Study reveals ultimate limits of space plates in optical systems

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Engineers working to miniaturize optical systems for modern electronics have found great success with the most familiar components, lenses and optical sensors. It has been more difficult to reduce the size of the third component of an optical system, the free space between the lens and the sensor necessary for the light waves to achieve focus.

Researchers have developed a technology to replace some or all of this free space with a thin, transparent device called a space plate. Now, Cornell researchers led by doctoral student Kunal Shastri and assistant professor Francesco Monticone, along with their collaborators, have for the first time defined the fundamental and practical limits of space plates in a paper published in the journal Optica titled “In what measure can space be compressed Bandwidth limits of spatial plates.

Francesco Monticone, assistant professor of electrical and computer engineering (right), and doctoral student Kunal Shastri discuss the science behind optical systems.

“In the quest for miniaturization of optical systems,” Shastri explains in the paper, “an often overlooked aspect is the large volume of free space between detector and lens, or between lenses, which is essential to allow the light to acquire a distance-dependent and angle-dependent phase and realize, for example, a focus at a certain distance.

The length of free space behind a lens is critical to the lens’ ability to focus an image onto the sensor or onto film, as was the case before digital cameras. The free space allows light waves coming from different directions after the lens to propagate and acquire enough phase to converge on the focal point: the sensor. This is one of the reasons why camera lenses designed to focus and magnify a distant subject, for example telephoto lenses, are so long. Space plates are designed to mimic the optical phase response of free space over a much shorter length.

Monticone, in collaboration with former doctoral student Aobo Chen, had previously used computer simulations to design scalable space plates and to demonstrate how they would work in an optical system. This new work extends this research by defining the limits of a spatial plate’s ability to maximize three fundamental optical parameters: compression ratio, numerical aperture and bandwidth.

“It’s very complicated to achieve these three objectives at the same time”, explained Monticone, “to have a maximum compression ratio and, at the same time, also to maximize the numerical aperture and the bandwidth. In this article, we Let’s try to clarify the general physics mechanism behind any space compression effect, however you implement the space plate.

Previous research into space plate technology had yielded functional but impractical or inefficient designs that worked for a single color, or for a small range of angles, or had to be immersed in a high refractive index material, such as l ‘oil. These devices could not be used to miniaturize typical optical systems.

“There’s a lot of interest in whether space plates would work for the full visible spectrum of light and in free space, and no one was sure we could do that,” Shastri said. “So we really wanted to see if there were any physical limitations that would prevent space plates from working for real cameras over the entire visible bandwidth.”

Shastri explained that the limits they define in this recently published paper will tell other engineers working in the field how far or how close they are to the overall fundamental limits of the space plate devices they are designing. “And that’s, I think, very valuable,” Shastri said. “That’s why we wrote this article.”

Space plates can be designed using the same materials that conventional imaging systems are made from, whether layers of glass and other transparent materials with different refractive indices, a patterned surface or photonic crystal slab – any structure that provides sufficient contrast in the index of refraction to switch from one material to another. The key factor is that the space plate must be highly transmissive; you don’t want it to absorb light.

“In the simplest possible implementation,” Monticone said, “a space plate could be fabricated as a stack of layers, and the layers would have at least two different refractive indices. By optimizing the thickness and spacing, you can optimize the optical response.

Applications of space plate technology are not limited to cameras. Space plates could miniaturize searchlights, telescopes and even antennas using a wider range of the electromagnetic spectrum. Monticone and Shastri are eager to go beyond the computer models they use and design physical experiments with fabricated space plates.

“The next step will be the experimental demonstration of a space plate operating in free space at optical frequencies,” Monticone said. “Using computational design methods, we will seek to optimize the spatial plates to work as close to our fundamental limits as possible. Perhaps we can combine a flat lens and a space plate in a single device, realizing ultra-thin, monolithic, and planar optical systems for a variety of applications.

Eric Laine is a communications specialist for the College of Engineering.

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