Systematic studies of cooperative upconversion and ${\mathrm{Yb}}^{3+}–{\mathrm{Er}}^{3+}$ energy transfer in newly developed phosphate glasses were performed by a rate-equation formalism. The cooperative-upconversion coefficients of the ${ }^4{I}_{13/2}$ level for different ${\mathrm{Er}}^{3+}$ concentrations were determined from the luminescence-decay curves for high pump intensities. A small cooperative-upconversion coefficient of $1.1\times {10}^{-18} {\mathrm{cm}}^3/\mathrm{s}$ was obtained for a high ${\mathrm{Er}}^{3+}$ concentration of $4\times {10}^{20}\hspace{0.5em}\mathrm{ions}/{\mathrm{cm}}^3.$ ${\mathrm{Yb}}^{3+}–{\mathrm{Er}}^{3+}$ energy-transfer coefficients for an ${\mathrm{Er}}^{3+}$ concentration of $2\times {10}^{20}\hspace{0.5em}\mathrm{ions}/{\mathrm{cm}}^3$ codoped with different ${\mathrm{Yb}}^{3+}$ concentrations were calculated from the lifetime measurements of the ${ }^2{F}_{5/2}$ level of ${\mathrm{Yb}}^{3+}\hspace{0.5em}\mathrm{ions}.$ For ${\mathrm{Er}}^{3+}$ codoped with an ${\mathrm{Yb}}^{3+}$ concentration of $6\times {10}^{20}\hspace{0.5em}\mathrm{ions}/{\mathrm{cm}}^3,$ an energy-transfer coefficient of $1.1\times {10}^{-16} {\mathrm{cm}}^3/\mathrm{s}$ and an energy-transfer efficiency higher than 95% were determined from our measurements under weak excitation. The cooperative-upconversion coefficients of ${\mathrm{Yb}}^{3+}–{\mathrm{Er}}^{3+}$-doped samples were found to be consistent with that of an ${\mathrm{Er}}^{3+}$-doped sample with the same ${\mathrm{Er}}^{3+}$ concentration. The weak cooperative-upconversion effect of high ${\mathrm{Er}}^{3+}$ concentrations and efficient ${\mathrm{Yb}}^{3+}–{\mathrm{Er}}^{3+}$ energy transfer indicate that these newly developed ${\mathrm{Er}}^{3+}$- and ${\mathrm{Yb}}^{3+}–{\mathrm{Er}}^{3+}$-doped phosphate glasses are excellent for active device applications.

The continuous development of internet of things (IoT) infrastructure and applications is paving the way for advanced and innovative ideas and solutions, some of which are pushing the limits of state-of-the-art technology. The increasing demand for Wireless Sensor Nodes (WSNs) able to collect and transmit data through wireless communication channels, while often positioned in locations that are difficult to access, is driving research into innovative solutions involving energy harvesting (EH) and wireless power transfer (WPT) to eventually allow battery-free sensor nodes. Due to the pervasiveness of radio frequency (RF) energy, RF EH and WPT are key technologies with the potential to power IoT devices and smart sensing architectures involving nodes that need to be wireless, maintenance free, and sufficiently low in cost to promote their use almost anywhere. This paper presents a state-of-the-art, ultra-low power 2.5 $\mathsf{\mu }$ W highly integrated mixed signal system on chip (SoC), for multi-source energy harvesting and wireless power transfer. It introduces a novel architecture that integrates an ultra-low power intelligent power management, an RF to DC converter with very low power sensitivity and high power conversion efficiency (PCE), an Amplitude-Shift-Keying/Frequency-Shift-Keying (ASK/FSK) receiver and digital circuitry to achieve the advantage to cope, in a versatile way and with minimal use of external components, with the wide variety of energy sources and use cases. Diverse methods for powering Wireless Sensor Nodes through energy harvesting and wireless power transfer are implemented providing related system architectures and experimental results.

With reference to the mask-constrained power synthesis of shaped beams, we extend the Spectral Factorization method to both uniformly and non-uniformly spaced linear arrays wherein mutual coupling and mounting-platform effects are present. The overall design procedure does not rely on any restriction on the nature and the shape of the fields, and it is cast as a couple of convex optimizations plus a polynomial factorization. Moreover, it allows one to ascertain the actual feasibility of the radiating system before performing the synthesis, and it is able to identify very many different arrayexcitation solutions all corresponding to the desired power pattern. The given theory is supported by numerical experiments concerning the full-wave synthesis of realistic arrays affected by mutual coupling and mounting-platform effects.