Fabricating THz spiral zone plate by high throughput femtosecond laser air filament direct writing.

The sixth-generation wireless communication will exploit the radio band with frequencies higher than 90 GHz, reaching terahertz (THz) band, to achieve huge signal bandwidths. However, the cost-effective fabrication methods of the key components in THz band, which can compromise large scale, high precision, and high efficiency, remain great challenges at present. In this work, we have developed a high throughput fabrication method based on the femtosecond laser filament direct writing.

Femtosecond filament emergence between π-shifted beamlets in air.

By rotating the four-section π-shifted phase plate in the transverse plane relatively to the axes of the elliptical beam of 800-nm, 1.1-mJ, 35-fs pulse propagating in air, we switch between the regime of four parallel plasma channels and the regime of spatial symmetry breakup followed by on-axis plasma channel formation identified on the burnt paper images of the beam. Relaxation of the π-phase shift for 45 phase plate rotation is demonstrated explicitly in 3D+time carrier wave resolved numerical simulations yielding the initial step-like phase distribution degradation along the plasma region.

Effect of Molecular Orbital Angular Momentum on the Spatial Distribution of Fluorescence during Femtosecond Laser Filamentation in Air.

Nonuniform azimuthal distribution of N fluorescence emitted from the femtosecond laser filament in air was discovered. The fluorescence is stronger when the detector is placed perpendicular or parallel to the laser polarization. The experimental results have been confirmed by the theoretical calculation that the azimuthal distribution of fluorescence is reproduced by the convolution of the transition of the dipole and the molecular alignment in the strong laser field.