Previous blogs have mentioned the need for improved tools and the goal of at least one order of magnitude greater capability than the objects attempting to be evaluated. That development is beginning to become available. The May/June issue of Photonic Focus (page 15) has a summary of a paper [Ref. 1] that addresses enhanced resolution of x-rays. As mentioned in a few blogs including last month’s, the resolution limit of light is determined by its wavelength. The shorter the wavelength, the smaller the image that can be resolved. That is one of the reasons that the semiconductor industry invested so much time and funds in getting EUV lithography to work. With a wavelength of 13.5nm, the limit for just resolving images is only a few nanometers. The article mentioned refers to an “Achromat”, which is an optical device that separates a light beam and then recombines the images to produce the results. The results are sharp optical images in photography and microscopy. Still size limited by Snell’s Law. The article mentioned describes a similar type arrangement at the Achromat, but employs x-rays as the source. The proof-of-concept microscopy employed a synchrotron. Not practical for most cases. However, the initial efforts in EUV Lithography were built on X-ray Lithography which employed a synchrotron. Something to keep an eye on for developments.
The second reference describes a method of improving the stability and imaging time of Tip-Enhanced Raman Spectroscopy (TERS). One of the challenges that drove this development is the need for developing minute details across large samples. (Large in this case is micron sized surfaces.) Why is this important? As the size of the materials that are being employed in specific applications shrinks, the need to guarantee that the surface is exactly as specified becomes critical. From experience, I know of a superior sensor that was developed about 15 years. It employed sheets of graphene. That never became a product due to the fact that the graphene had random defects, and there was no instrumentation available to detect the defects. The need for tip enhancement is due to the fact that in the typical Raman Spectroscopy process for surfaces that can be deformable, the stability time is on the order of milliseconds. The authors have developed the methodology to permit longer imaging times with increased scanning areas, and better resolution. Obviously, much more work is required, but a direction has been demonstrated for additional efforts.
The following is about work performed at Duke University [Ref/3]. Their work has been referenced with respect to metamaterials in blogs on optics. To fully appreciate the work they have done, it is necessary to explain some of the background on their work. In order to control light, it is necessary to create structures that enable conditions that can be considered negative indices of refraction. When lights impinges on a metasurface, the light frees electrons in the metal so that it creates an oscillation. With the appropriate structure the light is effectively absorbed. Their efforts have trapped light beneath the surface. These metasurfaces consist of a base metal layer with a nanometer layer of transparent material in specific shapes. The top of this three layer structure is a lay of silver nanocubes. The entire structure is only several nanometers thick. Colloidal chemistry enables the ability of synthesis of shaped nanomaterials across large areas – even wafer sized ones. Their efforts inverted the layers and created nanosized indents in the surface. This process permits the construction of different sized/shaped increases the wavelengths that can be modified with one structure.
Tools and processes are being developed to work at scales that we impossible to even observe only one or two decades ago. There is interesting work [Ref. 4] on using the chirality of materials to enable an entirely new field for controlling the properties of electromagnetic waves. That is left to the reader to explore if interested.
- A. Kubec, et al., Nat. Comm., 2022, doi: 10.1038/s41467-022-28902-8)
- (Xu et al., Adv Photon., 2022, doi: 10.1117/1. AP.4.4.046004)