For centuries, scientists have been studying methods to overcome the diffraction limit of light. Currently, most optical instruments use traditional, photon-based lenses that magnify objects by refracting light. These lenses are diffraction limited--that is, their maximum magnification is restricted to half a wavelength of light. This diffraction limit prevents scientists from viewing living matter in its natural state. The limit results from the destructive interference of light rays with other light rays as they propagate through air, which causes some rays to gradually decay before they can be used to magnify objects. Destructive interference becomes especially pronounced when light rays are less than 250 nanometers apart and prevents scientists from viewing cells, viruses and nanostructures in detail.  It is anticipated that the introduction of non-diffraction limited optics to molecular biology can have the same revolutionary effect that optical microscopy had on the field.

Illustration of a diffraction limited system: As the two points come closer together, interference from the light rays makes it difficult to distinguish the  points4

Illustration of a diffraction limited system: As the two points come closer together, interference from the light rays makes it difficult to distinguish the  points4

Some nanomaterials can circumvent the diffraction limit via an alternative lensing mechanism based on phonon-polaritons to produce super resolution images of diffraction limited systems. This lensing technique--known as "hyper" or "super" lensing--uses a subatomic particle called a phonon-polariton to capture evanescent light rays before they decay. Thus, the super lens  retains resolution that is normally lost in traditional lenses. Phonon-polaritons are particles created by the vibration of bonds in crystals as they interact with light. Similar to particles of light, phonon-polaritons have a unique frequency and wavelength, which allows them to retain the image information from the decaying light-rays. Once the image information is collected, the polaritons can propagate all the way through the crystal and be re-emitted again as photons of light on the other side of the super-lens. Through this method, scientists can conserve image information from incredibly small systems and overcome the diffraction limit of light.

Nanomaterial based super-lenses offer a novel way to overcome the diffraction limit of light.


Nanomaterial based super-lenses offer a novel way to overcome the diffraction limit of light. The new era of optical microscopy that results from this discovery will have a profound impact on every area of science but will be particularly significant in molecular biology. Stay tuned to this blog series to learn more about the quest for developing new super-lenses and the exciting physics behind phonon-polaritons.

References

  1. Giles, A.J.; Dai, S.; Glembocki, O.J.; Kretinin, A.V.; Sun, Z.; Ellis, C.T.; Caldwell, J.D. Imaging of anomalous internal reflections of hyperbolic phonon-polaritons in hexagonal boron nitride. Nano Letters 201663858-3865.

  2. Li, P.; Lewin, M.; Kretinin, A.V.; Caldwell, J.D.; Novoselov, K.S.; Taniguchi T.; Taubner, T. Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing. Nature Communications201567507-7515.

  3. Caldwell, J.D.; Kretinin, A.V.; Chen, Y.; Giannini, V.; Fogler, M.M.; Francescato, Y.; Novoselov, K.S. Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride. Nature Communications201455221-5230.

  4. Murphy, D.B.; Davidson, M.W. Fundamentals of light microscopy and electronic imaging. Wiley-Blackwell, 2012, 2.

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