In June’s blog, invisibility cloaking was covered. While that is a type of optical metamaterial, the advantage of optical metamaterials is that they can provide the ability to extend the range of traditional optics. (While the work has been ongoing for years, there is not the impact of the invisibility cloak.) There is a rule based on the wavelength of light called Rayleigh’s Limit that provides the limit of the smallest objects that can be defined. Blue light is in the range of 450nm and green light is 550nm. The Rayleigh Limit predicts that the smallest separation between two points that can be detected is 56nm using blue light and 69nm using green light. This is the theoretical limit at which two points can be separately identified by perfect optics. This is not the minimum that structure can be identified, which is much larger.
So what is the big deal? Semiconductors devices have features in the low nanometer range and they can be inspected. Yes, features smaller than 10 nm can be visualized employing various types of electron microscopy because the material being “viewed” is a solid surface. The limitations of optical microscopy have the greatest impact on biological work. Many of the investigations in this filed work with objects that are small, transparent, and have little contrast difference in the object. This includes viruses and DNA molecules. Bright field microscopy limits the resolution to approximately 200nm. [Ref. 1]
The challenges in manufacturing the optical metamaterial are a combination of both finding the proper materials to create a negative index of refraction and creating layers of the required thickness to become a metamaterial. Work done and published in 2007 [Ref. 2] indicated that using positive and negative layers of refractive index material, they were able to achieve a resolution of 70nm. Work presented in Reference 3 provides more information on the state of the effort in 2014. “By using 15 nm TiO2 nanoparticles as building blocks, the fabricated 3D all-dielectric metamaterial-based solid immersion lens (mSIL) can produce a sharp image with a super-resolution of at least 45 nm under a white-light optical microscope, significantly exceeding the classical diffraction limit and previous near-field imaging techniques.” Additional work in 2016 [Ref. 4] demonstrated 3D resolution of sub 50nm across the plane and 10nm in depth. Current research efforts include the application of metamaterials and the inclusion of immersion techniques.
The focus of the metamaterial enhanced lenses is to provide a better understanding of the interaction of biostructures that are beyond the limit of optical microscopy. The challenges moving forward are numerous. The application of various layers to create the negative index is dependent on the material being employed and achieving the proper thickness of each layer. Defects in the layers reduce the resolution of the image. Fortunately, the production of precise layer thickness can be accomplished by available tools. Atomic Layer Deposition (ALD) is available with existing semiconductor manufacturing tools. Even the ability to create structures can be accomplished with existing tools. The question that remains is how small a dimension will be able to be analyzed optically. Progress is needed to advance biological/medical research.