# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 7 Metal-Waveguide-Integrated Nano-LEDs for Surface Plasmon Polariton (SPP) Sources  
Another successful scheme for generating SPPs is coupling a light-emitting nanomaterial to passive plasmonic components [42; 43]. In this scheme, the nanomaterial is electrically doped and forms a spatially localized \(p\)-\(n\) or \(p\)-\(i\)-\(n\) junction so that it can serve as a current-injectable nanoscale LED. A passive plasmonic component, such as a metal waveguide, is then coupled to the nano-LED in such a way that a major portion of the generated photons can interact with bound charges in metal. This process naturally helps to convert the photons into SPPs. Representative examples are shown in Figure 10. In Figure 10a, a bottom-up synthesized core-shell GaAs NW with a radial \(p\)-\(i\)-\(n\) junction is connected to \(n\)- and \(p\)-type contacts [42]. Two metallic strip waveguides, making a right-angle to each other at their ends, are coupled to the NW LED by covering the junction area where the carrier recombination and light generation occur. When the NW LED was electrically pumped, linearly polarized light emission parallel to the direction of each waveguide was observed at the end of the waveguide, which is a clear demonstration of electrically driven SPP generation and its propagation. Figure 9: Electrically driven metal-encapsulated nanocavity nanolasers. (**a**) Metallic nanocavity laser. Schematic showing Au-encapsulated InP/InGaAs/InP pillar cavity structure (left). SEM image of fabricated laser (right, top) and IR camera image of lasing mode (right, bottom) [40]. Figure reproduced with permission from Springer Nature Limited. (**b**) Electrically driven metal-insulator-metal (MIM) sub-wavelength plasmonic waveguide lasers. Schematic of the device layer structure (top). A vertically \(p\)-\(i\)-\(n\)-doped rectangular InP/InGaAs/InP semiconductor pillar coated with silicon nitride is contact-metallized and encapsulated in silver. SEM image of vertical rectangular pillar before silicon nitride coating (left, bottom). IR camera image of the lasing mode for above threshold current of 320 \(\upmu\)A (right, bottom) [41]. Figure reproduced with permission from Optical Society of America. Top-down fabricated active nanomaterials can also be employed for nano-LEDs, provided that the materials are properly pumped through current injection [43]. The SEM images in Figure 10b show that a top-down fabricated GaAs NW with a vertical _p-n_ junction is connected to metal contacts. One of the contacts (_p_-contact) is connected with two individual metal strips. While the NW is located at the center of the two ends of the strips, the coupled metal strips extend along the direction of the NW axis and form a double-strip plasmonic waveguide. Since the waveguide supports several propagating waveguide modes, the light emitted from the NW LED can couple to the modes and propagate. The polarization-dependent EL measurement, again, confirms the SPP generation and propagation. The metal-waveguide-integrated nano-LED, however, suffers from several limitations, mostly originating from the coupling geometry and ohmic loss. Since the SPP generation strongly depends on how efficiently the light from the nano-LED is coupled to the metal waveguide, the coupling geometry should be judiciously designed to maximize the interaction between the light and bound charges in metal. For example, the light coupling efficiency from the structures in Figure 10b was estimated at only few percentage points, which shows a clear limitation of such a scheme. In addition, the passive plasmonic components should support various plasmonic modes to which the light can couple, but at the same time, they should be designed to minimize the metallic loss. These two requirements are often contradictory to each other because increasing the size of the plasmonic waveguide can not only increase the number of plasmonic modes, but also cause more metallic loss.