# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 6 Electrically Pumped Metal Nanocavity Lasers  
Semiconductor-based optical devices and their integrated circuits are fundamentally limited in terms of their sizes because the diffraction of light plays a dominant role when the size of an optical device becomes comparable to, or smaller than, the wavelength of light in the material. Surface plasmon polaritons (SPPs), surface-bound electromagnetic waves supported by the metal dielectric interface, can offer an exclusive solution that addresses the diffraction of light by concentrating electromagnetic fields in nanoscale regions that are far smaller than the wavelength of the material [38]. For more than a decade, researchers have made tremendous efforts to propose new designs of essential plasmonic elements, and they have experimentally demonstrated the feasibility of their respective functions [39]. In particular, plasmonic light sources, together with other key components including plasmonic waveguides, modulators, and detectors, have been the mainstream research of nanophotonics. However, despite the numerous investigations on the optical properties of plasmonic nanocavities, the realization of an electrically driven plasmonic source remains a great challenge because there have been limited schemes satisfying the conditions for simultaneous carrier injection, SPP generation, and confinement with minimized ohmic loss in a subwavelength volume.  
To circumvent these difficulties, several strategies have been proposed. For example, the metal encapsulation of a vertical semiconductor nanostructure is a successful scheme [40; 41]. Figure 9a shows the first electrically driven metal cavity nanolaser, the gold-finger nanocavity laser [40]. The nanocavity comprises an inner \(n\)-InP/\(i\)-InGaAs/\(p\)-InP heterostructure vertical nanopillar, a thin nitride isolation layer on the sides of the pillar, and an outer gold cavity encapsulating the entire semiconductor/insulator structure. The vertically injected carriers are recombined in the intrinsic InGaAs region where a slight index contrast between InP and InGaAs allows for a resonant cavity mode. In the experiment, lasing oscillations were observed at low-threshold currents of 3.5 and 6.0 \(\upmu\)A at cryogenic temperatures of 10 and 77 K, respectively. However, lasing operation failed at room temperature due to the increased metallic loss, reduced material gain, and decreased \(Q\)-factor. In this study, the author did not specifically identify the origins of the observed lasing modes (e.g., plasmonic or photonic lasing mode). It remains questionable whether it is the first electrically driven plasmonic laser, although it is clearly the first metallic nanocavity nanolaser. In their follow-up paper, the same authors reported modified cavity structures and realized plasmonic lasing [41]. In Figure 9b, the rectangular semiconductor active core with a length of few \(\upmu\)m (indicated as \(l\)) is employed. This plasmonic waveguide cavity supports Fabry-Perot-type resonant modes in the axial direction. In addition, the reduced width of 130 nm in the rectangular core (indicated as \(d\)) only allowed a transverse magnetic (TM) mode (or gap plasmon mode) in a deep sub-wavelength region. The optical microscope image shows the plasmonic lasing mode, revealing the subwavelength confinement of SPPs. The lasing threshold current was measured as \(\sim\)40 \(\upmu\)A at a low temperature of 78 K, which was an order of magnitude higher than that of the gold-finger nanocavity laser. It is believed that the increased metal/insulator or metal/semiconductor interfaces, where the light\(-\)matter interaction strongly occurred, inevitably produced optical loss and increased the lasing threshold.