As clearly mentioned in the acronym laser (light amplification by stimulated emission of radiation), a stimulated emission process in the gain medium is involved in the generation of a laser pulse, and therefore the gain bandwidth of the emission limits the possible wavelength range of laser pulses. For more than a half-century, scientists and engineers have put great effort in the search for the gain media and have developed clever ways to expand the wavelength range from the UV to the far infrared. Currently, these efforts have allowed the wavelength of lasers to cover the range of 0.2 - 27 μm almost entirely. However, as the authors note, particularly for the mid-infrared region (3 - 8 μm), the generation of laser pulses still suffers from a low working temperature requirement and a low plug efficiency. One way to overcome these issues is to use nonlinear frequency conversion techniques such as different frequency generation (DFG) and optical parametric amplification (OPA), where the energy of photons generated by stimulated emission processes in the near infrared is split into two, so that one of the two resulting photons can fall into the mid-infrared. These mid-infrared photons then undergo amplification to increase their number.

In this Letter, the authors design a new way of generating broadband mid-infrared femtosecond (fs) laser pulses at a central wavelength of 3.75 μm via two stages of non-collinear OPA (NOPA) in BBO and 4H-Silicon carbide crystals. First, a train of 52-fs pulses at a central wavelength of 800 nm (near infrared) is used to generate white light continuum (WLC) using a YAG crystal. Following WLC generation, in the first stage of NOPA the 1 μm component of the WLC is collimated and parametrically amplified within a thick BBO crystal pumped with a second harmonic of fs pulses (400 nm). Next the amplified WLC pumps the 4H-Silicon carbide crystal simultaneously with 800 nm fs pulses for the generation of the mid-infrared femtosecond pulses in the second stage of NOPA. Finally, the pulse duration of the generated mid-infrared fs pulses is confirmed via a sum frequency generation technique. During these two NOPA processes, the spatial and temporal overlaps of the pulses involved in nonlinear frequency conversion within the crystal are extremely important, since these significantly affect the efficiency of the processes. To achieve these overlap requirements, the authors carefully choose the phase matching angle where the walk off angle is close to zero, and control a non-collinear angle to make the group velocity mismatch zero, as well as compensating angular dispersion by using 4H-silicon carbide prisms.

In summary, the authors engineer the efficient way to generate 3.75 μm mid-infrared fs laser pulses by cleverly arranging the two stages of NOPA with BBO and 4H-Silicon carbide crystals. This technique attract a lot of attention in the mid-infrared ultrafast optics community, since, compared to the visible and the near infrared, the mid-infrared spectral region has not clearly explored in spite of many important applications such as molecular dynamics, remote sensing, and process monitoring.

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