Periodic Reporting for period 1 - ENOSIS (ENhancing and prObing Strong light-matter Interactions in 2D materials by ultrafaSt optical techniques)
Periodo di rendicontazione: 2021-06-01 al 2023-05-31
Light technologies are the key enabler for broad trends of society such as Internet of Things, big data, artificial intelligence, autonomous transportation and robotics. For example, IBM predicts that future computing hardware will not be based on standard silicon transistors but on optical circuits, enabling significant advances in computing speed, with lower energy dissipations, creating entirely new digital technologies, which rely on the classical and quantum properties of light. Strong light-matter coupling in microcavities comprising atomically thin transition metal dichalcogenides (TMDs) and their stacked bilayers could lead to radically innovative photonic devices which merge the properties of polaritons, arising from the mix of excitons and photons confined in microresonators, with the outstanding properties of TMD excitons. Highly non-linear polariton devices, potentially covering large areas, could be building blocks of future photonic circuits. The main goal of this project is to enable and enhance the most advantageous properties of polaritons in TMD-based devices, by developing novel structures to be characterized with advanced optical spectroscopies. To this aim, microcavities embedding TMD monolayers (MLs) and bilayers (BLs) have been fabricated during this project and characterized by innovative hyperspectral microscopy techniques. Moreover, ultrafast spectroscopy revealed intriguing valley polarization phenomena in TMDs and distinctly different dynamics of hybridized interlayer excitons in bilayers. The SC between hybridized excitons in TMD bilayers and microcavity photons allowed to merge their favourable properties, leading to increased polaritonic interactions.
During the work-packages carried out during this project, monolithic cavities on TMD monolayers in strong coupling regime have been successfully fabricated exploiting two different methods; the polarization behaviour of TMD monolayer cavities have been explored in photoluminescence (PL); valley depolarization dynamics have been analysed in the monolayers out of the cavity by means of circular dichroism pump probe experiments, observing valley selective Stark shift and inverse valley polarization between A and B excitons. Moirè heterostructures of MoSe2/WS2 with a small twist angle and TMDs homobilayers, all encapsulated in hBN, have been fabricated and fully characterized using static spectroscopy measurements; a hyperspectral microscope was optimized to perform widefield PL imaging of such structures; the ultrafast exciton behaviour for interlayer hybridized excitons has been explored by means of an ultrafast spectro-microscopy pump probe setup, finding different population times for the interlayer excitons. Strong coupling have been successfully reached in microcavities embedding both hBN-encapsulated moirè HSs of MoSe2/WS2 and MoS2 homobilayers; angle-resonant polariton amplification has not been observed in such systems, but enhanced optical nonlinearity has been observed in microcavity strongly coupled to hybridized interlayer excitons due the new effect of tunnelling-enabled phase space filling. Regarding the knowledge transfer between the researcher and the host institution and viceversa, the fellow has considerably broadened his scientific background on the ultrafast optics and photonics, hyperspectral microscopy, ultrafast dynamics of TMDs monolayers and bilayers and non-linear optical properties of 2D materials in microcavities. The fellow has worked with state-of-the-art femtosecond optical setups, learning how to generate ultrashort pulses using optical parametric amplifiers and characterize them, work with ultrafast transient absorption and circular dichroism setups at room and cryogenic temperatures, use common-path birefringent interferometers for hyperspectral imaging. The fellow has been trained in management skills, team working, time management, leadership and presentation skills, taking active part in the scientific and financial management of the project.The fellow transferred his knowledge by supervising 2 bachelor students, 3 master students and currently 1 PhD student. He has been also giving periodic seminars during the group meetings to transfer his previous knowledge in the field of exciton-polaritons and to show the latest results of the research to the group. Concerning the exploitation and dissemination of the results, a total of 6 papers have been published in international peer-reviewed journals, 1 submitted and uploaded on a public repository and 3 other manuscripts in preparation. The fellow gave a total of 9 oral contributions in international conferences and a seminar on 2D materials for the general audience during an outreach event. The fellow gave a seminar on 2D materials for the general audience during an outreach event, spread some of the results of the project on the national media, opened a twitter account for the project and updated the group website with the project results.
During the project, a novel approach for k-space hyperspectral microscopy based on a common-path birefringent interferometer has been developed, capable of measurements on a wide angular field of view; the proposed method has high throughput, angular resolution of 0.3° and spectral resolution of 3 THz (4 nm at lambda=635nm), while the maximum resolution in terms of spectral shift is less than 0.1 nm. We fully characterize the optical behavior of a planar microcavity and a dielectric nanodisks metasurface by measuring the hyperspectral reflectivity in k-space. The compact footprint and the superior stability of the interferometer allow an easy implementation in custom microscopy setups or even in commercial microscopes equipped with a Bertrand lens. With further optimization, it could be also used for measuring the frequency dependent linear and circular Stokes parameters of the analyzed samples. The proposed method can serve as a fast and reliable characterization tool for a broad range of materials and devices with non-trivial angle/wavelength-dependent optical properties, such as 2D semiconductors and metasurfaces. We believe that our technique has strong potential for commercial exploitation and may in the long run have an impact on a wide range of optical technologies. Moreover, the scientific results obtained from the studies on excitons and polaritons nonlinearities will pave the way for future technological advances in the field of optical computing and neural networks, having also the possibility to reach quantum regimes. Concerning the impact of the prject on the researcher career, owing to the extensive training on a state-of-the-art ultrafast laser system and polarization-sensitive optical techniques, the researcher extended his broad experience on ultrafast optical spectroscopy significantly. The researcher also got extensive experience working with 2D semiconductor materials (in particular the 2D moiré heterostructures) which are important for advanced optoelectronic and spintronics applications. The acquired soft skills and further supervision experiences he made at Politecnico di Milano supported his goals of becoming an independent researcher. By the end of the fellowship, the researcher has received several invitations to present his work at international conferences and reputed international institutions, which attests to the researcher’s journey to an independent career.