Minghuan Liu

A comprehensive analysis on progressive reset transitions in RRAMs

Reset processes in resistive random-access memory devices have been studied in depth. In particular, progressive transitions, where no clear current reduction steps are seen, are analysed by using a previously developed simulator and by comparing with experimental data of devices based on HfO2 oxides. It has been reported that the characterization of progressive reset processes can be performed by separately considering devices with a single conductive filament or devices with more than one conductive filament. In addition, making use of the experimental measurements shown, different numerical methods are proposed to extract the reset voltage. These methods are applied to different I–V reset curves and discussed.

Enhancement of pump efficiency for an organic distributed feedback laser based on a holographic polymer dispersed liquid crystal as an external light feedback layer

We report a low threshold, high energy conversion organic distributed feedback (DFB) laser based on a holographic polymer dispersed liquid crystal (HPDLC) grating as an external light feedback layer, specifically, by adopting an acrylate-based monomer with low functionality and a rubbed polyimide (PI) alignment layer. In such configuration, the phase separated LCs were aligned along the preferred direction, which gave an increased refractive index difference between the LC and the polymer, so it can provide better light feedback in the HPDLC layer. The pump efficiency for the laser, such as lasing output threshold and conversion efficiency, can be enhanced. The light loss, diffraction efficiency and driving voltage were also investigated for the HPDLC structures to identify the effects of the rubbing layer and monomer functionality.

A new compact model for bipolar RRAMs based on truncated-cone conductive filaments—a Verilog-A approach

A new model for bipolar resistive random-access memories (RRAMs) is presented in this article. Redox and diffusion processes are used to describe in detail the physics behind the filamentary resistive switching (RS) mechanisms of the RRAMs under study. The model includes truncated-cone shaped filaments which are known to be close to the real conductive filament (CF) geometry and a detailed thermal approach, where two temperatures are considered to describe the rupture process at the CFs narrowest part and also the main CF bodys electrical conductivity variations. Ti/ZrO2/Pt RRAM devices have been fabricated and measured, and the model has allowed us to reproduce the experimental data for all the cases analyzed. Finally, the model has been implemented in Verilog-A code within the ADS circuit simulator, and the response of a device to pulsed external voltages within a characterization circuit has been simulated, producing good results when compared with experimental measurements.

Tunable surface-emitting dual-wavelength laser from a blended gain layer with an external holographic grating feedback structure

A tunable surface-emitting dual-wavelength laser emitted from the blended organic gain layer based on a holographic polymer dispersed liquid crystal (HPDLC) transmission grating feedback structure was reported. The organic blended gain layer was formed from Poly (2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and Poly (2-methoxy-5-(3′, 7’-dimethyloctyloxy)-1, 4-phenylenevinylene) (MDMO-PPV) with a weight ratio of 2:1. The dual-wavelength laser located at 629.9 nm and 640 nm was obtained in a single beam. The optical characteristics of these two organic semiconducting materials and the dual-wavelength laser performance under an electric field are investigated and illustrated. This simple tunable dual-wavelength laser shows the potential to extend the development of organic lasers.

Improving the conversion efficiency of an organic distributed feedback laser by varying solvents of the laser gain layer

ABSTRACT Control experiments were performed to improve the slope conversion efficiency of the organic distributed feedback laser by varying the dissolution solvents of the laser gain layer, a conjugated polymer poly(2-methoxy-5-(2ʹ-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) in this work. The distributed feedback configuration of the laser was prepared by holographic photopolymerisation of the polymer/liquid crystal (HPDLC) mixture. Experimental results showed that the tetrahydrofuran (THF) solvent cast laser gain layer had a lower lasing threshold (0.28 μJ/pulse) and a higher slope conversion efficiency (7.8%) than that of the xylene solvent cast laser gain layer (0.5 μJ/pulse, 4.9%). Thin film waveguide characterisation demonstrated that the THF-cast film possessed a smaller waveguide loss (5.3 cm−1) and larger net gain (17.1 cm−1) than the xylene-cast film (8.3 cm−1, 15.7 cm−1). Absorbance and photoluminescence spectra indicated that the THF-cast film showed brighter luminescence at 620 nm and larger absorbance at 532 nm, indicating that the interchain interactions of the MEH-PPV is different, which plays the vital role in improving the optical performance of our organic DFB lasers. GRAPHICAL ABSTRACT

Organic Solid-State Tri-Wavelength Lasing from Holographic Polymer-Dispersed Liquid Crystal and a Distributed Feedback Laser with a Doped Laser Dye and a Semiconducting Polymer Film

Organic solid-state tri-wavelength lasing was demonstrated from dye-doped holographic polymer-dispersed liquid crystal (HPDLC) distributed feedback (DFB) laser with semiconducting polymer poly[-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and laser dye [4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran] (DCM) by a one-step holography technique, which centered at 605.5 nm, 611.9 nm, and 671.1 nm. The temperature-dependence tuning range for the tri-wavelength dye-doped HPDLC DFB laser was as high as 8 nm. The lasing emission from the 9th order HPDLC DFB laser with MEH-PPV as active medium was also investigated, which showed excellent s-polarization characterization. The diffraction order is 9th and 8th for the dual-wavelength lasing with DCM as the active medium. The results of this work provide a method for constructing the compact and cost-effective all solid-state smart laser systems, which may find application in scientific and applied research where multi-wavelength radiation is required.

Controlling Morphology and Aggregation in Semiconducting Polymers: The Role of Solvents on Lasing Emission in Poly[2-methoxy-5-(2?-ethyl-hexyloxy)-1,4-phenylene-vinylene]

Systematic experiments were performed to investigate solvent-dependent morphology and aggregation of the semiconducting polymer film poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV), which was span-cast from nonaromatic strong polarity solvents tetrahydrofuran (THF), trichloromethane (TCM) and aromatic weak polarity solvents chlorobenzene (CB), toluene, and p-xylene. The results indicated that the conformation of the spin-cast MEH-PPV films with weak aggregation such as THF and TCM demonstrated excellent lasing emission performances because of inhibiting the fluorescence quenching induced by bi-molecule process. The Atomic Force Microscope (AFM) images confirmed the distinct morphologies of the spin-cast MEH-PPV films. The amplified spontaneous emission (ASE) was investigated in a simple asymmetric slab planar waveguide structure by methods of variable stripe length (VSL) and shifting excitation stripe (SES). The amplified spontaneous emission (ASE) experiments confirmed the distinct polymer chain conformation. The conformation, which preserved from the spin-cast process, indicated the distinct interactions between solvents and MEH-PPV polymer chains. The pure film spectra were performed to confirm the effect of distinct conformation on the material energy level. This work provides insights into the morphology and aggregation effect of the spin-cast polymer films on the performances of lasers.