Nanoplasmonics deals with collective electron dynamics on the surface of metal nanostructures, which arises due to excitations called surface plasmons. The surface plasmons localize and concentrate optical energy in nanoscopic regions creating highly enhanced local optical fields. They undergo ultrafast dynamics with timescales as short as a few hundred attoseconds. Nanoplasmonics has numerous applications in science, technology, biomedicine, environmental monitoring, and defense.
Until recently, all the effects, elements, and devices in nanoplasmonics have been passive: they use external optical energy, always losing a fraction of it to heat and leakage radiation. There is an all-important need in active devices capable of generating and amplifying coherent optical fields on the nanoscale analogous to lasers and amplifiers of the conventional optics or transistors of microelectronics. Such an active device generating energy directly on the nanoscale has been spaser (surface plasmon amplification by stimulated emission of radiation), which is a quantum amplifier and generator of coherent nanolocalized fields. We will present quantum theory of spaser as an ultrafast quantum generator and amplifier of nanoplasmonic fields. We will briefly consider application of spaser to loss compensation by gain in metamaterials. We will review extensive experiments on spasers.
In perspective, the spasers will have applications as ultrafast nanoamplifiers (the same size as the field-effect transistors, they are orders of magnitude faster) for petahertz processors, nanoscale sources of coherent and intense optical fields, non-saturable nano-labels, and others.
