Lasers present the means to deliver powerful, coherent, and directional high-frequency electromagnetic energy. However, the diffraction limit of light imposes fundamental constraints on how compact such photonic devices can be and their potential for integration with electronic circuits, which are orders of magnitude smaller.

The room-temperature Plasmon Laser. (a) Schematic diagram of the plasmon laser showing a thin CdS square atop a silver substrate separated by a 5-nm MgF2 gap, where the most intense electric fields of the device reside. (b) SEM micrograph of the 45-nm-thick, 1-μm-length CdS square plasmon laser.
Surface plasmon polaritons (SPP), the collective electronic oscillations of metal-dielectric interfaces, show great promise for a new class of light source capable of reconciling photonic and electronic length scales. Furthermore, SPPs are capable of extremely strong confinement in one or two dimensions, enabling plasmon lasers to deliver intense, coherent, and directional optical energy well below the diffraction barrier.

Room-temperature plasmon laser operation below the diffraction limit demands effective cavity feedback, low metal loss, and high gain — all within a single nanoscale device. A semiconductor plasmon laser operating at room temperature has been developed. A 45-nm-thick cadmium sulphide (CdS) nanosquare atop a silver surface separated by a 5-nm-thick magnesium fluoride gap layer provides the sub-diffraction-limited mode confinement and low metal loss. Although the high-index material is only 45-nm thick, the surface plasmons of this system carry high momentum — even higher than light waves in bulk CdS or plasmonic nanowire lasers. This leads to strong feedback by total internal reflection of surface plasmons at the cavity boundaries.

The close proximity of the high-permittivity CdS square and silver surface enables modes of the CdS square to hybridize with SPPs of the metal-dielectric interface, leading to strong confinement of light in the gap region with relatively low metal loss. The coupling is extremely strong and causes a dramatic increase in the momentum with respect to the modes of the CdS square alone. As the dominant magnetic-field component of the waves is always parallel to the metal surface, these are called transverse magnetic (TM) waves. On the other hand, waves with dominant electric field parallel to the metal surface (transverse electric or TE) cannot hybridize with SPPs. Consequently, they become increasingly delocalized as the gap size decreases and are effectively pushed away from the metal surface with a corresponding decrease in momentum with respect to TE waves of the CdS square alone.

Although both wave polarizations are free to propagate in the plane, only TM waves have sufficient momentum to undergo total internal reflection and achieve the necessary feedback for lasing. Although CdS squares thicker than about 60 nm can support TE waves with sufficient momentum to undergo total internal reflection, they are scattered out of the plane more effectively than TM waves because they are delocalized from the metal surface.

This laser can be considered to be a SPASER (surface plasmons amplified by stimulated emission of radiation), since it generates plasmonic cavity eigenmodes and only emits light to the farfield as a side-effect of scattering. The current device exhibits multiple laser peaks attributed to the number of available modes in the square cavity configuration. Single-mode plasmon lasing was observed in irregularly shaped devices with lower symmetry where only a limited number of modes can undergo total internal reflection.

The intense fields that are generated and sustained in the gap region make such lasers highly useful for investigating light-matter interactions. Namely, an emitter placed in this gap region is expected to interact strongly with the laser light. Such light-matter interaction enhancements are also observable to a lesser extent in the CdS gain medium; under weak pumping, the CdS bandedge transitions of this plasmon laser show a spontaneous emission lifetime reduced by a factor of 14.

This work was done by Ren-Min Ma, Rupert F. Oulton, Volker J. Sorger, and Guy Bartal of the National Science Foundation Nanoscale Science and Engineering Centre, University of California, Berkeley; and Xiang Zhang of Lawrence Berkeley National Laboratory for the Air Force Office of Scientific Research. AFOSR-0001

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Room-Temperature Sub-Diffraction-Limited Plasmon Laser

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This article first appeared in the August, 2011 issue of Defense Tech Briefs Magazine.

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