Pressure-induced enhancement of optoelectronic properties in PtS2

PtS2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressuredependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ～ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to26.8 GPa, which indicates a stable lattice structure of PtS2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.

Watt-level,green-pumped optical parametric oscillator based on periodically poled potassium titanyl phosphate with high extraction efficiency

We report a compact,efficient optical parametric oscillator(OPO)based on a periodically poled potassium titanyl phosphate(PPKTP)crystal pumped by a 532 nm laser,which generated 1.51 W of average power at the signal wavelength of 709 nm with the pulse duration of~1.0 ns.The extraction efficiency was up to 59%.To the best of our knowledge,this is the first report on Watt-level green-pumped PPKTP-based singly resonant oscillator OPO(SRO-OPO).The precise build-up time of OPO was determined to be 1.6 ns benefitting from the characteristic of twin-peak pulse profile of pump beam.The spectrum width of the idler was also measured to be 4.2 nm with the central wavelength of 2134 nm at 0.2 nm spectral resolution of optical spectrum analyzer.In addition,the beam quality of M^(2)<1.9 of generated signal exhibits a good consistency with M^(2)<1.5 for the pump source.

Existence and Multiplicity of Solutions for Nonlocal Systems with Kirchhoff Type

Firstly,we use Nehari manifold and Mountain Pass Lemma to prove an existence result of positive solutions for a class of nonlocal elliptic system with Kirchhoff type.Then a multiplicity result is established by cohomological index of Fadell and Rabinowitz.We also consider the critical case and prove existence of positive least energy solution when the parameter β is sufficiently large.

Existence of Positive Solutions for a General Nonlinear Eigenvalue Problem

Let Ω R n be a bounded domain, H = L 2 （Ω）, L ： D（L） H → H be an unbounded linear operator, f ∈ C（ × R, R） and λ∈ R. The paper is concerned with the existence of positive solutions for the following nonlinear eigenvalue problem Lu = λf （x, u）, u ∈ D（L）, which is the general form of nonlinear eigenvalue problems for differential equations. We obtain the global structure of positive solutions, then we apply the results to some nonlinear eigenvalue problems for a second-order ordinary differential equation and a fourth-order beam equation, respectively. The discussion is based on the fixed point index theory in cones.