Keynote Speakers

Lin Wang

Lin Wang
Professor
Shanghai Institute of Technical Physics, Chinese Academy of Sciences, China

Speech title: Topological Semimetals for Long Wavelength Photodetection
Abstract: Photodetectors are playing pivotal roles in communication and sensing systems nowadays, that have become ubiquitous in our daily life. In particular, the rapid growth of information technology in areas of all-weather surveillance, intelligent discrimination and high-throughput communications, require highly sensitive photodetectors operational in long wavelength band with the frequency defined between microwave and far-infrared regimes. However, current available technologies suffer from several drawbacks with high manufacturing costs, requires cryogenic operation, and low speed. Innovative approaches will be needed to achieve fast, broadband, room temperature detection in this long wavelength regions.
Beyond early landmarks of band-structure classification, understanding the role played by the geometrical or topological properties of quantum wavefunction, and their effects on the transport of photoexcited carriers, showing alternative advantages beyond traditional band engineering. Since the fundamental limitation have been arrived in the ear of microwave electronics and infrared photonics, it is of particular importance to capitalize novel properties led by the topology in order to circumvent technical bottlenecks. In this speech, we will portray opportunities that topological semimetals bring in photodetection across terahertz gap, and reviewing recent efforts that our group has invested for this area. We have shown that topological semimetal could be ideal candidates to achieve highly sensitive, broad operation and ultrafast speed at ambient environment, benefiting from its inherent properties led by Dirac/Weyl physics.

Wen Xu

Wen Xu
Professor
Institute of Solid State Physics, Chinese Academy of Sciences, China

Speech title: Spin polarization in bi-layer MoS2 in the presence of proximity induced interactions
Abstract: Few-layer MoS2 is a typical valleytronic material proposed for, e.g., future information storage and processing, which has been intensively investigated both theoretically and experimentally in recent years. Here, we demonstrate theoretically that when bi-layer MoS2 is placed on a dielectric or magnetic substrate, proximity-induced interactions, such as the Rashba spin-orbit coupling and the Zeeman-like exchange interaction, can induce: i) the in-plane spin orientation; ii) the further splitting of the conduction and valence bands in different valleys; and iii) the lifting of the energy degeneracy for electrons in different valleys. More interestingly and importantly, the electronic interaction between two MoS2 layers can break the symmetry of the sample geometry. As a result, an asymmetric electronic energy spectrum, dependent upon the linear terms of ky, can be observed and, consequently, the in-plane spin polarization in bi-layer MoS2 is much stronger than that in mono-layer MoS2. Such an effect can be utilized for experimental observation of novel valleytronic phenomena such as the valley Hall effect and the terahertz optical Hall effect in the absence of an external magnetic field.

Martin Oberlack

Martin Oberlack
Professor
Technische Universität Darmstadt, Germany

Speech title: Statistical symmetries and arbitrary velocity moments of wall-bounded turbulent shear flows

Bobo Tian

Bobo Tian
Professor
East China Normal University, China

Speech title: Ferroelectric transistors for neuromorphic computing
Abstract: The multiple ferroelectric polarization tuned by external electric field could be used to simulate the biological synaptic weight. Ferroelectric synaptic devices have two advantages compared with other reported ones: One is the intrinsic switching of ferroelectric domains without invoking of defect migration as in resistive oxides contributes reliable performance in these ferroelectric synapses. Another tremendous advantage is the extremely low energy consumption because the ferroelectric polarization is manipulated by electric field which eliminate the Joule heating by current as in magnetic and phase change memory. Ferroelectric synapses are potential for the construction of low-energy and effective brain-like intelligent networks. Here I summarize our recent work of ferroelectric transistors as synapses for neuromorphic computing [AIP Adv. 7, 065121 (2017); Adv. Electron. Mater. 5, 1970006 (2019); npj 2D Materials and Applications 3, 31 (2019); Nat. Electron. 3, 43 (2020)].

Jiahao Yan

Jiahao Yan
Associate professor
Jinan University, China

Speech title:The near-field photothermal effect and the far-field Kerker effect of silicon-based Mie resonators
Abstract: Nano-resonators made from high-refractive-index dielectric materials are called Mie resonators because of the strong Mie resonances in visible or near-infrared ranges. The unique magnetic dipole resonances make dielectric Mie resonators different from plasmonic counterparts and exhibit different performances in nanophotonic applications. Therefore, it is important to understand the basic optical properties of dielectric Mie resonators in both near-field and far-field. In this work, we have studied the near-field photothermal effect and the far-field Kerker effect thoroughly based on the typical Mie resonator: silicon nano-resonators (nanoparticles or nanostripes). For the near-field study, to realize sensitive photothermal measurement of Si nanoparticles, we combined the Mie resonances of Si nanoparticles and the phase change of vanadium dioxide (VO2) film to obtain the thermally controlled dark-field scattering. Through adding tungsten disulfide (WS2) flakes between VO2 films and Si nanoparticles and adjusting the thickness of WS2 from monolayer to bulk (>30nm), the slab modes in WS2 and VO2 films gradually modify the Mie resonances of Si nanoparticles. Specifically, Si nanoparticles on thick WS2 (>30nm) can produce Fabry-Perot (F-P) mode assisted Mie resonances, which convert the conventional intensity change of scattering spectrum under two states of VO2 to obvious wavelength shift. The new type thermally controlled scattering greatly enhances the sensitivity on temperature sensing. If the surrounding of Si nanoparticles is pre-heated to the starting point of the phase change region of VO2 (60 oC), additional temperature increase because of the light-induced photothermal effects of Si nanoparticles will affect the phase transition of VO2 and the final dark-field scattering spectrum measured from the Si nanoparticles. In experiment, we first demonstrated different local heating performances of Si nanoparticles with different aggregation states. A thermal resolution less than 1 oC was realized. For the far-field study, we observed the directional PL manipulation of WS2 monolayers based on the Kerker condition of Si nanostripes. Kerker condition was first proposed by M. Kerker et al. to study the zero backward or forward scattering arising from the interference between electric and magnetic dipole modes based on the Mie coefficients. This theory can be expanded to study the directional scattering from dielectric nanostructures with arbitrary shapes containing multipole electric and magnetic resonant modes. Moreover, Kerker condition brings new mechanism on the coupling between 2D excitons and dielectric nanostructures. However, it still remains unclear if we can realize significant PL enhancements based on pure far-field Kerker effects. Therefore, it is very important to study the Mie-exciton coupling and obtain strong PL enhancements based on pure Kerker effects. In this work, we proposed a Si nanostripe with a 135 nm top oxide layer. Hybrid structures were fabricated through placing monolayer and bilayer WS2 on Si nanostripes with different widths. The presence of oxide spacer blocks the near-field enhancements and heating effects of Si nanostripes and helps obtain the pure Kerker effect PL enhancements. In experiments, the measured PL enhancement factors are comparable to the best enhancement performance of single dielectric nanostructures. Moreover, the pure far-field effect greatly inhibited the Joule heating which is inevitable for plasmonic platforms. In theory, dipole sources were used to simulate the excitonic emission from WS2 layers, and we found the Kerker conditions and top/bottom radiation ratios change a lot with widths of Si nanostripes and the thickness of oxide spacer. Through optimizing the Kerker condition, we observed strong PL enhancements without near-field enhancements and large Q. The proposed new characterizations on the near-field and far-field optical properties of Mie resonators will greatly inspire the design of all-dielectric nanophotonic and optoelectronic devices in the future.

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