This year is the 50th anniversary of nonlinear optics, which started with the first experimental observation of second-harmonic generation by Franken et al. in 1961. In the focus issue of Optics Express on Nonlinear Optics we show that second-harmonic generation can be used as a novel and surprisingly efficient method for generating energetic few-cycle mid-infrared pulses. Such pulses are attractive for investigating ultrafast vibrations of chemical bonds. A current obstacle is that today’s ultrafast laser technology is almost exclusively located in the near-infrared regime, and while parametric amplification is standard it always comes at a price of generating pulses with multi-cycle duration.
Through numerical simulations we show how to tackle this problem by using a standard phase-mismatched lithium niobate frequency conversion crystal. The nonlinear process is second-harmonic generation, but instead of converting then pump to the second-harmonic wavelength a cascaded nonlinearity is achieved due to the nonzero phase mismatch. This cascaded nonlinearity will compress the near-infrared pump pulse and generate a few-cycle soliton. This soliton can then shed radiation in the linear dispersion regime through a so-called Cherenkov phase-matching condition. This process occurs also in supercontinuum generation, and is responsible for the blue part of the broad spectrum. Here we are able to generate few-cycle dispersive waves in the mid-infrared due to the peculiar interplay of dispersion and nonlinearity. Surprisingly the conversion efficiency can be 20% or more, and the dispersive wave has a duration of a few optical cycles.
The focus issue on Nonlinear Optics contains peer-reviewed invited submission from participants of the 2011 Nonlinear Optics conference, which took place in July in Kaua’i, Hawai’i. The conference and the focus issue celebrate 50 years of nonlinear optics, starting with the original observation of second-harmonic generation. Fittingly, our study uses exactly second-harmonic generation to generate few-cycle pulses.
M. Bache, O. Bang, B.B. Zhou, J. Moses, and F. Wise
Optical Cherenkov radiation by cascaded nonlinear interaction: an efficient source of energetic few-cycle near- to mid-IR pulses
Optics Express 19, 22557-22562 (2011). Special issue for Nonlinear Optics 2011.
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