Generation of high-OAM ultraviolet twisted light for RF-photoinjector applications

This paper demonstrates the generation of high-purity, high-orbital-angular-momentum (OAM) ultraviolet vortex beams at 266 nm using three distinct diffractive optical elements, successfully integrating them into an operational RF-photoinjector beamline to enable the production of relativistic vortex electron beams for accelerator applications.

The Gist

Imagine you are trying to spin a tiny, invisible top made of light. In the world of physics, this "spinning light" is called twisted light, and the amount it spins is called Orbital Angular Momentum (OAM). Scientists want to use this spinning light to kick-start electrons in giant machines called particle accelerators.

However, there's a catch: to do this job effectively, the light needs to be ultraviolet (UV) and spin very fast (high OAM). Making UV light spin is like trying to spin a top made of glass that is also on fire—it's extremely difficult because UV light is so energetic it can melt or damage the tools usually used to twist it.

This paper is a report on how the researchers built three different "specialized tools" to twist this high-energy UV light without breaking them, and they tested which tool works best.

The Three Tools (The "Twisters")

The researchers created three different types of optical devices to twist the light. Think of them as three different ways to spin a pizza dough:

1. The Fork Grating (The "Stenciled Sieve")

• What it is: A tiny mirror with a pattern etched into it that looks like a fork.
• How it works: When the light hits the fork, it splits into different beams. Some beams spin slowly, some spin faster, depending on which "fork tine" the light bounces off.
• The Result: It's like a Swiss Army knife. You can easily switch between different spin speeds (low to medium OAM) just by looking at different parts of the beam. It's robust and easy to make, but it's not the most efficient at keeping the light pure.

2. The Spiral Phase Plate (The "Helical Slide")

• What it is: A piece of glass carved into a perfect, continuous spiral staircase.
• How it works: As the light travels up this spiral staircase, it gets twisted. Because the staircase is smooth and continuous, the light comes out spinning very cleanly and tightly.
• The Result: This was the champion of the experiment. It created the highest spin speed tested (up to 64 spins per photon) with 80% efficiency. It's like a perfectly engineered slide that sends the light spinning without losing much energy. The downside? It is incredibly hard and expensive to carve this glass with the precision of a hair's width.

3. The Binary Axicon (The "Pixelated Cone")

• What it is: A cone-shaped lens, but instead of being smooth, it's made of tiny, stepped rings (like a digital staircase).
• How it works: It forces the light into a ring shape that looks like a Bessel beam (a beam that doesn't spread out easily).
• The Result: This tool creates a beam that is a "mixture" of different spins. Instead of one pure spin speed, it's like a choir singing slightly different notes at the same time. It creates a very stable, low-spread beam, but the "spin" isn't a single, pure number. It's a controlled mix.

The Experiment: Putting it to the Test

The researchers took these three tools and installed them into a real, working machine (an RF photoinjector) that uses UV lasers. They didn't just simulate this on a computer; they actually shone the laser through the tools and took pictures.

• The Fork Grating worked exactly as predicted, creating clear spinning beams.
• The Spiral Plate produced a beautiful, clean ring of light with a dark hole in the middle, spinning at record speeds for UV light.
• The Axicon created a ring of light that looked like a flower with distinct petals (lobes), which is a signature of its "mixed" spin nature.

Why Does This Matter?

The paper claims this is the first time anyone has successfully made these high-speed spinning UV beams using these specific tools inside a real accelerator system.

The main takeaway is that they now have a "menu" of options for scientists:

• If you need maximum spin speed and purity, use the Spiral Plate (but be prepared for the cost and difficulty).
• If you need flexibility to switch between different spins, use the Fork Grating.
• If you need a stable, non-spreading beam and don't mind a mix of spins, use the Axicon.

This work paves the way for creating "vortex electron beams"—streams of electrons that are also spinning. The paper suggests this could help scientists study the internal structure of protons (the building blocks of atoms) and potentially lead to better electron microscopes in the future.

In short: They built three different "light spinners" that can survive the harsh UV environment, tested them in a real machine, and proved they can create the high-speed spinning light needed for the next generation of particle physics experiments.

Technical Summary

Technical Summary: Generation of High-OAM Ultraviolet Twisted Light for RF-Photoinjector Applications

Problem Statement
The generation of relativistic vortex electron beams via photoemission requires ultraviolet (UV) laser beams with well-controlled orbital angular momentum (OAM) compatible with radio-frequency (RF) photoinjector drive-laser systems. While OAM generation is routine at visible and infrared wavelengths, achieving high-OAM beams in the deep UV (specifically 266 nm) remains technically challenging. Standard devices like spatial light modulators (SLMs) and digital micromirror devices (DMDs) suffer from strong absorption, laser-induced damage, and low diffraction efficiency in the UV range. Existing diffractive solutions present trade-offs: binary spiral zone plates (SZPs) are limited to ~40% efficiency; fork gratings offer flexibility but restrict OAM values to discrete multiples and often require tight focusing; and spiral phase plates (SPPs) demand nanometer-scale surface accuracy over large apertures while withstanding high UV fluences. No single existing method provides a universal, high-fidelity, and robust solution for high-order OAM generation under realistic photoinjector conditions.

Methodology
The authors experimentally demonstrated the generation of deep-UV twisted light by integrating three distinct types of fabricated diffractive optical elements (DOEs) into an operational RF photoinjector drive-laser system. The experimental setup utilized a 266 nm laser beam, which was spatially filtered and directed onto the DOEs. The generated beams were characterized using cylindrical-lens mode conversion (transforming Laguerre–Gaussian modes to Hermite–Gaussian modes) and radial intensity analysis.

The three DOE types investigated were:

1. Reflective Fork Grating: A binary mask with a topological charge $m=2$ laser-engraved onto an aluminum mirror. This element generates optical vortices in diffraction orders where the OAM $\ell = n \cdot m\hbar$.
2. Spiral Phase Plate (SPP): A high-topological-charge ($m=64$) element fabricated in fused silica using multi-level photolithography and high-precision plasma etching. It features a helical surface relief with a total height variation of approximately 34 µm to induce a $2\pi$ phase shift per sector.
3. Binary Axicons: Elements with topological charges $m=3$ and $m=10$ fabricated in SU-8 photoresist on leucosapphire substrates. These generate quasi–Bessel beams interpreted as controlled superpositions of multiple OAM states.

Numerical simulations using an angular-spectrum propagation framework and Rayleigh–Sommerfeld integrals were employed to model the field propagation, verify expected OAM values, and compare theoretical intensity distributions with experimental results.

Key Results

• High-OAM Generation: The system successfully generated UV vortex beams with OAM values up to $\ell = 64\hbar$ at a wavelength of 266 nm.
• Spiral Phase Plate (SPP) Performance: The SPP produced a high-purity Laguerre–Gaussian mode with a conversion efficiency of approximately 80%. The beam exhibited a well-defined annular structure with a central singularity and low divergence, suitable for photocathode illumination.
• Fork Grating Performance: The reflective grating provided flexible access to lower-order OAM states ($\ell = \pm2\hbar, \pm4\hbar, \dots$) corresponding to diffraction orders. It demonstrated robust modal diagnostics, with observed Hermite–Gaussian modes confirming the relation $N = |\ell| + 1$.
• Binary Axicon Performance: These elements generated low-divergence quasi–Bessel beams characterized by segmented ring-shaped intensity profiles. The OAM spectrum for these beams was broadened, representing a superposition of multiple OAM states with a finite bandwidth, rather than a single pure eigenstate.
• Fabrication and Robustness: All elements were fabricated using scalable processes compatible with fused silica, SU-8 resists, and metal-coated substrates. The devices demonstrated stable operation under high-intensity UV irradiation without damage.

Significance and Claims
The paper claims to present the first experimental realization of deep-UV high-OAM beams based on multiple types of fabricated DOEs integrated into a single RF photoinjector drive-laser system, validated through mode-conversion diagnostics. The work provides a comparative, application-driven framework for selecting UV-compatible OAM generation techniques, addressing the specific material limitations of the deep UV band.

The authors position these results as a practical and scalable solution for implementing controlled OAM states in high-brightness photoinjectors. This capability is directly relevant to accelerator-based light sources, including free-electron lasers, synchrotron radiation facilities, and Compton $\gamma$-ray sources, where precise control of transverse beam structure is required. The ultimate scientific goal identified is the generation of relativistic vortex electron beams carrying quantized OAMs for fundamental physics applications, such as analyzing proton spin structure and developing quantum electron microscopes. The study establishes a direct connection between diffractive phase design, numerical propagation, and structured light generation in the deep-UV regime, offering a route toward structured photocathode illumination.