Silica nanowires

Recent advances in the fabrication and manipulation of sub-wavelength optical fibers provide new methods for building chemical and biological sensors, generating supercontinuum light by nonlinear pulse propagation, and constructing microphotonic components and devices.

We propose to use bent silica wires with nanometric diameters to guide light as optical waveguide bend. We bend silica wires with scanning tunneling microscope probes under an optical microscope, and wire bends with bending radius smaller than 5 m are obtained. Light from a He-Ne laser is launched into and guided through the wire bends, measured bending loss of a single bend is on the order of 1 dB. Brief introductions to the optical wave guiding and elastic bending properties of silica wires are also provided. Comparing with waveguide bends based on photonic bandgap structures, the waveguide bends from silica nanometric wires show advantages of simple structure, small overall size, easy fabrication and wide useful spectral range, which make them potentially useful in the miniaturization of photonic devices.

Silica waveguides with diameters larger than the wavelength of transmitted light are widely used in optical communications, sensors and other applications. Minimizing the width of the waveguides is desirable for photonic device applications, but the fabrication of low-loss optical waveguides with subwavelength diameters remains challenging because of strict requirements on surface roughness and diameter uniformity. Here we report the fabrication of subwavelength-diameter silica wires for use as low-loss optical waveguides within the visible to near-infrared spectral range. We use a two-step drawing process to fabricate long free-standing silica wires with diameters down to 50 nm that show surface smoothness at the atomic level together with uniformity of diameter. Light can be launched into these wires by optical evanescent coupling. The wires allow single-mode operation, and have an optical loss of less than 0.1 dB/mm. We believe that these wires provide promising building blocks for future microphotonic devices with subwavelength-width structures.

We use tapered silica fibers to inject laser light into ZnO nanowires with diameters around 250 nm to study their waveguiding properties. We find that high-order waveguide modes are frequently excited and carry significant intensity at the wire surface. Numerical simulations reproduce the experimental observations and indicate a coupling efficiency between silica and ZnO nanowires of 50%. Experimentally, we find an emission angle from the ZnO nanowires of about 90, which is in agreement with the simulations.

Based on the exact solutions of Maxwells equations, we have studied the basic theoretical properties of submicron and nano-diameter air-cladding silica-wire waveguides. The single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of micrometer to over 1 millimeter have been fabricated. SEM examination shows that these wires have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling, and guiding light through a bent wire has also been demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.

Subwavelength-diameter silica wires fabricated using a taper-drawing approach exhibit excellent diameter uniformity and atomic-level smoothness, making them suitable for low-loss optical wave guiding from the UV to the near-infrared. Such air-clad silica wires can be used as single-mode waveguides; depending on wavelength and wire diameter, they either tightly confine the optical fields or leave a certain amount of guided energy outside the wire in the form of evanescent waves. Using these wire waveguides as building blocks we assembled microscale optical components such as linear waveguides, waveguide bends and branch couplers on a low-index, non- dissipative silica aerogel substrate. These components are much smaller than comparable existing devices and have low optical loss, indicating that the wire-assembly technique presented here has great potential for developing microphotonics devices for future applications in a variety of fields such as optical communication, optical sensing and high-density optical integration. Keywords: Subwavelength, silica, nanowire, microphotonics, nanophotonics, optical components

Silica nanowires provide strong mode confinement in a cylindrical silica-core/ air-cladding geometry and serve a model system for studying nonlinear propagation of short optical pulses inside fibers. We report on the fiber diameter dependence of the supercontinuum generated by femtosecond laser pulses in silica fiber tapers with average diameters in the range of 200 nm to 1200 nm. We observe a strong diameter-dependence of the spectral broadening, which can be attributed to the fibers diameter-dependent dispersion and nonlinearity. The short interaction length (less than 20 mm) and the low energy threshold for supercontinuum generation (about 1 nJ) make tapered fibers with diameters between 400 nm and 800 nm an ideal source of coherent white-light source in nanophotonics.

Low-loss optical wave guiding along a subwavelength-diameter silica wire leaves a large amount of the guided field outside the solid core as evanescent wave and at the same time maintains the coherence of the light, making it possible to develop sensitive and miniaturized optical sensors for physical, chemical and biological applications. Here we introduce, for the first time to our knowledge, a scheme to develop optical sensors based on evanescent-wave- guiding properties of subwavelength-diameter wires. Optical wave guiding properties of these wires that are pertinent to a waveguide sensor, such as single-mode condition, evanescent field, Poynting vector and optical loss are investigated. By measuring the phase shift of the guided light, we propose a Mach-Zehnder-type sensor assembled with two silica wires. The sensitivity and size of the sensor are also estimated, which shows that, subwavelength- diameter silica wires are promising for developing optical sensors with high sensitivity and small size.

We have fabricated silica fibers with diameters less than one micrometer and molecularly smooth side walls. We modeled the waveguide properties and demonstrated low-loss light propagation over tens of millimeters, sufficient for microphotonic device applications. We manipulated silica nanowires into geometries to demonstrate waveguiding around tight bends as small as 5 micrometers, evanescent coupling, and wavelength filtering as a ring resonator. The linear waveguiding properties produced by the large index contrast between silica and air yield a tight confinement of the mode, which, when combined with the high electric field intensity in an ultrashort laser pulse, can produce significant nonlinear effects over lengths of about one millimeter. We studied nonlinear optical properties by observing the spectral broadening as a probe for the diameter-dependent nonlinearity of the silica nanowire. Our measurements confirmed the dependence of the generated spectra on the theoretically calculated effective nonlinearity and diameter-dependent dispersion. Our results reveal a diameter range for silica nanowires with an enhanced nonlinearity which may be employed in nonlinear devices. We fabricate a nonlinear Sagnac interferometer using silica nanowires for optical switching and discuss possibilities for optical logic. Our results confirm light-by-light modulation, with pulse energies less than a couple of nJ. Combining top-down and bottom-up fabrication techniques, we use tapered silica fibers to couple light directly into the waveguiding modes of ZnO nanowires. We experimentally confirm simulations for the coupling efficiencies and propagation of higher order modes. We also excite ZnO nanowires with ultrashort laser pulses and compare the spectrum transmitted along the nanowire waveguide with the spectrum at the excitation spot. Our results show a shifting of the band edge that indicates a local heating of the nanowire by a few hundred degrees, which is confirmed by finite-element simulation. Finally, we discuss the outlook for nanoscale nonlinear optical devices.

Subwavelength- and nanometer-diameter glass nanofibers have been fabricated using a high-temperature taper-drawing process. As-drawn nanofibers show extraordinary uniformities in terms of diameter variation and surface roughness and are suitable for single-mode optical wave guiding. Measured optical losses of these nanofibers are typically below 0.1 dB/mm. Photonic devices such as linear waveguides, optical couplers and microrings assembled with nanofibers are also demonstrated. Our results show that taper-drawn glass nanofibers are promising building blocks for micro- and nano-scale photonic devices.