The authors report the experimental observation of a critical diameter (CD) of III-V compound semiconductor epitaxial nanowires (NWs) grown on lattice-mismatched substrates using Au-catalyzed vapor-liquid-solid growth. The CD is found to be inversely proportional to the lattice mismatch. NWs with well-aligned orientation are synthesized with catalysts smaller than the CD. Well-aligned InP NWs grown on a Si substrate exhibit a record low photoluminescence linewidth (5.1meV) and a large blueshift (173meV) from the InP band gap energy due to quantization. Well-aligned InAs NWs grown on a Si substrate are also demonstrated.

We report a catalyst-free, self-assembled growth mode generating single-crystal wurtzite phase ultrasharp GaAs/ AlGaAs nanoneedles on both GaAs and Si substrates via low-temperature metal-organic chemical vapor deposition. The needles exhibit record-narrow tip diameters of 2-4 nm wide and sharp 6°-9°taper angles. The length is dependent on growth time and up to 3-4 m nanoneedles are attained. The structures do not exhibit twinning defects, contrary to typical GaAs nanowires grown by vapor-liquid-solid catalyzed growth. AlGaAs layered nanoneedle structures are also demonstrated.

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A novel long-wavelength vertical-cavity surface-emitting laser (VCSEL) structure based on a subwavelength high-contrast grating (HCG) as the output mirror has been realized. By design, these devices are highly polarization stable, are single mode at large apertures, and solve the VCSEL-mirror problem at long wavelengths in an elegant way. With cost-effective mass fabrication in mind, the top HCG reflector consists of amorphous silicon on isolator (amorphous silica). The single-mode laser emission is tailored to be around 1320-nm wavelength, targeting applications in high-speed optical data transmission, particularly those for passive optical networks. We report single-mode emission for devices with apertures as large as 11 m operating in continuous wave with output powers in excess of 0.4 mW. Pulsed operation with output powers up to 4 mW at room temperature is demonstrated as well. This is the first electrically pumped VCSEL structure realized in this wavelength regime utilizing an HCG mirror.

Optical-fi ber-based, hollow-core waveguides (HCWs) have opened up many new applications in laser surgery, gas sensors, and non-linear optics. Chip-scale HCWs are desirable because they are compact, light-weight and can be integrated with other devices into systems-on-a-chip. However, their progress has been hindered by the lack of a low loss waveguide architecture. Here, a completely new waveguiding concept is demonstrated using two planar, parallel, silicon-on-insulator wafers with high-contrast subwavelength gratings to refl ect light in-between. We report a record low optical loss of 0.37 dB/cm for a 9-µ m waveguide, mode-matched to a single mode fi ber. Two-dimensional light confi nement is experimentally realized without sidewalls in the HCWs, which is promising for ultrafast sensing response with nearly instantaneous fl ow of gases or fl uids. This unique waveguide geometry establishes an entirely new scheme for low-cost chip-scale sensor arrays and lab-on-a-chip applications.Keywords: hollow-core waveguide; high-contrast subwavelength grating; gas-sensing; silicon photonics.Conventional light guiding is achieved in a geometry where a high-refractive-index core is surrounded by a low-refractiveindex cladding. In the past decade, the opposite schemeguiding light through a low-index core surrounded by high-index cladding layers has emerged as a new tool for applications. In particular, hollow-core optical waveguides/fi bers are desirable for gas sensors and gas-based non-linear optics because of the increased lengths for light-matter interaction [1,2] , and for laser surgery to guide light in mid-to far-infrared wavelength regimes that lack low-absorption materials [3,4] . Chip-scale hollow-core waveguides (HCWs) are desirable because they enable cost-effective manufacturing of on-chip systems with the potential to monolithically integrate light sources, detectors and electronics. Chip-scale HCWs have been reported using metal [5] , distributed Bragg refl ectors [6,7] and anti-resonant refl ection layers [8,9] as the guiding refl ectors. However, their use is limited due to large optical losses because of insuffi cient refl ection.A hollow-core waveguide is best understood by the ray optics model, with an optical beam guided by zig-zag refl ections from the guiding walls [6,7,10] . The propagation loss is strongly dependent on the refl ectivity of the walls [6,7,10] due to the large number of refl ections for a given length (the number of refl ections is L λ /2 d 2 , where L is the length of the waveguide, d is the waveguide core height and λ is the wavelength of light used). Low losses can be obtained for HCW with core size in the tens of µ m [7] . However, a core of this size does not lend itself to a low bending loss or effi cient fi ber coupling. High-contrast subwavelength gratings (HCGs) have been found to offer very high refl ection for surface-normal incident light [11 -14] . Recently, we reported numerical simulation results of a one-dimensional (1D) waveguide guided by two parallel layers...