Tighter hologram lights with nano zinc

Researchers out of China and Singapore have discovered that zinc oxide nanoparticles may be the secret to finally achieving high-resolution 3D holograms that are often promised in science fiction but yet to be delivered in reality.

Although rudimentary 3D light shows that play off particles in the air, reflections to trick the eye, spinning lights, or projectors to present an illusion of depth have become more prevalent in recent years, a true-blue 3D hologram has seemingly escaped the realms of possibility for decades.

While the technology has advanced enough to resurrect legendary performers like Frank Sinatra, Elvis Presley, Michael Jackson, and Tupac Shakur, to name a few, for convincing stage shows, these holographic projections typically lack convincing three-dimensionality.

In technical terms, low axial resolution – which is equivalent to the distance from the nearest image plane in focus to the farthest field in focus, also called depth of field – and high levels of crosstalk interference between projection planes have long prevented 3D holograms from achieving finer depth control.

In simpler terms, we can't stop light within a specific space. At least, not without something physically bouncing the photons around, ultimately crushing the dream of stabilized light projections.

Now, however, a research team from the University of Science and Technology of China and the National University of Singapore have reported a new technique that possibly solves the problem.

A lesson in light

"Our work presents a new paradigm towards realistic 3D holograms that can deliver an exceptional representation of the 3D world around us," said the senior author of the accompanying paper, Lei Gong, associate professor of optical engineering at the University of Science and Technology of China.

Gong and colleagues have taken to calling this new technology scattering-assisted dynamic holography.

"The new method might benefit real-life applications such as 3D printing, optical encryption, imaging and sense, and more," Gong continued.

Large-scale 3D holograms are typically created by scattering a projection across many planes to create a stack of pixels that, when viewed together, give the impression of a virtual, 3D object. Stacking these image planes close together generally creates high-density images; however, increasing the plane density can also generate interference in the form of crosstalk, the senior author noted.

"In short, cross talk is the mutual-intensity interference between images projected at different depths,"

he added.

Even though a projection is broken into separate image fields, interference can occur when light from one plane filters through into others – for example, like a pen bleeding through from the front of a sheet of paper to the back. This filtering can cause interference because the light for each plane contains unique pixel information; as a result, blurred images are created when this light bleeds through.

Essentially the light bounces off each other, and the further from the projector, the more scattered they become.

"[This] restricts the depth control of 3D projections," Gong said.

In order to increase image-plane depth without creating blurry images, Gong and team focused on how to shape the projection photons before they become scattered into the air.

Particle-vision

Typically, a hologram projection is controlled by passing it through a spatial light modulator, which is a medium made of physical and reflective barriers that modulate a light beam's amplitude, phase, and intensity. A prime example of this is liquid crystal displays or LCD televisions, which is also a type of SLM.

However, these SLMs alone can limit the final hologram's resolution. To solve this and the crosstalk issue, the team introduced a scattering medium made from zinc oxide nanoparticles to help further scatter the projection's light.

Introducing this additional medium served to increase the total amount of scattering the light experienced and created a greater range of diffractive angles.

This greater range of angles, thus, made it possible to decorrelate, or disentangle, the image fields to reduce pixel correlation between planes, i.e., the light would not bounce off other photons but nanoscale mirrors.

As a result, crosstalk interference via light bleeding through the image layers was reduced.

With this handled, the team was then able to stack the image plans more densely with less depth between them which enabled an increase in the 3D experience of the projection.

To verify the effectiveness of this approach, the researchers used both simulated and experimental scenarios.

Where other scattering approaches limited the number of image layers to roughly 32 at a depth of 3.75 millimeters, simulations of the new scattering approach could generate 125 image layers at a depth interval of just under 1 millimeter.

While no metrics yet exist to quantify holograms, one can imagine the difference between 720 pixels and 1080p resolution – a much sharper image.

So far, in experimental trials, the researchers have reported reduced depth intervals and plane density compared to existing scattering techniques while generating minimal crosstalk – thus, a much sharper hologram.

In addition to improving naked-eye 3D hologram visualizations and improving the viewing angle of holographic virtual reality headsets, Gong said that this technology could also be implemented beyond VR as well.

"For the biomedical field, it might be used to project the 3D medical images that help the diagnosis and treatment."

Nevertheless, before these advancements can truly take place, the researchers say the technology will need to move from point-cloud projections to solid ones. In order to bridge this gap, the team says it will be necessary to develop new algorithms to handle the complexity of these 3D images.

"A much higher pixel-count hologram is required to project complicated 3D scenes," the senior author said. "New algorithms, such as learning-based methods, should be developed for this purpose."