Si Wang

Stability of phase locking in coupled semiconductor laser arrays

It is shown that amplitude phase coupling (as described by the linewidth enhancement factor α) leads to unstable phase locking in semiconductor laser arrays with evanescent coupling.

Dynamics of phase‐locked semiconductor laser arrays

Time‐dependent coupled mode theory is used to investigate the stability of phase‐locked semiconductor laser arrays. The output of individual array elements is dynamically unstable and exhibits large amplitude chaotic pulsations. The total output initially exhibits damped relaxation oscillations and then settles down to a quasi‐steady state characterized by small amplitude fluctuations. The theory predicts both the pulsation frequency and the phase lock‐in time of the array.

Concerted Rattling in CsAg5Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S(2) σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p-type thermoelectric material, CsAg5 Te3 , is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18u2005Wm(-1) u2009K(-1) ) and a high figure of merit of about 1.5 at 727u2005K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.

Picosecond optical properties of a grating surface emitting two-dimensional coherent laser array

The picosecond dynamical properties of a two-dimensional grating surface emitting (GSE) laser array are investigated by recording with a streak camera the optical emission under low duty cycle, 200-ns pulsed operation. The elements of the two-dimensional GSE arrays are index-guided ridge lasers. The array (a 10*2 array) consists of two groups of ten laterally coupled laser elements which are coupled longitudinally by an intervening grating, and gratings on either end of the two groups which provide the necessary feedback for the laser cavities. A variety of nonlinear dynamic behavior regimes, ranging from quiescent through periodic, to erratic is observed during the course of a single-drive current pulse. The transition from one regime to another may be due to transient heating during the pulse.<<ETX>>

Grain boundary scattering effects on mobilities in p-type polycrystalline SnSe

The extremely high ZTs of p-type SnSe single crystals have attracted much attention. However, due to the high cost of preparation, SnSe single crystals are difficult to be commercialized. On the other hand, the biggest challenge facing more cost-effective polycrystalline SnSe samples are their inferior electronic properties compared to single crystals. It has been proposed that the crystal orientation is responsible for the difference between the electronic properties of polycrystalline and single crystalline SnSe. To explore the role of the crystal orientation, we synthesized textured pure and Ag-doped polycrystalline SnSe and found that the electronic properties of our most highly oriented polycrystalline SnSe are still not higher than single crystals of SnSe oriented along the a-axis (the least favorable orientation). In this study, we compared the temperature-dependent mobility of Ag-doped polycrystalline samples with Ag-doped single crystals of SnSe. We found that grain boundary scattering is the dominant scattering mechanism in polycrystalline SnSe, and this mechanism is substantially absent in single crystals of SnSe. We conclude that grain boundary scattering, and not an averaging effect of the random grain distribution, is the major reason for the poor performance of polycrystalline SnSe compared to single crystals. Based on our results, improving the thermoelectric performance of polycrystalline SnSe will require identifying a synthesis process that minimizes grain boundary scattering.

Low temperature thermoelectric properties of p-type doped single-crystalline SnSe

SnSe single crystals have been widely studied lately as a result of their record high ZT and controversial low thermal conductivity. Much research has focused on the high-temperature properties of single crystals and polycrystalline SnSe, but few studies were carried out on the low-temperature properties of doped single-crystalline SnSe. To study the mechanism of the charge carrier and phonon scattering, and to eliminate the ambiguity of the high temperature thermal conductivity measurement, we performed low temperature transport characterization of Na-doped and Ag-doped single-crystalline SnSe by a longitudinal steady-state technique. The electronic transport property measurements suggest that Na is a more efficient p-type dopant in SnSe than Ag. In the thermal conductivity data, we observe pronounced dielectric peak around 10u2009K with magnitude dependent on the doping level. In the p-type doped samples, we found that our room temperature lattice thermal conductivities (>1.74u2009W m−1u2009K−1) are in general higher than those previously reported. Based on these findings, our study implies that the lattice thermal conductivity values of doped and pure single-crystalline SnSe were underestimated.SnSe single crystals have been widely studied lately as a result of their record high ZT and controversial low thermal conductivity. Much research has focused on the high-temperature properties of single crystals and polycrystalline SnSe, but few studies were carried out on the low-temperature properties of doped single-crystalline SnSe. To study the mechanism of the charge carrier and phonon scattering, and to eliminate the ambiguity of the high temperature thermal conductivity measurement, we performed low temperature transport characterization of Na-doped and Ag-doped single-crystalline SnSe by a longitudinal steady-state technique. The electronic transport property measurements suggest that Na is a more efficient p-type dopant in SnSe than Ag. In the thermal conductivity data, we observe pronounced dielectric peak around 10u2009K with magnitude dependent on the doping level. In the p-type doped samples, we found that our room temperature lattice thermal conductivities (>1.74u2009W m−1u2009K−1) are in general high...

Nonlinear dynamics of semiconductor laser arrays

The simulated output is shown of a 2-stripe index guided array and a 4-stripe gain guided array under constant pump conditions. The results of the partial differential equation (PDE) model for the strongly-index-guided array confirm the predictions of coupled-mode theory. The pulsations observed in laser arrays have several origins. One is the breakdown of phase locking due to amplitude-phase coupling. Another is the beating between multiple lateral modes which can drive the relaxation oscillations. A third mechanism is related to saturable absorption in the unpumped inter-element regions. We elucidate these mechanisms and present experimental results for index guided, antiguided, and gain guided arrays.

Facile room temperature solventless synthesis of high thermoelectric performance Ag2Se via a dissociative adsorption reaction

Simultaneous control of the stoichiometry, microstructure, and compositional homogeneity is a prerequisite for understanding the properties of Ag2Se. These are difficult to attain because of the highly mobile Ag+ ions above the superionic phase transition at 407 K. Here we report on a novel synthesis of well crystallized orthorhombic Ag2Se carried out at room temperature, which requires no expensive instrumentation, and yields a single-phase material in a very short time. Our facile reaction process is a self-sustaining room temperature synthesis driven by the dissociative adsorption of Se by Ag and promoted by stirring and intermittent grinding under ambient conditions. Systematic experimental and theoretical studies of chemical reactions between Ag and Q (Te, Se, and S) revealed that the reaction mechanism between Ag and Q is in line with the Hard Soft Acid Base (HSAB) scheme (rate order Ag2Te > Ag2Se > Ag2S). The low carrier concentration achieved ∼1018 cm−3 and the optimized weighted majority-to-minority carrier mobility ratio observed in the samples as corroborated by the state-of-the-art thermoelectric performance of ZT ∼1.2 at 390 K attest to the superiority of the synthesis route in yielding highly stoichiometric Ag2Se samples.