Light can carry angular momentum in two distinct ways. One comes from polarization, which describes how the electric field rotates. The other comes from the shape of the wavefront itself, which can twist like a corkscrew as it travels. This second form, known as orbital angular momentum, has attracted wide interest because it allows light to encode information, interact with matter in new ways, and probe physical and biological systems. Despite this promise, producing well-defined twisted light in free space remains technically challenging, especially when the light originates from small or localized sources.
Recent research reported in Advanced Photonics Nexus demonstrates a route to generating twisted light beams by combining a dielectric multilayer with a patterned metallic surface. The work shows that surface-bound light waves can be converted into free-space beams with controlled angular momentum and polarization. Importantly, the approach avoids several limitations of earlier designs and points toward future integration with single-photon emitters.
From surface waves to free-space beams
Many existing methods for generating orbital angular momentum rely on reshaping a laser beam using holograms, liquid‑crystal plates, or patterned films known as metasurfaces. While effective for large, externally illuminated beams, these approaches struggle when light must be generated directly on a chip or from nanoscale emitters such as quantum dots or single molecules. Such sources cannot uniformly illuminate a structure or arrive at a precisely defined angle, making efficient beam shaping difficult.
The authors address this challenge by using Bloch surface waves—a type of electromagnetic wave that travels along the surface of a carefully designed dielectric stack. Unlike surface plasmons, which propagate along metal interfaces and suffer from significant absorption losses, Bloch surface waves can guide light with much lower losses. This makes them attractive as intermediaries between microscopic light sources and free‑space radiation.
In the reported platform, a multilayer stack of tantalum pentoxide and silicon dioxide supports Bloch surface waves at visible wavelengths. On top of this stack, the researchers fabricate a chiral metasurface made of gold nanorods arranged in concentric rings or spiral patterns. The gradual rotation of the nanorods introduces a defined handedness into the structure, allowing it to control how surface waves are scattered into free space.
How the system works
The process unfolds in three steps. First, an incoming laser beam is shaped to efficiently couple into Bloch surface waves on the dielectric stack. The illumination is circularly polarized (with a ring-shaped beam to favor the coupling with the surface wave and focused onto a flat region at the center of the structure, avoiding direct illumination of the metasurface itself. The video animation explains the working principle.
Second, the excited Bloch surface waves spread radially across the surface. Because of the way they are launched, these waves already carry a structured phase pattern linked to the polarization of the incident light.
Third, when the surface waves reach the surrounding chiral metasurface, they are diffracted upward into free space. The geometry of the metasurface determines both the polarization and the orbital angular momentum of the outgoing beam. By adjusting the number of spiral arms and the orientation of the nanorods, the researchers can select which angular momentum states are produced and which are suppressed.
A key feature of the design is its strong polarization selectivity. The metasurface favors one circular polarization over the other, channeling most of the optical energy into a single twisted‑light state. Experiments show that about 80 percent of the emitted light carries the desired circular polarization, improving beam purity and simplifying the output.
"Our goal was to build a bridge between tiny light sources and the rich world of structured light," says senior author Emiliano Descrovi. "By guiding the light with Bloch surface waves, we can shape it with great precision while avoiding the heavy losses of metallic systems."
Modeling, fabrication, and measurements
The team first validated the concept through numerical simulations, which predicted that most of the diffracted power would be carried by light with the selected polarization and orbital angular momentum. The simulations also confirmed that the vortex charge of the beam follows directly from the combination of the incident polarization and the metasurface design.
To test the approach experimentally, the researchers fabricated the multilayer stacks and patterned the metasurfaces using electron‑beam lithography and gold deposition. They then characterized the devices using a custom optical microscope that images the angular distribution of light in the back focal plane of a high‑numerical‑aperture objective.
The measurements closely matched the simulations. The diffracted beams displayed the ring‑shaped intensity profiles and spiral interference patterns characteristic of light carrying orbital angular momentum, with vortex charges ranging from zero to several units. Crucially, twisted light appeared only when Bloch surface waves were excited, confirming that the effect arises from surface‑wave mediation rather than direct scattering from the metasurface.
Why it matters
The study demonstrates a low‑loss, surface‑wave‑based route to generating free‑space light with controlled angular momentum and polarization. Because the approach relies on dielectric materials to guide the surface waves, it avoids many of the efficiency limits associated with plasmonic structures.
The authors note that the platform could be adapted for use with nanoscale light sources placed directly on the surface. If such emitters can be positioned accurately, the structure could convert their emission into well‑defined twisted beams, potentially even at the single‑photon level. More broadly, the work helps address a longstanding challenge in photonics: linking microscopic emitters to macroscopic structured beams in compact, integrated systems.
For details, see the original Gold Open Access article by N. Marcucciet al., " Chiral geometric-phase metasurface for Bloch surface wave out-coupling in free space ," Adv. Photon. Nexus 5(2), 026008 (2026), doi: 10.1117/1.APN.5.2.026008
