2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid-infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α-MoO . The α-MoO supports strong hyperbolic PhPs in the mid-infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ /120, which can be tailored by altering the thicknesses of the α-MoO 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal-ion-intercalation strategy. The results demonstrate α-MoO as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid-infrared nanophotonic devices.

Cu 1.8 S quantum dots were prepared by using a single-source-precursor type method and investigated in the light of opto-electronic applications. With femtosecond time-resolved transient absorption measurements, the electron relaxation as well as their trapping dynamics could be evaluated. The measurements reveal that the largest and the smallest QD samples prepared exhibit the longest mobility lifetimes, and that the electron-hole relaxation dynamics is strongly dependent on the occurrence of trapping sites. Based on the argument of optical response, it appears that the largest prepared Cu 1.8 S QDs with band gap energy of 2.35 eV are preferred candidates for opto-electronic device fabrication.y Dedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

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