High-OAM Deep Ultraviolet Twisted Light Generation for RF-Photoinjector Applications

TL;DR

This work tackles the generation of stable, high-order orbital angular momentum (OAM) beams in the deep-UV for photocathode-based accelerators. It demonstrates three fabricated DOEs—reflective fork gratings, spiral phase plates, and binary axicons—integrated into an RF-photoinjector drive-laser to produce and diagnose UV twisted light at 266 nm with high OAM up to 64. The spiral phase plate delivers the best performance (approximately 80% conversion efficiency for an OAM of 64), while axicons provide robust quasi-Bessel beams and fork gratings enable efficient generation of lower-order states; all results agree with numerical simulations and LG-HG mode-conversion diagnostics. The findings offer a practical route toward structured photocathode illumination and the generation of relativistic vortex electrons in accelerator facilities, with direct relevance to next-generation high-brightness light sources.

Abstract

We report on the generation and characterization of ultraviolet (wavelength 266 nm) twisted light with high orbital angular momentum (OAM) using three types of fabricated diffractive optical elements (DOEs): a reflective fork grating, a high-charge spiral phase plate (SPP), and binary axicons. All elements were integrated into a drive-laser beamline of an electron RF-photoinjector, enabling direct evaluation under accelerator-relevant conditions. The SPP produced a high-purity Laguerre-Gaussian mode with OAM l = 64 and a measured conversion efficiency of approximately 80%. Binary axicons generated quasi-Bessel twisted light with topological charges up to m = 10, exhibiting low divergence and stable multi-lobe ring structures. The fork grating reliably produced lower-order modes, l = 2-8, with good agreement between simulations and cylindrical-lens diagnostics. These results constitute, to our knowledge, the first comprehensive experimental demonstration of deep-UV high-OAM beams generated with fabricated DOEs and validated through mode-conversion measurements. The demonstrated techniques are compatible with high-power UV laser systems used in RF-photoinjectors and offer a practical route toward structured photocathode illumination and the generation of relativistic vortex electrons at a particle accelerator facility.

Figures (14)

• Figure 1: A binarized fork-grating mask.
• Figure 2: A microscope image of the fork grating.
• Figure 3: Two-dimensional phase topology of the spiral phase plate (SPP) featuring 64 azimuthal steps within an aperture of $D = 25.6\ \mathrm{mm}$. The topological charge is $m = 64$. The step height is $h_s \approx 532\ \mathrm{nm}$, calculated for $\lambda = 266\ \mathrm{nm}$ and $n = 1.49$.
• Figure 4: The atomic force microscope image of the fabricated SPP. Dust particles trapped between the etched sectors and the striped etching pattern do not degrade the beam quality. The measured step height is $\sim$500 nm, consistent with the design.
• Figure 5: (a) Phase mask of an axicon with topological charge $m$ = 3. The micrograph of a binary planar axicon with an aperture diameter of 6 mm and $m$ = 3 in a SU-8 photoresist layer; (b) full view; (c) enlarged view obtained in refractive mode, covered with a gold layer