Practical microstructured and plasmonic terahertz waveguide

The terahertz frequency range, with frequencies lying between 100 GHz and 10 THz, has strong potential for various technological and scientific applications such as sensing, imaging, communications, and spectroscopy. Most terahertz (THz) sources are immobile and THz systems use free-space propagation in dry air where losses are minimal. Designing efficient THz waveguides for flexible delivery of broadband THz radiation is an important step towards practical applications of terahertz techniques. THz waveguides can be very useful on the system integration level when used for connection of the diverse THz point devices, such as sources, filters, sensor cells, detectors, etc.;The most straightforward application of waveguides is to deliver electromagnetic waves from the source to the point of detection. Cumbersome free-space optics can be replaced by waveguides operating in the THz range, which could lead to the development of compact THz time domain spectroscopy systems. Other promising applications of THz waveguides are in sensing and imaging. THz waveguides have also been shown to operate in subwavelength regimes, offering mode confinement in waveguide structures with a size smaller than the diffraction limit, and thus, surpassing the resolution of free-space THz imaging systems.;In order to design efficient terahertz waveguides, the frequency dependent loss and dispersion of the waveguide must be minimized. A possible solution would be to increase the fraction of mode power propagating through air. In this thesis, the usage of planar porous air/dielectric waveguides and metal wire/dielectric hybrid terahertz fibers will be discussed.;First, I present a novel design of a planar porous low-loss waveguide, describe its fabrication, and characterize it in view of its potential applications as a low-loss waveguide and sensor in the THz spectral range. The waveguide structure features a periodic sequence of layers of thin (25-50 mum) polyethylene film that are separated by low-loss air layers of comparable thickness. A large fraction of the modal fields in these waveguides is guided in the low-loss air region, thus effectively reducing the waveguide transmission losses. I consider that such 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 waveguide is the convenient access to its 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 (< 1 s filling of a 10 cm long waveguide with an analyte). The transmission and absorption properties of such waveguides have been investigated both experimentally using THz-TDS spectroscopy and theoretically using finite element software. The modal refractive index of porous waveguides is smaller compared to pure polymer and it is easy to adjust by changing the air spacing between the layers, as well as the number of layers in the core. The porous waveguide exhibits considerably smaller transmission losses than bulk material.;In the following chapters I review another promising approach towards designing of low-loss, low-dispersion THz waveguides. The hybrid metal/dielectric waveguides use a plasmonic mode guided in the gap between two parallel wires that are, in turn, encapsulated inside a low-loss, low-refractive index, micro-structured cladding that provides mechanical stability and isolation from the environment. I describe several promising techniques that can be used to encapsulate the two-wire waveguides, while minimizing the negative impact of dielectric cladding on the waveguide optical properties. In particular, I detail the use of low-density foams and microstructured plastic claddings as two enabling materials for the two-wire waveguide encapsulation.;The hybrid fiber design is more convenient for practical applications than a classic two metal wire THz waveguide as it allows direct manipulations of the fiber without the risk of perturbing its core-guided mode. I present a detailed analysis of the modal properties of the hybrid metal/dielectric waveguides, compare them with the properties of a classic two-wire waveguide, and then present strategies for the improvement of hybrid waveguide performance by using higher cladding porosity or utilizing inherently porous cladding material. I study coupling efficiency into hybrid waveguides and conclude that it can be relatively high (>50%) in the broad frequency range ∼0.5 THz.;Not surprisingly, optical properties of such fibers are inferior to those of a classic two-wire waveguide due to the presence of lossy dielectric near an inter-wire gap. At the same time, composite fibers outperform porous fibers of the same geometry both in bandwidth of operation and in lower dispersion. I demonstrate that hybrid metal/dielectric porous waveguides can have a very large operational bandwidth, while supporting tightly confined, air-bound modes both at high and low frequencies. This is possible as, at higher frequencies, hybrid fibers can support ARROW-like low-loss air-bound modes, while changing their guidance mechanism to plasmonic confinement in the inter-wire air gap at lower frequencies.;Finally, I describe an intriguing resonant property of some hybrid plasmonic modes of metal / dielectric waveguides that manifests itself in the strong frequency dependent change in the modal confinement from dielectric-bound to air-bound. I discuss how this property can be used to construct THz refractometers. Introduction of even lossless analytes into the fiber core leads to significant changes in the modal losses, which is used as a transduction mechanism. The resolution of the refractometer has been investigated numerically as a function of the operation frequency and the geometric parameters of the fiber. 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.