Femtosecond laser filamentation is a unique nonlinear optical phenomenon when high-power ultrafast laser propagation in all transparent optical media. During filamentation in the atmosphere, the ultrastrong field of 10¹³–10¹⁴ W/cm² with a large distance ranging from meter to kilometers can effectively ionize, break, and excite the molecules and fragments, resulting in characteristic fingerprint emissions, which provide a great opportunity for investigating strong-field molecules interaction in complicated environments, especially remote sensing. Additionally, the ultrastrong intensity inside the filament can damage almost all the detectors and ignite various intricate higher order nonlinear optical effects. These extreme physical conditions and complicated phenomena make the sensing and controlling of filamentation challenging. This paper mainly focuses on recent research advances in sensing with femtosecond laser filamentation, including fundamental physics, sensing and manipulating methods, typical filament-based sensing techniques and application scenarios, opportunities, and challenges toward the filament-based remote sensing under different complicated conditions.

Two-dimensional arrays of periodic nanostructures are fabricated on bulk tungsten surface within a single step using collinear propagation of two time-delayed femtosecond laser beams with orthogonal polarizations. It is surprisingly found that the geometric profile of the structure unit exhibits a triangle shape in hundred nanometer scales, and its spatial dimension can be modulated by the ambient air pressure ranging from 1 atm to 10⁻³ Pa. As the ambient air pressure decreases, the obtained surface structures display a large depth covered with nanowires. Physically, the formation of such triangle structures is originated from the transient physical correlations between the two laser-matter interaction processes, and also affected by the heat transfer effects of the surrounding air. In addition, the experimental measurements reveal that the minimum reflectivity of the nanotriangle surface structures is unprecedentedly reduced to as low as ~2.9% especially within the visible-infrared range.

Highly oriented pyrolytic graphite is ablated by femtosecond laser in air and vacuum, respectively. The morphologies and material components of the ablated target surface are examined by scanning electron microscopy and micro-Raman spectroscopy. For laser fluence no less than $0.5\mathrm{J}/{\mathrm{cm}}^2$, the laser-induced periodic surface grooves formed in air are found to be much less clearer than those formed in vacuum; beneath the amorphous carbon debris, the surface ablated in air is composed of nanocrystalline graphite, but in vacuum it retains the ordered graphite structure. The thermal relaxation of the ablated target induced by air-assisted heat transfer is considered to be the main reason for the differences. The transportation of air molecules along the surface, the classical size effect of heat conduction and high solid-air heat transfer coefficient in nanoscale enhance the air-assisted heat transfer.

Using two beams of femtosecond laser pulses linearly polarized in different directions, we demonstrate a new phenomenon of eliminating the periodic subwavelength surface structures recorded on Fe-based metallic glass. It is found that such femtosecond laser erasing process can be efficiently controlled by varying the temporal delay between two laser beams while maintaining the amorphous properties of the sample surface. The underlying mechanisms are substantially attributed to the transient enhancement of the surface mobility of the sample by two laser-matter interactions. These investigations may be helpful in high precision manipulation of the material surface for rewritable applications.