中红外脉冲在分子共振附近的成丝现象
Filamentation of Mid-IR pulses in ambient air in the vicinity of molecular resonances
摘要：
通过监测等离子体密度和损耗以及不同聚焦强度下的光谱动力学和光束轮廓演化，实验研究了由中心为3.9 μm的数毫焦耳、90 fs中红外脉冲点燃的灯丝的特性。通过将聚焦从强（f=0.25 m）软化到松散（f=7 m），我们观察到从等离子体辅助聚焦到具有低等离子体密度的细丝的转变。在后一种情况下，双折射通过光束自对称化和空间自沟道来表现。在散聚焦情况下，非线性拉曼频移控制了光谱动力学，导致在4.2 μm附近与CO2共振峰重叠。在3.9 μm等离子体含量低的情况下，动态CO2吸收是能量损失的主要机制，并且单独或与其他非线性过程一起有助于强度的捕获。
英文摘要：
Properties of filaments ignited by multi-millijoule, 90-fs mid-IR pulses centered at 3.9 μm are examined experimentally by monitoring plasma density and losses as well as spectral dynamics and beam profile evolution at different focusing strengths. By softening the focusing from strong (f=0.25 m) to loose (f=7 m) we observe a shift from plasma assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the non-linear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2 μm. The dynamic CO2 absorption in the case of 3.9-μm filaments with their low plasma content is the main mechanism of energy losses and either alone or together with other nonlinear processes contributes to the arrest of intensity.
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