Failed silicon laser leads to surprising insight in light scattering

Together with researchers of the California Institute of Technology (Caltech), AMOLF researcher Albert Polman of the Photonic Materials Group has attempted to build a silicon laser. The desired outcome, the laser application of silicon by use of a silicon laser, wasn’t established. However, the measurements did lead to a surprising new insight into light scattering: The light rarely scattered out of the microcavity, even though normally light scatters in every direction. Light scattering is of great importance in the telecommunication industry, in medical diagnostics and in the development of new solar cells. The results were published on July 13th in the prestigious journal Physical Review Letters.

In 2000 Italian researchers published an article in the renowned journal Nature in which they claimed to have measured light amplification in silicon. This appeared to be a breakthrough, because researchers had long been looking for a light source that is made of the basic material of the computer chip industry. With a silicon laser it would become possible to integrate the internet (which works with glass fibre and light pulses) with the computer technology (which works with electricity) in a natural way. However, the evidence in the Italian article was indirect; real proof was lacking.

The Dutch-American team started looking for genuine evidence and tried to build a silicon laser. At Caltech they had already developed a special microcavity in which the light is trapped in a ring, and is thus recycled a million times. In such a structure, light can amplify itself and lasing can occur. The researchers applied small silicon particles to the microcavity. However, whatever they tried, lasing in silicon was never found.

Silicon particles, measuring only a few nanometers, are very strong light scatterers. Each time light collides with a silicon particle, it is partly reversed. The outcome is an immense chaos of light waves running back and forth. The surprising aspect, however, was that the light almost never scattered out of the microcavity, even when normally it scatters in every direction. The sky, for instance, is blue because light from the sun collides with dust particles and molecules in the air, and is then scattered in all directions. In a microcavity that doesn’t appear to be the case: The light only scatters into the cavity and doesn’t escape.

The effect strongly resembles what the American Nobel Prize winner Edward Purcell predicted for light sources over fifty years ago. A light source being applied into a microcavity mainly emits its light into the microcavity, and not beyond. This effect has been confirmed in experiments several times. For instance, lasers in DVD players are based on this principle. The researchers now found that exactly the same is true for light scattering: Light collides with a scatterer and has such a strong interaction with the microcavity, that the scattered light remains locked within the microcavity. Therefore, the scattering process is not only determined by the scatterer itself, but also mainly by the surroundings (the microcavity).

From failure to surprise scientific finding
Albert Polman: "The Italian researchers’ suggestion that a silicon laser might be made turned out incorrect. Unfortunately, in the past years many researchers worldwide started working towards this goal, with no result. But thanks to their work, we started working with silicon particles and microcavities, with surprising results. That’s often how it works in science: One failure can lead to a surprise scientific finding.”

Reference
Purcell Factor Enhanced Scattering from Si Nanocrystals in an Optical Microcavity,
T. J. Kippenberg, A.L. Tchebotareva, J. Kalkman, A. Polman, and K.J. Vahala, Physical Review Letters, 103, 027406 (2009).

Figure 1. Ring of light. Light continuously turns round in a ring of glass in which silicon nano particles have been inserted (Design: Caltech).