Manipulation of femtosecond laser multi-filament array by spatiotemporal phase modulation

High-intensity (∼1 TW/cm2 and higher) region formed in the propagation of ∼60 GW, 90 fs Ti:Sapphire laser pulse on a ∼100 m path in air spans for several tens of meters and includes a plasma filament and a postfilament light channel. The intensity in this extended region is high enough to generate an infrared supercontinuum wing and to initiate laser-induced discharge in the gap between the electrodes. In the experiment and simulations, we delay the high-intensity region along the propagation direction by inserting metal-wire meshes with square cells at the laser system output. We identify the presence of a high-intensity region from the clean-spatial-mode distributions, appearance of the infrared supercontinuum wing, and occurrence of the laser-induced discharge. In the case of free propagation (without any meshes), the onset of the high-intensity zone is at 40–52 m from the laser system output with ∼30 m extension. Insertion of the mesh with 3 mm cells delays the beginning of the high-intensity region to 49–68 m with the same ∼30 m extension. A decrease in the cell size to 1 mm leads to both delay and shrinking of the high-intensity zone to 71–73 m and 6 m, respectively. Three-dimensional simulations in space confirm the mesh-induced delay of the high-intensity zone as the cell size decreases.

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Nonlinear Propagation and Filamentation on 100 Meter Air Path of Femtosecond Beam Partitioned by Wire Mesh

Modulating the length and position of plasma filament is desired for many filament-based applications. We proposed a novel method to manipulate the properties of filament along the propagation direction by introducing asymmetric focusing with a wedge plate in the forced focusing region. By simply varying the inclination degree of a wedge and the distance between the wedge and lens, a plasma filament can break into two sub-filaments along the propagation direction, and the separation between sub-filaments can be controlled. The experiment results are in agreement with the results of simulation by the ray-tracing method. Furthermore, the effect of the wedge angle and rotation angle on the control of the filament is studied. It is found that the separation of the sub-filaments and the azimuth angle of the filament around the original optical axis can be controlled by the wedge angle and rotation angle of the wedge, respectively. When combined with a phase plate, the control method also shows the ability to modulate the multi-filament array with wavefront modification. Our research provides a simple and flexible method for 3-D control of femtosecond laser filaments for practical applications. 

Multi-dimensional control of femtosecond laser filaments by inserting a wedge plate in the forced focusing region

Flexible manipulation of the onset and terminal positions of femtosecond laser filamentation in fused silica via controlling beam profile before axicon

Stable propagation of large, multifilament arrays over long distances in air paves new ways for microwave-radiation manipulation. Although, the dynamics of a single or a few filaments was discussed in some of the previous studies, we show that the stability of large plasma filament arrays is significantly more complicated and is constrained by several trade-offs. Here, we analyze the stability properties of rectangular arrays as a function of four parameters: relative phase of the generating beams, number of filaments, separation between them, and initial power. We find that arrays with alternating phase of filaments are more stable than similar arrays with all beams in phase. Additionally, we show that increasing the size of an array increases its stability, and that a proper choice of the beam separation and the initial power has to be made in order to obtain a dense and regular array of filaments.

Dynamics of large femtosecond filament arrays: possibilities, limitations, and trade-offs

Manipulation of femtosecond laser filamentation is essential for many potential applications. We report the simulations of the manipulation of femtosecond laser filamentation by introducing a novel gaseous lattice medium with the alternating positive and negative refractive index distribution at different stages of filamentation. The results show that the filament length has greatly been extended and a multi-filament array can be formed by the gas lattice medium. It has been found that additional focusing and discrete diffraction provided by the gas lattice medium contribute to a new dynamic equilibrium in the filamentation. As a result, a varied cross-section pattern, higher field intensity, and electron density along the filamentation are obtained. Our approach provides a new way to manipulate filamentation for many practical photonic applications.

Manipulation of femtosecond laser filamentation by a gaseous lattice

White-light supercontinuum from a femtosecond laser filamentation is essential for many applications due to its broadening spectrum and remote sensing ability. We propose to enhance the supercontinuum intensity by presetting the gaseous lattice in the path of femtosecond laser filamentation. Our results show that the introduction of a gaseous lattice can increase the spectral intensity in both visible and infrared wings, up to about ten times over the no lattice case in a gas medium. We find that the gaseous lattice dramatically affects the time-domain evolutions of the optical intensity and the electron density during the nonlinear propagation of the pulse, thereby resulting in spectral intensity enhancement. The results provide a new way to enhance the supercontinuum generation in the ultra-intense nonlinear process by using a gaseous pre-lattice with a special spatial refractive index profile.

Jianji Wang

Papers

Manipulation of femtosecond laser filamentation by a gaseous lattice

Manipulation of femtosecond laser multi-filament array by spatiotemporal phase modulation

Gaseous pre-lattice assisted supercontinuum enhancement of femtosecond laser filamentation