Bimetallic photonic metamaterials

Atmatzakis, Evangelos (2016) Bimetallic photonic metamaterials. University of Southampton, Faculty of Physical Sciences and Engineering, Doctoral Thesis, 156pp.

Record type: Thesis (Doctoral)

Abstract

In this thesis, I introduce bimetallic ring resonator arrays as a platform for enhancing and controlling magneto-optical and thermoelectric effects. By taking advantage of differences in the physical (optical, electronic, thermal, magnetic) properties of the two elements comprising the resonators, I have demonstrated tailoring of the optical absorption and the Faraday effect, and generation of THz magnetic pulses. In particular:

I have theoretically predicted for the first time the light-driven generation of intense, highly-localised, THz magnetic pulses in thermoelectric metamaterials. Excitation of bimetallic plasmonic resonators by ultrafast electromagnetic pulses, raises the electron energy and causes them to diffuse towards colder regions due to the thermoelectric effect. In addition, the resonator design and the excitation polarization create a situation in which electrons are forced to migrate in the azimuthal direction along the circumference of the ring, giving rise to a quasistatic magnetic field of up to 0.35 T, localized in an area with diameter of 70 nm. The time dynamics of the pulse generation process are controlled by electron energy relaxation to lattice through electron-phonon collisions, which lead to pulse duration of ~ 1.8 ps.

I have demonstrated experimentally for the first time enhanced Faraday rotation in bimetallic single-layer metamaterials. The design of the unit cell resonator allows light confinement in the vicinity of the magneto optically active ferromagnetic metal which results in enhancement of the Faraday effect. Tailoring the size of the ferromagnetic component, which simultaneously controls the magneto-optical response and the dissipation losses in the system, results in a design which rotates the polarization azimuth of an incident electromagnetic wave by up to 1 mrad.

I have demonstrated for the first time resonant absorption in single-layer Au/Ni photonic metamaterials and investigated its dependence on wavelength and polarization. A hybrid system consisting of materials that exhibit different levels of dissipation losses can be excited in ways which enable control over the spectral position, bandwidth and level of the optical absorption. The plasmonic modes mediating the metamaterial absorption are identified by a detailed computational analysis, which is in good agreement with the far-field experimental observations.

I have fabricated for the first time single-layer, bimetallic ring resonator arrays consisting of noble and ferromagnetic metals. An optimized, multi-step electron beam lithography process has been developed in order to bring together, with sub-10 nm accuracy, the two metallic sectors and form 100 x 100 µm² ring resonator arrays.

I have developed a sensitive polarimeter apparatus to probe polarization rotation in metamaterial arrays. Following the principles of polarization modulation polarimetry, I have built a state of the art experimental setup that resolves angles as low as 10^-5 radians in finite size, strongly scattering samples which have dimensions down to 50 x 50 µm².