Recently, the State Key Laboratory of High Field Laser Physics of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences has made progress in the research of spatiotemporal characterization of ultrafast optical-vortex pulses. The new method, TERMITES, developed by the research team, effectively characterizes ultrafast light beams with a topological charge of |l| = 1, 2, or 3 through self-referencing. It utilizes a tilted shearing interferometer and a type-II FROG set-up. The relevant research results were published in Laser & Photonics Reviews under the title of "Self-Referencing 3D Characterization of Ultrafast Optical-Vortex Beams Using Tilted Interference TERMITES Technique", and this work was selected as the inside cover article.

Ultrafast light fields carrying optical angular momentums (OAMs), as the most typical representatives of ultrafast optical-vortex beams, have attracted tremendous interest in recent several years. While ultrafast vortex light beams are widely recognized as particularly important in multiple laser and nonlinear-optics experiments such as laser-frequency conversion, high-harmonic generation, laser micro-machining, and strong-field physics, three-dimensional characterization of ultrafast light beams carrying OAMs in the spatiotemporal domain has, however, proved difficult to achieve. Conventional characterization schemes rely on the use of reference light pulses which need to be well-characterized in their phase fronts and should have sufficient overlap and coherence with beams under test, largely limiting their practical applications.

Self-referencing technique, being capable of reconstructing three-dimensional, spatiotemporal information of optical-vortex beams without using any reference light, is highly demanded in studies of nonlinear optics and ultrafast lasers. It can, on one hand, help researchers to understand the mechanisms of the ongoing nonlinear processes, on the other hand, the development of ultrafast OAM-carrying light sources will be greatly promoted if the electrical fields of the output laser beams can be clearly characterized. Although being urgently demanded, the advanced self-referencing scheme for 3D ultrafast optical-vortex beam characterization has escaped experimental realization so far.

The research team demonstrated a self-referencing set-up based on a tilted interferometer that can be used to measure complete spatiotemporal information of OAM-carrying femtosecond pulses with different topological charges. Through scanning one interferometer arm, the spectral phase over the pulse spatial profile can be obtained using the tilted interference signal, and the temporal envelope of the light field at one particular position around its phase singularity can be retrieved simultaneously, enabling three-dimensional beam reconstruction. This self-referencing technique, capable of measuring spatiotemporal ultrafast optical-vortex beams, may find many applications in fields of nonlinear optics and light-matter interactions.

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Postdoctoral Program for Innovative Talents, the National Natural Science Foundation of China Youth Science Foundation Project, the China Postdoctoral Science Foundation, the Shanghai Science and Technology Innovation Action Plan, the National High-level Talent Youth Project, the Zhangjiang Laboratory Construction and Operation Project, the National Natural Science Foundation of China.

Fig 1. (a) Schematic of the diagnosis setup, (b) polarization direction of each beam during spatial and temporal measurements, (c) the tilted shearing-interference signal and FROG sampling position.(Image by SIOM)

Fig 2. Reconstructed three-dimensional electrical fields of the optical-vortex pulses with respective topological charges of (a) l = -1, (b) l = 2, (c) l = 3.(Image by SIOM)

Fig 3. Back inside cover