Previous blogs have covered metamaterials for optics including an invisibility cloak. Reference 1 moves into the shorter wavelengths of extreme ultraviolet (EUV) light. It is interesting in that the original work waw focused on attosecond physics. (atto is 10-18) This field of physics is working to understand physical processes like photoelectric effect by creating short pulses of EUV. The image area they were working on was to incorporate a single focal point of 10mm. They were using 50nm EUV light. The final design consisted of a 200nm film etched wit million holes. The key accomplishment is they produced a membrane for 50nm EUV light that behaves like a lens with optical light. Current semiconductor EUV lithography is in the 12nm wavelength range and uses reflective optics. This is not a simple transition to take the existing work and make it into semiconductor masks. But, it shows that it is possible to use metamaterials to focus wavelengths that we currently do not have a good method for achieving the results. This is not a complete project for semiconscious. The researchers indicated the fabrication of this metalens required creating images that were five time smaller than they have previously done to create the focal point. Semiconductors would require a further reduction by a factor of four and be able to design structures that have multiple type of structure images.
An article [Ref. 2] in the July 2023 issue of IEEE Spectrum magazine addressed the challenges in today’s shrinking cameras that exist in phones and many other products. It points out that the most space-consuming part of the camera is the lens. It is typically a difficult trade off. A shorter focal length (small distance to the imaging device) requires a thick center part of the lens (more thickness equals more space). This does not consider that stronger curved create aberrations that distort the different wavelengths of the image, which require additional optics. SO the solution was to replace conventional optical technology with a new technology – the metalens. The metalens is manufactured using semiconductor processing technology to create structures a few hundred microns thick. The article employs an example of a shallow marsh with grass standing in water. The grass moves with the incoming water and changes the position of the grass. Then different height grass would have different effect on the overall picture due to the motion of each individual stalk of grass. The following is directly from the article [Ref. 2]:
“The objects in the scene bounce the light all over the place. Some of this light comes back toward the metalens, which is pointed, pillars out, toward the scene. These returning photons hit the tops of the pillars and transfer their energy into vibrations. The vibrations—called plasmons—travel down the pillars. When that energy reaches the bottom of a pillar, it exits as photons, which can be then captured by an image sensor. Those photons don’t need to have the same properties as those that entered the pillars; we can change these properties by the way we design and distribute the pillars.”
The design incorporates both height and thickness of the ”stalks” to change the characteristic of the incoming energy. There is always more work to create finer structures and more precise heights. However, the metalens is an engineered material construct that can provide wave control functions previously unavailable.