In this work, we introduce an electro-absorption (EA)-based retro-modulator for effective realization of free-space optical communications via passive downlinks. Demands for deep modulation and broad directionality in such links are met by its corner-cube assembly of EA-modulators. The EA-modulators use semi-insulating InP as its band edge absorption exhibits an Urbach tail near the 980-nm wavelength of the laser light. This enables Urbach-edge-assisted EA, which allows the field-induced absorption to be optimized via temperature. The theory, from a uniting of the Einstein model and Franz–Keldysh effect, and experiments, from a prototype, show good agreement with deep (greater than 15%) modulation depths. Such functionality can meet the key demands of emerging free-space optical communication links.

In this work, we put forward a rigorous study on ultraviolet (355-nm) laser irradiation of polyimide for the realization of high-quality laser-induced graphene (LIG) with micron-scale features. High-quality material and micron-scale features are desirable—but often at odds—given that small features demand tightly focused beam spots, with a predisposition to ablation. As such, we investigate the synthesis of LIG by correlating the material characteristics, as gleaned from scanning electron microscopy and Raman spectroscopy, to the incident optical fluence, as a measure of applied optical energy per unit area. The study reveals that high-quality LIG, with ratios of Raman 2D-to-G peak heights approaching 0.7, can be synthesized with micron-scale features, down to 18 ± 2 μm, given suitable attention to the optical fluence. Optimal characteristics are seen at optical fluences between 40 and 50 J/cm2, which promote graphenization and minimize ablation. It is hoped that these findings will lay a foundation for the application of LIG in future integrated technologies.