It’s almost commonplace in technology that when there seem to be limits, somebody will figure out a way to exceed them. In a way, Moore’s Law about the seemingly never ending increase in the power of computer processors is built on constantly – somehow – extending the limits. That’s also, apparently, the situation for lasers. Lasers as we know them in DVD players, eye surgeries (etc.) have limits; they cannot project a beam smaller than one-half the wavelength of their light. For some applications, notably in the growing field of nanoelectronics, this isn’t small enough.

The answer, or at least one answer that a number of researchers are pursuing is the Surface Plasmon Amplification by Stimulated Emission of Radiation, which fortunately acronymizes (sic) to SPASER, or just plain spaser. The spaser is a laser that uses plasmons rather photons. That’s a big, or rather ultra-small distinction. The spaser uses surface plasma waves – plasmons, which are formed by the oscillation of free ions between one polarity of a metal to the other. A plasmon wave can generate light (photons) but is itself much smaller than light, less than 100 nanometers in wavelength compared to the wavelength of light at about 250 nanometers. Plasmons also move faster than photons, about a 100 times faster.

Faster and smaller – those are magic words for electrical engineers. That’s where the interest in spasers is generated. In 2003 David Bergman (Tel Aviv University, Israel) and Mark Stockman (Georgia State University, USA) laid out the theory behind the spaser. In 2009 a team of researchers in the United States made the first spaser prototype, which however only functions at -250 degrees centigrade. [SciTechStory, October 19, 2009: It’s a spaser as in laser] Now a new prototype created by a team at the University of California, Berkeley (USA) has demonstrated a spaser using metals and semiconductors (common components of standard electronic equipment) that can operate at room temperature.

This new spaser, built from a 1-micrometer square of cadmium sulphide, a relatively commonplace semiconductor, which sits on a 5-nanometer slice of magnesium fluoride on top of sheet of silver catches the light of a conventional laser and forces it to reflect from the edges of the cadmium sulphide. Less than 5 percent of the radiation escapes, which allows the buildup of plasmon waves (called pumping) until they ‘lase’ or give off a high-energy radiation – at room temperature.

This high-energy radiation at a very short wavelength is what gives the spaser its potential usefulness in applications as diverse as photolithography (etching microchips), packing more data into DVDs, or single-molecule detection sensors.

However (how often there is a however), researchers will need to demonstrate that this semiconductor configuration can work with electrical rather than optical pumping (translation: the current prototype uses pulses of light to build the plasmon waves. In the future to make a spaser practical it will need to be pumped by the application of electrical pulses). In other forms of lasers, for example photonic crystal lasers, this change in energy source required many years of experimentation. That means even optimistic predictions for commercially ready spasers runs into several years.

Nevertheless, the promise (both scientific and commercial) of spasers is great enough to attract research teams all over the world.