Control of electromagnetic wave propagation and emission with metamaterials and plasmonic structures
Student thesis: Doctoral Thesis
The control of light is important in different areas of applications, for example, in information processing and in communication. Metamaterials, being constructed with artificial subwavelength structural units, have been recently proved to be very effective to manipulate light in exotic ways. Negative refraction or a negative refractive index is a prominent example. While the first negative-index metamaterial is based on wires and split-ring resonators, there are huge efforts in searching for alternative routes, which may provide a larger bandwidth, a lower loss or a larger scalability to bulk metamaterials. Here, we have formulated a geometric approach called space-coiling to achieve both a negative refractive index and a conical dispersion without using local-resonating units. Proof-of-concept experiments are carried out to construct two-dimensional isotropic negative index metamaterials, with current experimental figure-of-merit (|Re(n)/Im(n)) around 10. This can give us insight into the loss issue, the possibility of scaling down a unit cell to the deep sub-wavelength limit and the broadband control of slow light. Recent developments of metamaterials point to lower symmetries to gain additional control of light impinging on a metamaterial slab. For example, by engineering the cross-coupling between electric and magnetic fields, artificial chirality, strong optical activity and asymmetric transmission, in addition to a negative refractive index, can be achieved. However, the commonly employed S-parameters retrieval method for extracting the effective medium parameters is more difficult to be applied due to the need of solving a simultaneous set of equations involving all the eigenmodes for each particular case. Here, we have developed an extended S-parameter retrieval method for extracting the effective medium parameters of thin metamaterials with generally low symmetry. A closed-form expression relating the S parameters and the general constitutive matrix is obtained, with radiative correction and with anti-resonance artifact for the extracted constitutive parameters being avoided. It can act as a guide for future designs of metamaterials with cross-coupling and with various symmetry breaking effects. Metamaterials, due to their flexibility in dispersion engineering and hence on the photon density of states, are also very useful for manipulating emission properties. To an extreme, we have developed a magnetic metamaterial to enhance magnetic dipole emission with a special magnetic artificial atom, which is constructed with double gold patches. It is found that the spontaneous emission of a magnetic dipole transition can be significantly enhanced through a magnetic hot-area. The Purcell factor of nearly 2000 can be obtained at optical frequencies together with a low sensitivity in spatial and spectral mismatches between the light emitter and the resonance mode. The associated resonance can be tuned from the visible to the infrared frequencies, enabling efficient control of forbidden transitions using plasmonic structures. Consequently, by using metamaterials, we can now not only steer the far-field propagations of electromagnetic waves, but also engineer the near-fields and hence the emission properties of light.
- Electromagnetic waves, Plasmons (Physics), Transmission, Metamaterials