H. K. Tyagi

Polarization-dependent coupling to plasmon modes on submicron gold wire in photonic crystal fiber

We present experimental results on coupling to surface plasmon modes on gold nanowires selectively introduced into polarization-maintaining photonic crystal fibers. Highly polarization- and wavelength-dependent transmission is observed. In one sample 24.5 mm long, the transmission on and off resonance differs by as much as 45 dB. Near-field optical images of the light emerging from such a gold-filled fiber show light guided on the wire at surface plasmon resonances. Finite element simulations are in good agreement with the experimental results. These gold-filled fibers can be potentially used as in-fiber wavelength-dependent filters and polarizers and as near-field tips for sub-wavelength-scale imaging.

Pressure-assisted melt-filling and optical characterization of Au nano-wires in microstructured fibers

We report a novel splicing-based pressure-assisted melt-filling technique for creating metallic nanowires in hollow channels in microstructured silica fibers. Wires with diameters as small as 120 nm (typical aspect ration 50:1) could be realized at a filling pressure of 300 bar. As an example we investigate a conventional single-mode step-index fiber with a parallel gold nanowire (wire diameter 510 nm) running next to the core. Optical transmission spectra show dips at wavelengths where guided surface plasmon modes on the nanowire phase match to the glass core mode. By monitoring the side-scattered light at narrow breaks in the nanowire, the loss could be estimated. Values as low as 0.7 dB/mm were measured at resonance, corresponding to those of an ultra-long-range eigenmode of the glass-core/nanowire system. By thermal treatment the hollow channel could be collapsed controllably, permitting creation of a conical gold nanowire, the optical properties of which could be monitored by side-scattering. The reproducibility of the technique and the high optical quality of the wires suggest applications in fields such as nonlinear plasmonics, near-field scanning optical microscope tips, cylindrical polarizers, optical sensing and telecommunications.

Plasmon resonances on gold nanowires directly drawn in a step-index fiber

High quality metallic wires (diameters down to 260nm) are fabricated using direct fiber drawing from a gold-filled cane. Measurements show coupling of light from the glass-core to plasmonic resonances on the wire, causing dips in the transmission at specific wavelengths.

Optofluidic refractive-index sensor in step-index fiber with parallel hollow micro-channel.

We present a simple refractive index sensor based on a step-index fiber with a hollow micro-channel running parallel to its core. This channel becomes waveguiding when filled with a liquid of index greater than silica, causing sharp dips to appear in the transmission spectrum at wavelengths where the glass-core mode phase-matches to a mode of the liquid-core. The sensitivity of the dip-wavelengths to changes in liquid refractive index is quantified and the results used to study the dynamic flow characteristics of fluids in narrow channels. Potential applications of this fiber microstructure include measuring the optical properties of liquids, refractive index sensing, biophotonics and studies of fluid dynamics on the nanoscale.

Transmission properties of selectively gold-filled polarization-maintaining PCF

We report on the optical properties of a polarization-preserving PCF in which two enlarged hollow channels on opposite sides of the core are filled with gold. Surface plasmon resonances and intriguing polarisation effects are observed.

Plasmon resonances on gold nanowires directly drawn in step-index fiber

High quality metallic wires (diameters down to 260nm) are fabricated using direct fiber drawing from a gold-filled cane. Measurements show coupling of light from the glass-core to plasmonic resonances on the wire, causing dips in the transmission at specific wavelengths.

Selective excitation of guided surface plasmons on uniform and conicallytapered Au nanowires

We report the precise excitation of guided surface plasmon modes (SPMs) on Au nanowires placed adjacent to a single-mode fibre core (20 mol% Ge, diameter 1.1 µm). The nanowires were produced by pressure-assisted melt-filling of a hollow channel (diameter 510 nm) running parallel to the core with a centre-centre core-wire spacing of 3.6 µm [1, 2]. An unintended byproduct of the fabrication technique was frequent breaks a few µm wide along the nanowires, every 100 µm or so. As a result, when broad-band light was launched into the core, bright red-coloured scattering spots could be seen at the end of each nanowire (Fig. 1(a)). These are caused by radiation from an m = 2 SPM on the wire [2], excited by phase-matched coupling at a particular resonant wavelength. The coupling between core and nanowire is, however, unbalanced because the loss on the nanowire is many orders of magnitude higher than in the glass core, and greater than the coupling constant (estimated value: 3.5 mm−1). As a result the supermodes are highly asymmetrical, one of them (with field predominantly in the wire) having extremely high loss and the other (field mainly in the core) very low loss. The scattered light from the spots was collected using a multimode fibre and delivered to an optical spectrum analyzer. The data was then used to work out the attenuation rate of the low-loss supermode (Figs. 1(b)&(c)), which at 7 dB/cm centre-band is remarkably low for a plasmonic device. The attenuation rate can of course be adjusted by varying the geometry of the core and channel.

From plasmonics to supercontinuum generation: Subwavelength scale devices based on hybrid photonic crystal fibers

Photonic crystal fibers (PCFs) consist of arrays of micrometer-size hollow channels extending along the entire fiber length. A new approach to modify the optical properties and to improve the design flexibility of such fibers is to fill the air holes of the array by with different materials, realizing all-solid hybrid fiber structures. In this talk, recent results on silica PCF filled with noble metals, semiconductors or soft glasses are presented. Depending on the actual material being filled into the holes, we observed plasmonic excitations, Mie-resonances and photonic band gaps.

Metal Nanowire Arrays in Photonic Crystal Fibres

Nanowire arrays are produced by pumping molten metal into the holes of silica PCF. Distinct dips in the transmitted spectra coincide with the coupling of the core-guided light to leaky plasmonic resonances in the nanowires.