Air-hole collapse and mode transitions in microstructured fiber photonic wires

We demonstrate robust, low bend loss photonic wires made from air-clad microstructured "grapefruit" fiber. By tapering the fiber and collapsing the air-holes, the guided mode evolves from being fully embedded within the fiber to a spatially-localized evanescent regime a few millimeters in length, where the mode is strongly influenced by the external environment. We show that in the embedded regime there is negligible loss when the taper is immersed in index-matching fluid, while in the evanescent regime the attenuation increases by over 35 dB. Furthermore, we show that an 11 microm wire in the embedded regime can be bent to a radius as small as 95 microm with bend-loss of 0.03 dB in a 500 nm band. The combination of spatial localization, strong dependence on the external environment andsmall bend radius make the device ideally suited for bio-photonic sensing.