Formation of axially modulated plasma strings by filamentation of interfering femtosecond Bessel beams

We numerically investigate the formation of axially modulated plasma strings through the filamentation of two interfering femtosecond Bessel beams. The constituent Bessel beams have different central spot sizes and propagate collinearly. Our results show that filamentation of these beams in air leads to the formation of intense axially modulated light filaments, and corrugated plasma strings with high modulation depths and tunable modulation periods. We find that the modulation periods of the intensity and the plasma density are identical and comparable to the modulation period of the intensity in the linear propagation regime. Furthermore, we show that, in the nonlinear propagation regime, the modulation period is independent of the pulse energy and can be tuned by selecting appropriate central spot sizes for the interfering Bessel beams. Finally, we propose a simple interferometric arrangement using a single axicon to generate two interfering Bessel beams with adjustable spot sizes enabling the creation of axially-modulated plasma strings with tunable periodicity at high input powers.