Terahertz (THz) radiation from laser plasmas features with ultrabroad bandwidth, which is attractive for many applications. Significant efforts have been made to enhance its conversion efficiency, tune its frequency spectrum, and even manipulate its polarization.
However, little attention has been paid to beam structure shaping, for example, the generation of Bessel THz beams. Traditional beam shaping techniques often rely on some optical devices (such as spatial light modulators) or specially designed materials, which cannot be well applied to broadband THz sources either due to insufficient adaptive bandwidth or high absorption of the materials. Recently, the research team led by Yanping Chen and Zhengming Sheng from Shanghai Jiao Tong University has realized Bessel THz radiation with ultrabroad bandwidth from a laser plasma filament by manipulating its spatial-temporal structure with tailored femtosecond lasers.
In their experiment, a 1 kHz femtosecond laser pulse composed of a fundamental wave and its second harmonic was focused by a convex lens together with an axicon in air, forming a superluminal plasma filament. Based on the cutting knife method, the terahertz radiation from the superluminal plasma filament is demonstrated to have a typical high-order Bessel beam profile. They proposed a theoretical model called "nonlinear dipole model", which can well explain the generation of the THz Bessel beam radiation from a superluminal plasma filament.
This approach for generating THz Bessel beams does not require any terahertz optics, thus free from possible spectral narrowing or energy absorption associated with traditional beam shaping techniques. It perfectly combines together the features of both ultra-broad terahertz radiation and Bessel beam profiles, which may be interesting for applications, such as organic material excitation, subwavelength image with high depth of fields, and high-speed communications. In addition, different from common Bessel beams that are limited in the interference region, THz radiation from such superluminal plasma filaments can remain its Bessel pattern while propagating for long distance, suitable for far-field applications.
The research team has been working on the generation and the manipulation of terahertz radiation from laser-plasma sources for many years. "THz radiation with Bessel beam profiles may be particularly interesting for medical and biological imaging" said Zhelin Zhang, the first author of the paper, "Our objective is to enhance the overall controllability of THz beams from laser plasma filaments for different applications."