Generation of 1.4-cycle, 1.85 μm pulses by multi-plate post-compression in ambient air

We demonstrate near-single-cycle pulse generation at 1.85 μm using an ambient-air, all-solid post-compression scheme that combines a broadband collinear-spectrum-synthesis optical parametric amplifier (OPA) with two fused-silica multi-plate stages. The 20.3-fs, 0.72-mJ idler pulses from the OPA are compressed to 8.5 fs (1.4 cycles) with 0.56 mJ output energy, corresponding to 78% post-compression throughput. Efficient compression is achieved using only fused-silica optics by exploiting the favorable dispersion at 1.85 μm to compensate for nonlinear phase accumulated from self-phase modulation and self-steepening. The setup requires no vacuum chamber, gas handling, or waveguide coupling, and uses only silver-coated mirrors and fused-silica plates. The output preserves passive carrier-envelope-phase stability (458 mrad, average of 4 shots, over 20 min) and shows 0.68% energy fluctuation over 1 h. This alignment-tolerant and infrastructure-light approach provides a practical route to CEP-stable near-single-cycle short-wave infrared drivers for various applications.