title: Multifrequency third-harmonic generation by red-shifting solitons in a multimode photonic-crystal fiber.
abstract: While the standard scenario of third-harmonic generation (THG) by a dispersive-wave pump involves the emission of light with a frequency 3omega, thrice the frequency omega of the input pump field, solitons undergoing a continuous shift of their central frequency omega due to the Raman effect in a multimode optical fiber can generate the third harmonic in a different fashion. In the experiments reported here, we provide the first direct experimental evidence of THG by a continuously red-shifting soliton pump by studying the third-harmonic buildup in relation to the spectral evolution of the soliton pump field in a silica photonic-crystal fiber (PCF). We show that solitons excited in a PCF by unamplified femtosecond pulses of a Cr:forsterite laser sweep through the spectral range from 1.25 to 1.63 microm , scanning through a manifold of THG phase-matching resonances with 3omega dispersive waves in PCF modes. As a result, intense third-harmonic peaks build up in the range of wavelengths from 370 to 550 nm at the output of the fiber, making PCF a convenient fiber-format multifrequency source of short-wavelength radiation. Time-resolved fluorescence measurements with photoexcitation provided by the third-harmonic PCF output are presented, demonstrating the high potential of PCF sources for an ultrafast photoexcitation of fluorescent molecular systems in physics, chemistry, and biology.
authors.affiliation: Department of Physics, M. V. Lomonosov Moscow State University, Vorob'evy gory, Moscow 119992, Russia.
authors.forename: A B
authors.initials: AB
authors.lastname: Fedotov
authors.forename: A A
authors.initials: AA
authors.lastname: Voronin
authors.forename: E E
authors.initials: EE
authors.lastname: Serebryannikov
authors.forename: I V
authors.initials: IV
authors.lastname: Fedotov
authors.forename: A V
authors.initials: AV
authors.lastname: Mitrofanov
authors.forename: A A
authors.initials: AA
authors.lastname: Ivanov
authors.forename: D A
authors.initials: DA
authors.lastname: Sidorov-Biryukov
authors.forename: A M
authors.initials: AM
authors.lastname: Zheltikov