Unscrambling holograms gains high-res images

David L. Shenkenberg, david.shenkenberg@laurin.com

There’s a catch-22 to cameras. Either they have high resolution and a narrow field of view, or they have a wide field of view at a loss of resolution.

One reason why resolution is poor is that many light rays never make it to the detector because they are too weak or they go off to the side instead of straight through the lens. Some cameras have been rigged to scan very close to objects to capture more of these light rays. Then these images can be pieced together to get a wide field of view.

This means putting a camera really close to an object and taking a bunch of pictures. This course of action is not always wise or feasible, especially when that subject is dangerous, hostile or otherwise hard to reach.

In the case of a microscope and camera setup, putting the objective really close to the sample can damage both the sample and the objective if the objective inadvertently touches the sample. Moreover, taking bits of images at different time points and stitching them together is no good if the experiment is time-sensitive.

What if high resolution and a wide field of view could be obtained all at once without the objective’s being so close to the object? That’s what assistant professor Jason Fleischer of Princeton University is wondering. He and grad students Christopher Barsi and Wenjie Wan decided to project a hologram of an object under study because this can be done relatively far away from the object.

Jason Fleischer, an assistant professor at Princeton University, adjusts the green laser setup that was used to create holograms. Courtesy of Frank Wojciechowski.

The researchers not only created a hologram but also used an unusual material to do it: a crystal of strontium barium niobate. This material mixes the light non-linearly, which results in a scrambled hologram.

At this point, an ordinary person might suppose that the hologram is worthless because it’s all mixed up, but the researchers realized that it contains more light rays than an ordinary lens can pick up. All they have to do is unscramble the hologram.

In a way, these light rays can be thought of as optical information and the hologram as a message. The researchers decoded the message mathematically.

There are some more details. The researchers fired a green laser through a chart and then through the crystal to create the hologram. They determined the amplitude and phase of the light field and input those variables into Schrödinger’s equation to unscramble the image. The setup also involved a reference laser beam that helped them determine the amplitude and phase. This experiment is detailed in the April issue of Nature Photonics.

At this point, the work is preliminary, and it could go down several different paths. The principle of encoding and decoding the image could be used for data encryption or for tomography. The method also improve microscopy or lithography. Or it could simply help them understand how weird materials like strontium barium niobate mix up light.