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Tunable Nanomaterials and their Applications for Terahertz Devices

Carbon Nanotubes and Silver Nanowires

Time: Mon 2021-06-07 10.00

Location: zoom link for online defense (English)

Doctoral student: Serguei Smirnov , Mikro- och nanosystemteknik

Opponent: Professor Hartmut Roskos, Goethe-Universität Frankfurt am Main

Supervisor: Docent Dmitri Lioubtchenko, Mikro- och nanosystemteknik; Professor Joachim Oberhammer, Mikro- och nanosystemteknik

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The interest in terahertz (THz) technologies is growing in academia and industry. The design of electronic components at THz frequencies is relevant to application areas such as telecommunication, radar, material spectroscopy, and medical imaging and diagnosis. Even though high-performance THz instrumentation becomes more available, the systems are not commonly found outside of the laboratory environment. Researchers have recently demonstrated a platform based on dielectric rod waveguides (DRWs) that is suitable for integrating THz electronics. The electromagnetic waves propagate inside a low-loss dielectric structure, a concept similar to optical fibres. DRWs have seen many advances for THz electronics in recent years. However, the platform still lacks essential active and passive components for building complete systems. 

This thesis investigates ways to integrate tunable nanomaterials to dielectric waveguides in order to design novel terahertz devices. First, silicon rectangular DRWs are investigated at 75 GHz to 500 GHz frequencies. In particular, the interface with the measurement instrumentation and the tapered transitions to hollow metallic waveguides are considered by electromagnetic simulations. Additionally, ways to implement phase shifters and attenuators are explored using thin layers of nanomaterials that are modelled by an impedance surface in the simulations. 

Second, several nanomaterials are studied by optical spectrophotometry, Raman spectroscopy, and terahertz time-domain and frequency- domain spectroscopy. Thin layers of silver nanowires are fabricated with increasing densities, ranging from individual nanowires to nanowire networks at the percolation threshold, to thick semi-continuous layers. This technique allows the manufacturing of optically transparent samples with a tunable THz conductivity. Additionally, thin layers of single-walled carbon nanotubes are investigated. Their dielectric properties are shown to be tunable by light illumination, supported by measurements at low frequencies and in the terahertz range. 

Finally, tunable THz devices based on dielectric waveguides are designed, manufactured, and characterized. Thin layers of carbon nanotubes are integrated with DRWs and used as a surface impedance to modify the wave propagation in the waveguide. The presented phase shifters are tunable by light with wideband operation at sub-THz and potentially higher frequencies, and further device improvements are proposed.