This paper presents the modeling and experimental characterization of the microwave properties of laser-induced solid-state plasma in silicon dielectric waveguides at 140-220 GHz. Lasers with wavelengths of 1550, 1064, 980, 852, 785, 685, 520, and 405 nm are used at different laser intensities to investigate their effects on electromagnetic wave propagation. An improved analytical model is developed to accurately predict the behavior of laser-induced solid-state plasma, taking into account wavelength and intensity-dependent excess carrier generation, and the resulting depth-dependent conductivity and permittivity. The analytical results are implemented in a multi-layer simulation model in CST Studio Suite to simulate full-wave electromagnetic propagation. The 980 nm laser achieves the largest attenuation, reaching up to 67 dB at 20 W/cm2 at 200 GHz. The simulation results show excellent agreement with experimental measurements, confirming the validity the analytical and simulation models.

Terahertz waveguide crossings are critical for compact, integrated signal routing in monolithic platforms, but simple waveguide intersections suffer from high losses and crosstalk due to mode mismatch in the regions where the waveguide channels overlap. The Maxwell fisheye lens with its inherent imaging properties is an excellent solution for multichannel intersections, however its circularshape is not easily integrated with common planar input/output waveguides. Here, we introduce all-silicon waveguide crossings basedon Maxwell fisheye lenses reshaped via conformal transformation optics for improved planar waveguide integration in the terahertzrange. Using effective medium techniques with subwavelength air inclusions, we design and fabricate 2 × 2 and 3 × 3 crossings operating over the 220-330 GHz frequency band. The transformed lenses enable aberration-free imaging without mode mismatch, implemented through a single deep reactive ion etching step. Experimental characterization reveals average insertion losses of 1.2 dB andcrosstalk below -50 dB for the fundamental quasi-TE mode, with a 40% bandwidth across the entire 220-330 GHz band, while thequasi-TM mode is also supported for dual-polarization applications. The transformed lenses have a diameter of just 4 mm (3.66λ₀),while the total device footprint including input and output tapers is 11.5 × 11.5 mm². This approach is scalable to N × N waveguidecrossings, providing a broadband and compact solution for low-loss terahertz integrated optics. 

The development of future 6G communication systems necessitates advanced materials for efficient electromagnetic interference shielding in the sub-terahertz frequency range. This study presents the preparation, porosimetry analysis, compositional and electromagnetic characterization of a highly efficient hierarchically porous carbon-silica composite suitable for shielding and stealth applications in this frequency regime. The composite, fabricated using a mixture of carbon powder and tetraethoxysilane, possesses a highly porous structure with high surface area, which facilitates multiple reflections and scattering of electromagnetic waves. Electromagnetic characterization was conducted using a free-space semi-optical method at 140-220 GHz, focusing on reflection-only measurements due to the sample's large thickness. The results demonstrate that the composite exhibits a qualified bandwidth of 83% over the measured frequency band, with a maximum reflection loss of 35 dB at 187 GHz. Furthermore, measurements demonstrate that electromagnetic power within the sample's volume is effectively attenuated. The composite's shielding efficiency due to reflection is on average 0.26 dB across the band, highlighting its potential for high frequency EMI shielding and stealth applications.

The practical deployment of terahertz systems for future applications requires a comprehensive toolkit of high performance, compact passive components that overcome several limitations innate to terahertz waves. A primary challenge is minimizing attenuation in signal routing, a problem especially critical when terahertz power is scarce. At the same time, effective signal management and device characterization require components that can efficiently absorb power to minimize reflections and crosstalk. This thesis addresses both of these needs, presenting novel low-loss all-silicon devices for terahertz applications and introducing advanced loss engineering techniques to create highly effective integrated and free space absorbers.

The first part of this thesis introduces high performance, all-silicon passive devices fabricated using silicon micromachining techniques. A perforation-free, mechanically robust planar parabolic reflector antenna is presented utilizing slab optics and total internal reflection to achieve a flat gain response over a broad bandwidth of 220-330 GHz. Furthermore, a very low crosstalk waveguide crossing is demonstrated by applying transformation optics to a Maxwell fisheye lens. This approach resolves the fundamental mode mismatch problem inherent to conventional lens designs, enabling dense and complex terahertz circuit integration.

The second part focuses on lossy structures for both dielectric and metallic waveguides. For open dielectric waveguides, ultrathin single-walled carbon nanotube films are deposited to create compact, broadband and reflectionless terminations without altering the geometry of the guide, drastically attenuating the propagating waves over short distances by evanescent field interaction. For enclosed hollow metallic waveguides, integrated absorbers are developed using highly porous nanomaterials, including randomly oriented and aligned graphene-coated nanofibers, as well as carbon nanotube aerogels. Characterized using a novel multi-band measurement methodology, these materials demonstrate broadband stealth and shielding performance across a wide frequency range (67-500 GHz). The investigation is also extended to free-space applications, demonstrating a hierarchically porous carbon-silica composite as a low reflectivity absorber in 140-220 GHz.

Collectively, this thesis expands the component toolkit for terahertz integrated systems, providing practical and high performance solutions for waveguiding, radiation and termination that are crucial for the next generation of high-frequency applications.

Den praktiska driftsättningen av terahertssystem för framtida tillämpningar kräver en heltäckande uppsättning högpresterande, kompakta passiva komponenter som övervinner flera begränsningar som är inneboende för terahertzvågor. En primär utmaning är att minimera dämpning i signalvägarna, särskilt kritiskt när tillgänglig terahertzkraft är knapp. Samtidigt kräver effektiv signalhantering och komponentkarakterisering lösningar som kan absorbera effekt på ett kontrollerat sätt för att minimera reflektioner och överhörning. Denna avhandling adresserar båda behoven: den presenterar nya lågdämpande helkiselbaserade komponenter för terahertztillämpningar och introducerar avancerade metoder för förluststyrning för att skapa mycket effektiva integrerade och fri-rum-absorbatorer.

I avhandlingens första del introduceras högpresterande, helkiselbaserade passiva komponenter framställda med kiselmikromaskinering. En perforationsfri, mekaniskt robust plan parabolisk reflektorantenn presenteras, som utnyttjar slab-optik och totalreflektion för att uppnå en jämn förstärkningsrespons över det breda bandet 220–330 GHz. Vidare demonstreras en vågledarkorsning med mycket låg överhörning genom att tillämpa transformationoptik på en Maxwell fisheye-lins. Detta angreppssätt löser det fundamentala modmisspassningsproblemet hos konventionella linsdesigner och möjliggör tät och komplex terahertzkrets-integrering.

Den andra delen fokuserar på förluststrukturer för både dielektriska och metalliska vågledare. För öppna dielektriska vågledare deponeras ultratunna filmer av enkelväggiga kolnanorör för att skapa kompakta, bredbandiga och i praktiken reflektionsfria terminer utan att ändra vågledarens geometri; de dämpar kraftigt den fortskridande vågen över korta sträckor genom interaktion med det avklingande fältet. För inkapslade ihåliga metallvågledare utvecklas integrerade absorbatorer baserade på högporösa nanomaterial, inklusive slumpmässigt orienterade och alignerade grafenbelagda nanofibrer samt aerogeler av kolnanorör. Med en ny multibandsmätmetodik karakteriseras dessa material och uppvisar bredbandiga stealth- och skärmningsegenskaper över ett stort frekvensområde (67–500 GHz). Studien utvidgas även till fri-rum-tillämpningar, där en hierarkiskt porös kol–silika-komposit demonstreras som en absorber med låg reflektivitet i 140–220 GHz.

Sammantaget utökar denna avhandling uppsättningen komponenter för integrerade terahertssystem och tillhandahåller praktiska och högpresterande lösningar för vågledning, strålning och terminering som är avgörande för nästa generations högfrekventa tillämpningar.

Dielectric waveguides are an emerging platform for terahertz (THz) integrated circuits, but a key challenge for dense integration is the realization of terminations that enable both multi-port device characterization and elimination of electromagnetic interference. Here, we demonstrate a compact, broadband termination by coating silicon waveguides with ultrathin single-walled carbon nanotube (SWCNT) films. Fabricated via a floating-catalyst (aerosol) chemical vapor deposition process, film thicknesses vary from 2 to 53 nm and are characterized in 140-220 GHz. A 53 nm thick film introduces up to 47 dB of attenuation while maintaining over 20 dB reflection loss, confirming nearly reflection-free absorption. Shielding analysis shows absorption dominates over reflection, and a record specific shielding efficiency of 5.5 x 109 dB cm2 g-1 is achieved. This approach offers a footprint-efficient solution for high-density THz circuits without bulky, radiative terminations.

Here, we present the design and experimental directional coupler operating in the low THz frequency range of 60-330 GHz enabled by silicon micromachining technology. Macromachining offers very low losses and high performance due to absence of metals and high precision of manufactured devices. This technology allows fabrication in a single etching step and provides a method for engineering the coupling coefficient. The fabrication of such high-performance directional couplers in the sub-THz frequency range is described, and the measurement results of the fabricated prototypes are reported and discussed.

In this paper, we present a THz 3-dB evanescent wave directional coupler, operating at 300 GHz based on the silicon micromachining waveguides surrounded by effective-medium with subwavelength perforations. This technique offers very low losses due to the absence of metals, allows fabrication in a single etching step and provides a method for engineering the permittivity of the cladding, thus allowing wave manipulation in all-dielectric platforms, extending the concepts of photonics to the THz region.

This article investigates the dichroic properties of aligned γ-Al2O3 nanofibers coated with graphene in the terahertz (THz) regime, revealing significant variance in absorption based on the orientation of the electric field in relation to the nanofibers, arising from the anisotropic nature of the material. Samples are prepared in a hot-wall chemical vapor deposition reactor with varying growth times, resulting in 5 samples with increasing graphene content. Compositional characterization is carried out using scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The samples are characterized electromagnetically using two distinct measurement techniques. First, a novel waveguide measurement setup is deployed, wherein square waveguide cassettes are used to capture the anisotropic behavior of the material and equally measure both polarization states in 67–500 GHz. Then, the samples are characterized using terahertz time-domain spectroscopy up to 4 THz. Both techniques highlight absorption enhancement when the electric field is parallel to the fibers, opening new possibilities for THz devices using polarization filtering.

We report on the performance of dielectric rod waveguide-coupled CMOS-based sources and detectors. The 252 GHz resonant field-effect-transistors-based THz detector is coupled to a Si rod. A similar rod is attached to a voltage control oscillator based on a Colpitts oscillator topology with optimized third-harmonic emission frequency at the same 252 GHz. A dielectric rod, implemented in this work, is employed as an antenna for coupling into a free space, as a transition element from the source or detector to the standard metal waveguide, or to enable the rod-to-rod coupling of a source-detector system. A dielectric rod-coupled system enables it to reach >60 dB signal-to-noise ratio for an equivalent noise bandwidth of one Hz. The knife-edge scans revealed the minimum half-width at half maximum of 0.24 mm at 252 GHz enabling high-resolution imaging applications. The obtained results demonstrate the high-efficiency coupling CMOS elements and Si rods. This concept can be applied to both sensing and THz imaging.

This work presents the preparation and the frequency domain measurements of graphene augmented inorganic nanofibers oriented across the longitudinal axis of the fibers, embedded in a hollow WR-10 waveguide cassette. A simple measurement setup is employed, allowing for rapid, broadband characterization of the material under test. Due to the orientation of the fibers, highly anisotropic behavior is revealed.