Subwavelength Silica Wires Display <0.1 dB/mm Optical Loss
Researchers at Harvard University in Cambridge, Mass., Zhejiang University in Hangzhou, China, and Tohoku University in Sendai, Japan, have produced subwavelength-diameter silica waveguides that display optical losses of less than 0.1 dB/mm. The wires, which they reported in the Dec. 18/25 issue of Nature, may find uses in optical communications and sensing applications and as micro- and nanoscale research tools.
The scientists have developed a two-step fabrication process to create the wires. They begin by drawing wires with diameters on the order of a micron from a silicon fiber. They then draw these wires onto a sapphire taper that is heated with a methanol torch so that the temperature at the tip around which the wire is wound remains stable at approximately 2000 K. Drawing speeds of 1 to 10 mm/s yield wires tens of millimeters long with diameters as small as 50 nm.
Microscopic analysis of the waveguides fabricated in the study indicated that they display variations in diameter only on the order of 2 × 10-6 and sidewall rms roughness of less than 0.5 nm. Their tensile strength was higher than 5.5 GPa, enabling the researchers to bend the wires into rings with diameters of less than 6 µm.
The investigators calculated that the critical diameters for single-mode operation are 450 and 1100 nm for 633- and 1550-nm radiation, respectively. Measurements at these wavelengths with wires of these sizes revealed optical losses of less than 0.1 dB/mm. The researchers noted that, because some of the energy propagates evanescently outside the wire, the waveguides may be of particular interest for the development of optical sensors.
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