Metasurfaces and bound states in the continuum (BICs) have attracted considerable attention due to their unique optical properties, infinite quality factors (Q-factor), and the extremely localized fields that drastically enhance light–matter interactions and offer great potential in topological photonics and quantum optics. In this study, the Taiwan-Russia research team demonstrated the bound state in the continuum (BIC) directional lasing with the hybrid surface lattice resonances (SLRs) metasurface and the ultra-low threshold at room temperature. The novel features of metasurfaces provide the way for engineering BICs. The developed device can be used in various applications, including quantum photonics, optical sensing, nonlinear optics, and topological photonics.

Conventional optical components (such as lenses, wave plates, filters, etc.) have existed for centuries, and most of the optical components are limited in terms of component size and function. In recent years, based on the development of topological photonics and the advance of nano-fabrications, metasurfaces have attracted attention due to their unique capabilities to fully control light within a subwavelength layer.

The project’s main PI is Prof. Kuo-Ping Chen, from National Yang Ming Chiao Tung University - NYCU (originally, National Chiao Tung University - NCTU). In their research, the team successfully developed the world's first silicon nitride metasurfaces with gain for the ultra-low threshold lasing at room temperature. This research was mainly conducted by a Ph.D. student from NYCU, Dr. Jhen-Hong Yang. The research results were published in Aug 2021, Laser and Photonics Review (IF = 13.14, Journal Ranking: 4th/97 in Optics).

Fig. 1 Difference in resonance state and bound state in the continuum (BIC)
 

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 thresholds on 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.  

In the future development of quantum applications, metasurfaces could also play an important role, including photon statistics, quantum state superposition, quantum entanglement, and single-photon detection. Especially, research in quantum optics usually requires single-photon sources, entangled photon sources, or other types of non-classical light source. Quantum states can be based on different degrees of freedom of light polarizations, directions, and orbital angular momentums. Therefore, metasurfaces have great potential for manipulating photon for quantum physics. For example, metasurface BIC can be used to enhance the efficiency of single photon emitters based on quantum dots or solid-state color centers. Metasurfaces can also be used to generate spontaneous parametric down-conversion (SPDC), and can be further applied in quantum interference and quantum entanglements.

Fig. 2: Ultra-low threshold laser with bound state in the continuum (BIC)