A fiber‐based approach that generates mid‐infrared femtosecond pulses in the 3–4
m spectral region with microjoule‐level single pulse energy is demonstrated. This is realized in a piece of gas‐filled antiresonant hollow‐core fiber that is pumped by a two‐micron light source. A rapid variation of the dispersion near a structural resonance of the fiber creates a phase‐matching point in mid‐infrared,...