Spectroscopic characterization of Er3+, Yb3+ co-doped UC single crystals : the influence of host and sensitizer concentrations

The energy transfer upconversion (ETU) mechanism is known to be the most efficient route for the conversion of near infrared (NIR) light to visible emission in Ln3+-co-doped systems. In this work, we examined these energy transfer (ET) processes in Yb3+,Er3+-co-doped fluoride single crystals. Because of their low phonon energy, high thermal dissipation, chemical stability and high transmission in the ultraviolet, visible, and NIR, these materials are ideal systems to study such processes. Here, we focus on the influence of the concentration of the sensitizer Yb3+ on the optical and upconversion properties of three different fluoride hosts doped with Yb3+ and Er3+ as a function of excitation power density and compare direct Ln3+-excitation and excitation in the NIR via ET

Thickness-dependent elastic softening of few-layer free-standing MoSe2

Few-layer van der Waals (vdW) materials have been extensively investigated in terms of their exceptional electronic, optoelectronic, optical, and thermal properties. Simultaneously, a complete evaluation of their mechanical properties remains an undeniable challenge due to the small lateral sizes of samples and the limitations of experimental tools. In particular, there is no systematic experimental study providing unambiguous evidence on whether the reduction of vdW thickness down to few layers results in elastic softening or stiffening with respect to the bulk. In this work, micro-Brillouin light scattering is employed to investigate the anisotropic elastic properties of single-crystal free-standing 2H-MoSe as a function of thickness, down to three molecular layers. The so-called elastic size effect, that is, significant and systematic elastic softening of the material with decreasing numbers of layers is reported. In addition, this approach allows for a complete mechanical examination of few-layer membranes, that is, their elasticity, residual stress, and thickness, which can be easily extended to other vdW materials. The presented results shed new light on the ongoing debate on the elastic size-effect and are relevant for performance and durability of implementation of vdW materials as resonators, optoelectronic, and thermoelectric devices

Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials

Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime

Thermal conductivity of MoS2 polycrystalline nanomembranes

Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. Afavorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS₂ nanosheets in a nanoscale grain-size limit.Wereport thermal conductivity measurements on 5 nm thick polycrystalline MoS₂ by means of 2-laser Raman thermometry. The free-standing, drum-like MoS₂ nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS₂ and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 W m⁻¹ K⁻¹. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed

Milliwatt terahertz harmonic generation from topological insulator metamaterials

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi$_2$Se$_3$ topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW - an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications

Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials [Dataset]

6 pages. -- Supplementary Note 1, Sample Preparation. -- Supplementary Note 2, Experimental. -- Supplementary Note 3, Calculations of electron temperature. -- Supplementary Note 4, THz fluence and intensity. -- Supplementary Figures. -- Supplementary References.Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime.Peer reviewe

Unraveling Heat Transport and Dissipation in Suspended MoSe 2 from Bulk to Monolayer

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications

Spectroscopic characterization of Er3+, Yb3+ co-doped UC single crystals : the influence of host and sensitizer concentrations

The energy transfer upconversion (ETU) mechanism is known to be the most efficient route for the conversion of near infrared (NIR) light to visible emission in Ln3+-co-doped systems. In this work, we examined these energy transfer (ET) processes in Yb3+,Er3+-co-doped fluoride single crystals. Because of their low phonon energy, high thermal dissipation, chemical stability and high transmission in the ultraviolet, visible, and NIR, these materials are ideal systems to study such processes. Here, we focus on the influence of the concentration of the sensitizer Yb3+ on the optical and upconversion properties of three different fluoride hosts doped with Yb3+ and Er3+ as a function of excitation power density and compare direct Ln3+-excitation and excitation in the NIR via ET

Spectroscopic characterization of Er3+, Yb3+ co-doped UC single crystals : the influence of host and sensitizer concentrations

The energy transfer upconversion (ETU) mechanism is known to be the most efficient route for the conversion of near infrared (NIR) light to visible emission in Ln3+-co-doped systems. In this work, we examined these energy transfer (ET) processes in Yb3+,Er3+-co-doped fluoride single crystals. Because of their low phonon energy, high thermal dissipation, chemical stability and high transmission in the ultraviolet, visible, and NIR, these materials are ideal systems to study such processes. Here, we focus on the influence of the concentration of the sensitizer Yb3+ on the optical and upconversion properties of three different fluoride hosts doped with Yb3+ and Er3+ as a function of excitation power density and compare direct Ln3+-excitation and excitation in the NIR via ET

Elastic Properties of Few Nanometers Thick Polycrystalline MoS2 Membranes : A Nondestructive Study

The performance gain-oriented nanostructurization has opened a new pathway for tuning mechanical features of solid matter vital for application and maintained performance. Simultaneously, the mechanical evaluation has been pushed down to dimensions way below 1 μm. To date, the most standard technique to study the mechanical properties of suspended 2D materials is based on nanoindentation experiments. In this work, by means of micro-Brillouin light scattering we determine the mechanical properties, that is, Young modulus and residual stress, of polycrystalline few nanometers thick MoS membranes in a simple, contact-less, nondestructive manner. The results show huge elastic softening compared to bulk MoS, which is correlated with the sample morphology and the residual stress