Fig. 1. Schematic of the experimental setup of the laser system and cloud chamber. The femotosecond laser beam (red line) is generated by a Ti-sapphire chirped-pulse amplification laser system and then launched into the cloud chamber via a focusing lens (f=300  mm), an 800 nm highly reflective mirror, and a 3 mm-thick window (fused silica). The green laser beam (green line) expanded in diameter by Lens 2 (f=5  mm) and Lens 3 (f=300  mm) and truncated by a slit was used for in situ light scattering measurements by a camera (Nikon D7000).

Fig. 2. Images of filaments in air and helium, which were captured by a digital camera (Nikon D7000: f number (F)=5.6, light sensitivity (ISO)=800, and shutter speed (S)=1/13  s). The dashed line shows the position of the geometric focus. The arrows indicate the propagation direction of the femtosecond laser.

Fig. 3. Side Mie scattering images of airflow in (a) air and (b) helium, respectively, which were captured by a digital camera (Nikon D7000: F=5.6, ISO=800, S=1/13  s). The white dashed curves are guidelines for the eyes for the rotation direction of the vortex pairs.

Fig. 4. Video frame figures of airflow patterns at the probe laser power of (a) 0.25 and (b) 2.5 W in the cloud chamber filled with air.

Fig. 5. Two-dimensional simulation results for airflow in the X–Z cross section (the bottom X–Y section (Z=0) was the cold plate) containing laser filament in helium. The velocity vectors are colored based on their velocity magnitude (m/s).