Mid-infrared ultrafast soliton molecules from a few-cycle Cr:ZnS laser

Soliton molecules, or soliton bound states, are envisioned to make far-reaching changes in both fundamental research and applications. Here, we report on the generation and precision manipulation of soliton molecules based on a Kerr-lens mode-locked single-crystal Cr:ZnS laser at 2.4 µm. In the classical soliton regime, self-starting near-transform-limited pulses with a duration of 37 fs, less than 5 optical cycles, have been obtained at a repetition frequency of 173 MHz and an average output power of 572 mW. By fine-tuning the cavity group-delay dispersion profile, bi-soliton states with pulse durations between 55 fs and 98 fs with temporal separations between 348 fs and 604 fs have been observed and characterized. These are the shortest pulse duration and separation of soliton molecules reported so far in the mid-infrared region, to the best of our knowledge. With the ability of precision manipulation of soliton molecules generated on a sub-100-fs timescale, the tunable mid-infrared soliton molecule source paves the way for applications in the fields of telecommunications and ultrafast laser technologies.