Bound State in the Continuum (BIC) is a special eigenstate, which energy lies in the continuous spectrum of propagating modes of the surrounding space. However, the state does not interact with any of the states of the continuum; therefore, it cannot emit and cannot be excited by any wave that came from the infinity. BIC resonance can theoretically reach an infinite quality factor (Q-factor), if there is no absorption in the system. BIC has the potential application in the design of optical resonators, lasers, filters and sensors.

Concept of bound state in the continuum (BIC) with 1D photonic crystals


This project is supported by the Ministry of Science and Technology (MOST) “Outstanding Young Scholars Program” and the MOST-RFBR international joint project (Taiwan – Russia). The project’s main PI is Prof. Kuo-Ping Chen, from Institute of Imaging and Biomedical Photonics, College of Photonics, National Yang Ming Chiao Tung University -NYCU (originally, National Chiao Tung University – NCTU). The collaborated research team is led by Dr. Ivan Timofeev from Kirensky Institute of Physics, Russia. In this project, we successfully developed the world’s first tunable BIC combining liquid crystals and photonic crystal. This research was mainly conducted by Bing-Ru Wu, Jhen-Hong Yang, Pavel S. Pankin, and Chih-Hsiang Huang. The research results have been published in March 2021, Laser and Photonics Review (IF = 10.655, Journal Ranking: 4th/97 in Optics). In this research, we study both BIC and quasi-BIC in nematic liquid crystal (LC) layer embedded between a distributed Bragg reflector (DBR) and a metal layer. We experimentally demonstrate a special class of true BICs between the DBR and the metal film with the strong field localization provided by the Brewster TE reflection in the DBR.

Ultra-low threshold laser with bound state in the continuum (BIC)


In addition, we also successfully developed the world’s first silicon nitride metasurfaces with R6G dye for the ultra-low threshold lasing. This research was mainly conducted by the Ph.D. student from NYCU, Dr. Jhen-Hong Yang. The research results have been published in Aug 2021, Laser and Photonics Review. In this research, Si3N4 metasurfaces with hybrid SLRs are used to investigate BICs with complete dark resonance modes. To determine the design rule and mechanism of BICs, simulations and experiments were performed with external and internal excitation. Compared to the nanolaser, the BIC metasurface laser possesses directional radiation and a large emission volume, and the high Q-factor resonance overcomes the limitation of large mode volume in achieving thresholdless lasing. In addition, a proposed design rule can eliminate the wavelength shift when the Q-factor changes, which makes the comparison of lasing threshold in different BIC metasurfaces possible. We successfully use the high localization ability of BICs to demonstrate a low-threshold (1.25 nJ) BIC laser at room temperature. Also, the large spontaneous emission coupling factor (β = 0.9) and S-curve in the “light in-light out” diagram are rigorously discussed and demonstrated in both simulation and experiment. Interestingly, due to the high Q-factor resonance of BICs, the laser signals and images can be observed in almost transparent samples. The novel features of metasurfaces provide the way for engineering BICs. The developed device can be used in various applications, including novel light sources, optical sensing, nonlinear optics, and topological photonics.

The Taiwan research team includes Prof. Kuo-Ping Chen (NYCU), Prof. Wei Lee (NYCU), Prof. Tien-Chang Lu (NYCU), Prof. Tzy-Rong Lin (NTOU), and Prof. Chan-Shan Yang (NTNU). The Russian research team includes Dr. Ivan V. Timofeev, Dr. Almas. Sadreev, Dr. Dmitrii N. Maksimov, Dr. Pavel S. Pankin, and Dr. Rashid Bikbaev. The collaboration research will continue to develop the BIC with metasurfaces for quantum photonic application. This research also has the strong potential in development of quantum source and topological photonics.
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