A study finds the ultimate limits of space plates in optical systems

Engineers working to miniaturize optical systems for modern electronics have had great success with the more familiar components, optical lenses and sensors. It has been more challenging to reduce the size of the third component of an optical system, the clearance between the lens and sensor needed for light waves to achieve focus.

Researchers have been developing technology to replace some or all of that free space with a thin, transparent device known as a space plate. Now, Cornell researchers led by PhD student Kunal Shastri and Assistant Professor Francesco Monticone, along with their collaborators, have defined the fundamental and practical boundaries of space plates for the first time in a paper published in the journal. Optics titled “How Far Can Space Be Compressible? Bandwidth Limits of Space Plates”.

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

The length of free space behind a lens is critical to the lens’s ability to focus an image on the sensor or on film, as was the case before digital cameras. free space allows light waves coming from different directions after the lens to spread out 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 at a much smaller length.

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

“It is very difficult to meet these three objectives at the same time,” Monticone explained, “to have the maximum compression ratio and, at the same time, also maximize the numerical aperture and bandwidth. In this article we try to clarify the physical mechanism behind any spatial compression effect, regardless of how you implement the spatial plate.”

Previous research into space plate technology had produced functional but impractical or inefficient designs that worked for a single color, or for a small range of angles, or that needed to be immersed in a material with a high refractive index, such as 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 entire visible spectrum of light and in free space, and no one was sure we could do it,” Shastri said. “So we really wanted to see if there were any physical limits that prevented space plates from working for real cameras across the full visible bandwidth.”

Shastri explained that the limits they define in this recently published article will tell other engineers working in the field how far or how close they are to the global fundamental limits of the space plate devices they are designing. “And that is, 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 of, be it layers of glass and other transparent materials with different refractive indices, a patterned surface or a photonic crystal slab, whatever structure provides sufficient contrast in the refractive index. passing 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 thickness and spacing, you can optimize optical response” . “

Spaceplate technology applications are not limited to cameras. Space plates could miniaturize projectors, telescopes, and even antennas by making use of a broader range of the electromagnetic spectrum. Monticone and Shastri are eager to go beyond the computer models they have been using and design physical experiments with manufactured space plates.

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