasmon in the gold core. In the tiny space, the dye molecules and the gold core are coupled by electric fields, explains Purdue co-author Vladimir Shalaev. Oscillations of the plasmon in turn trigger more dye molecules to release their energy, which further pumps up the plasmon, creating a "spaser" (surface plasmon amplification by stimulated emission of radiation). When the energy of the system reaches a threshold the electric field collapses, releasing its energy as a photon. The size of the core--14 nm in diameter--is chosen to set up a resonance that reinforces a wave corresponding to the desired 531 nm light output.
For more information, go to www.news.cornell.edu/stories/Aug09/nanolaser.html .