When designing materials and structures at the nanoscale, it is important to be able to characterize and measure them. Getting tools to measure on these very small length scales can be challenging as it often means performing very high-resolution imaging techniques such as electron microscopy or super-resolution optical microscopy.
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Microscopy images can be used to try and extract information such as distances between objects and the sizes of objects. Using easy-to-use tools that could rapidly profile the dimensions of nanoscale objects, such as nanoparticles, would be useful for materials synthesis and manufacture.1 Many properties of nanomaterials, including optical, are dependent on the size of the nanostructure, and so it is important the dimensions can be well characterized.
Recently, a new approach has been used to measure nanometric displacements using a so-called optical ruler.2 The optical ruler is an electromagnetic analog of a traditional, physical ruler but the key difference is an optical ruler is capable of making measurements at the nanoscale.
When measuring distances as short as nanometers, the measuring tool needs to be both very accurate and precise. At these length scales, even techniques like certain kinds of electron microscopy that are theoretically capable of achieving 0.1 nm spatial resolutions, it becomes essential to start accounting for even small environmental fluctuations such as vibrations or scanning noise to avoid image blurring and worsening the resolution.3,4
In highly accurate metrological measurements, interferometers have become a staple tool. One of the most famous examples of an interferometer being used to detect incredibly small spatial displacements – 10-18 m – is the LIGO interferometer which is used to detect gravitational waves.
An interferometer works by using the interference of multiple waves to make measurements. Depending on the phase relationship between the waves interfered, a pattern of constructive or destructive interference can be measured.
In the simplest interferometer designs, a beamsplitter is used to split the incident electromagnetic light that passes down two arms of the interferometer. Usually, the path length of one arm can be varied to change the relative phase relationship between the waves. One beam passes through the object of interest and the other acts as a reference. The two beams are recombined onto a detection screen to provide an interferogram.
There are many different interferometer designs, from the classic Michelson geometries to more complex Sagnac interferometers. What makes interferometers such powerful tools in metrology is the accuracy with which they can measure various parameters.
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For the optical rulers, the interferometer was capable of achieving a resolving power of better than 1 nm.2 This gives an overall resolving power of ~ λ/4000, which starts to open the possibilities of being able to resolve single atoms. Even for many electron microscopy methods, which are normally the standard approach to measuring nanostructures, achieving sub-nm resolution is challenging.
The advantage of this new optical ruler method is that it is relatively cost and size efficient compared to the bulky and highly-expensive electron microscopy apparatus. The interferometer set-up consisted of an 800 nm semiconductor laser and then the key optical component – the Panchartatnam-Berry phase metasurface.
The Panchartatnam-Berry phase metasurface was used to shape the gradient of the phase of the incident light. This shaping to form a high phase gradient was essential to the application as it is necessary for the modulus of the local wave vector to exceed the free-space wavevector to be able to resolve zones of local wavevectors and achieve the necessary wavelength resolution.
The overall device performed well against a more standard super-resolution metrology set-up consisting of a micrometer scale monolithic interferometer. The team is optimistic that their new device will offer superior performance as it is less affected by thermal and mechanical instabilities that larger interferometers are subject to. The device also uses optical light and, compared to similar super-resolution microscopy methods, does not rely on heavy data processing or the need for long acquisitions or high intensities.
There are a number of expansive international collaborations and competitions underway to continue the nanoscale technological revolution.5 Some developments include the creation of new nanoparticles to act as highly efficient catalysts of chemical reactions or as antimicrobial particles that can be added to drinking water. In contrast, others focus on the development of tools like the aforementioned optical rulers that can be used to characterize these new nanomachines and technologies.
The behavior and properties of many nanoparticles are highly size-dependent. Small, efficient metrological devices that can be integrated into synthesis and manufacturing procedures will help rapidly characterize and sort particle types.
As it becomes more straightforward to manufacture some of the specific metasurfaces required for the optical components used to shape light beams are required for these types of interferometers, it may be that optical rulers become something of a staple characterization method in the way dynamic light scattering and electron microscopy currently are.
References and Further Reading
West, P., & Mecartney, Æ. M. L. (2008). A comparison of atomic force microscopy ( AFM ) and dynamic light scattering ( DLS ) methods to characterize nanoparticle size distributions. J Nanopart Res, 10, 89–96. https://doi.org/10.1007/s11051-008-9435-7
Yuan, G. H., & Zheludev, X. I. (2019). Detecting nanometric displacements with optical ruler metrology. Science, 775(May), 2–5. https://doi.org/10.1126/science.aaw7840
Smith, D. J. (2008). Ultimate resolution in the electron microscope? Materials Today, 11(SUPPL.), 30–38. https://doi.org/10.1016/S1369-7021(09)70005-7
Jones, L., & Nellist, P. D. (2013). Identifying and correcting scan noise and drift in the scanning transmission electron microscope. Microscopy and Microanalysis, 19(4), 1050–1060. https://doi.org/10.1017/S1431927613001402
Crew, B., Payne, D., & Plackett, B. (2022). How cross-border collaboration underpins the nanoscience revolution. Nature, 608(7922), S4–S5. https://doi.org/10.1038/d41586-022-02148-2
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