Frequency-Selective Surfaces for Microwave and Terahertz Spectra
Date
2021
Authors
Lv, Xiaojing
Editors
Advisors
Fumeaux, Christophe
Withayachumnankul, Withawat
Withayachumnankul, Withawat
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
FREQUENCYselective surfaces (FSSs) made of subwavelength periodic structures
have been broadly applied in various electromagnetic applications. Their
main function is to tailor the frequency response to incident waves, or to obtain
electromagnetic (EM) properties that do not exist in homogeneous natural materials.
When increasing the design complexity to enhance performance, however, the
computation cost hikes dramatically in analysis and synthesis as additional design
variables are introduced. In contrast to such complexity increase, this thesis aims at
systematically developing effective and efficient design and optimization approaches
for FSS-based structures adopting fundamental unit-cell patterns, such as rectangular
patches, rings and grids. Additionally, impedance matching to free space is thoroughly
investigated and adapted as a means towards performance improvement in both absorbers
and filters. Hereby, multiple designs are demonstrated with realizations from
the microwave to the terahertz (THz) frequency spectrum. In spite of their simplicity,
the proposed designs outperform the state-of-the-art counterparts in the literature by
fully exhausting the potentials of their spatial structures and material attributes.
Specifically, Chapter 3 challenges a common belief that adding an impedance matching
superstrate to an absorber will broaden its operation bandwidth at the cost of increased
total thickness profile. This Chapter proves that it is possible to increase the
bandwidth-to-thickness ratio. The concept is firstly demonstrated at the circuit level,
and then verified by full-wave simulations. The optimization process can be illustrated
with an admittance Smith chart. The distinctive performance of the proposed single-
FSS-layer absorber is justified with a figure of merit (FoM) which comprehensively
involves the relative bandwidth, the normalized thickness and the level of reflectivity.
In Chapter 4, a semi-analytical approach for absorber design is developed by systematically
combining analytical, empirical and numerical techniques. The optimization
space can be simplified from millions of exhaustive search cases to merely a few hundreds
of seed simulations, by exploiting insights into the linearity, scalability and independence
regarding the major components of an absorber. For any specified level of
absorption and operation bandwidth, the obtained semi-analytical algorithm enables
fast synthesis of an absorber with a minimal thickness. Both absorbers proposed in the above chapters have been realized using patterned resistive layers and experimentally
validated under oblique angles of incidence for transverse-electric (TE) and transversemagnetic
(TM) modes. The design methods can be readily expanded for structures of
multiple FSS layers.
In the terahertz frequency range, common microfabrication technologies do not accommodate
those resistive inks used for silk-printing lossy FSS patterns. As an alternative,
a sub-skin-depth metal layer with nanoscale thickness is proposed in Chapter 5 to meet
this requirement. The Drude model is adopted to simulate the ultra-thin metallic FSS,
as it satisfactorily describes the frequency dependent properties of noble metals. The
proposed absorber is robust to dimensional tolerance in fabrication and attains a stable
absorption bandwidth under oblique impinging waves.
In Chapter 6, a frequency reconfigurable terahertz bandpass filter is proposed and experimentally
verified. It includes two identical double-layer FSSs separated by an air
spacer which can be mechanically tuned. The filter allows a highly selective transmission
sweeping across a wide spectrum. The underlying mechanism can be explained
from two perspectives, namely through interpretation as Fabry-Perot resonant cavity
and through consideration of the admittance Smith chart. The designed device is
insensitive to fabrication tolerances and stable to oblique angle of incidence. The fabricated
filter confirms a 40% tuning range and less than 1 dB insertion loss. This design
is among the first few practical reconfigurable terahertz bandpass filters reported in
the literature.
Overall, the research outcomes suggest that developing complicated FSS patterns with
a large number of degrees of freedom is unnecessary in many cases if the potential of
fundamental geometries is fully exploited through rigorous algorithmic optimization
methods. The design approaches illustrated in this thesis are generic to all FSS-based
structures and can potentially be extended to multi-FSS layers and impedance surfaces,
to satisfy performance requirements in specific applications.
School/Discipline
School of Electrical and Electronic Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 2021
Provenance
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