5. Enseñanza y aprendizaje
5.2. Currículo escolar
In this section I would like to explain how the proposed waveguides different from those developed earlier and mentioned in the literature review. I will emphasize the novelty of their design and state the new functionalities or conveniences that they bring.
The first waveguide that will be described in this thesis has a planar porous structure and consists of multiple layers of thin polyethylene film that are separated by low-loss air layers of comparable thickness. High porosity of the waveguide together with the low absorption of polyethylene makes this waveguide low-loss in the terahertz region. The lack of confinement in the direction perpendicular to polyethylene layers results in the absence of the low frequency cutoff enabling the broad bandwidth of the waveguide. So far multilayered dielectric waveguides in the terahertz range have been proposed only in the context of their anisotropic properties, and the first prototypes of wave plates have been demonstrated. In this thesis I will present the first thorough
investigation of the optical properties of such waveguides in the broad frequency range as a function of the waveguide geometry. Moreover, multilayered dielectric waveguides can be useful not only for low-loss THz wave delivery but also for sensing of biological and chemical specimens in the terahertz region by placing the recognition elements directly into the waveguide microstructure. The main advantage of the proposed planar porous waveguides is the convenient access to their optical mode, since the major portion of THz power launched into such a waveguide is confined within the air layers. Moreover, small spacing between the layers promotes rapid loading of the analyte into the waveguide due to strong capillary effect. The modal refractive index is easy to adjust by changing the air spacing between the layers, as well as the number of layers in the core.
Further along the text of the thesis several types of hybrid metal-wires/dielectric waveguides will be presented. Their structure is a combination of widely studied porous dielectric fibers and metal wires waveguides, yet it is the first attempt to combine these two types guiding structures. Previously reported composite metal-dielectric fibers are essentially hollow-core fibers not fully exploiting plasmonic guidance in a two metal-wire waveguide. Metal wires structure has been chosen as one of the most promising techniques of making cost efficient, low-loss, low- dispersion, and broadband waveguides in the terahertz range. And the porous dielectric cladding structure has been added to a two-wire waveguide to provide robust mechanical support in order to hold the metallic wires straight and parallel to each other, to optically separate the core region from the environment, and to provide encapsulation for the dry gas in the vicinity of the wires. In such a fiber design, a significant fraction of the modal power is propagating in the air-filled core, which reduces the modal absorption losses as well as the modal group velocity dispersion compared to an empty porous fiber of the same geometry (without metal wires).
Then I describe an alternative approach that allows mechanical support and encapsulation of the metal wires. Having a large fraction of power guided inside of dry low-loss gas is beneficial for reduction of the modal propagation loss. Foam is inherently highly porous, its pores are filled with gas and the pores can be sealed during fabrication. For the aforementioned reasons, foam is a good candidate for guiding of THz waves. To the best of my knowledge, no significant attention was given to foams as core materials for THz waveguiding. Polystyrene foam slabs have been used to support the two-wire waveguide structure before but they were not used as a core or cladding material. In this thesis I will describe demonstrate that foam can be utilized as cladding material
for two-wire waveguides providing mechanical stability, ease of manipulation, and insensitivity to the variations in the environment.
Finally, hybrid metal-wire/dielectric fibers described can also be useful for sensing in the terahertz region by placing the recognition elements directly into the fiber microstructure. Introduction of even lossless analytes into the fiber core leads to significant changes in the modal losses, which is used as a transduction mechanism. With a refractive index resolution on the order of ∼10−3 RIU, the composite fiber-based sensor is capable of identifying various gaseous analytes and aerosols or measuring the concentration of dust particles in the air. From the comparison of sensor resolutions it follows that the proposed device combines both the sensitivity comparable to that of the best demonstrated sensors while being cost effective and easy to manufacture.
I would like to make a small comment of the potential for future commercial exploitation of the THz field. I note that the prices of terahertz systems are steadily decreasing, whereas their variety and availability are booming. At low frequency THz region solid state oscillators, frequency multipliers and amplifiers are offering an efficient solution to reduce the cost of terahertz systems. At higher THz frequencies optical-to-THz down conversion and high temperature quantum cascade lasers are the two most intensively used technique with the former having the variations including usage of photoconductive antennas, photodiodes, and nonlinear optics crystals.