2018Optics letters 43(6), 1335 - 1338 (2018). doi:10.1364/OL.43.001335We report on a femtosecond ∼8.5 $\mu$m, ∼2 $\mu$J source based on the intrapulse difference-frequency generation (DFG) of 2.1 $\mu$m pulses in an AgGaSe$_2$ (AGSe) crystal. Compared to the conventional ∼ 0.8 or 1 $\mu$m near-infrared (IR) pulses, a ∼ 2 $\mu$m driver for intrapulse DFG can provide more efficient conversion into the wavelengths longer than 5 $\mu$m due to a lower quantum defect and is more suitable for the non-oxide nonlinear crystals that have a relatively low bandgap energy. Using 26 fs, 2.1 $\mu$m pulses for type-II intrapulse DFG, we have generated intrinsically carrier-envelope phase-stable idler pulses with a conversion efficiency of 0.8%, which covers the wavelength range of 7–11 $\mu$m. Our simulation study shows that the blueshift of intrapulse DFG is assisted by self-phase modulation of the driving pulses in AGSe. The idler pulses are particularly useful for strong-field experiments in nanostructures, as well as for seeding parametric amplifiers in the long-wavelength IR.Published by OSA, Washington, DC10.3204/PUBDB-2018-01486en2018Deutsches Elektronen-Synchrotron, DESY, Hamburg10.1364/OL.43.0013350146-95921539-4794Journal articlepages 1335 - 1338530Femtosecond 85 μm source based on intrapulse difference-frequency generation of 21 $\mu$m pulsesNovák, OndřejKrogen, PeterKroh, Tobias0000-0002-1958-7556Deutsches Elektronen-SynchrotronCentre for Free-Electron Laser ScienceMocek, TomášKärtner, Franz X.0000-0001-8829-5461Deutsches Elektronen-SynchrotronCentre for Free-Electron Laser ScienceHong, Kyung-Han0000-0001-5041-5210