This review is devoted to summarizing and discussing the scientific progress achieved during the past decade in diode-pumped ytterbium-doped solid-state lasers. The output radiation centered on the 1-μm region is of particular interest for the well-known applications of previously developed Nd3+ lasers. Several hosts have been studied and tested in recent years. Due to the enormous number of papers dealing with ytterbium, Yb3+ doped hosts, we will limit our report here to materials based on transparent polycrystalline sesquioxide laser ceramics, such as Lu2O3, Sc2O3 and Y2O3. The reason is basically due to the wider emission band of ytterbium-doped sesquioxides as compared with more commonly used hosts such as YAG. The higher thermal conductivity with respect to hosts such as CaF2 makes them very interesting for applications requiring both good thermal conductivity and a wide fluorescence emission band, as well as for short pulse generation at high average power levels. Furthermore, the replacement of trivalent Yb with sesquioxides does not require any charge compensation (as is the case of the replacement of divalent Ca in CaF2), which makes their fabrication easier, in particular at high doping levels.

Metasurfaces can be opportunely and specifically designed to manipulate electromagnetic wavefronts. In recent years, a large variety of metasurface-based optical devices such as planar lenses, beam deflectors, polarization converters, and so on have been designed and fabricated. Of particular interest are tunable metasurfaces, which allow the modulation of the optical response of a metasurface; for instance, the variation in the focal length of a converging metalens. Response tunability can be achieved through external sources that modify the permittivity of the materials constituting the nanoatoms, the substrate, or both. The modulation sources can be classified into electromagnetic fields, thermal sources, mechanical stressors, and electrical bias. Beside this, we will consider optical modulation and multiple approach tuning strategies. A great variety of tunable materials have been used in metasurface engineering, such as transparent conductive oxides, ferroelectrics, phase change materials, liquid crystals, and semiconductors. The possibility of tuning the optical properties of these metamaterials is very important for several applications spanning from basic optics to applied optics for communications, depth sensing, holographic displays, and biochemical sensors. In this review, we summarize the recent progress on electro-optical magnetic, mechanical, and thermal tuning of metasurfaces actually fabricated and experimentally tested in recent years. At the end of the review, a short section on possible future perspectives and applications is included.

This paper is devoted to reviewing the latest results achieved in solid-state lasers based on thulium-doped mixed-sesquioxide ceramics, i.e., (Lu,Sc,Y)₂O₃. The near- and mid-infrared regions are of interest for many applications, from medicine to remote sensing, as they match molecular fingerprints and cover several atmospheric transparency windows. These matrices are characterized by a strong electron–phonon interaction—which results in a large splitting of the ground state—and by a spectral broadening of the optical transition suitable for developing tunable and short-pulse lasers. In particular, the manuscript reports on the trivalent thulium laser transitions at 1.5, 1.9, and 2.3 µm, along with the thermal and optical characteristics of the (Lu,Sc,Y)₂O₃ ceramics, including the fabrication techniques, spectroscopic and optical properties, and laser performances achieved in different pumping regimes, such as continuous-wave (CW), quasi-CW, and pulsed modes. A comparison of the performance obtained with these mixed-sesquioxide ceramics and with the corresponding crystals is reported.

We report the first oscillation achieved on $1\text{\hspace{0.17em} at. \%}$ Yb-doped ${\mathrm{Sc}}_2{\mathrm{O}}_3$ and ${\mathrm{Lu}}_2{\mathrm{O}}_3$ ceramics under quasi-CW pumping at $968\mathrm{nm}$. With ${\mathrm{Sc}}_2{\mathrm{O}}_3$ we measured a maximum output power of $2.2\mathrm{W}$ with a slope efficiency of 59% at $1040.5\mathrm{nm}$. Comparable results are obtained with ${\mathrm{Lu}}_2{\mathrm{O}}_3$ at $1032.5\mathrm{nm}$, i.e., $1.8\mathrm{W}$ with a corresponding slope of 45%, while at $1078\mathrm{nm}$ we measured $1.5\mathrm{W}$ with 34% of slope efficiency. Finally, we present the range of tunability, which exceeds $41\mathrm{nm}$ for both samples. To the best of our knowledge, these are the first measurements reported in literature either in terms of laser emission or continuous tunability range achieved with $1\text{\hspace{0.17em} at. \%}$ doping level.