PLASMONICS
Small gain yields great control

Optical gain in subwavelength waveguides is generally used for compensating losses. But it may also enable control of dispersion and propagation of electromagnetic pulses at the nanoscale, perhaps even leading to a nanoscale optical transistor.

Plasmonics allows light to be confined to nanoscale dimensions where a typical waveguide cross section would be much smaller than the wavelength, according to Viktor Podolskiy, assistant professor of physics at Oregon State University (Corvallis, OR). But the plasmonic materials that confine the propagation of radiation, exceeding the diffraction limit within nanowaveguides, are often limited in effectiveness by propagation losses inherent in the plasmonic material. A major thrust of plasmonic research involves overcoming propagation losses by adding gain using

either bulk-scale lasers, very high-quality gain materials, or both, Podolskiy said. His research team, however, has been looking at the effects on plasmonics of levels of gain and material quality much lower than what would be required to compensate losses.

Gain materials themselves have dispersion, which has been used in experiments involving slow and fast light, Podolskiy said. At the macroscopic scale, material dispersion comes into play; at the nanoscale, however, the shape of the material and material dispersion come into play. In studying the use of gain to control the dispersive properties of active nanoscale waveguides, the researchers observed that even relatively weak material gain, which is unable to compensate losses, is capable of producing large variations of the group velocity, enabling generation of slow and fast light

on the nanoscale. The interplay between material- and geometry-induced modal dispersion appears to be the factor that enables such strong group-velocity modulation even when the material dispersion is relatively weak.

“It turns out that to moderate group velocity, only a slight change in material dispersion is required,” Podolskiy said. “So you don’t need high-quality gain materials; moderate quality is sufficient.”

His research team has developed and demonstrated a mathematical model for modulating group velocity of light from slow to superluminal, in excess of the diffraction limit, in subwavelength waveguides (see figure). ¹ Podolskiy described the basic system as a nonhomogeneous waveguide structure. “With nondispersive materials, only the geometric effects come into play. If you decrease size, the group velocity goes to infinity. With dis-

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