|M.Sc Student||Hanukayev Benyahu|
|Subject||Infrared Radiation Absorption Manipulation by Use|
of Two Dimensional Sub-wavelength Metallic
|Department||Department of Mechanical Engineering||Supervisor||Professor Erez Hasman|
|Full Thesis text - in Hebrew|
Metamaterials are periodic or quasi-periodic, sub-wavelength metal structures. The electro-magnetic material properties are derived from its structure rather than inheriting them directly from its material composition. This term is particularly used when the resulting material has properties that are not found in naturally formed substances. This research proposes to use metamaterials for absorption manipulation in thin films. These metamaterials are two dimensional sub-wavelength thin metal gratings, therefore can be treated as layers with artificial complex refractive index. This work shows manipulation of complex refractive index by the grating's parameters, e.g.: grating's period, duty cycle and different structure forms. Unlike other absorption methods where IR absorption manipulated by changing the thickness of uniform metal films, the advantage of using metamaterials is that by one element with constant uniform thickness, one can manipulate the space variant absorption level, by grating's parameters.
Furthermore, this work shows that absorption dynamic range can be manipulated from 0% to 100% absorption, in resonant structures.
Usually, regular uniform absorbing films are very thin, about few nanometers in thickness, what makes difficulties in mass production process. Because of a strong non-uniformity in these thicknesses, the absorption level in the absorbers exploits, what finally causes, e.g. in thermal detectors applications, to non-uniformity between different pixels in the detector, and between different batches of detectors. Using metamaterials avoids problems related with absorption non-uniformity and with mass production process difficulties, which occur in regular uniform, very thin absorbing films.
In order to understand the absorption mechanism in multiple layer structures, transmission line model and lumped circuit’s approximation were used. By use of lumped circuit's approximation, the maximum possible absorption in the absorber was determined. These were made for conductive layer as on semi-infinite dielectric layer, and as in the resonant structures. Optimal absorber was proposed, using metamaterials in microbolometer detector applications. The advantages of using metamaterials in resonant structures, such as absorption stability for thickness non-uniformity, were demonstrated.
Numerical method, called FDTD (Finite Difference Time Domain) was used for the reflection and transmission amplitude calculations of the sub-wavelength two-dimensional metal gratings.
In the theoretical part of this work, matrix method was demonstrated as an instrument for reflection and transmission intensity calculations in multiple absorbing layer configurations. Different methods for complex refractive index calculations were shown. Among these methods one method was chosen for complex refractive index calculations of metamaterials.