Nanowires, nanoneedles and sub-wavelength structures: Enabling a new era of optoelectronic technologies

We review the progress of synthesis, characterization and properties of III-V compound semiconductor nanowires and nanoneedles grown on lattice-mismatched substrates and a new family of optoelectronic devices based on high-contrast grating. There are intense interests in semiconductor structures with dimensions at the two ends of the nanometer regime. Materials with size on the order of a few nanometers, close to the de Broglie wavelength, are critical because their physical properties may be altered by its dimensions. In particular, they offer a tremendous opportunity to engineer properties of active materials. On the other hand, materials with dimensions on the order of one hundred nanometer scale are of enormous interests because they open up a new window to re-examine the wave guiding properties in sub-wavelength structures. In this talk, I will discuss recent progress in these two areas. We will discuss the synthesis and characteristics of III-V compound nanowires and nanoneedles that are monolithically grown on Si and sapphire despite of a very large lattice mismatches (over 50% for the latter) [1-4]. The structures show excellent crystalline and optical quality. The nanowires are promising for ultra low threshold lasers and high efficiency solar cells. The nanoneedles, on the other hand, have an extremely sharp tip of a few atoms in diameter. They are shaped in hexagonal pyramid and have a sharp 6-9deg angles. Core-shell GaAs/AIGaAs and GaAs/lnGaAs/GaAs layered nanoneedles are demonstrated with bright photoluminescence. They will find applications for field emission tips, atomic force microscopic probe tips and atto-liter liquid delivery. For the hundred-nm regime structures, we recently discovered a novel high index contrast subwavelength grating (HCG) structure, which showed an unprecedented effects on optical wave guiding and reflection properties [5-9]. We showed a single grating with an exceedingly large bandwidth (Deltalambda/lambda=35%) at high reflectivity (99.5%) for surfacenormal incident light. The broad bandwidth was accompanied by a very large tolerance where key grating parameters can vary by 50% to still yield VCSEL with similar light-current characteristics. By varying the grating parameters, we show a single layer of uniform grating exhibits high-Q (14,000) resonator characteristics - again for surface-normal emission. The HCG grating brings guided-wave optics to a totally untrespassed regime and will find many useful applications in lasers, filters, waveguides, sensors and detectors.