The doctoral dissertation in the field of Physics will be examined at the Faculty of Science, Forestry and Technology, Joensuu campus.
What is the topic of your doctoral research? Why is it important to study the topic?
My doctoral research investigates new ways to control terahertz (THz) radiation using metamaterials - artificial structures engineered to interact with light in unusual ways, and carbon-based thin films. The THz frequency range, known as the “THz gap,” remains one of the least explored regions of the electromagnetic spectrum, although, it offers exciting opportunities for imaging, sensing, and ultra-fast communication and enhancing quantum technologies. By carefully designing their geometry, metamaterials can create strong and selective interactions with THz radiation, useful for sensing, spectroscopy, and communications. A detailed investigation of multipole responses in metamaterials, accounting for asymmetry and source displacement was performed to investigate radiation of complex and compound metamaterial geometries. Carbon-based thin films allow for tunability and broadband absorption. Together, these materials open the path toward efficient, compact, and tunable THz devices.
What are the key findings or observations of your doctoral research?
Within this doctoral research, a theoretical framework was developed to analyze complex metamaterials, accounting for asymmetry and source displacement, leading to the discovery of modified anapoles and broadband non-radiating structures. Two free-standing metamaterials with tunable, high-Q resonances in sub-THz and THz frequency range have been developed, fabricated and experimentally investigated. Their properties were explained through multipole expansion by a dominant electric dipole moment and an electric quadrupole-driven quasi-bound state in the continuum. Three carbon thin films (pyrolyzed photoresist, graphitic films, and pyrolytic carbon) were investigated in THz frequencies, with applications in pollutant detectors and bolometers. Finally, an ultrabroadband broadband THz absorber combining pyrolytic carbon with moth-eye–structured silicon was proposed. These results provide both fundamental insights into light-matter interaction and practical pathways for THz devices.
How can the results of your doctoral research be utilised in practice?
The results of this research offer multiple practical applications in the THz domain. The modified multipole approach provides insights and design tools for complex and compound metamaterials required for the THz range. The development of high-Q, tunable metamaterials enables compact THz devices, antennas, and sensors that provide efficient energy confinement with minimal losses. These structures could improve THz imaging, communication, and sensing systems. The investigation of carbon-based thin films, demonstrated their potential for water pollutant detection and thermal sensing using bolometric detectors. These materials broaden the range of THz-compatible fabrication techniques and device designs. Finally, the hybrid ultrabroadband absorber combining pyrolytic carbon films with moth-eye–structured silicon achieves over 99.5% absorption across 1–1000 THz, making it a promising candidate for broadband blackbody sources, thermal imaging calibration, and spectroscopic applications.
What are the key research methods and materials used in your doctoral research?
Within this research new ways to control terahertz (THz) radiation were explored using metamaterials and carbon-based materials. Multipole responses in complex metamaterials were studied through simulations and multipole expansion. Free-standing sub-THz and THz metamaterials were designed, simulated, investigated with multipole expansion, fabricated using laser cutting, and experimentally characterized with THz Time-Domain Spectroscopy (TDS) and sub-THz VNA measurements. Carbon thin films, including pyrolyzed photoresist, graphitic films, and pyrolytic carbon, were fabricated via CVD for pollutant detection and bolometric sensing. Finally, pyrolytic carbon was integrated with patterned silicone to create an ultrabroadband THz absorber investigated in low THz with TDS.
The doctoral dissertation of Maria Cojocari, MSc, entitled Metamaterials and carbon-based thin films for terahertz absorption and bolometric detection will be examined at the Faculty of Science, Forestry and Technology, Joensuu campus. The opponent will be Research Professor Emeritus, Zoran Ivić, Vinča Institute of Nuclear Sciences, and the custos will be Professor Jyrki Saarinen, University of Eastern Finland. Language of the public defence is English.
For more information, please contact:
Maria Cojocari, [email protected], tel. 41 570 1468
