For lasers, small and thin are in

David L. Shenkenberg, david.shenkenberg@laurin.com

Lasers just keep getting smaller and smaller: Recently, only a couple of months after the report of the world’s thinnest laser, the world’s smallest laser was announced. Both developments barely precede the 50th anniversary of the laser, which will be celebrated worldwide next year.

Both the ultrathin and ultrasmall laser purport to enable all-photonics computers and Internet capability, which theoretically will be much faster than current computing and Internet speeds. A light-based system must have a source of light, and a very small laser can be integrated much more practically, with tiny computer chips, than a gigantic laser ever could.

Conceptual drawing of the ultrathin laser. Courtesy of Hill et al.

The smallest laser, which was made by a team of researchers from three US universities, consists of a spherical particle only 44 nm in diameter. It also is the first of its kind to emit visible light.

The team consisted of Mikhail A. Noginov of Norfolk State University, Vladimir M. Shalaev and Evgenii E. Narimanov of Purdue University in West Lafayette, Ind., and Ulrich B. Wiesner of Cornell University in Ithaca, N.Y., and their associates. The three groups contributed equally to the work.

Shown is the conceptual illustration of the particle on which the new laser is based (top), an SEM image of the particles (bottom, left) and a conceptual image of the emission of the particle (bottom, right). Courtesy of Noginov et al.

“This nanolaser may find a number of applications – maybe biomedical diagnostics or some sort of therapy,” Noginov said. “Maybe some sort of sensors. On the other hand, there is some ongoing new effort to develop a new generation of electronics that will operate at light frequencies; potentially the speed of computers can be increased tens of thousands of times.”

Photons from electrons

Noginov and company call this little laser the spaser, for “surface plasmon amplification by stimulated emission of radiation.” The size of a conventional laser in any one dimension is believed to be limited by one-half the wavelength of light, but the spaser operates in an unconventional way. Instead of amplifying photons, it amplifies surface plasmons, which are oscillating clouds of electrons on the surface of a metal that can emit photons of light when stimulated. For this reason, the researchers chose to give the particle a metal core; in this case, gold.

However, these surface plasmon oscillations usually dissipate too quickly to produce amplified light like a laser. The researchers discovered that they could make the plasma oscillations last long enough to do so when they encapsulated the gold core in a silica shell filled with a dye called Oregon Green 488 (OG-488).

Once the dye was excited with a laser, it transferred its energy to the gold core, generating surface plasmon oscillations sufficient to produce greenish laserlike light at 531 nm in all directions rather than a focused beam. “Our laser does not produce a beam, and this does not mean that the laser does not have a coherence,” Noginov explained.

The investigators generated the spaser beam with a 466-nm laser with <90-ps pulses at a 40-MHz repetition rate, as described in the Aug. 16, 2009, online publication of Nature. “Our laser is laser-pumped,” Noginov said. “The size of the pumping system is typically not accounted toward the size of the laser-pumped laser.”

However, to be practically incorporated into computers, the plasmons must be stimulated electrically in a semiconductor. “What we did – we demonstrated the proof of principle of the spaser,” Noginov explained. “The use in practical electronics schemes will, most likely, require an electrically pumped device. The groups of [Martin T.] Hill of [Technical University of Eindhoven in] the Netherlands and [Cun-Zheng] Ning of Arizona State University [in Tempe] successfully work in this direction.”

In the June 22, 2009, issue of Optics Express, just before the details on the world’s smallest laser were released, Hill, Ning and their associates reported developing the world’s thinnest laser. This laser runs on electricity instead of on an external laser source.

“We think that [the spaser project] is very interesting work,” Hill said. “For some applications, electrically pumping of the laser will be preferred. The structure we use is quite well suited for this electrical pumping.

“We think that these lasers could be used where a very fast modulation speed, very small, low-power laser is required; for example, short-distance ultrafast communications or ultrafast signal processing of optical signals. There may be new niche applications due to the high fields and high pumping densities available in these lasers and also the ability to locate many subwavelength-size devices close together.”

Likes it cold – for now

Their laser is an upright rectangular structure 3 or 6 µm in length with a semiconductor core width that varies from 90 (±20) to 350 nm. The rectangle is made of five layers: metal, insulator, semiconductor, insulator and metal. The semiconductor is InP with an InGaAs core, SiN is the insulator, and a thin layer of silver is the metal.

There are two contact points that the researchers connected to positive and negative electrodes. These contacts are made of a mixture of gold, platinum and titanium. The flow of the electricity from the negative to positive electrode excited the semiconductor core gain medium of InGaAs, which emitted infrared light at 1500 nm.

At extremely cold temperatures down to 10 K, the laser operated in plasmon mode, whereas it operated in transverse electron mode at room temperature.

“In the paper, we gave results for the smallest waveguides at lower temperatures in order to show the behavior clearly,” Hill said. “However, in the experiments, there were indications that these thinnest devices could still work up to, say, 100 to 200 K.

“Some of the wider devices, which don’t propagate a plasmonic mode, operated at room temperature. We expect in the future that we will succeed in having the smaller plasmonic mode device also operating at room temperature.”