Last time we discussed the physics behind the polaritonic lensing mechanism that can be used to beat the diffraction limit. Hexagonal Boron Nitride crystals can use these polaritons to limit-destructive interference by linearly propagating polaritons along their positive permittivity axis. Boron Nitride is famous for having a birefringent crystal configuration: one axis of the crystal has a positive permittivity and easily conducts polaritons, while the orthogonal axis has a negative permittivity and reflects light away. The presence of both these permittivities in the same crystal allows Boron-Nitride to have superlensing applications by coupling decaying light rays into polaritons and retaining image information that is normally lost.

Superlensing functions by placing a slab of Boron-Nitride directly on top of an object and illuminating the object using a high frequency ray. As soon as light rays reflect off of the object, the photons are coupled into polaritons in the Boron-Nitride. This mechanism retains information about the light rays in the form of polaritons before the photons can decay hence retaining greater image information. For use in a lens these crystals have to be oriented such that the positive permittivity axis is along the line of observation from the object to the detector in order for superlensing to take place.

A great advantage of superlensing is the ability to modulate magnification. Different frequencies of light produce polaritons with different permittivity values1. Generally, the higher the frequency of light, the higher the permittivity of the resulting polaritons1. When the permittivity of a polariton is exceptionally high, it is possible for the polaritons to “jump” along the negative permittivity axis between two parallel layers of Boron-Nitride1,2. Since higher frequency light rays are more likely to produce exceptionally high permittivity polaritons, these frequencies are more likely to travel further in the negative axis of the crystal2. The movement of polaritons in the negative permittivity axis “stretches”the image and causes it to appear larger than the object. This stretch results from a cone-like propagation of the polaritons in the crystal, a characteristic trait of a magnifying lens.

Figure 1: Magnification through a concave lens. The “stretched” image appears larger than the incident light source.

Figure 1: Magnification through a concave lens. The “stretched” image appears larger than the incident light source.

Super-lenses can beat the diffraction limit of light via a unique polaritonic lensing mechanism that limits destructive interference. This lensing method is only accessible to hyperbolic crystal structures like hexagonal Boron-Nitride that have opposite primitives along orthogonal axis. The minification capability of polaritonic lenses is dictated by the frequency of the observational light rays and can be increased by using higher frequency light rays. This modulation capability allows super-lenses to achieve infinitely high magnifications.

References:

  1. Decoopman, T.; Tayeb, G.; Enoch, S.; Maystre, D.; Gralak, B. Physical Review Letters 2006, 97 (7).
  2. Wypych, A.; Bobowska, I.; Tracz, M.; Opasinska, A.; Kadlubowski, S.; Krzywania-Kaliszewska, A.; Grobelny, J.; Wojciechowski, P. Journal of Nanomaterials 2014, 2014, 1–9.

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