Synthesis of tunable space-time beams toward structured light-matter interaction

Controlling the degrees of freedom of light is a topic of extensive experimental efforts. The characteristics of the light are determinant in accessing the desired underlying physical phenomena. Structured light emerged as a field of research dedicated to exploring and pushing the tailoring possibilities for light sources.  A light spring (LS) is a complex form of structured light created through the coupling between the topological charge and the frequency components of a pulsed beam.

In its most recent generation scheme, a circular diffractive axicon spectrally separates the pulse, which then receives different topological charges via phase modulations. Since these modulations are controlled with a spatial light modulator, the beam can be tailored to present different forms of space-time coupling. In this work, the generation of spatiotemporally coupled beams with tunable properties is explored. Such beams open the possibility of guiding light-matter interactions, including the generation of new sources of radiation and platforms for synthetic motion studies.

Initially, a new tunable property of LS is defined: the orbital group velocity ( ). The  describes the velocity with which the apparent motion of the LS hotspot completes a full spin around its axis, and it can be tailored to achieve sub- and superluminal values. The tunability of the   in such a range gives way to controlling the interaction between light and matter in a new way.

The sub- and superluminal regimes unlock the possibility of novel sources of radiation that could access different responses in light-matter interaction systems, as well as being a new platform for synthetic motion studies, where beams with superluminal properties play a central role in unlocking new interaction scenarios. Finally, the control of the space-time beam spectral phase is implemented. This is an additional parameter that unlocks unprecedented control of light sources, paving the way towards ultrabroadband control of ultrafast pulses.