Space-time beams with tunable orbital group velocity for plasma superradiance

Abstract

Light springs are space-time beams that have a helical wavepacket. Due to this special property, light springs result into a rotating pulse when intercepting a plane lying orthogonal to their propagation direction. Associated to this, we introduce here the orbital group velocity, an additional tunable property of light springs. The orbital group velocity quantifies the speed of the light spring intensity rotation, distinctly from the conventional longitudinal group velocity, which describes the motion of the wavepacket envelope along its propagation axis. We demonstrate experimentally by tunable Fourier synthesis that the orbital group velocity can assume sub- and superluminal values, thus becoming a new platform for synthetic motion studies and control of laser-matter interactions. Particularly, in the superluminal regime, when interacting with a thin overdense plasma, we reveal by particle-in-cell simulations that the light spring unlocks superradiant radiation, due to the coherent excitation of the electrons in the plasma acting as a quasiparticle. This superradiant source inherits the ultrafast temporal dynamics of the light springs while emitting in the terahertz region, thus creating a new source of terahertz radiation controlled by the properties of spatiotemporal coupling of the laser. Therefore, spatiotemporal tuning of light springs is at the frontier of controlling laser-matter interaction and generating new tunable sources of radiation.

Figures (9)

• Figure 1: The lighthouse effect in helical space-time beams. Connection between the apparent light motion observed in lighthouses and the orbital group velocity, $v_{og}$, in a light spring---a space-time coupled beam with a characteristic helical wavepacket. The maximum intensity of the wavepacket rotates at a tangential speed $v_{og}$ on an intercepting plane, as the beam propagates with a longitudinal group velocity $v_{g}$. Similarly, the light emitted by a lighthouse produces the same pattern. $\ell$ represents the topological charge and $\mathcal{}f$ the laser frequency.
• Figure 1: Reconstruction of the LS isosurface. The angle between the arms of the interferometer is decisive to the proper reconstruction of the LS. It directly impacts the FFT (a), which is used to reconstruct the intensity profile of the LS, used to plot the LS isosurface, (b) and (c). (b) Shows a LS isosurface with linear space-time correlation, and (c) shows a torus isosurface, a structure where the same topological charge is applied to all frequencies to create the spatiospectral correlation. In (b) and (c) $\ell$ represents the topological charges and $\mathcal{}f$ the laser frequencies.
• Figure 2: Tuning the orbital group velocity of light springs. The orbital group velocity, $v_{og}$, tunability of a light spring can be accessed through control over the spatiospectral correlation: by changing the slope of the linear relation---(a) to (b)---or by changing the average topological charge---(a) to (c). The control over the topological charges is experimentally achieved without any physical adjustments to the light spring generating setup (d), as it relies on changing the spatiospectral hologram (e) displayed on the spatial light modulator. In (a), (b), and (c) $\mathcal{}f$ represents the laser frequency.
• Figure 2: Experimental calibration of the axicon and SLM. To test the control of the spectral windows created with the axicon grating, in addition to the topological charges applied to each window, a linear diffraction grating was added to the hologram displayed at the SLM. This allows to quantify the control over the spectral independent points (7 (a) and 10 (b)) with the SLM.
• Figure 3: Experimental demonstration of subluminal to superluminal light springs. Tunability of the orbital group velocity: the $v_{og}$ can be experimentally modulated to achieve sub-(a) and superluminal (c) values via different spatiospectral correlations. In (b) the white squares represent the programmed $v_{og}$ (Eq. \ref{['eqOGV2']}), while the black circles represent the experimentally measured $v_{og}$ (Eq. \ref{['eqOGV1']}), and the continuous line corresponds to numerical simulations of light springs. The scale bar in (a) and (c) is 173 $\mu$m and $\mathcal{}f$ represents the laser frequency.