Sep 17, 2014
A new design for a cavity-free nanolaser has been proposed by physicists at Imperial College London. The design builds on a proposal from the same team earlier this year to reduce the group velocity of light of a particular frequency to exactly zero in a metal–dielectric–metal waveguide. The laser, which has yet to be built, makes use of two such zero-velocity regions, and would achieve population inversion and create a laser beam without the need for an optical cavity. The researchers suggest that the design could have important applications in optical telecommunications and computing, as well as theoretical implications in reconciling the physics of lasers with plasmonics.
Slowing light to a stop: nanolaser has no cavity
The traditional design for a laser involves encasing a gain medium such as a gas in a cavity containing two opposing mirrors. The gain medium contains two electronic energy levels, and, in the natural state, the lower energy level is the more populated. However, by injecting electrical or light energy into the cavity, some electrons can be “pumped” into the upper state. At low pumping levels, atoms pushed to the upper level decay spontaneously back to the ground state with the emission of a photon. However, above a certain threshold, transitions back to the ground state are predominantly caused by an excited atom’s absorption of a second photon. The two photons are emitted perfectly in phase, and go on to excite emission from more atoms. The resulting beam of phase-coherent photons is the laser beam.
Lasers have revolutionized modern science and technology, with tiny lasers can be found everywhere from cheap pointers to state-of-the-art telecommunications systems. While much smaller nanoscale lasers would be useful for creating chip-based optical circuits, the need for a cavity limits means that it is difficult to miniaturize a conventional laser beyond the wavelength of the light it produces. This limit is about one micron for the light used in telecommunications technologies.
Now, Ortwin Hess and colleagues have devised a new way of producing a sub-wavelength laser by removing the cavity altogether. The researchers designed a layered metal–dielectric–metal waveguide structure that supports plasmonic interactions between light and conduction electrons at the metal–dielectric interfaces. Such a plasmonic waveguide supports two “zero-velocity singularities” at closely spaced but distinct frequencies. Light of other frequencies will propagate through the semiconductor very slowly – allowing it plenty of time to interact with the gain material. While slow and stopped light might sound like unphysical concepts, they can occur when light interacts with plasmons. Injecting a pulse of this slow light, the researchers calculated, will pump carriers from a lower energy state to a higher state. This higher state would then decay to an intermediate state, which would then decay to produce the laser light. The presence of the zero-velocity singularities causes the laser light to be trapped in the material, where it drives the desired coherent stimulated emission.
To produce a laser beam, however, some of the laser light must be able to leave the device. In previous work (see “Plasmonic waveguide stops light in its tracks”), Hess and colleagues proposed exciting a zero-velocity mode by passing the light through the cladding in the form of an evanescent wave – a special type of wave the frequency of which is a complex number. Radiation incident on the cladding would excite an evanescent wave, which would in turn excite the stopped-light mode in the dielectric inside. In their new paper, Hess and colleagues turn this idea on its head and use the evanescent field to allow laser light to escape. By varying the precise properties and thickness of the cladding layer, the proportion of light allowed to escape could be tuned, producing a laser beam of variable intensity.
Nicholas Fang, a nanophotonics expert at the Massachusetts Institute of Technology, believes that, if such cavity-free nanolasers could be produced, they could have major practical implications not only in computation and signalling, but also in less-obvious fields such as prosthetics: he suggests they could be embedded in synthetic tissue to provide sensors with output signals detectable by the nervous system. “Here you’d have a laser source that could be directly implantable,” he explains.
Hess, meanwhile, is excited by the potential theoretical implications of the work. While the current research focuses on using plasmonic interactions to produce coherent light, he believes that it should also be possible to keep the plasmons themselves confined within the waveguide to produce a miniature surface plasmon laser or “spaser”.
The research is described in Nature Communications.
See the full article here.
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