Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-19-11-10326
Timestamp: 2019-04-24 19:09:15+00:00

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We report the fabrication and characterization of a cylindrically symmetric fiber structure that possesses significant and thermodynamically stable second-order nonlinearity. Such fiber structure is produced through nanoscale self-assembly of nonlinear molecules on a silica fiber taper and possesses full rotational symmetry. Despite its highly symmetric configuration, we observed significant second harmonic generation (SHG) and obtained good agreement between experimental results and theoretical predictions.
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Fig. 1 Nonlinear coating on a silica fiber. (a) The schematic structure of nonlinear fiber with full cylindrical symmetry. The radially aligned nonlinear molecules provide a second-order susceptibility tensor dominated by the χ r r r ( 2 ) component. (b) The AFM image of 10-bilayer nonlinear self-assembled film on a planar glass substrate shows highly uniform coating. (c) SEM image of a nonlinear fiber with 10-bilayer coating. (d) The transmission loss of a coated taper at wavelength of 1294 nm was recorded for every 10-bilayer film deposition.
Fig. 2 Schematic diagram of the experimental setup. An infrared, nanosecond-pulsed pump beam obtained from an OPO system is coupled into a multimode fiber. The beam propagates through a nonlinear taper, and interacts with the second-order nonlinear molecules on the taper surface. The output SHG signal is collimated, filtered and finally detected by a PMT.
Fig. 3 The THG (at 431 nm) and SHG (at 647 nm) spectra from a bare and a 10-bilayer coated taper, excited by pump pulse energy of 2 μJ at 1294 nm. The bare fiber curve is shifted both vertically and horizontally for clarity. The lower inset confirms a cubic dependency of the THG emitted from the bare taper. With nonlinear coating, we observe an increase in SHG power and decreased THG component due to the film absorption at 431 nm as indicated in the upper inset.
Fig. 4 SHG power as a function of pump pulse energy. The measurement was taken for a 3.8-µm-radius taper with 0, 10, 20, 30 and 40 bilayers of nonlinear film. Each data point is the result of an average of over 100 measurement data and the error bar is smaller than the symbol. The lines are quadratic fits to the data.
Fig. 5 Dependence of SHG on taper radius. The log-log plot shows the 1/a 4 behavior of SHG power with different taper radius. All samples are coated with 10-bilayer films and excited with pulse energy of 2 μJ. The error bars in the x axis indicate the estimated accuracy of taper radii measurements ( ± 0.2 µm) as determined by the optical microscope.
Fig. 6 (a) Typical real-time transmission of optical power (at 980 nm) during the taper pulling process. The taper pulling was stop at the taper radius ~0.5 µm. (b) The difference between the propagation constants of the TM01 mode and HE21 mode at the SHG wavelength of 647 nm. The inset shows the propagation constant of the TM01 (at 647 nm) and HE11 mode (at 1294 nm).
Fig. 7 (a) The profile of a 3.8-μm taper obtained by combining sequential images taken using an optical microscope (Leica DMI-6000 B). (b) Theoretical estimate of SHG power as a function of taper interaction length. The result is obtained by numerically integrating the coupled mode Eq. (2) while using the taper profile determined in (a). (c) Theoretical simulation and experimental result of SHG power versus pump wavelength. Both results are normalized with respect to their corresponding peak SHG power.

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