Nanolasers employing epitaxial plasmonic layers

Summary form only given. Miniaturization of semiconductor lasers holds the key to the development of compact, low-threshold, and fast coherent on-chip light sources/amplifiers, which are critically important for emerging applications in nanophotonics, integrated optics, and information technology. However, on-chip integration of nanoscale electronic components with conventional semiconductor lasers utilizing dielectric optical cavities is impeded by the diffraction limit-i.e., ~(λ/2n)
³
for three-dimensional (3D) cavities, where λ is the free-space wavelength and n is the refractive index of the dielectric. The recent advent of nanoplasmonics based on metallodielectric structures has led to the design of optical components and optoelectronic devices in the deep subwavelength regime. In particular, a new class of lasers based on surface plasmon amplification by stimulated emission of radiation (SPASER) has recently been proposed and experimentally demonstrated (16-19). In the SPASER operation, surface plasmons excited in noble-metal structures adjacent to gain media dramatically increase the optical mode density, shrink the optical mode volume, and provide the necessary feedback mechanism. Among the available plasmonic cavity materials in the visible and near-infrared ranges, silver (Ag) is the best choice due to minimal plasmonic damping. However, so far, most plasmonic devices are based on granular polycrystalline Ag films where strong surface roughness and grain boundaries lead to strong scattering losses of surface plasmon polaritons (SPPs). Thus, atomically smooth or single-crystalline plasmonic structures are desirable building blocks for low-loss applications. In polycrystalline metallic materials the lasing threshold of plasmonic nanolasers remains impractically high and the continuous-wave (CW) operation of a plasmonic laser has yet to be demonstrated. Having developed epitaxially grown, atomically smooth Ag films as a new plasmonic platform, we report a SPASER under CW operation with an ultralow lasing threshold at liquid nitrogen temperature and mode volume well below the 3D diffraction limit. The device is based on the plasmonic nanocavity formed between epitaxial Ag film and a single nanorod consisting of a gallium nitride (GaN) shell and an indium gallium nitride (InGaN) core acting as gain medium. The nanolaser emits in the green spectral region with perfectly polarized far-field radiation at the main lasing mode. In comparison with other semiconductor gain media, the large gain coefficient of InGaN plays a critical role in the ultralow lasing threshold observed for our experiment with a 3D subdiffraction cavity. The development of atomically smooth epitaxial Ag on Si as a new platform for plasmonics not only allows us to demonstrate the SPASER-enabled nanolaser, it should also promote the development of monolithically integrated plasmonics and Si-based electronics on a single platform.