Flexible dispersion engineering using polymer patterning in nanophotonic waveguides.

We demonstrate the engineering of waveguide dispersion by lithographically patterning the polymer cladding on silicon nitride waveguide resonators. Both normal and anomalous dispersion, ranging from - 462 to 409 ps/nm/km, can be achieved for the same waveguide dimension within an integrated photonic chip. In the meantime, this simple process shows no impact on the waveguide loss and the quality factor of the waveguide resonators, offering flexibility in tailoring designable dispersion for a universal photonic platform. In addition, by adjusting the coverage ratio of cladding, relatively low dispersion (≈ - 130 ps/nm/km) is also demonstrated in the same waveguide resonator, yielding the potentials for zero-dispersive waveguide resonators by a proper coverage ratio of the polymer cladding.

Stability analysis of mode-coupling-assisted microcombs in normal dispersion.

We theoretically study the stability of mode-coupling-assisted frequency comb generation in normal-dispersion microresonators. With the aid of mode coupling, quantitative analysis of the modulational instability is explored in the parameter space of pump power and detuning. By exploring the coupled mode number, dispersion, and coupling strength in the normalized Lugiato-Lefever model, the modulational stability gain exists and yields extended spatial structures within the regime of eigenvalue bifurcations. Moreover, the dynamics and efficiency of microcombs are discussed, providing the accessibility of high-efficient, stable, and controllable combs. This work offers universal guidelines for operating mode-coupling-assisted combs in a normal-dispersion system.

Dual-Axis Alignment of Bulk Artificial Water Channels by Directional Water-Induced Self-Assembly.

Approaching single-crystal-like morphology has always been important in driving materials toward their optimal properties. With only orientational order, liquid crystal (LC) materials require dual-axis orientational control to optimize their structural order in the bulk phase. However, current external guiding fields such as electrical, magnetic, and mechanical guiding fields are less effective in aligning amphiphilic LCs. In this study, water is developed as an excellent structural stabilizer and orientation-directing agent of an amphiphilic discotic molecule (AD) in the water-induced self-assembly (WISA) process. Thermal analysis and structural characterization results show that water increases the stability and domain sizes of the hexagonal columnar (Colh) phase of the AD by co-assembling with the ADs to form bulk artificial water channels (AWCs). Moreover, through control over the nucleation conditions (degree of supercooling and location of nucleation), dual-axis alignment in both the planar and vertical growth of the AWCs is achieved by applying water as the guiding field in the directional WISA. With precise control over the hierarchical structures, the bulk AWC array of the AD delivers excellent salt rejection properties and water permeability. Considering that all the amphiphilic LCs have hydrophilic segments, these new roles of water in the WISA process could launch the further development of functional amphiphilic LCs by providing a dynamic interaction and a readily available guiding field.

Study of microcomb threshold power with coupling scaling.

We model the generation threshold and conversion efficiency of microcombs by scaling the cavity coupling. With the Lugiato-Lefever equation (LLE), quantitative analysis of threshold is established in the parameter space of pump power and coupling. Considering the large detuning and Kerr-induced phase shift, the threshold power is numerically solved with the minimum at over-coupling, in agreement with that from the traveling wave theory. Furthermore, the coupling dependence on microcomb generation is discussed, providing the accessibility of high-efficient, stable combs (≥ 40%) around the threshold. This work offers universal guidelines for the design of microcombs with low-power and high-efficient operation.

A facile energy-saving route of fabricating thermoelectric Sb2Te3-Te nanocomposites and nanosized Te.

A facile energy-saving route is developed for fabricating Sb2Te3-Te nanocomposites and nanosized Te powders. The fabrication route not only avoids using organic chemicals, but also keeps the energy consumption to a minimum. The fabrication procedure involves two steps. Energetic precursors of nanosized powders of Sb and Te are produced at room temperature followed by hot pressing at 400°C under 70 MPa for 1 h. The resulting Sb2Te3-Te nanocomposite exhibits enhanced power factor. The dimensionless figure of merit zT value of the Sb2Te3-Te nanocomposite is 0.29 at 475 K.

Enhanced thermoelectric properties of hydrothermally synthesized Bi0.88-x Zn x Sb0.12 nanoalloys below the semiconductor-semimetal transition temperature.

Bi0.88-x Zn x Sb0.12 alloys with x = 0.00, 0.05, 0.10, and 0.15 were prepared using hydrothermal synthesis in combination with evacuating-and-encapsulating sintering. The effects of partial Zn substitution for Bi and different sintering temperatures on the thermoelectric properties of Bi0.88-x Zn x Sb0.12 alloys were investigated between 25 K and 425 K. Both the electrical conductivity and absolute thermopower are enhanced for the set of alloys sintered at 250 °C. The maximum power factor of 57.60 µW cm-1 K-2 is attained for the x = 0.05 alloy sintered at 250 °C. As compared with Zn-free Bi0.88Sb0.12, both the total thermal conductivity and lattice component are reduced upon Zn doping. Bipolar conduction is observed in both electronic and thermal transport. The maximum zT of 0.47 is attained at 275 K for the x = 0.05 alloy sintered at 250 °C.

Optimization and Analysis of Thermoelectric Properties of Unfilled Co(1-x-y)Ni(x)Fe(y)Sb3 Synthesized via a Rapid Hydrothermal Procedure.

A series of nanostructured co-doped Co(1-x-y)Ni(x)Fe(y)Sb3 were fabricated using a rapid hydrothermal method at 170 °C for a duration of 12 h, followed by evacuated-and-encapsulated heating at 580 °C for a short period of 5 h. The resulting samples were characterized using powder X-ray diffraction, field emission scanning electron microscopy, bulk density, electronic and thermal transport measurements. The power factor of Co(1-x-y)Ni(x)Fe(y)Sb3 is significantly enhanced in the high-temperature region due to significant enhancement of the electrical conductivity and absolute value of thermopower. The latter arises from the onset of bipolar effect being shifted to higher temperatures as compared with the non-doped CoSb3. The room temperature thermal conductivity falls in the range between 1.22 and 1.67 W m(-1) K(-1) for Co(1-x-y)Ni(x)Fe(y)Sb3. The thermal conductivity of both the (x,y) = (0.14,10) and (0.14,12) samples is measured up to 600 K and found to decrease with increasing temperature. The thermal conductivity of the (0.14,10) sample goes down to ∼1.02 W m(-1) K(-1). As a result, zT = 0.68 is attained at 600 K. The lattice thermal conductivity is analyzed to gain insight into the contribution of various scattering processes that suppress the heat transfer through the phonons in Co(1-x-y)Ni(x)Fe(y)Sb3. The effect of the simultaneous presence of Co, Ni, and Fe elements on the electronic structure and transport properties of Co(1-x-y)Ni(x)Fe(y)Sb3 is described using the quantum mechanical tunneling theory of electron transmission among the potential barriers.