Optical Vortex Discovery Could Lead to Microscopy, Fiber Optics Advances

3D ring-shaped light structures, generated by high-intensity lasers, have been identified and could lead to new opportunities for the use of lasers in microscopy and telecommunications.

Researchers at the University of Maryland have detected a novel type of vortex, called a spatiotemporal optical vortex (STOV), that has phase and energy circulation in a spatiotemporal plane. The STOV forms a ring around a self-focusing light pulse; and the light flows through the inside of the ring and then loops back around the outside. Light waves curl around the vortex, similar to air currents around a smoke ring.

The STOVs travel along with the laser pulse at the speed of light and control the energy flow around the laser. Unlike other laser vortices, STOVs are time dynamic.

Spatiotemporal optical vortices, or STOVs (thin, gray ringlike objects), are newly described three-dimensional light structures that strongly resemble smoke rings. Unlike other laser vortices, STOVs are time dynamic, which means that they travel along with the central laser pulse. Compared to other laser vortices, STOVs could prove more broadly useful for engineering applications. Courtesy of Howard Milchberg.

"A STOV is not just a spectator to the laser beam, like an angel's halo," said professor Howard Milchberg, noting the ability of STOVs to control the central beam's shape and energy flow. "It is more like an electrified angel's halo, with energy shooting back and forth between the halo and the angel's head. We're all very excited to see where this discovery will take us in the future."

In order to experimentally confirm the existence of STOVs, the researchers imaged the spatiospectral phase and intensity profiles of femtosecond (fs) laser pulses midflight during their precollapse and postcollapse evolution in air. They used an air-helium interface to image the in-flight beam intensity and phase profile. The air-helium transition layer was thin enough to enable distortion-free imaging of the air filament cross section. The interface allowed midflight filament intensity and phase imaging over the full propagation path.

The researchers found that STOVs form naturally as a consequence of arrested self-focusing collapse, and their dynamics influence subsequent pulse propagation. They learned that STOVs could be imposed linearly via prescribed spatiotemporal or spatiospectral phase shifts, making possible their engineering for applications.

The researchers expect that STOVs are a fundamental and ubiquitous element of nonlinear propagation of intense pulses.

"Lasers have been researched for decades, but it turns out that STOVs were under our noses the whole time," said Milchberg. "This is a robust, spontaneous feature that's always there. This phenomenon underlies so much that's been done in our field for the past 30-some years."

Orbital angular momentum (OAM) vortices (pink ringlike objects) are three-dimensional laser light structures that rotate around a central beam, much like water circles around a drain. Physicists and engineers have studied this type of laser vortex since the 1990s as a tool to help improve microscopy and telecommunications. Courtesy of Howard Milchberg.

Orbital angular momentum (OAM) vortices, where light energy circulates around the beam propagation direction much like water rotates around a drain, are well-known from prior research. Because they can influence the shape of the central beam, they have proven useful for advanced applications such as high-resolution microscopy.

"Conventional optical vortices have been studied since the late 1990s as a way to improve telecommunications, microscopy and other applications. These vortices allow you to control what gets illuminated and what doesn't, by creating small structures in the light itself," said researcher Nihal Jhajj.

The smoke ring vortices we discovered may have even broader applications than previously known optical vortices, because they are time dynamic, meaning that they move along with the beam instead of remaining stationary," Jhajj added. "This means that the rings may be useful for manipulating particles moving near the speed of light."

Jhajj and Milchberg acknowledge that more work needs to be done to understand STOVs; but they are excited about potential opportunities to apply their discovery of STOVs. With the potential to travel with the central beam at the speed of light, STOVs could have as-yet unforeseen advantages in technological applications, including the potential to expand the effective bandwidth of fiber-optic communication lines.

"All the evidence we've seen suggests that STOVs are universal," Jhajj said. "Now that we know what to look for, we think that looking at a high-intensity laser pulse propagating through a medium and not seeing STOVs would be a lot like looking at a river and not seeing eddies and currents."

The research was published in Physical Review X (doi: 10.1103/PhysRevX.6.031037).