Bo-Wen Liu

Theoretical investigation of A-element atom diffusion in Ti2AC (A=Sn, Ga, Cd, In, and Pb)

The phase stability and mechanical properties of Ti(2)AC (A=Ga, Cd, Sn, In, and Pb) have close relationship with the behavior of group-A element atoms in these compounds. This letter shows that although the Al, Ga, and In (or Si, Sn, and Pb) are in the same group, the migration energy and vacancy formation energy of group-A atom in Ti(2)GaC/Ti(2)InC (or Ti(2)SnC/Ti(2)PbC) are noticeable lower than those in Ti(2)AlC (or Ti(3)SiC(2)). The present results are helpful for a better understanding of easy out diffusion and self-extrusion of group-A atoms in these compounds.

Low-temperature instability of Ti(2)SnC: A combined transmission electron microscopy, differential scanning calorimetry, and x-ray diffraction investigations

Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD) investigations were conducted oil the hot-pressed Ti(2)SnC bulk ceramic. Microstructure features of bulk Ti(2)SnC ceramic were characterized by using TEM, and a needle-shaped beta-Sn precipitation was observed inside Ti(2)SnC grains with the orientation relationship: (0001) Ti(2)SnC // (200) Sn and [1 (2) over bar 10] Ti(2)SnC // [001] Sn. With the combination of DSC and XRD analyses, the precipitation of metallic Sri was demonstrated to be a thermal stress-induced process during the cooling procedure. The reheating temperature, even as low as 400 degrees C, Could trigger the precipitation of Sn from Ti(2)SnC, which indicated the low-temperature instability of Ti(2)SnC. A substoichiometry Ti(2)Sn(x)C formed after depiction of Sn from ternary Ti(2)SnC phase. Under electron beam irradiation, metallic Sri was observed diffusing back into Ti(2)Sn(x)C. Furthermore, a new Ti(7)SnC(6) phase with the lattice constants of a = 0.32 and c = 4.1 nm was identified and added in the Ti-Sn-C ternary system.

Transition of the polarization switching from extrinsic to intrinsic in the ultrathin polyvinylidene fluoride homopolymer films

Polyvinylidene fluoride homopolymer thin films have been prepared by the Langmuir-Blodgett technique, and their electrical properties have comprehensively been studied. The polyvinylidene fluoride homopolymer films show better ferroelectricity with higher polarization and higher breakdown electric field than that of the poly(vinylidene fluoride-trifluoroethylene) copolymer films. Inspection on the thickness dependence of the coercive field of the polyvinylidene fluoride films suggests an extrinsic polarization switching occurs in the thickness range from 200 to 45 nm, and a non-extrinsic switching is observed in the range between 45 and 11 nm, which is ascribed to the transition range from extrinsic to intrinsic switching.

Influences of carrier diffusion and radial mode field pattern on high speed characteristics for microring lasers

High-speed directly modulated microlasers are potential light sources for on-chip optical interconnection and photonic integrated circuits. In this Letter, dynamic characteristics are studied for microring lasers by rate equation analysis considering radial carrier hole burning and diffusion and experimentally. The coupled modes with a wide radial field pattern and the injection current focused in the edge area of microring resonator can greatly improve the high speed response curve due to the less carrier hole burning. The small-signal response curves of a microring laser connected with an output waveguide exhibit a larger 3 dB bandwidth and smaller roll-off at low frequency than that of the microdisk laser with the same radius of 15 μm, which accords with the simulation results.

Mode Characteristics of Unidirectional Emission AlGaInAs/InP Square Resonator Microlasers

Mode characteristics of AlGaInAs/InP square resonator microlasers laterally confined by a bisbenzocyclobutene layer, with an output waveguide connected to one vertex or the midpoint of one side, are investigated numerically and experimentally. Numerical simulation results indicate that the square resonator with the midpoint output waveguide has a better single-mode property, but a lower mode Q-factor than that with the vertex output waveguide. For a 20-μm-side-length square microlaser with a 1.5-μm-wide vertex output waveguide, lasing spectra with multiple transverse modes are observed with the mode wavelength intervals agreeing with the 2-D-finite difference time domain simulation results. In contrast, single-mode operation with a side-mode suppression ratio of 42 dB is obtained for a 20-μm-side-length square microlaser with a 1.5-μm-wide midpoint output waveguide. The results indicate that output waveguide can result in a mode selection in addition to realizing unidirectional emission.

Nonlinear Dynamics for Semiconductor Microdisk Laser Subject to Optical Injection

The whispering-gallery mode microlasers with the merits of small size and low power consumption are suitable for chip-based photonic integration. In this paper, the nonlinear dynamics for microdisk laser subject to optical injection is investigated for potential applications including photonic microwave generation and improved modulation characteristics. The four-wave mixing, period-one and period-two oscillations, and optical injection locking states are investigated based on the lasing spectra and generated microwave for a 10-μm-radius directional emission microdisk laser. Furthermore, direct modulation bandwidth enhancement from 5.6 to 17.6 GHz is observed for the injection-locked microdisk laser biased at 7 mA. The results reveal the necessity of further explore toward the potential applications of microdisk lasers under optical injection in photonic integrated chip.

Enhanced dielectric and ferroelectric properties in the artificial polymer multilayers

Multilayers consisting of alternating ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared. A superlattice-like structure is shown in the polymer multilayer as the periodic thickness is lower than a critical value. The dielectric constant of the multilayer with a small periodic thickness is two times higher than that of the P(VDF-TrFE) copolymer over a temperature range between 300 K and 350 K. The multilayer also shows a good ferroelectricity in the same temperature range. The enhanced electrical properties of the multilayers are due to the long-range ferroelectric coupling.

Integrated semiconductor twin-microdisk laser under mutually optical injection

We experimentally study the characteristics of an integrated semiconductor twin-microdisk laser under mutually optical injection through a connected optical waveguide. Based on the lasing spectra, four-wave mixing, injection locking, and period-two oscillation states are observed due to the mutually optical injection by adjusting the injected currents applied to the two microdisks. The enhanced 3 dB bandwidth is realized for the microdisk laser at the injection locking state, and photonic microwave is obtained from the electrode of the microdisk laser under the period-two oscillation state. The plentifully dynamical states similar as semiconductor lasers subject to external optical injection are realized due to strong optical interaction between the two microdisks.

Dual-transverse-mode microsquare lasers with tunable wavelength interval

A dual-transverse-mode microsquare laser with a tunable wavelength interval is designed and realized by using a square-ring-patterned contact window. For a 30-μm-side-length microsquare laser with the square-ring width of 4 μm, the wavelength interval varies from 0.25 to 0.37 nm with the intensity ratio less than 2.5 dB as the injection current increases from 89 to 108 mA. Based on the dual-transverse-mode microsquare laser, the microwave signals with the frequencies of 30.56, 32.70, 35.12, and 39.51 GHz and the 3-dB bandwidths of 47, 53, 54, and 47 MHz are obtained at the injection currents of 90, 95, 100, and 105 mA, respectively.

The creep process of the domain switching in poly(vinylidene fluoride-trifluoroethylene) ferroelectric thin films

The polarization switching behavior in poly(vinylidene fluoride-trifluoroethylene) thin films is studied by using a pulse transient current method. The dependence of the domain switching current on the coercive electric field was investigated. The charging current around the coercive field was found to be limited by domain switching instead of the series resistor in the measurement circuit because of the slow polarization switching in the films. The domain-switching process was explained by a creep model wherein the two-dimension domain walls motion in the transverse direction dominates the polarization switching process.