Efficient polarization-insensitive quasi-BIC modulation by VO2 thin films.

Bound states in the continuum (BIC) offer great design freedom for realizing high-quality factor metasurfaces. By deliberately disrupting the inherent symmetries, BIC can degenerate into quasi-BIC exhibiting sharp spectra with strong light confinement. This transformation has been exploited to develop cutting-edge sensors and modulators. However, most proposed quasi-BICs in metasurfaces are composed of unit cells with Cs symmetry that may experience performance degradation due to polarization deviation, posing challenges in practical applications. Addressing this critical issue, our research introduces an innovative approach by incorporating metasurfaces with C4v unit cell symmetry to eliminate polarization response sensitivity. Vanadium Dioxide (VO2) is a phase-change material with a relatively low transition temperature and reversibility. Here, we theoretically investigate the polarization-insensitive quasi-BIC modulation in Si-VO2 hybrid metasurfaces. By introducing defects into metasurfaces with Cs, C4, and C4v symmetries, we enable the emergence of quasi-BICs characterized by strong Fano resonance in their transmission spectra. Via numerically calculating the multipole decomposition, distinct dominant multipoles for different quasi-BICs are identified. A comprehensive investigation into the polarization responses of these structures under varying directions of linearly polarized light reveals the superior polarization-independent characteristics of metasurfaces with C4 and C4v symmetries, a feature that ensures the maintenance of maximum resonance peaks irrespective of polarization direction. Utilizing the polarization-insensitive quasi-BIC, we thus designed two different Si-VO2 hybrid metasurfaces with C4v symmetry. Each configuration presents complementary benefits, leveraging the VO2 phase transition's loss change to facilitate efficient modulation. Our quantitative calculation indicates notable achievements in modulation depth, with a maximum relative modulation depth reaching up to 342%. For the first time, our research demonstrates efficient modulation using polarization-insensitive quasi-BICs in designed Si-VO2 hybrid metasurfaces, achieving identical polarization responses for quasi-BIC-based applications. Our work paves the way for designing polarization-independent quasi-BICs in metasurfaces and marks a notable advancement in the field of tunable integrated devices.

High-temperature-insensitivity high-power Yb-doped fiber laser directly in-band pumping by 1018 nm laser diodes.

In this Letter, we demonstrate a high-power ytterbium-doped fiber laser (YDFL) based on a directly in-band pumping scheme (DIPS) which employs 1018 nm laser diodes (LDs) as pump sources. The wavelength of the LDs is designed theoretically, considering the distribution of Yb3+ absorption cross section (σa) as well as quantum defect (QD). The flat distribution of σa around 1018 nm ensures excellent temperature insensitivity and flexibility for the YDFL. Besides, lower QD and more compact structure promise higher optical-to-optical (O-O) and electrical-to-optical (E-O) efficiencies. Based on the experimental setup, as the cooling temperature of the 1018 nm LDs ranges from 6 to 23°C, an output power of 2 kW level is achieved that varies by only 2.01% without adjusting the operating current of the LDs subjectively. The output power is then scaled up to 5 kW level. Furthermore, there is a great potential to achieve higher output power and E-O efficiency in YDFLs based on the DIPS.

Photonic Bound States in the Continuum in Nanostructures.

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Functionalizing nanophotonic structures with 2D van der Waals materials.

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.