Showing papers by "Wei Fang published in 2011"

On-chip three-dimensional high-Q microcavities fabricated by femtosecond laser direct writing

Abstract: We report on the fabrication of three-dimensional (3D) high-Q whispering gallery microcavities on a fused silica chip by femtosecond laser microfabriction, enabled by the 3D nature of femtosecond laser direct writing The processing mainly consists of formation of freestanding microdisks by femtosecond laser direct writing and subsequent wet chemical etching CO2 laser annealing is followed to smooth the microcavity surface Microcavities with arbitrary tilting angle, lateral and vertical positioning are demonstrated, and the quality (Q)-factor of a typical microcavity is measured to be up to 107x10^6, which is currently limited by the low spatial resolution of motion stage used during the laser patterning and can be improved with motion stages of higher resolutions

Wavelength-scale deformed microdisk lasers

Abstract: We investigate lasing and output directionality of lima\ifmmode \mbox{\c{c}}\else \c{c}\fi{}on-shaped microdisk lasers of dimensions comparable to the emission wavelength. The far-field patterns are shown to differ between lasing modes, unlike in large cavities where lasing modes exhibit universal emission directionality determined by chaotic ray dynamics. Unidirectional emission is obtained for certain modes in the wavelength-scale cavities. It results from weak coupling of nearly isotropic high-quality resonances to anisotropic low-quality resonances, combined with chiral symmetry breaking of clockwise and counterclockwise propagating waves. The latter is described by an extended ray dynamics which includes the Goos-H\"anchen shift and the Fresnel filtering. Mode hybridization and wave effects in open cavities make it possible to control the output properties of individual lasing modes in wavelength-scale lasers.

Broad spectral response in composition-graded CdSSe single nanowires via waveguiding excitation

Abstract: We report broad spectral response in composition-graded CdSSe single nanowires (NWs) via waveguiding excitation The excitation light is coupled to CdSSe single nanowires via silica fiber tapers The guided photons in the nanowire with energies ranging from 174 to 244 eV can be efficiently captured at different sites along the NW length Compared to single-composition nanowires, the composition-graded CdSSe nanowires exhibit much broader spectral response range, while retaining the detection limit down to sub-pW level Utilizing the photoconductive quenching effect, the spectral range can be further extended beyond 15 μm with detection limit down to sub-nW level at room temperature

Longitudinal Lorentz force on a subwavelength-diameter optical fiber

Abstract: We analyze the longitudinal Lorentz forces that a propagating continuous-wave light exerts on a subwavelength-diameter optical fiber. Our theoretical results show that, during the propagating process, the guided light exerts no net time-averaged force on the fiber. Via numerical simulation, we find a significant overall pull force of 0.4 pN/mW acting on a 450-nm-diam fiber tip at a wavelength of 980 nm due to the scattering of the end face and a calculated force distribution reveals the feature of a near-field accumulation. Our results may be helpful to the configuration of optomechanical components or devices based on these fibers.

Large defect-induced sub-bandgap photoresponse in semiconductor nanowires via waveguiding excitation

Abstract: A large defect-induced sub-bandgap photoresponse over a broad spectral range is observed in semiconductor single nanowires via optical waveguiding excitation. Using an evanescent coupling technique, the excitation sub-bandgap light is efficiently transferred from a silica fiber taper into a CdS single nanowire (bandgap ~ 2.46 eV), and is tightly confined and guided through the whole length of the nanowire, which significantly enhances the light–defect interaction compared with the conventional irradiation excitation scheme. Under 593 nm wavelength (~2.09 eV) waveguiding excitation with an input power of 10 pW level at room temperature, a 350 nm diameter 150 µm-length CdS nanowire shows a responsivity of 250 A W − 1, offering a sub-bandgap photosensitivity five orders of magnitude larger than by irradiation excitation. These results may open opportunities for noninvasive characterization of defect states in semiconductor nanowires, as well as for enabling novel sub-bandgap nanowire devices.