Invisibility
radiation to be masked. Microwaves are much easier to manipulate due to their larger wavelengths. Implants for manipulating microwaves are relatively large and can use conventional engineering, whereas implants for manipulating visible light are much smaller and must utilize nanotechnology, 19 a technology that is still in its infancy. Meanwhile, these implants can only manipulate specific wavelengths of light, which will make it difficult to render an object invisible to the entire visible light spectrum. Another difficulty is that a metamaterial invisibility cloak would be extremely thick and impractical for macroscopic objects. 20 Active metasurface cloaking is a method which seeks to circumvent this problem (Fig.8). Metamaterials is still a relatively new field, yet it is experiencing rapid progress. Further developments could be accelerated as it could benefit from separate research into
photonic crystals – photonic crystals also seek to manipulate light at the nano-level. 21 Certainly, metamaterials are the most promising avenue for creating invisibility technology to narrow bands of light within this century. However, it will be difficult to create a metamaterial that can manipulate the entire visible wavelength spectrum.
Figure 7: Concept for metamaterials for invisibility. Humans can observe an object since light is scattered from the surface. However, if light canmanipulate the light around the object and recombined afterwards on their original trajectories, then to the human eye, the object is rendered invisible. 22
Figure 8: Active metasurface cloaking. Scattered light is what allows us to see an object; if the scattered waves are cancelled, then we can render an object invisible. The source antennas create a negative scattered wave which is out of phase – destructive interference cancels out the scattered waves. 23
A different technique of approaching metamaterials involves plasmons, in a technique known as ‘plasmonics’. Surface plasmons (delocalized electron oscillations on the surface of a material), are coherent with an incident light beam under certain circumstances. 24 In other words, plasmons can oscillate at the same frequency as an incident light beam. As a result, the plasmons carry the same ‘information’ as the light beam, except with a much smaller wavelength. Theoretically, light can be ‘squeezed’ , 25 which would allow the squeezed waves to be passed through nanowires. 26 In fact, one Cal Tech group constructed a metamaterial with a silicon-nitrogen insulator sandwiched between two 19 Kaku, M. , p23. 20 TEDx Talks (2014), Extreme manipulation of electromagnetic waves with metamaterials: George Eleftheriades at TEDxUofT, 6 May, Available at: https://www.youtube.com/watch?v=LFxQEUDIAuk&list=WL&index=43&t=0s (Accessed 29 July, 2020): 12:33. 21 Kaku, M., p26.
22 TEDx Talks: 11:09. 23 TEDx Talks: 13:40. 24 Kaku, M., p26-27. 25 Ibid., p26-27. 26 Ibid. , p27.
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