Optics of Metamaterials and Metasurfaces
Figure 1 - Hyperbolic metamaterial based on nanorod medium. Extraordinary modes are excited via scattering of Gaussian beam by microobject located on the surface of the metamaterial.
This area gives absolutely new spin for the fields of metamaterials and opto-mechanics by interfacing between them. Historically, one of the key objectives of metamaterial research was achieving negative index of refraction by artificial structuring. Since then, the scope is changing and new applications come into consideration. One of the approaches is to utilize unique optical properties of metamaterials towards opto-mechanical manipulation. In particular, we currently study light scattering and optical forces on small objects, situated in the close proximity, or embedded inside metamaterials, based on metal dielectric layers on vertically aligned metal nanorods. Unit cells of both structures have deep subwavelength nature. One of the recent proposals shows the ability of complete scattering suppression from objects, situated inside layered structures with effectively vanishing dielectric permittivity along the layers. Tailoring scattering patterns in this way makes objects to be invisible to an external observer, and gives rise to variety of intriguing and practical applications. Another outcome of the ability to control scattering with metamaterials is directly related to optical forces. We recently showed that ‘tractor beams’ could be straightforwardly realized with the help of metamaterials by utilizing their ability to enhance optical density of states. ‘Tractor beams’ attract particles to illumination sources by overcoming radiation pressure and provide additional degree of freedom to manipulation techniques. The picture above demonstrates the realization of ‘tractor bema’ – extraordinary waves inside nanorod metamaterial are excited via scattering from a big particle, situated on top of the layer. Small particle inside (blue sphere), being situated in the field of highly confined extraordinary wave exhibits attraction.
Figure 2 - Antireflecting metasurface based on 2-dimensional array of subwavelength tapered-cone micropores in substrate. AFM-microphotography and pores profile.
Another research topic deals with metasurfaces for photo-voltaic applications. Antireflection coatings based metasurfaces are investigated for enhancing photo-absorption and, as the result, for improving efficiencies of thin solar cells.
We showed that nano-structured film, made of periodic array of holes, enables suppression of scattering in the broad spectral range, in contrary to common intuition suggesting narrowband operation. The effect is based on carefully designed frequency-dependent phase shift, mimicking broadband quarter wave plate. This approach, in fact, relies on absolutely new concept, and gives rise to a new family of anti-reflection coatings.