Optics InfoBase: Spotlight on Optics - Linewidth enhancement in spasers and plasmonic nanolasers

Spotlight on Optics

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January 2013

Linewidth enhancement in spasers and plasmonic nanolasers

Published in Optics Express, Vol. 21 Issue 2, pp.2147-2153 (2013)
by Pavel Ginzburg and Anatoly V. Zayats

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Spotlight summary: What’s the difference between a laser and an incandescent lightbulb? From an optical physicist’s point of view, it’s about a micron. White light from a lightbulb is made up of colors from all across the visible spectrum, and hence has a broad linewidth, or optical bandwidth. In contrast, a laser emits light very close to a single wavelength, making it look purely red, or green, or whichever color it is. It has a narrow linewidth, which in fact directly causes some of the properties we commonly associate with lasers, like their ability to create interference patterns. The difference in these two linewidths is typically about one micron.

In this paper, Ginzburg and Zayats consider the linewidth of a spaser, which is closely related to a laser. They derive an equation that predicts the linewidth of the spaser in terms of its physical parameters, using a more accurate method than has been used before.

Several factors can affect the linewidth of a source. Light is emitted when a source undergoes some kind of transition. An electron might change energy states, or a molecule might change vibrational states, or two protons might be fused together. Each such transition is associated with a change in energy, and the resulting wavelength of light is dictated by that energy change. So if the source can be subjected to transitions at many different energies, the linewidth tends to be large. In general, effects like this are called broadening or linewidth enhancement.

While lasers emit only photons, spasers emit surface plasmon polaritons, which are photons coupled to electrical waves, typically confined to the surface of a metal object. Just like laser light, the surface plasmons have an associated energy and wavelength. So just like lasers, spasers have associated linewidths. If you wanted to create intense surface plasmons for an interference experiment, you might want a spaser with a narrow linewidth so that the emission is all close to one frequency. In contrast, if you wanted a broad spectrum of surface plasmons, say for a spectroscopy experiment, or to send high-information-content messages, you would prefer a spaser with a broad linewidth.

Until now, all derivations of spaser linewidth have relied on the so-called quasistatic approximation, which says that all of the photons and electrons in the device must react to an input at exactly the same time. But Ginzburg and Zayats find that under this assumption, some important mechanisms for linewidth enhancement are ignored. They therefore present a time-dependent theory, which is more precise. The linewidths they derive are several times larger than in the quasistatic case, which agrees much better with recent experimental results. This works should aid in the design of new spaser devices that are better tailored to specific applications.

--Brad Deutsch

Technical Division: Optoelectronics
ToC Category: Lasers and Laser Optics

OCIS Codes:	(140.4780) Lasers and laser optics : Optical resonators
	(240.6680) Optics at surfaces : Surface plasmons
	(350.4238) Other areas of optics : Nanophotonics and photonic crystals

Posted on January 23, 2013