Asymmetric light diffraction of two-dimensional electromagnetically induced grating with PT symmetry in asymmetric double quantum wells.

An asymmetric double semiconductor quantum well is proposed to realize two-dimensional parity-time (PT) symmetry and an electromagnetically induced grating. In such a nontrivial grating with PT symmetry, the incident probe photons can be diffracted to selected angles depending on the spatial relationship of the real and imaginary parts of the refractive index. Such results are due to the interference mechanism between the amplitude and phase of the grating and can be manipulated by the probe detuning, modulation amplitudes of the standing wave fields, and interaction length of the medium. Such a system may lead to new approaches of observing PT-symmetry-related phenomena and has potential applications in photoelectric devices requiring asymmetric light transport using semiconductor quantum wells.

Research Progress on the Microstructure Evolution Mechanisms of Al-Mg Alloys by Severe Plastic Deformation.

Al-Mg alloys are widely used as important engineering structural materials in aerospace engineering, transportation systems, and structural constructions due to their low density, high specific strength, corrosion resistance, welding capability, fatigue strength, and cost-effectiveness. However, the conventional Al-Mg alloys can no longer fully satisfy the demands of practical production due to difficulties caused by many defects. The high strength of Al-Mg alloys as non-heat treatment precipitation-strengthened alloys is achieved primarily by solid solution strengthening along with work hardening rather than precipitation strengthening. Therefore, severe plastic deformation (SPD) techniques can be often used to produce ultrafine-grained structures to fabricate ultra-high strength aluminum alloys. However, this approach often achieves the strengthening of material at the cost of reduced ductility. This paper comprehensively summarizes the various approaches of ultrafine/nanocrystalline materials for enhancing their plasticity, elaborates on the creation of a bimodal microstructure within the alloy, and discusses the formation of a nanotwin microstructure within the alloy and the incorporation of dispersed nanoparticles. The mechanisms underlying both the strengthening and toughening during large plastic deformation in aluminum alloys are summarized, and the future research direction of high-performance ultrafine crystalline and nanocrystalline Al-Mg aluminum alloys is prospected.

The Main Failure Modes of Hot-Work Die Steel and the Development Status of Traditional Strengthening Methods and Nano-Strengthening Technology.

As an important part of die steels, hot-work die steels are mainly used to manufacture molds made of solid metal or high-temperature liquid metal from heating to recrystallization temperature. In view of the requirements for mechanical properties and service life for hot-work die steel, it is conducive to improve the thermal fatigue resistance, wear resistance, and oxidation resistance of hot work die steel. In this review, the main failure modes of hot-work die steel were analyzed. Four traditional methods of strengthening and toughening die steel were summarized, including optimizing alloying elements, electroslag remelting, increasing the forging ratio, and heat treatment process enhancement. A new nano-strengthening method was introduced that aimed to refine the microstructure of hot-work abrasive steel and improve its service performance by adding nanoparticles into molten steel to achieve uniform dispersion. This review provides an overview to improve the service performance and service life of hot work die steel.

The Role of Lithium in the Aging Precipitation Process of Al-Zn-Mg-Cu Alloys and Its Effect on the Properties.

It is well known that the development of lightweight alloys with improved comprehensive performance and application value are the future development directions for the ultra-high-strength 7xxx series Al-Zn-Mg-Cu alloys used in the aircraft field. As the lightest metal element in nature, lithium (Li) has outstanding advantages in reducing the density and increasing the elastic modulus in aluminum alloys, so Al-Zn-Mg-Cu alloys containing Li have gained widespread attention. Furthermore, since the Al-Zn-Mg-Cu alloy is usually strengthened by aging treatment, it is crucial to understand how Li addition affects its aging precipitation process. As such, in this article, the effects and mechanism of Li on the aging precipitation behavior and the impact of Li content on the aging precipitation phase of Al-Zn-Mg-Cu alloys are briefly reviewed, and the influence of Li on the service properties, including mechanical properties, wear resistance, and fatigue resistance, of Al-Zn-Mg-Cu alloys are explained. In addition, the corresponding development prospects and challenges of the Al-Zn-Mg-Cu-Li alloy are also proposed. This review is helpful to further understand the role of Li in Al-Zn-Mg-Cu alloys and provides a reference for the development of high-strength aluminum alloys containing Li with good comprehensive properties.

Development Trend in Composition Optimization, Microstructure Manipulation, and Strengthening Methods of Die Steels under Lightweight and Integrated Die Casting.

In the general environment of lightweight automobiles, the integrated die-casting technology proposed by Tesla has become the general mode to better achieve weight reduction in automobiles. The die-casting mold required by integrated die-casting technology has the characteristics of large scale and complexity. Hence, higher requirements are put forward for the comprehensive performance of the die steel. Despite the stagnation in the progress of conventional strengthening methods, enhancing the performance of die steel has become increasingly challenging. Indeed, it necessitates exploring novel die steel and optimizing heat treatment and reinforcement technologies. This article summarizes and analyzes the development status of die steel and corresponding heat treatment and microstructure manipulation as well as strengthening methods and elaborates on an excellent nano-strengthening technology. Furthermore, this review will aid researchers in establishing a comprehensive understanding of the development status of die steel and the processes utilized for its strengthening. It will also assist them in developing die steel with improved comprehensive performance to meet the high demand for mold steel in the integrated die-casting technology of the new era.

The Synthesis, Structure, Morphology Characterizations and Evolution Mechanisms of Nanosized Titanium Carbides and Their Further Applications.

It is widely known that the special performances and extensive applications of the nanoscale materials are determined by their as-synthesized structures, especially their growth sizes and morphologies. Hereinto, titanium carbides, which show brilliant comprehensive properties, have attracted considerable attention from researchers. How to give full play to their potentials in the light-weight manufacture, microwave absorption, electromagnetic protection, energy conversion and catalyst areas has been widely studied. In this summarized article, the synthesis methods and mechanisms, corresponding growth morphologies of titanium carbides and their further applications were briefly reviewed and analyzed according to their different morphological dimensions, including one-dimensional nanostructures, two-dimensional nanosheets and three-dimensional nanoparticles. It is believed that through the investigation of the crystal structures, synthesis methods, growth mechanisms, and morphology characterizations of those titanium carbides, new lights could be shed on the regulation and control of the ceramic phase specific morphologies to meet with their excellent properties and applications. In addition, the corresponding development prospects and challenges of titanium carbides with various growth morphologies were also summarized.

Parity-time symmetry in coherent asymmetric double quantum wells.

A coherently prepared asymmetric double semiconductor quantum well (QW) is proposed to realize parity-time (PT) symmetry. By appropriately tuning the laser fields and the pertinent QW parameters, PT-symmetric optical potentials are obtained by three different methods. Such a coherent QW system is reconfigurable and controllable, and it can generate new approaches of theoretically and experimentally studying PT-symmetric phenomena.

Enhancing Third- and Fifth-Order Nonlinearity via Tunneling in Multiple Quantum Dots.

The nonlinearity of semiconductor quantum dots under the condition of low light levels has many important applications. In this study, linear absorption, self-Kerr nonlinearity, fifth-order nonlinearity and cross-Kerr nonlinearity of multiple quantum dots, which are coupled by multiple tunneling, are investigated by using the probability amplitude method. It is found that the linear and nonlinear properties of multiple quantum dots can be modified by the tunneling intensity and energy splitting of the system. Most importantly, it is possible to realize enhanced self-Kerr nonlinearity, fifth-order nonlinearity and cross-Kerr nonlinearity with low linear absorption by choosing suitable parameters for the multiple quantum dots. These results have many potential applications in nonlinear optics and quantum information devices using semiconductor quantum dots.

Extracting more light for vertical emission: high power continuous wave operation of 1.3-µm quantum-dot photonic-crystal surface-emitting laser based on a flat band.

For long distance optical interconnects, 1.3-µm surface-emitting lasers are key devices. However, the low output power of several milliwatts limits their application. In this study, by introducing a two-dimensional photonic-crystal and using InAs quantum dots as active materials, a continuous-wave, 13.3-mW output power, 1.3-µm wavelength, room-temperature surface-emitting laser is achieved. In addition, such a device can be operated at high temperatures of up to 90 °C. The enhanced output power results from the flat band structure of the photonic crystal and an extra feedback mechanism. Surface emission is realized by photonic crystal diffraction and thus the distributed Bragg reflector is eliminated. The proposed device provides a means to overcome the limitations of low-power 1.3-µm surface-emitting lasers and increase the number of applications thereof.

Microstructures and Compressive Properties of Al Matrix Composites Reinforced with Bimodal Hybrid In-Situ Nano-/Micro-Sized TiC Particles.

Bimodal hybrid in-situ nano-/micro-size TiC/Al composites were prepared with combustion synthesis of Al-Ti-C system and hot press consolidation. Attempt was made to obtain in-situ bimodal-size TiC particle reinforced dense Al matrix composites by using different carbon sources in the reaction process of hot pressing forming. Microstructure showed that the obtained composites exhibited reasonable bimodal-sized TiC distribution in the matrix and low porosity. With the increasing of the carbon nano tube (CNT) content from 0 to 100 wt. %, the average size of the TiC particles decreases and the compressive strength of the composite increase; while the fracture strain increases first and then decreases. The compressive properties of the bimodal-sized TiC/Al composites, especially the bimodal-sized composite synthesized by Al-Ti-C with 50 wt. % CNTs as carbon source, were improved compared with the composites reinforced with single sized TiC. The strengthening mechanism of the in-situ bimodal-sized particle reinforced aluminum matrix composites was revealed.

Creation and Transfer of Coherence via Technique of Stimulated Raman Adiabatic Passage in Triple Quantum Dots.

We propose a scheme for creation and transfer of coherence among ground state and indirect exciton states of triple quantum dots via the technique of stimulated Raman adiabatic passage. Compared with the traditional stimulated Raman adiabatic passage, the Stokes laser pulse is replaced by the tunneling pulse, which can be controlled by the externally applied voltages. By varying the amplitudes and sequences of the pump and tunneling pulses, a complete coherence transfer or an equal coherence distribution among multiple states can be obtained. The investigations can provide further insight for the experimental development of controllable coherence transfer in semiconductor structure and may have potential applications in quantum information processing.