14 February 2007 Semiclassical dynamics of light beams supported by adiabatically tapered nanofibers
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Abstract
An optical microfiber with the diameter significantly less than the radiation wavelength ~ 1 micron is often called a nanofiber (NF). The fundamental mode of a NF consists primarily of an evanescent field propagating in the ambient medium outside a NF. Any deformation of a NF changes the evanescent field. If a NF is thin enough, even a very small deformation may cause dramatic changes of the evanescent field structure. The simplest types of deformation of a uniform NF are bending and tapering. The structure of evanescent field and optics of radiation loss in bent uniform microfibers is understood quite well. It is determined by an effective potential barrier terminated by a caustic surface, which separates the tunneling and classically allowed regions. Tunneling through the barrier determines the radiation loss. Alternatively, for an adiabatically tapered microfiber, called a nanotaper (NT), a similar potential barrier of finite width cannot be introduced. Instead, the radiation loss in a NT takes place in small neighborhood of focal circumferences of the evanescent field, while a NT is lossless elsewhere. More specifically, for a NT, the mentioned caustic surface becomes complex and can intersect real space along certain lines only. These lines are the focal circumferences where the radiating modes and the guiding mode are split off. As examples, conical and biconical NTs with characteristic shapes are considered. The theoretically predicted interference between the guiding and radiating components of the evanescent field are confirmed by the beam propagation method (BPM) numerical modeling. The derived analytical expressions for radiation loss are in a good agreement with BPM calculations. Finally, a simple estimate formula for radiation loss of a NT is suggested.
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M. Sumetsky, M. Sumetsky, } "Semiclassical dynamics of light beams supported by adiabatically tapered nanofibers", Proc. SPIE 6452, Laser Resonators and Beam Control IX, 64520R (14 February 2007); doi: 10.1117/12.714258; https://doi.org/10.1117/12.714258
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