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(Color) (a) Experimental setup for demonstrating cascaded harmonic generation and Raman lasing in the lithium niobate whispering-gallery resonator. (b) Whispering-gallery resonator resonance, implying a quality factor of 2 × 107 at 1550 nm. (c) Energy-level diagrams illustrating various nonlinear processes involved in the demonstrated pump wavelength extension, including the cascaded harmonic generation, Raman scattering, cascaded Raman scattering and/or multi-photon hyper-Raman scattering in the lithium niobate whispering-gallery resonator (ħωP and ħωR represent the pump photon and Raman phonon energy levels, respectively).
(Color) Measured emission spectrum of the whispering-gallery resonator at the pump wavelength of 1536 nm and pump power of 200 mW at (a) infrared, (b) near-infrared, (c) visible, and (d) ultraviolet spectral ranges. The spectral lines indicate generation of the 2nd, 3rd, and 4th harmonics as well as the stimulated Raman, two-photon, three-photon, and four-photon Raman scattering corresponding the molecular vibrational wavenumbers 632 and 255 cm−1 in z-cut lithium niobate.
(Color) Whispering-gallery resonator out-coupled light observed on a color CCD camera at a pump wavelength of 1538 nm. The red, green, and purple spots correspond to the 1st, 2nd, and 3rd harmonics. The less bright yellow and orange spots correspond to the three-photon Raman lines, yellow-shifted with respect to the third harmonic.
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