Mridha, Manoj Kumar
(2014).
Low-dispersion, broadband two-wire waveguide for THz and its application.
Mémoire.
Québec, Université du Québec, Institut national de la recherche scientifique, Maîtrise en sciences de l'énergie et des matériaux, 83 p.
Résumé
Terahertz (THz) radiation (0.1-10 THz) is a part of the electromagnetic spectrum lying in
between the electrical domain of microwaves and the optical domain of infrared radiation. For
the last two decades, this region has been regarded to as a gap due to the lack of sources and
detectors. With the development in semiconductor and photonic technologies, several sources
and detectors are now becoming available for THz radiation. As a result, THz gap is vanishing
and this radiation is finding a lot of applications in material characterization, imaging of
biological tissues, security check and is stimulating further fundamental investigations on lightmatter
interaction. Terahertz radiation may play a major role in the future of "communications".
Terahertz is indeed higher in frequency than microwaves, and may provide larger bandwidth for
transmitting information. The availability of larger bandwidth and higher frequency could
significantly increase the data transfer rates with respect to the available microwave based
wireless communication system.
In order to propagate the electromagnetic signal required for communications purposes
over long distances, guided wave approaches must be developed to counteract the natural
diffraction of the radiation. To this end, THz waveguides are extremely important and in this
thesis we discuss the possible implementation and the characteristics of a two-wire THz
waveguide.
The first part of the thesis discusses the characterization and applications of the metallic
two-wire THz waveguide. The waveguide is made up of two copper wires of 125 /Jm radius
each, with a wire separation of 300 µm and over 20 cm of length. It has been demonstrated to
carry a low-dispersive Transverse Electromagnetic Mode (linearly polarized TEM) with about 2
THz of bandwidth. The TEM mode can be excited by a Photoconductive (PC) antenna, which
emits a linearly polarized beam with a quasi-Gaussian envelope. For the waveguide to carry the
TEM mode, the separation between the wires should be close to the wavelength of operation
(300 µm in our case). The low-dispersive characteristic, associated with the confinement of the
TEM mode between the two wires, opens up new prospects for THz signal processing and
enhanced THz-Time Domain Spectroscopy (THz-TDS).
Hence, consistently with the idea of signal processing, we demonstrate spectral filtering
in the two-wire waveguide by inserting a pol ymer Bragg grating in its guiding region. The
grating creates a notch of almost 23 dB with a hne width of ~16 GHz in the waveguide spectrum at 0.5823 ± 0.0003 THz. Furthermore, one of the consequences ofa notch in the spectrum is the
appearance of the "slow-light effeet", whieh is assoeiated with low group veloeity at the edge of
a resonance. Due to the slow-light effect induced by the polyrner grating, a strong enhancement
in the water absorption line at 0.557 THz was observed.
Finally, the possibility of inereasing the coupling of the THz signal into the two-wire
waveguide was investigated. This was done by generating the THz radiation directly inside the
waveguide. A piece of GaAs with breadth and width equal to the separation between the two
wires was inserted in between the wires. The combination of voltage applied to the wires (as the
electrodes) and the femtosecond laser pulse focused onto the GaAs piece (as the semiconductor)
allowed the system to behave lîke a PC antenna and a THz transmitter. The peak amplitude
measured in this configuration was five tîmes larger than the peak amplitude measured when the
waveguide was coupled to an external PC antenna for similar excitation power and applied
voltage.
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