Femtosecond laser sources and optical frequency combs in the molecular fingerprint region of the electromagnetic spectrum are crucial for a plethora of applications in natural and life sciences. Here we introduce ${\mathrm{Cr}}^{2+}$-based lasers as a convenient means for producing super-octave mid-IR electromagnetic transients via optical rectification (or intra-pulse difference frequency generation, IDFG). We demonstrate that a relatively long, 2.5 μm, central wavelength of a few-cycle ${\mathrm{Cr}}^{2+}\text{:}\mathrm{ZnS}$ driving source (20 fs pulse duration, 6 W average power, 78 MHz repetition rate) enabled the use of highly nonlinear ${\mathrm{ZnGeP}}_2$ crystal for IDFG with exceptionally high conversion efficiency ($>3\%$) and output power of 0.15 W, with the spectral span of 5.8–12.5 μm. Even broader spectrum was achieved in GaSe crystal: 4.3–16.6 μm for type I and 5.8–17.6 μm for type II phase matching. The results highlight the potential of this architecture for ultrafast spectroscopy and generation of broadband frequency combs in the longwave infrared.

中文翻译：

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几个周期的2.5μm脉冲的光整流产生的超八度长波中红外相干瞬变

飞秒激光源和电磁频谱的分子指纹区域中的光学频率梳对于在自然科学和生命科学中的大量应用至关重要。这里我们介绍${铬}^{\mathrm{2个}+}$激光器作为通过光整流（或脉冲内差分频率生成，IDFG）产生超倍频程中红外电磁瞬变的便捷手段。我们证明了几个周期的相对较长的2.5μm中心波长${铬}^{\mathrm{2个}+}\text{：}\mathrm{硫化锌}$ 驱动源（20 fs脉冲持续时间，6 W平均功率，78 MHz重复频率）可以使用高度非线性 ${\mathrm{锌锗磷}}_{\mathrm{2个}}$ IDFG的晶体具有极高的转换效率（$>3％$），输出功率为0.15 W，光谱范围为5.8–12.5μm。GaSe晶体的光谱范围更大：I型相位匹配为4.3–16.6μm，II型相位匹配为5.8–17.6μm。结果突出了该架构在超快光谱学和长波红外中产生宽带频率梳的潜力。