Single crystal erbium compound nanowires as high gain material for on-chip light source applications

Integrated photonics requires high gain optical materials in the telecom wavelength range for optical amplifiers and coherent light sources. Erbium （Er） containing materials are ideal candidates due to the 1.5 μm emission from Era＋ ions. However, the Er density in typical Er-doped materials is less than 10^20 cm-3, thus limiting the maximum optical gain to a few dB/cm, too small to be useful for integrated photonic applications. Er compounds could potentially solve this problem since they contain much higher Era＋ density. So far the existing Er compounds suffer from short lifetime and strong upconversion effects, mainly due to poor crystal qualities. Recently, we explore a new Er compound： erbium chloride silicate （ECS, Er3（SiO4）2C1） in the form of nanowire, which facilitates the growth of high quality single crystal with relatively large Era＋ density 0.62 ×10^22 cm^-3）. Previous optical results show that the high crystal quality of ECS material leads to a long lifetime up to 1 ms. The Er lifetime-density product was found to be the largest among all the Er containing materials. Pump-probe experiments demonstrated a 644 dB/cm signal enhancement and 30 dB/cm net gain per unit length from a single ECS wire. As a result, such high-gain ECS nanowires can be potentially fabricated into ultra-compact lasers. Even though a single ECS nanowire naturally serves as good wavegnide, additional feedback mechanism is needed to form an ultra-compact laser. In this work, we demonstrate the direct fabrication of 1D photonic crystal （PhC） air hole array structure on a single ECS nanowire using focused ion beam （FIB）. Transmission measurement shows polarization-dependent stop-band behavior. For transverse electric （TE） polarization, we observed stop-band suppression as much as 12 dB with a 9 μm long airholed structure. Through numerical simulation, we showed that Ω-factor as high as 11000 can be achieved at 1.53 μm for a 1D PhC micro-cavity on an ECS nanowire. Such a high Q cavity combined with the high material gain of ECS nanowires provides an attractive solution for ultra-compact lasers, an important goal of this research.

Excitonic complexes and optical gain in two-dimensional molybdenum ditelluride well below the Mott transition

Semiconductors that can provide optical gain at extremely low carrier density levels are critically important for applications such as energy efficient nanolasers.However,all current semiconductor lasers are based on traditional semiconductor materials that require extremely high density levels above the so-called Mott transition to realize optical gain.The new emerging 2D materials provide unprecedented opportunities for studying new excitonic physics and exploring new optical gain mechanisms at much lower density levels due to the strong Coulomb interaction and co-existence and mutual conversion of excitonic complexes.Here,we report a new gain mechanism involving charged excitons or trions in electrically gated 2D molybdenum ditelluride well below the Mott density.Our combined experimental and modelling study not only reveals the complex interplay of excitonic complexes well below the Mott transition but also establishes 2D materials as a new class of gain materials at densities 4-5 orders of magnitude lower than those of conventional semiconductors and provides a foundation for lasing at ultralow injection levels for future energy efficient photonic devices.Additionally,our study could help reconcile recent conflicting results on 2D materials:While 2D material-based lasers have been demonstrated at extremely low densities with spectral features dominated by various excitonic complexes,optical gain was only observed in experiments at densities several orders of magnitude higher,beyond the Mott density.We believe that our results could lead to more systematic studies on the relationship between the mutual conversion of excitonic species and the existence of optical gain well below the Mott transition.

Nonvolatile electrical switching of optical and valleytronic properties of interlayer excitons

Long-lived interlayer excitons(IXs)in van der Waals heterostructures(HSs)stacked by monolayer transition metal dichalcogenides(TMDs)carry valley-polarized information and thus could find promising applications in valleytronic devices.Current manipulation approaches for valley polarization of IXs are mainly limited in electrical field/doping,magnetic field or twist-angle engineering.Here,we demonstrate an electrochemical-doping method,which is efficient,in-situ and nonvolatile.We find the emission characteristics of IXs in WS2/WSe2 HSs exhibit a large excitonic/valley-polarized hysteresis upon cyclic-voltage sweeping,which is ascribed to the chemical-doping of O2/H2O redox couple trapped between WSe2 and substrate.Taking advantage of the large hysteresis,a nonvolatile valley-addressable memory is successfully demonstrated.The valley-polarized information can be non-volatilely switched by electrical gating with retention time exceeding 60 min.These findings open up an avenue for nonvolatile valley-addressable memory and could stimulate more investigations on valleytronic devices.