Showing papers by "Zheyu Fang published in 2022"

Giant excitonic upconverted emission from two-dimensional semiconductor in doubly resonant plasmonic nanocavity

Abstract: Phonon-assisted upconverted emission is the heart of energy harvesting, bioimaging, optical cryptography, and optical refrigeration. It has been demonstrated that emerging two-dimensional (2D) semiconductors can provide an excellent platform for efficient phonon-assisted upconversion due to the enhanced optical transition strength and phonon-exciton interaction of 2D excitons. However, there is little research on the further enhancement of excitonic upconverted emission in 2D semiconductors. Here, we report the enhanced multiphoton upconverted emission of 2D excitons in doubly resonant plasmonic nanocavities. Owing to the enhanced light collection, enhanced excitation rate, and quantum efficiency enhancement arising from the Purcell effect, an upconverted emission amplification of >1000-fold and a decrease of 2~3 orders of magnitude in the saturated excitation power are achieved. These findings pave the way for the development of excitonic upconversion lasing, nanoscopic thermometry, and sensing, revealing the possibility of optical refrigeration in future 2D electronic or excitonic devices.

Engineering of single-photon emitters in hexagonal boron nitride [Invited]

Abstract: Quantum information technology requires bright and stable single-photon emitters (SPEs). As a promising single-photon source, SPEs in layered hexagonal boron nitride (hBN) have attracted much attention recently for their high brightness and excellent optical stability at room temperature. In this review, the physical mechanisms and the recent progress of the quantum emission of hBN are reviewed, and the various techniques to fabricate high-quality SPEs in hBN are summarized. The latest development and applications based on SPEs in hBN in emerging areas are discussed. This review focuses on the modulation of SPEs in hBN and discusses possible research directions for future device applications.

Inverse Design of Unidirectional Transmission Nanostructures Based on Unsupervised Machine Learning

Abstract: Structural design is an important driving force for technological development in nanophotonics. To achieve better performance of nanophotonic devices, the freedom of structural design space needs to be expanded. Unsupervised learning algorithm in deep learning provides a great platform to expand design freedom of structures and avoid the “curse of dimensionality” effectively. It performs well on extracting important features from high‐dimensional data and excavating potential rules. In this work, an unsupervised convolutional neural network is built to inversely design nanostructures with unidirectional transmission. Near‐field information with high dimensions is recognized and extracted into a 2D feature space which maintains high physical continuity and maps to far‐field transmittance effectively. The feature space is further expanded to the whole space by optimistic Bayesian multisampling, from which nanostructures with transmittance over 95% forward while less than 40% backward are inversely designed. Moreover, the relation between near‐field information and far‐field transmittance is explored. A feasible design method of nanostructures is proposed based on unsupervised learning with design space expanded. This design mentality exhibits a way of extracting near‐field features to analyze far‐field spectra with deep learning algorithms, which is suitable for more abundant physical design and can be extended to other similar systems.

Study of a High-Index Dielectric Non-Hermitian Metasurface and Its Application in Holograms

Abstract: We demonstrate that a high-index dielectric Si metasurface with a designed chiral unit structure possesses an exceptional point (EP) when it is described by a non-Hermitian Hamiltonian associated with the transmission matrix. By encircling any path in the parameter space around the EP, topologically protected 2π-phase accumulation occurs. These typical non-Hermitian properties are ascribed to complex scattering phenomena related to the coupling between electric and magnetic dipolar modes from the high-index dielectric Si metasurface. The topologically guaranteed entire 2π-phase accumulation and chiral distinction around the EP open up many promising possibilities in nanophotonic device designing; for instance, phase-only and polarization multiplexing holograms are realized in this work.

Molding 2D Exciton Flux toward Room Temperature Excitonic Devices

Abstract: Devices operating with excitons have promising prospects for overcoming the dilemma of response time and integration in current generation of electron‐ or/and photon‐based elements and devices. In combination with the advantages of emerging twistronics and valleytronics, the atomically thin transition metal dichalcogenide semiconductors open up new opportunities for pursuing practical excitonic devices, where the strong exciton binding energy enables operating exciton at room temperature. The essential and foremost step toward exciton devices is the control of spatiotemporal exciton flux, which is density‐dependent and affected by the complex many‐body interactions. It can be effectively controlled by the strain, electric field, electron‐doping, and local dielectric environment. Intriguingly, exotic phenomena such as exciton condensation, electron‐hole liquid, exciton Hall effects, and exciton halo effects can be occurred in 2D exciton system, providing new possibilities for excitonic devices. Up to now, the proof‐of‐principle of room temperature exciton devices, including excitonic switching and transistor, exciton guides, and excitonic nanolaser, have been realized. Here the authors review the recent advances in molding 2D exciton flux from basic principle, manipulation, exotic phenomena to promising applications and discuss the opportunities and challenges in pushing the frontiers of room temperature excitonic devices.

Polarization-Dependent Purcell Enhancement on a Two-Dimensional h-BN/WS2 Light Emitter with a Dielectric Plasmonic Nanocavity.

Abstract: Integrating two-dimensional (2D) transition-metal dichalcogenides (TMDCs) into dielectric plasmonic nanostructures enables the miniaturization of on-chip nanophotonic devices. Here we report on a high-quality light emitter based on the newly designed 2D h-BN/WS2 heterostructure integrated with an array of TiO2 nanostripes. Different from a traditional strongly coupled system such as the TMDCs/metallic plasmonic nanostructure, we first employ dielectric nanocavities and achieve a Purcell enhancement on the nanoscale at room temperature. Furthermore, we demonstrate that the light emission strength can be effectively controlled by tuning the polarization configuration. Such a polarization dependence meanwhile could be proof of the resonant energy transfer theory of dipole-dipole coupling between TMDCs and a dielectric nanostructure. This work gains experimental and simulated insights into modified spontaneous emission with dielectric nanoplasmonic platforms, presenting a promising route toward practical applications of 2D semiconducting photonic emitters on a silica-based chip.

Enhancement of photoluminescence of monolayer transition metal dichalcogenide by subwavelength TiO2 grating

Abstract: Monolayer transition metal dichalcogenide (TMDC) has a direct band gap and can produce strong photoluminescence(PL), thereby possessing a wide application prospect in photoelectric devices and photoelectric detection fields. However, its PL efficiency needs further improving because its monolayer is of atomic thickness only, besides, it has non-radiative recombination of excitons. In this study, a combination structure of a gold film, titanium dioxide subwavelength gratings and monolayer TMDCs is designed, which can greatly improve PL efficiency of monolayer TMDC. The spontaneous emission rate can be controlled by the Purcell effect, and the maximum enhancement of photoluminescence is as high as 3.4 times. In this paper, the PL signal of monolayer WS2 and monolayer WSe2 on the designed structure are studied. The feasibility of the enhancement of PL of monolayer TMDC in the coupling structure of monolayer TMDC and the subwavelength grating is verified experimentally, which provides a new idea for the application of two-dimensional materials to optoelectronic devices.

Ultra-Thin Wood-Based Acoustic Diaphragms Fabricated via an Environmentally Friendly Strategy.

Abstract: An acoustic diaphragm is a crucial component that regulates sound quality in earphones and loudspeakers. Natural wood with inherent good acoustic resonance and vibration spectrum is widely used in sound devices. However, using natural wood to produce an acoustic diaphragm is still a big challenge because making ultra-thin wood is hard and it warps easily. Therefore, this study introduces a new method for preparing ultra-thin wood acoustic diaphragms less than 10 μm in thickness, relying on delignification, sulfonation, and densifying techniques. The innovative sulfonation process increased the intermolecular hydrogen bond force, which significantly improved the tensile strength and Young's modulus of the wood diaphragm, up to 195 MPa and 27.1 GPa, respectively. Compared with the commonly used diaphragms in the market, this wood diaphragm exhibits an excellent specific dynamic elastic modulus up to 95.1 GPa/g cm3, indicating better acoustic properties. Also, the resonance frequency was up to 1240 Hz, 4.5 times higher than the titanium diaphragm among high-end products. Besides, the drying shrinkage rate of the ultra-thin wood diaphragm is only 1.2%, indicating excellent dimensional stability. This high-quality wood acoustic diaphragm has a very high application prospect and outstanding attributes for promoting the development of acoustic devices. Moreover, the reaction reagent can be recycled after preparation, and the selected reagents are green and environmentally friendly.