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Designing and building unique cage assemblies attract increasing interest from supramolecular chemists but remain synthetically challenging. Herein, we propose the use of a flexible vertex with adjustable angles to selectively form highly distorted tetrahedral and octahedral cages, for the first time, in which the flexible vertex forms from the synergistic effect of coordination and covalent interactions. The inherent interligand angle of the vertex can be modulated by guest anions present, which allows for the fine-tuning of different cage geometries. Furthermore, the reversible structural transformation between tetrahedral and octahedral cages was achieved by anion exchange monitored by mass spectrometric technique, the smaller anions favoring tetrahedral cages, while the larger anions supporting octahedral cages. Additionally, the KBr-based cage thin films exhibited prominent enhancement of their third-order NLO responses in two or three orders of magnitude compared to those obtained for their corresponding solutions. This work not only provides a new methodology to build irregular polyhedral structures in a controlled and tunable way but also provides access to new kinds of promising functional optical materials.

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Optical limiting (OL) materials are of great importance for protecting optoelectronic sensors and human eyes from high power laser illumination. Despite extensive efforts on various OL materials (organic molecules, metal clusters, low dimensional semiconductors, etc.), however, a candidate OL material with ultrafast response speed, broad laser protection window and large transmission modulation remains elusive. Gallium oxide (β-Ga2O3) is one of the third generation wide-band gap semiconductors. Due to its large band gap (~4.8 eV), high breakdown field strength, low preparation cost and compatibility with other III-N semiconductors, Ga2O3 has huge potential for high-power and lowloss photonic devices. In this study, we investigated the ultrafast optical limiting response in β-Ga2O3 (100) single crystal by using the femtosecond pump-probe technique. Using a 355 nm femtosecond laser pulse as the modulation source, ultrafast broadband transmission modulation was achieved across the white-light continuum probe spectrum (450-750 nm) via the non-degenerate two-photon absorption in Ga2O3. Furthermore, a slow Drude free-carrier plasma effect was observed <5 ps after pump excitation, which is related to the non-degenerate two-photon absorption induced carrier absorption. Our results indicate that Ga2O3 is a promising material platform for broadband optical limiting and highspeed all-optical signal processing.