Focus
BIT has made important progress in the research of Angle photonics
News Source & Photographer & Editor: School of Physics；
Translator: Wu Shanshan; News Agency of BIT
Recently, Professor Duan Jiahua, a member of Professor Yao Yugui's team from the School of Physics in Beijing Institute of Technology, collaborated with Professor Pablo Alonso Gonzalez from the University of Oviedo in Spain and Professor Alexey Yu Nikitin from the Center for International Research in Physics in Spain. Important progress has been made in the experimental verification of multiple optical magic angles in Angle photonics and in the regulation of mid-infrared nanometer light fields. The relevant results were published in the international authoritative journal Nature Materials (Editor Featured), and the editor invited relevant experts to report on the News&View column with the title of "A twist for nanolight".
The frontier of photonic chip research in nanophotonics is about the realization of optical path and its on-chip integration by using high-field local polarized polariton waves (semi-light-semi-matter electromagnetic mode generated by photon coupling with other particles). Duan Jiahua has long been engaged in the study of infrared nanoparticle light field regulation based on polaritons of two-dimensional quantum materials, and has made a series of achievements in experimental observation of novel optical phenomena of polaritons [Science Advances 2022, 8, eabp8486; Science advances 2021, 7, abj0127; Nature Communications 2021 12, 4325 ]and nanoscale light field regulation and its application [Nature Reviews Physics 2022, 4, 578; Science Advances 2021, 7, eabf2690; Nature Materials 2020, 19, 964]. In 2020, together with other research groups in the world, the concept of "Angle photonics" was first proposed [Nano Letters 2020, 20, 5323]. It was found that when the Angle between two layers of anisotropic two-dimensional materials is a fixed value (optical magic Angle), the Poynting vector corresponding to all wave vector components of polariton wave points in the same direction, that is, the light field energy propagates along a specific direction with low loss and no diffraction, which is the natural nanowaveguide of infrared light. However, the same two-layer Angle device only has an optical magic Angle at a certain frequency, that is, a natural waveguide for a single frequency photon. At the same time, the energy of the optical field propagates in a fixed direction under the optical magic Angle, and the traditional control techniques (such as constructing the refraction interface and changing the dielectric environment, etc.) can not achieve the non-diffractive control of the nano-optical field propagation.
In order to solve this problem, the researchers found multiple optical magic angles in the three-layer angular molybdenum oxide crystals, and realized the in-plane full Angle regulation (0-360°) of the non-diffractive propagation direction of the nano-light field through the angular reconstruction, and covered a wide spectrum frequency. As shown in Figure 1, the degree of rotation in the three-layer molybdenum oxide crystal can be precisely controlled through the self-built micro-control platform, and the prepared three-layer molybdenum oxide structure can be split through the micro-region picking technology, and multiple reconstruction of the rotation can be achieved by repeating the above process. Based on the infrared nanoimaging technique, the modulation effect of the Angle of the same sample on the non-diffraction propagation of infrared nanofield has been studied. It is found that the propagation direction of infrared nano light field changes from φc°="50° to φc°=90° when the Angle changes.
Figure 1 Optical magic Angle in a three-layer angular molybdenum oxide crystal, where the polariton propagates in a highly directional direction (without diffraction loss).
By constructing a universal theoretical model of multi-layer angular structure, we found that the non-diffraction propagation direction of polarions in three-layer angular molybdenum oxide crystals strongly depends on the Angle. As shown in Figure 2, when the Angle changes, the normal direction of the isofrequency line also changes (θ1-2 is the Angle between the first layer and the second layer of molybdenum oxide crystals, θ1-3 is the Angle between the first layer and the third layer of molybdenum oxide crystals)(For example, the Angle between normal and vertical direction is φc=50° when θ1-2 ="30°," θ1-3 =""-40°, and φc=80° when θ1-2 =30°, θ1-3 =""-60°), that is, by changing the Angle of the three-layer molybdenum oxide crystals, the in-plane full Angle control of the nano infrared light field with low loss and no diffraction propagation can be achieved. It can be seen that the non-diffraction propagation direction of nanometer light field can only be controlled in the range of 0-30° by changing the thickness of material layer and incident light frequency in the double-angle molybdenum oxide crystal. However, in-plane full Angle (0-360°) control of nanometer infrared light field without diffraction propagation direction can be achieved by changing the Angle of three-layer molybdenum oxide crystal (same three-layer molybdenum oxide crystal, without changing the material layer thickness).
Figure 2 Multiple optical magic angles and polarimetric polarions in a three-layer angular molybdenum oxide crystal are controlled at all angles without diffraction.
In order to observe the modulation effect of Angle on non-diffraction propagation of nanometer infrared light field in three-layer molybdenum oxide crystals, the scattering scanning near-field optical microscope (s-SNOM) was used to characterize the near-field optical distribution of three-layer molybdenum oxide crystals. It can be seen from Figure 3 that there are multiple optical magic angles in the three-layer Angle molybdenum oxide crystal, and the nano infrared light field propagates in different directions with low loss and no diffraction under different optical magic angles, which is consistent with previous theoretical studies. In other words, by changing the Angle of rotation, it is possible to realize the in-plane full Angle control of the nanometer light field without diffraction propagation direction in the three-layer molybdenum oxide crystal.
Figure 3 Near-field optical images of three-layer molybdenum oxide crystals at different angles. When the Angle changes, polarions propagate in different directions without diffraction.
In addition, theoretical studies have also shown that the polariton-isofrequency lines in three-layer angular molybdenum oxide crystals can appear as parallel lines (without diffraction propagation) over a wide frequency range (870 cm-1-940 cm-1). As shown in Figure 4, the researchers used s-SNOM to obtain near-field optical images of three-layer angular molybdenum oxide crystals at different incident light frequencies. When the incident light frequency changes from 901 cm-1 to 930 cm-1, the near-field optical image is consistent with the theoretical prediction. The polariton excited by the gold nanoantenna behaves as a highly directional propagation (without diffraction loss) along the direction of φc=""50°. This shows that the optical magic Angle in the three-layer angular molybdenum oxide crystal has spectral robustness, and can achieve highly directional propagation of infrared nanometric light field in a wide spectral range.
Figure 4 Optical magic Angle in a three-layer angular molybdenum oxide crystal can realize diffraction-free propagation of polaritons of wide spectral frequencies.
Links to relevant papers are as follows: