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Timestamp: 2019-04-25 10:55:57+00:00

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Compared with conventional lasers, the random laser is realized through strong multiple scatterings in disordered gain system. In this paper, random lasing (RL) in one-dimensional metal surface plasmon (SP) waveguide with gold-plated self-formed silicon pyramids was investigated comprehensively. Consequently, the emission intensity of RL was enhanced dramatically and the RL threshold was reduced significantly through Au-coated Si spiky tips. Meanwhile, one-dimensional metal SP channel waveguides confined the emitting light in a certain direction with a small divergence angle. Using FDTD simulations, it was found that the enhancement effect for RL is likely attributed to the localized surface plasmon (LSP) field. In addition, the LSP field nearby the spiky tips can enhance field-molecule interaction, which was benefit for lasing in small scale. The results in this letter supplied a feasible method to realize the application of RL in subwavelength optical elements.
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Fig. 1 (a) Surface morphology of an etched channel waveguide with randomly distributed Si pyramids in it. (b) Enlarged single Si pyramid, which has eight smooth top surfaces. (c) Cross-section SEM image of the facet of channel waveguide. The waveguide exhibits a ladder-like shape with top (bottom) width of 25 μm (5 μm) and height of 15 μm. (d) Experimental setup for RL measurement. The pulsed pumping light (532 nm) was generated from frequency-doubled Nd:YAG laser (repetition 5 Hz, pulse duration 17 ns) and focused on the waveguide by a cylindrical lens. The emission light was collected through an objective lens and a focus lens and then was analyzed by Princeton spectrometer. (e) Emission spectra of composite active media (R6G/SU-8) spin-coated on bare Si plate. (f) Lasing spectra of gain medium (R6G/SU-8) from pure silicon channel waveguide. (g) Emission intensity and FWMH of peaks versus the excitation density. The threshold of RL that extracted from the L-L curve is about 390 kW cm−2.
Fig. 2 Comparison of emission characteristics from different waveguide structures. (a) Emission spectra of R6G/SU-8 film on bare Si waveguide under different pumping intensity. (b) Lasing spectra of gain media on silver-plated channel waveguide. (c) Lasing emission spectra of gain media on gold-plated waveguide. Insets of Fig. 2(a)-(c) display the SEM images of the cross-section for above three structures. (d)-(f) The intensity of emission peak as a function of pumping intensity for bare Si waveguide, silver-plated and gold-plated structure respectively. The typical kink point can be seen from three light-out versus light-in (L-L) curves corresponding to (a)-(c). Note, the lasing threshold of RL is reduced to 400 kW cm-2 through gold surface plasmon waveguides. For all measured samples, the thickness of gain media is kept unified for above three types of structures.
Fig. 3 Characteristics of near-/far-field and divergence angle of RL. (a) CCD images of near-field patterns (NFP) for emissions from the end facet of bare Si waveguide with different excitation intensity. (b) Far-field patterns of emission from the bare Si channel structure above threshold. (c) The photograph of NFP from gold-plated channel waveguide under a series of pumping powers. (d) Far-field patterns of RL above threshold from the channel waveguide with gold film. (e) The dependence of emission intensity on output angle along x-axis. Inset, the coordinate for divergence spectra measurement. (f) The emission intensity versus detection angle along y-axis.
Fig. 4 Simulation Results of optical field via FDTD. (a) Schematic diagram of bare silicon channel waveguide with composite R6G gain media. The disordered silicon pyramids are also shown. (b) The optical field distribution along the longitudinal direction (z-axis). Here, the boundary between the gain media and air was indicated. (c) The simulated result of optical field at cross-section of the waveguide. (d) The enlarged image of field intensity distribution nearby an individual pyramid. (e) Diagram of metal SP waveguide for FDTD simulation. (f) The optical field distribution of metal SP waveguide along the z-axis. (g) Calculated result about the optical field at cross-section of metal SP waveguide. (h) Results of field intensity distribution nearby single spiky tip with Au coating.

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