Final Report Summary - GRAB-A-META (Graphene Based Active Metamaterials) This project of Dr. Kim aimed at developing graphene metamaterials that manifest the potential to dynamically control optical properties including polarization and speed of light by exploiting chiral metamaterials and electromagnetically induced transparency (EIT) metamaterials. The proposed hybrid metamaterial system features very thin, free-standing and flexible platform and is capable of full electrical control, which will not only control novel optical properties, but also overcome most of the limitations of current semiconductor-based metamaterials.Recently, progress has been made over a class of new artificial media – so called metamaterials, which consist of the arrangement of periodic subwavelength optical elements that exhibit unusual optical properties beyond what natural materials can offer. Since their effective electromagnetic properties can be engineered by designing subwavelength scale metallic structures, called ‘meta-atoms’, these artificially constructed materials have promised a vast variety of otherwise unexpected physical phenomena such as negative index of refraction, gigantic chirality, and analogue EIT. Over last several years, graphene metamaterials have been realized from terahertz (THz) to optical frequencies, opening way to powerful active control of significant transmission or reflection of amplitude and phase modulation in deep subwavelength scale. Although significant effort has been devoted to conceiving and demonstrating various designs of graphene-based metamaterials for active control of light, previous work mainly focuses on the distinct resonance features and their spectral tuning. In this project, Dr. Kim have studied basic chiral metamaterials with extreme chirality and developed the electrically controllable active graphene polarizer by collaborating with Dr. Sangsoon Oh (Imperial College London) and Prof. Bumki Min (KAIST). It has been experimentally demonstrated that a new class of bi-layer metamaterial structures that substantially enhance optical activity through strong inter-molecular coupling between adjacent planar chiral metamolecules. Based on a chiral metamaterials combined with a gated single-layer graphene it has been shown that transmission of terahertz wave with one circular polarisation can be electrically controlled without affecting that of the other circular polarisation, leading to remarkably large modulation depths (> 63%) with a low gate voltage. From the two-port resonator model, Dr. Kim theoretically demonstrated that this effective control of polarisation originates from the transition from an underdamped to overdamped resonator at a critical damping condition when increasing the intrinsic losses in graphene. It has been also shown that the linear polarisation of terahertz waves can be electrically controlled with a maximum rotation modulation of about 10 degrees, which means that the rotation modulation angle per wavelength reaches 72°/λ. Manuscript preparation for this work is almost finished and Dr Kim is planning to submit in a high-ranked peer-reviewed journal. Furthermore, it has been also experimentally shown that polarization memory operations with 10 years retention time can be possible by hybridizing the graphene, a ferroelectric material, and the chiral metamaterials. Benefitting from the electric control of polarisation, the graphene chiral metamaterials concept may lead to various applications in the THz technologies such as ultra-compact active polarisation modulator for telecommunications and imaging devices and ultrasensitive sensors for identification of the chirality and structures of macromolecules or bio-molecules. Another aspect of Dr. Kim’s work was developing active graphene EIT metamaterials slow light devices which can control light speed by gate-voltages. By integrating a single layer of graphene onto designed diatomic metamaterials composed with two split ring resonators pair and a cut-wire, a giant switching of the transparency window experimentally measured under a low gate voltage, allowing for an electrically tunable group delay of the terahertz light. Theoretical analysis based on the radiating two oscillating model varies that this active EIT switching is attributed to the suppression of dissipative loss of the radiative resonator by electrically tuning the conductivity of the integrated graphene layer. Thus, Dr Kim’s work for electrically active tuning of EIT metamaterials may open up avenues for designing active slow light devices, ultrasensitive sensors and nonlinear devices. Manuscript for this work is now under preparation.Alongside his main projects, Dr. Kim has started research into dynamic control of PT (Parity Time)-symmetry by changing coupling or loss coefficients of metallic metamaterials by sdjusting graphene’s conductivity. For this work, he applied European Research Council (ERC) Starting grant 2017.From an individual point of view Dr. Kim has received valuable trainings which will be very useful for his future career. He has acquired a deep intellectual knowledge in quantum physics and graphene photonics and has obtained practical skills through use of various simulation programs, micro/nano fabrication, sophisticated analytical instruments and various research and laboratory technique. This project has also allowed him to improve his grant application and workshop organizing skills. Dr. Teun-Teun Kim applied ERC Starting grant 2016 & 2017. From the two times of mock interviews and interview at Brussels he could have grants presentation skills and as a result, he has been awarded Young Scientist Fellowship from IBS (Institute of Basic Science) Korea. As a co-coordinator, he has organized “UK-Korea Focal Point workshop on Development of ultra-precision 3D electron-beam lithography for 3D metamaterials and metadevices” successfully at Birmingham (10/09/2015, 21/10/2016). His participation in the supervision of undergraduate students (Physics Lab, Year2 and Photonic Group Study, Year3) has enhanced his capabilities for teaching and communication in English.In conclusion, from this project, Dr. Kim has been able to achieve the all research objectives he proposed in the application. In addition, Dr. Kim has acquired a strong intellectual knowledge and practical skills from valuable trainings and experiences which have allowed him to consolidate his future career.
