20 research outputs found
Effect of Size and Shape on Electronic and Optical Properties of CdSe Quantum Dots
In this paper, we used the 8-band kp model with valence force field
considerations to investigate the effect of size and shape on electronic and
optical properties of cadmium selenide quantum dots. Major factors related to
their properties including band mixing probabilities, spatial charge
distributions, transition matrix elements and Fermi factors were studied.
Volumetrically larger CdSe dots were found to have smaller band-gaps but higher
transition matrix elements and Fermi factors. The maximum optical gain for dots
was observed to have an initially positive and then negative correlation with
their real-space size as a result of combined effects of various factors. For
the shape effects, cubic dots were found to have smaller band-gaps, Fermi
factors and transition matrix elements than spherical dots due to higher level
of asymmetry and different surface effects. Consequently, cubic dots have lower
emission energy, smaller amplification. The occurrence of near E1-H1 transition
broadens the gain spectrum of cubic dots. Cubic and spherical dots are both
proven to be promising candidates for optical devices under visible range. We
have demonstrated that size and shape change could both effectively alter the
properties of quantum dots and therefore recommend consideration of both when
optimizing the performance for any desired application.Comment: Published in Optik - International Journal for Light and Electron
Optics (8 pages, 10 figures), 201
Terahertz spin dynamics in rare-earth orthoferrites
Recent interest in developing fast spintronic devices and laser-controllable
magnetic solids has sparked tremendous experimental and theoretical efforts to
understand and manipulate ultrafast dynamics in materials. Studies of spin
dynamics in the terahertz (THz) frequency range are particularly important for
elucidating microscopic pathways toward novel device functionalities. Here, we
review THz phenomena related to spin dynamics in rare-earth orthoferrites, a
class of materials promising for antiferromagnetic spintronics. We expand this
topic into a description of four key elements. (1) We start by describing THz
spectroscopy of spin excitations for probing magnetic phase transitions in
thermal equilibrium. While acoustic magnons are useful indicators of spin
reorientation transitions, electromagnons that arise from dynamic
magnetoelectric couplings serve as a signature of inversion-symmetry-breaking
phases at low temperatures. (2) We then review the strong laser driving
scenario, where the system is excited far from equilibrium and thereby subject
to modifications to the free energy landscape. Microscopic pathways for
ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore,
we review a variety of protocols to manipulate coherent THz magnons in time and
space, which are useful capabilities for antiferromagnetic spintronic
applications. (4) Finally, new insights on the connection between dynamic
magnetic coupling in condensed matter and the Dicke superradiant phase
transition in quantum optics are provided. By presenting a review on an array
of THz spin phenomena occurring in a single class of materials, we hope to
trigger interdisciplinary efforts that actively seek connections between
subfields of spintronics, which will facilitate the invention of new protocols
of active spin control and quantum phase engineering
Phorbol ester induced ex vivo expansion of rigorously-defined phenotypic but not functional human cord blood hematopoietic stem cells: a cautionary tale demonstrating that phenotype does not always recapitulate stem cell function
The Remaining Buddhist Architecture in Fu’an, the Core Hinterland of the Changxi River Basin
The Changxi River Basin is a small root-like watershed, surrounded by mountains on three sides and facing the sea to the southeast. It is located on the border between Fujian and Zhejiang on the southeast coast of China. The area gave rise to the Changxi Culture that began in the Sui and Tang Dynasties and flourished in the Song Dynasty. Buddhism in the Changxi Basin was introduced no later than the 9th century. As the core hinterland of the Changxi Basin, Fu’an has always been an important center for Buddhism in Eastern Fujian. It reached its peak in the 10th to 13th centuries during the Song Dynasty. This article conducts a comprehensive investigation and study of the existing Buddhist temple sites and relics in Fu’an. It highlights these structures’ single-bay pattern of construction, based on rectangular plans in which the longitudinal axis extends along the plan’s direction of depth. This is a pattern rarely seen in the history of Chinese Buddhist architecture. The paper also summarizes a common element in these temples, their petal-shaped corrugated stone pillars which are divided into eight segments. Lastly, it illustrates the evolution of the temples in the Changxi River Basin from single-bay layouts to those with widths of multiple bays and indicates the unique status and associated values of single-bay Buddhist temples in the history of southern Buddhist architecture. The study examines new local findings and ideas for the study of Chinese Buddhist architectural history, providing academic support for the protection and research of Buddhist architectural heritage in Southeast China
Numerical simulations of transverse liquid jet to a supersonic crossflow using a pure two-fluid model
A pure two-fluid model was used for investigating transverse liquid jet to a supersonic crossflow. The well-posedness problem of the droplet phase governing equations was solved by applying an equation of state in the kinetic theory. A k-ε-k p turbulence model was used to simulate the turbulent compressible multiphase flow. Separation of boundary layer in front of the liquid jet was predicted with a separation shock induced. A bow shock was found to interact with the separation shock in the simulation result, and the adjustment of shock structure caused by the interaction described the whipping phenomena. The predicted penetration height showed good agreement with the empirical correlations. In addition, the turbulent kinetic energies of both the gas and droplet phases were presented for comparison, and effects of the jet-to-air momentum flux ratio and droplet diameter on the penetration height were also examined in this work
Low-Photon Counts Coherent Modulation Imaging via Generalized Alternating Projection Algorithm
Phase contrast imaging is advantageous for mitigating radiation damage to samples, such as biological specimens. For imaging at nanometer or atomic resolution, the required flux on samples increases dramatically and can easily exceed the sample damage threshold. Coherent modulation imaging (CMI) can provide quantitative absorption and phase images of samples at diffraction-limited resolution with fast convergence. When used for radiation-sensitive samples, CMI experiments need to be conducted under low illumination flux for high resolution. Here, an algorithmic framework is proposed for CMI involving generalized alternating projection and total variation constraint. A five-to-ten-fold lower photon requirement can be achieved for near-field or far-field experiment dataset. The work would make CMI more applicable to the dynamics study of radiation-sensitive samples
Structural Topology Optimization of Reflective Mirror Based on Objective of Wavefront Aberration
Due to the increasing requirements for the imaging quality of optical systems, the design method for optical–mechanical structures has become a research hotspot in recent decades. To improve the imaging performance of the reflective system, it is often necessary to increase the aperture of the mirror. To meet the imaging quality and lightweightedness requirements of the mirror, the topology optimization method aiming at the minimization of wavefront aberration is proposed. The optical–mechanical coupling relationship is established by ray tracing of the deformed mirror surface fitted by orthogonal bases. The topology optimization model is established by the solid isotropic material with penalization model (SIMP). Additionally, the adjoint method is used to analyze the sensitivity of the objective and the constraints. To illustrate the rationality and effectiveness of the method, the mirrors of the Cassegrain system have been optimized under the action of gravity with the objective of the weighted sum of squares of wavefront aberration coefficients under the constraints of the mass of the design domain, the rigid body displacement of the mirror surface, and the residual of deformation fitting. The results show that the proposed method can effectively improve imaging performance under the condition of satisfying the constraints. In addition, the optimization method with wavefront aberration as the objective is a concrete application of the idea of opto-mechanical integration, which can improve optical performance more directly and effectively
Structural Topology Optimization of Reflective Mirror Based on Objective of Wavefront Aberration
Due to the increasing requirements for the imaging quality of optical systems, the design method for optical–mechanical structures has become a research hotspot in recent decades. To improve the imaging performance of the reflective system, it is often necessary to increase the aperture of the mirror. To meet the imaging quality and lightweightedness requirements of the mirror, the topology optimization method aiming at the minimization of wavefront aberration is proposed. The optical–mechanical coupling relationship is established by ray tracing of the deformed mirror surface fitted by orthogonal bases. The topology optimization model is established by the solid isotropic material with penalization model (SIMP). Additionally, the adjoint method is used to analyze the sensitivity of the objective and the constraints. To illustrate the rationality and effectiveness of the method, the mirrors of the Cassegrain system have been optimized under the action of gravity with the objective of the weighted sum of squares of wavefront aberration coefficients under the constraints of the mass of the design domain, the rigid body displacement of the mirror surface, and the residual of deformation fitting. The results show that the proposed method can effectively improve imaging performance under the condition of satisfying the constraints. In addition, the optimization method with wavefront aberration as the objective is a concrete application of the idea of opto-mechanical integration, which can improve optical performance more directly and effectively
Deep learning-enabled prediction of 2D material breakdown
10.1088/1361-6528/abd655NANOTECHNOLOGY322