Coherent and tunable light radiation from nanoscale surface plasmons array via an exotic Smith-Purcell effect.

We demonstrate that surface plasmons on a nanoscale metallic array can be transformed into radiation waves via an exotic Smith-Purcell effect. Although the radiation frequency and direction satisfy the Smith-Purcell relation, it is coherent radiation with directions specified, which is essentially different from the ordinary Smith-Purcell radiation. Its radiation spectral density is an order of magnitude higher. By adjusting the material and structure of the array, the radiation frequency can be tuned from an infrared to ultraviolet region. Its remarkable advantages in intensity, coherence, tunability, and miniature size indicate new prospects in developing nanoscale light sources and related techniques.