Impressive laser intensity increase at the trailing stage of femtosecond laser filamentation in air

The longitudinal distribution of the laser peak intensity inside a half meter long femtosecond laser filament in air is studied by measuring the signal ratio of two nitrogen fluorescence lines, 391 nm and 337 nm. The experimental results reveal that laser peak intensity initially remains almost constant (~4.3 × 1013 W/cm2) inside the filament. However, before the end of the filament, surprisingly the laser intensity undergoes dramatic increase. A maximum intensity as high as 2.8×1014 W/cm2 could be reached. The experimental result is unexpected by the conventional intensity clamping scenario, according to which the laser peak intensity would feature low variation along a filament. The experimental result is then interpreted as being due to the generation of a short pulse at trailing stage of the filamentation with reduced diameter. This phenomenon might be of great interest owing to its potential application in high-order-harmonic generation and producing isolated single attosecond laser pulse through simple experimental approach.