For the first time, long-distance fidelity transmission and directional distribution of valley photons are realized

Recently, the joint team composed of Professors Chen Yang, Wu Dong and Chu Jiaru of the University of Science and Technology of China, the research group of Professor Wang Kai and Professor Lu Peixiang of Huazhong University of Science and Technology and the research group of Professor Qiu Chengwei of the National University of Singapore have made important progress in the intersection of valley electronics and micro-nanophotonics, and for the first time realized the long-distance fidelity transmission and directional distribution of WS2 valley photons based on hybrid nanoguides. The research results were published in Nature Nanotechnology on October 3, 2022 under the title “Chirality-dependent, unidirectional routing of WS2 valley photons in a nanocircuit.”

As the cornerstone of modern technological development, integrated circuit (IC) technology has achieved great success in the past fifty years, doubling the number of components it can accommodate per unit area every 18 months, which is known as Moore’s law. Today, thanks to mature silicon-based lithography technology, the feature size of chip components has reached the order of several nanometers, which also lays the foundation for the development of portable electronic devices, wearable devices, and large-scale storage and computing industries. However, conventional integrated circuits rely on the charge freedom of electrons, and their chip size is already close to the theoretical limit due to the influence of energy consumption and quantum effects. In order to further reduce the chip size, continue Moore’s Law, find new degrees of electronic freedom and develop new electronic devices has become an important research direction in the scientific research community and industry.

In fact, in addition to the charge degrees of freedom, electrons also have internal degrees of freedom such as spin and valley. Among them, the energy valley refers to the extreme point of the crystal Bloch electronic energy band, and the electronic devices based on the energy valley are expected to achieve lower energy consumption, less heat generation and faster processing speed than traditional devices. However, due to the extremely short depolarization life and very small mobility of the energy valley, the long-distance fidelity transmission problem of the energy valley information has become a key bottleneck in the development of the energy valley device. In past studies (Science359, 443-447 (2018); Nat Photonics13,180-184 (2019); ACS Nano15, 18163-18171 (2021)), researchers usually based on the valley-direction locking between the energy valley and a single waveguide mode transmission direction (valley-direction locking), to achieve the separation of two different energy valleys, but in the separation process of the energy valley information is also lost, can not achieve the energy valley information post-processing.

In this work, we innovatively designed and prepared an Au-WS2-SiO2-TiO2 hybrid waveguide (Figures 1a and b) that simultaneously supports two propagation modes (630 nm) at the resonant wavelength (630 nm) of the WS2 excitor, both of which are locally distributed in the SiO2 gap layer known as gap mode, and have symmetrical and antisymmetric electric field distributions respectively (Figure 2c). When the WS2 monolayer is stimulated in the same gap layer, its energy valley degrees of freedom (K or K’) as a pseudospin can be equivalent to a circular polarized electric dipole with opposite rotation, and both gap modes can be excited at the same time. Since the two gap modes have different effective wave vector neffs, they superimpose in transmission to produce beating waves with a beat frequency period of l= 2π/ (kGM1–kGM2) = 1261 nm. Corresponding to the opposite pseudospin of K and K’ energy valleys, the resulting beat frequency waves have a pattern distribution of mirror symmetry (Figure 1d), so that the energy valley information of the exciton is deterministically encoded and stored in the chiral distribution of the beat wave photons, and transmitted forward with low loss, and the energy valley fidelity (FVP) can be calculated to reach more than 98%. As a control, if the wavelength of a circularly polarized electric dipole is set to a wavelength of 810 nm for a pumped laser, the hybrid waveguide supports only one gap mode and therefore does not generate a beat wave (Figure 1e).

This chiral beat frequency mode that carries Nenggu information lays the foundation for the post-processing of Noh Valley information. We have built a single-in, dual-output valley photonic router to realize the directional selective distribution of energy valley information. By modulating the circular polarization of the incident pump light, we can selectively excite K or K’valley excitons at the input. When K’valley excitons are excited, the resulting valley photons are distributed directionally to output B; conversely, when the K valley excitons are excited, the resulting valley photons are distributed directionally to output A (Figure 2). Through simulation calculations, the energy valley path selection ratio can reach 0.92, and the actual measured value also reaches 0.46. After analysis, the deviation between the theoretical value and the measured value is mainly caused by the size of the incident pump spot, while the influence of the energy valley depolarization effect caused by phonon-assisted intervalley scattering in WS2 is relatively weak, which is due to the small mode area and large Purcell factor of the nano cavity formed by the hybrid waveguide in the gap, so the valley exciton exciton initiates the gap mode through the coupling of the near-field non-radiant energy transfer process before the depolarization process occurs.

Further, we also show the one-way transmission of Nenggu information in a three-port circulator, where the corresponding valley photon can only be transmitted in the circulator in the counterclockwise direction for K’Nenggu excitons, and in the circulator in the clockwise direction for K’Nenggu excitons (Figure 3).

This study is the first to realize the long-distance fidelity transmission and directional distribution of Nenggu information, although the function of the Nenggu device demonstrated is still in its infancy, but it provides a solution for the next step of building a large-scale valley electronic device network. More importantly, this valley electron-photonic hybrid device provides a new idea for the simultaneous integration of valley electronic devices, spintronic devices and on-chip photonic devices on the chip to build a spin-energy valley-photonic hybrid system.

The first author of the paper is Professor Chen Yang of the University of Science and Technology of China and Qian Shuhang, a doctoral student of Huazhong University of Science and Technology, the corresponding authors are Professor Wang Kai, Professor Lu Peixiang and Professor Qiu Chengwei of the National University of Singapore, of which Professor Qiu Chengwei of the National University of Singapore led the work, Professor Lu Peixiang of the Central China University of Science and Technology gave full support in the experiment, and Professor Wu Dong and Professor Chu Jiaru of the University of Science and Technology of China also gave important guidance in the research process.

Thesis Links:https://www.nature.com/articles/s41565-022-01217-x

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