Researcher Han Dezhuan from Chongqing University, in collaboration with research groups led by Professor Chen Ziting from Hong Kong University of Science and Technology, Professor Zheng Chunxiong from Tsinghua University and Professor Zi Jian from Fudan University, has theoretically constructed the first strictly solvable bound state in the continuum (BIC) model in August 2020. The related research findings have been published in Science Advances, a sub-journal of Science, under the title “Coexistence of a new type of bound state in the continuum and a lasing threshold mode induced by PT symmetry”. Chongqing University is the first organization and corresponding organization of the paper. Song Qianju of Chongqing University, Hu Jiashun of Tsinghua University and Dai Shiwei of Chongqing University are first co-authors. Researcher Han Dezhuan of the School of Physics, Chongqing University, Professor Chen Ziting from Hong Kong University of Science and Technology, and Professor Zheng Chunxiong from Tsinghua University are corresponding co-authors.

BIC is a kind of special resonance state. Although its momentum and frequency match the free space mode, it has an infinite quality factor. Therefore, it has attracted great research interest in the past two decades. The traditional view holds that since BIC is located in the continuous spectrum and does not radiate outward, it cannot be excited by the wave from the far field. Therefore, earlier optical and acoustic literature called this mode a deaf mode. Since it cannot radiate into free space, the more accurate description of BIC should be deaf mute mode.

However, the research group found a new BIC induced by PT symmetry, which we call pt BIC. Similar to the traditional BIC, this pt-BIC has a pure real frequency and it does not radiate outward, but what makes it special is that it can be excited by waves from the far field. In other words, this pt-BIC is a mute but not dead mode. Based on the theoretically constructed BIC model that was strictly solvable, we proved this counter-intuitive phenomenon under PT symmetry disturbance. It is worth mentioning that the BIC and pt-BIC in question are not limited by material and geometric parameters, and can exist stably in the parameter space. Moreover, we also found that PT symmetry can induce pt-BIC for other periodic structures that support BIC.

In addition, the existence of pt-BIC also revealed some new physical phenomena in PT symmetric system. In PT symmetric systems, most studies focused on the merging of two states (the corresponding eigenvalues and eigenstates) in order to produce EP points. In this paper, we introduced gain and loss into BIC systems, and we found a novel phenomenon: PT symmetry would lead to state splitting. A BIC will split into two states with pure real energy: Pt-BIC and laser threshold state. In fact, we also proved that pt-BIC and laser threshold state corresponded to the left vector and right vector of the non-Hermitian matrix respectively. Therefore, this phenomenon of state splitting was universal and could be directly extended to other periodic systems.

These novel physical phenomena are completely different from the phenomena based on EP point, which has attracted much attention. Therefore, the results provided a new direction for the study of PT symmetry and non-Hermitian physics. At the same time, this bound state which can be excited by far field also provides new possibilities for BIC applications in laser, hypersensitive detection and so on.

Link of the paper:

https://advances.sciencemag.org/content/6/34/eabc1160

DOI: 10.1126/sciadv.abc1160

This research has been supported by the National Natural Science Foundation of China, the Earmarked President Funds of Chongqing University and Research Grant Committee of Hong Kong.