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.

This work presents a filtenna concept based on cascaded singlet filters, which inherits independent controllability of attenuation poles from the filter design and has high stopband performance. Two fourth-order filters with a center frequency of 270 GHz, a 14 GHz bandwidth, and in-band return losses of 15 dB and 18 dB, respectively, are manufactured and measured to verify the proposed concept. The filtennas with a single-slot and a double-slot configuration are fabricated by the silicon deep reactive ion etching technology and have the size of 3.65x2.6 mm2. Detailed explanations of the synthesis procedure, which was validated through EM simulation, and measurement results are provided. The measured single-slot and double-slot filtennas exhibit broadside radiation gains of approximately 4.5 dBi and 6.5 dBi at 270 GHz, respectively. Moreover, the challenges and details of silicon micromachining in fabricating the two filtering antennas are discussed.

This communication introduces a graded index (GRIN) Fresnel zone planar lens (FZPL) antenna operating with high gain in the 500–750-GHz frequency range. The main innovation involves achieving a gain of 35.9 dBi by incorporating a GRIN dielectric perforated disk. This perforated disk acts as a distributed spatial delay for beam focusing, ensuring gain improvement and circular polarization simultaneously without needing an extra polarizer. The antenna exhibits high efficiency, with an average radiation efficiency of −1.05 dB, achieved through the optimization of a modified FZPL for silicon-on-insulator (SOI) micromachining technology. The antenna maintains a return loss below −15 dB across the entire 500–750-GHz band, achieving a 40% fractional bandwidth, with circular polarization maintained and an axial ratio consistently below 2.8 dB. The fabricated chip, sized 12.18×12.18 mm with a thickness of 526 μ m, enhances practicality. The feeding arrangement involves a standard open waveguide, and direct mounting into a standard WM-380 waveguide flange is facilitated. This communication discusses the prototype’s design, fabrication, and measurement, emphasizing the excellent agreement between the antenna’s performance and simulated data.

This paper introduces a novel non-invasive skin cancer detection system using a high-resolution millimeter-wave time reversal imaging approach. Operating in the 2-30 GHz frequency range, the system utilizes two moving Vivaldi antennas to scan the imaging area, significantly reducing hardware complexity and cost compared to conventional systems. The system features a skin-removal technique that eliminates surface reflections, improving subsurface imaging accuracy. Through time-reversal methods, particularly the DORT algorithm, the system achieves high-resolution imaging capable of detecting both single and multiple tumors. In tests with phantoms, the system successfully identified single tumors with diameters of 5 mm and 8 mm and multiple tumors of 8 mm in close proximity. The worst-case positioning error for single tumor detection was 4 mm, and for multiple targets, 14 mm. The system also exhibited a high signal-to-clutter ratio (SCR) of up to 10.8 dB, demonstrating its effectiveness in distinguishing between benign and malignant tissues.

This paper presents the first demonstration of a high-gain planar THz lens antenna with beam-steering capability by frequency-orthogonal spatial spreading, operating in the 610–685 GHz range. The antenna is based on integrating a Fresnel zone planar lens with a graded-index silicon interposer. The concept enables passive beamforming of four simultaneous beams from a single feeding port, covering a field of view from -30° to -14° for 20 GHz bandwidth with a 4° separation of the beam direction. The beams can be swept continuously over that field of view by mapping the signal to different frequencies. Furthermore, when using four feeds, 16 simultaneous beams can be created, of which two frequency-orthogonal beams are separated by 40 GHz in frequency can be mapped into the same spatial direction. Thus, it is demonstrated that high-gain multibeam beam steering can be achieved without the hardware complexity of conventional phased-array antenna systems requiring a large number of RF chains. An antenna prototype is implemented using silicon micromachining, resulting in a compact 15.8 mm × 15.8 mm device with a thickness of 526μm, which is directly mounted on a standard WM-380 waveguide feed. Measurement results include a realized gain of 32.1 dBi, only a 0.8 dB beam steering loss across the field of view, an effective side-lobe suppression better than -22 dB, and a high radiation efficiency of -1.25 dB. The measurements are in excellent agreement with simulations, and the worst-case deviation of the measured beam direction from the simulated one is only 0.1° out of 16 beams.

The increasing demands for high-speed wireless communication, intelligent sensing, and high-resolution imaging have driven interest toward the underutilized sub-terahertz (sub-THz) frequency spectrum. This region offers large bandwidths and high spatial resolution, making it a promising candidate for next-generation communication and sensing systems. However, realizing practical sub-THz systems presents several challenges, including severe path loss, low power efficiency, and significant hardware complexity due to frequency-dependent losses and fabrication constraints.

This thesis addresses these challenges by proposing a set of passive, highperformance components and system-level architectures focused on antenna design, beamforming techniques, channel modeling, and imaging methods. These components are fabricated using silicon micromachining, a scalable technology that enables the realization of compact, high-frequency passive structures with low loss and micrometer feature size. The thesis begins by developing a ray-tracing-based statistical channel model that captures essential propagation phenomena, including molecular absorption, reflection,scattering, and diffraction. The model evaluates the root-mean-square delay spread, coherence bandwidth, and subchannel stability under varying link distances, antenna gains, and misalignment scenarios. The results reveal that higher-frequency bands exhibit reduced delay spread variability and allow for more robust multi-carrier communication through channel bonding, forming the foundation for hardware-aware THz link design.

Two high-gain silicon micromachined lens antennas are introduced next. The first design is an elliptical Fresnel zone planar lens antenna that achieves circular polarization across the 500–750 GHz band. The return loss remains better than -15 dB across this range, with a measured gain of 25.7 dBi and an axial ratio below 2.5. The second design uses a circular Fresnel lens enhanced by a graded-index dielectric perforated disc, fabricated using DRIE on a silicon-on-insulator wafer. With 13 optimized Fresnel zones, this antenna achieves 35.9 dBi gain and maintains circular polarization with an axial ratio below 2.8 dB across 40% bandwidth. These antennas demonstrate state-ofthe-art performance in compact, planar form factors. Specifically, a single silicon wafer is etched on both sides using deep reactive ion etching (DRIE)and two lithographic masks, forming structures with fixed thickness and precise vertical profiles. This makes the fabrication process simple for purely dielectric-based lens antennas.

To address the limitations of wideband beamforming at sub-THz, the thesis presents a spatial-spreading approach using frequency-orthogonal passive beam steering. A multi-feed Fresnel lens system is designed to steer each frequency sub-band into a distinct spatial direction. Using four feeds and 75 GHz of total bandwidth, 16 beams are generated to cover a 32° field of view.

Experimental results show only 0.9dB steering loss, sidelobe suppression below –22dB, and a realized gain of 32.1 dBi. The lens is compact (15.8×15.8 mm, 526 μm thick) and requires only a single RF chain per feed, significantly reducing system complexity. The spatial-spreading antenna is then paired with a convolutional neural network for adaptive beam and power allocation. The CNN estimates user location using amplitude-only received signal features and dynamically assigns beam and transmit power. The system achieves up to 61% improvement in direction-of-arrival estimation accuracy, a 94% increase in data rate, and a 30% reduction in transmit power compared to static strategies.

The final chapter investigates the effect of antenna dispersion in wideband imaging. A comparison between silicon lens and metallic horn antennas reveals that the former enables higher effective bandwidth and preserves the time-domain pulse shape. Experimental results show that lens-based antennas reduce range and cross-range localization errors by up to 64% and 68%, respectively, and improve signal-to-clutter ratio by 2.7 dB. The system achieves millimeter-level resolution and resolves targets as close as 2mm in cross-range and 3mm in range.

Using this insight, a full imaging system is demonstrated by combining frequencyorthogonal beams and a time-reversal DORT algorithm. The system reconstructs images of multiple targets without mechanical scanning. Experimental reconstructions verify resolution of 0.6 mm-radius objects and accurate discrimination between targets spaced only 2mm apart, affirming the impact of dispersion-aware design for high-resolution THz imaging.

The thesis demonstrates how silicon-micromachined, high-gain antennas and passive beamforming can be effectively combined with machine learning and wideband imaging strategies to address key limitations in sub-THz systems. The proposed components are validated across communication and sensing contexts, establishing a robust framework for compact, scalable THz frontend architectures.

De ökande behoven av högkapacitets trådlös kommunikation, intelligent avkänning och högupplöst avbildning har drivit intresset mot det underutnyttjade sub-terahertz (sub-THz) frekvensområdet. Detta område erbjuder stora bandbredder och hög rumslig upplösning, vilket gör det till en lovande kandidat för nästa generations kommunikations- och sensorsystem. Realiseringen av praktiska sub-THz-system medför dock flera utmaningar, inklusive kraftiga förluster i fri rymd, låg energieffektivitet och betydande hårdvarukomplexitet på grund av frekvensberoende förluster och tillverkningsbegränsningar.

Denna avhandling tar itu med dessa utmaningar genom att föreslå en uppsättning passiva, högpresterande komponenter och systemarkitekturer med fokus på antenndesign, strålformningstekniker, kanalkarakterisering och avbildningsmetoder. Komponenterna tillverkas med kiselmikromekanik, en skalbar teknik som möjliggör kompakta, högfrekventa passiva strukturer med låg förlust och mikrometerskala.

Avhandlingen inleds med utvecklingen av en statistisk kanalmodell baserad på strålspårning som fångar upp centrala propagationsfenomen, inklusive molekylär absorption, reflektion, spridning och diffraktion. Modellen används för att utvärdera root-mean-square-fördröjningsspridning, koherensbandbredd och delkanalers stabilitet vid olika länklängder, antennvinster och feljusteringar. Resultaten visar att högre frekvensband ger lägre variation i fördröjningsspridning och möjliggör mer robust flerbärarkommunikation genom kanalkoppling, vilket utgör grunden för hårdvaruanpassad THz-länkutformning.

Därefter introduceras två högvinst-linsantenner tillverkade med kiselmikromekanik. Den första designen är en elliptisk Fresnel-zonlins som uppnår cirkulär polarisation över 500–750GHz-bandet. Returförlusten är bättre än –15dB över hela bandet, med en uppmätt vinst på 25,7dBi och en axialkvot under 2,5. Den andra designen använder en cirkulär Fresnel-lins förbättrad med en graderad dielektrisk perforerad skiva, även den tillverkad med djupreaktiv jonetsning (DRIE) på ett SOI-substrat. Med 13 optimerade Fresnel-zoner uppnår denna antenn en vinst på 35,9dBi och bibehåller cirkulär polarisation med en axialkvot under 2,8dB över 40% bandbredd. Dessa antenner uppvisar topprestanda i kompakta, plana formfaktorer. I synnerhet etsas ett enda kiselchip från båda sidor med DRIE och två litografimasker, vilket skapar strukturer med fast tjocklek och exakt vertikal profil, vilket förenklar tillverkningen av rent dielektriska linsantenner.

För att hantera begränsningarna i bredbands-strålformning vid sub-THzfrekvenser presenterar avhandlingen en spatial-spridningsmetod baserad på frekvensortogonal passiv strålstyrning. Ett multifött Fresnel-linssystem är utformat för att styra varje frekvenssubband i en unik riktning. Med fyra matningar och totalt 75GHz bandbredd genereras 16 strålar som täcker ett synfält på 32°. Experimentella resultat visar endast 0,9dB styrförlust, sidlobdämpning bättre än –22dB och en realiserad vinst på 32,1dBi. Linsen är kompakt (15,8×15,8mm, 526μm tjock) och kräver endast en RF-kedja per matning, vilket kraftigt reducerar systemkomplexiteten.

Den spatiala spridningsantennen kombineras därefter med ett konvolutionellt neuronnät för adaptiv strål- och effektallokering. Genom att använda endast amplitudbaserade mottagarsignaler uppskattar nätverket användarens position och tilldelar dynamiskt optimal stråle och sändareffekt. Systemet uppnår upp till 61% förbättrad noggrannhet i riktninguppskattning, 94% ökning i datatakt och 30% minskning i sändareffekt jämfört med statiska strategier.

Avhandlingens sista kapitel undersöker effekten av antenndispersion i bredbandsavbildning. En jämförelse mellan kiselbaserade linser och metallhornantenner visar att de förra möjliggör högre effektiv bandbredd och bevarar pulsformen i tidsdomänen. Experimentella resultat visar att linsantenner minskar fel i avstånds- och tvärlägeslokalisering med upp till 64% respektive 68% och förbättrar signal-till-störförhållandet med 2,7dB. Systemet uppnår millimeterupplösning och kan särskilja mål med 2mm avstånd i sidled och 3mm i längsled.

Med dessa insikter demonstreras ett komplett bildsystem som kombinerar frekvensortogonala strålar och en tidsreversalbaserad DORT-algoritm. Systemet rekonstruerar flera mål utan mekanisk skanning. Experimentellaåteruppbyggnader bekräftar en upplösning på 0,6mm och korrekt identifiering av objekt separerade med endast 2mm, vilket visar på effekten av dispersionsanpassad design för högupplöst THz-avbildning.

Sammanfattningsvis visar avhandlingen hur kiselmikromekaniska högvinstantenner och passiv strålformning effektivt kan kombineras med maskininlärning och bredbandsavbildning för att hantera centrala begränsningar i sub-THz-system. De föreslagna komponenterna valideras i både kommunikations-och sensorsammanhang, och etablerar en solid grund för kompakta och skalbara THz-fronter.

This paper presents, for the first time, a novel highgain beam-steering lens antenna operating in the 610 to 645 GHz range. The antenna, fabricated using silicon micromachining, is based on incorporating a Fresnel lens layer in combination with a frequency-dependent graded-index interposer that acts as a passive beamformer. Frequency-dependent narrow beams are generated using an open waveguide feed. The prototype implemented in this paper features the generation of 8 frequencydependent narrow beams with just two open waveguide feeds. These beams can be steered from -30° to 2° in the azimuth. The prototype achieves a measured realized gain of 32.1 dBi with low beam-steering losses of just 0.8 dB. The antenna also exhibits a high radiation efficiency of -1.25 dB. The lens antenna is very compact, measuring only 15.8 mm × 15.8 mm × 0.526 mm. This beam-steering antenna offers a promising solution for future high-gain, multi-beam THz communication systems.

This study investigates the influence of laser parameters on electromagnetic wave attenuation within a silicon waveguide over a wide frequency range of 67–220 GHz using 3-D full-wave simulations. A 10-layer cylindrical model mimicking the energy distribution of a Gaussian laser beam shape is utilized for the analysis. The conductivity of each layer is calculated, and the S-parameters are simulated via CST Studio Suite. Significantly different attenuation levels are observed for different laser wavelengths used in this study. A 980 nm laser resulted in a substantially higher attenuation comparison to a 405 nm laser which had a minimal impact. Furthermore, by increasing the laser intensity, an increase in attenuation is observed. Moreover, the low level of simulated return loss indicates that solid-state plasma absorption dominates the reflections.

This article experimentally demonstrates a frequency-sweeping notch-beam sub-THz radar frontend based on a two-line array antenna featuring computational imaging. Operating within 237.5 GHz and 250 GHz with 12.5 GHz bandwidth, the radar utilizes a 12 λc delay line to achieve frequency-sweeping capabilities. This configuration allows dynamic notch-beam scanning across angular ranges from − 26.5 ∘ to 28 ∘ . The radar frontend is highly compact with a total size of 20 mm× 14.3 mm× 1.2 mm, including the beam-steering network, a magic-tee for creating the 180 ∘ phase shift required for creating the notch-beam, and the antenna array, and is implemented by silicon micromachining. The radar was evaluated with single and dual-target scenarios utilizing and benchmarking different computational imaging algorithms, i.e., matched filter (MF), fast iterative shrinkage-thresholding algorithm (FISTA), and multiple signal classification (MUSIC). It was found that the MUSIC algorithm outperforms MF and FISTA in range and angular resolution in single-target scenes, achieving a range resolution of 7.8 mm and an angular resolution of 15.7 ∘ , with detection errors of less than 6.6 mm and 3.5 ∘ , respectively. Although the MUSIC algorithm maintains reliable range resolution in dual-target scenarios, it performs poorly in providing angular information.

This article experimentally demonstrates a frequency-sweeping notch-beam sub-THz radar frontend based on a two-line array antenna featuring computational imaging. Operating within 237.5 GHz and 250 GHz with 12.5 GHz bandwidth, the radar utilizes a 12 λc delay line to achieve frequency-sweeping capabilities. This configuration allows dynamic notch-beam scanning across angular ranges from − 26.5 ∘ to 28 ∘ . The radar frontend is highly compact with a total size of 20 mm× 14.3 mm× 1.2 mm, including the beam-steering network, a magic-tee for creating the 180 ∘ phase shift required for creating the notch-beam, and the antenna array, and is implemented by silicon micromachining. The radar was evaluated with single and dual-target scenarios utilizing and benchmarking different computational imaging algorithms, i.e., matched filter (MF), fast iterative shrinkage-thresholding algorithm (FISTA), and multiple signal classification (MUSIC). It was found that the MUSIC algorithm outperforms MF and FISTA in range and angular resolution in single-target scenes, achieving a range resolution of 7.8 mm and an angular resolution of 15.7 ∘ , with detection errors of less than 6.6 mm and 3.5 ∘ , respectively. Although the MUSIC algorithm maintains reliable range resolution in dual-target scenarios, it performs poorly in providing angular information.