Near‐Field Energy Transfer Using Nanoemitters For Optoelectronics

Effective utilization of excitation energy in nanoemitters requires control of exciton flow at the nanoscale. This can be readily achieved by exploiting near‐field nonradiative energy transfer mechanisms such as dipole‐dipole coupling (i.e., Förster resonance energy transfer) and simultaneous two‐way electron transfer via exchange interaction (i.e., Dexter energy transfer). In this feature article, we review nonradiative energy transfer processes between emerging nanoemitters and exciton scavengers. To this end, we highlight the potential of colloidal semiconductor nanocrystals, organic semiconductors, and two‐dimensional materials as efficient exciton scavengers for light harvesting and generation in optoelectronic applications. We present and discuss unprecedented exciton transfer in nanoemitter–nanostructured semiconductor composites enabled by strong light–matter interactions. We elucidate remarkably strong nonradiative energy transfer in self‐assembling atomically flat colloidal nanoplatelets. In addition, we underscore the promise of organic semiconductor–nanocrystal hybrids for spin‐triplet exciton harvesting via Dexter energy transfer. These efficient exciton transferring hybrids will empower desired optoelectronic properties such as long‐range exciton diffusion, ultrafast multiexciton harvesting, and efficient photon upconversion, leading to the development of excitonic optoelectronic devices such as exciton‐driven light‐emitting diodes, lasers, and photodetectors.     

Wannier-Mott excitons in semiconductors with a superlattice

The effect of the motion of a Wannier-Mott exciton in semiconductors with a superlattice formed by heterojunctions on the exciton binding energy and wave function is analyzed. This effect arises as a result of the fact that the dispersion laws of the electron and hole that form an exciton in a superlattice differ from the quadratic law. The investigated one-dimensional superlattice consists of alternating semiconductor layers with different energy positions of the conduction and valence bands, i.e., with one-dimensional wells and barriers. The exciton state in a superlattice consisting of quantum dots is analyzed. It is demonstrated that the closer the electron and hole effective masses, the greater the dependence of the binding energy on the exciton quasi-momentum. The possibility of replacing the tunneling excitation transfer between superlattice cells with the dipole-dipole one at certain exciton quasi-wave vector values is investigated.

     

Exciton states in semiconductor spherical nanostructures

The results of theoretical studies of the energy spectra of excitons moving in semiconductor spherical quantum dots are described. The contributions of the kinetic electron and hole energies, the energy of the Coulomb interaction between an electron and hole, and the energy of the polarization interaction between them to the energy spectrum of an exciton in a quantum dot with a spherical (quantum dot)-(insulator medium) interface is analyzed.     

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass μ = μ(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E _ex(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a _ex, the exciton binding energy E _ex(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.     

Excitons and Electron–Hole Liquid State in 2D γ‐Phase Group‐IV Monochalcogenides

Different dispersion near the electronic band edge of a semiconductor can have great influence on its transport, thermoelectric, and optical properties. Using first‐principles calculations, it is demonstrated that a new phase of group‐IV monochalcogenides (γ‐MX, M = Ge, Sn; X = S, Se, or Te) can be stabilized in monolayer limit. γ‐MXs are shown to possess a unique band dispersion—that is, camel's back like structure—in the top valence band. The band nesting effect near the camel's back region induces a large excitonic absorbance and significantly different exciton behaviors from other 2D materials. Importantly, the small effective mass and the indirect characteristics of lowest‐energy exciton render it advantageous for the generation of electron–hole liquid state. After careful evaluation of the electron–hole dissociation temperature and the Mott critical density, it is predicted that a high‐temperature exciton gas to electron–hole liquid phase transition can be achieved in these materials with a low excitation power density. The findings open up new opportunities for both the fundamental research on exciton physics and design of excitonic devices based on 2D materials with distinct band dispersion.     

Influence of the stark effect on the resonance excitation transfer between quantum dots

The resonance energy transfer between exciton states in a system comprised of two semiconductor quantum dots is studied theoretically. A model Hamiltonian is constructed to describe the influence of the laser pulse, Coulomb interaction, the static Stark effect, and the relaxation of exciton states on the dynamics of the system. Specific calculations of the efficiency of energy transfer under different excitation conditions and different positions of energy levels are exemplified. It is shown that the transfer process can be controlled by shifting the levels in a constant electric field.     

The influence of the triplet exciton and charge transfer state energy alignment on organic magnetoresistance

Recently, it was discovered that the current through an organic semiconductor, sandwiched between two non-magnetic electrodes, can be changed significantly (up to 25%) by applying a small (a few millitesla) magnetic field. At present, the microscopic mechanisms underlying this so-called organic magnetoresistance (OMAR) are intensively being debated. One of the mechanisms which can successfully describe the magnetic field effects on the current in pristine organic semiconductor devices uses the reactions of triplet excitons and polarons. Here, we present a proof of concept study in which we tune these interactions in the device by deliberately doping our devices with fullerene, creating additional charge transfer states (CTS). By engineering devices with different energetic alignments of the CTS and triplet exciton, we can influence the triplet exciton density in the device. We correlate pronounced changes in the magnetic field effect magnitude and lineshape to the energy of the CTS with respect to the triplet exciton.     

Transmission of 2-ps optical pulses at 1550 nm over 40-km standardfiber using midspan optical phase conjugation in semiconductor opticalamplifiers

We demonstrate transmission of 2-ps optical pulses at 1550 nm over 40 km of standard fiber by employing midspan optical phase conjugation in semiconductor optical amplifiers (SOAs). The second-order group-velocity dispersion of the fiber is completely compensated and the third-order dispersion becomes a major transmission limitation. This experiment shows that the midspan optical phase conjugation system using SOAs is applicable to ultrahigh bit rates greater than 100 Gb/s     

Effect of surface on the excitonic characteristics of semiconductors

The effect of surface treatments on the main characteristics of excitons in the subsurface region of semiconductors (for GaAs), as well as the spatial distribution of main characteristics of excitons (for CdS), was studied. An analysis of experimental data showed that the deposition of insulator layers with a lower dielectric constant on the surface of the semiconductor resulted in an enhancement of the exciton-phonon interaction and an increase in the exciton binding energy. The appearance of the surface layer with a higher defect concentration increasing after some surface treatments results in the lowering of the exciton binding energy in the subsurface region and also in the weakening of the exciton-phonon interaction.     

半导体量子点的生长机理及特性

阐述了半导体量子点自组织生长方法的基本原理,并介绍了半导体量子点的一些特性,如激子束缚能、载流子动力学、排列规则和光学性质。     

Photomultiplication in Disordered Unipolar Organic Materials

Photomultiplication in conventional inorganic semiconductors has been known and used for decades, the underlying mechanism being multiplication by impact ionization triggered by hot carriers. Since neither carrier heating by an electric field nor avalanche multiplication are possible in strongly disordered organic solids, charge multiplication seems to be highly unlikely in these materials. However, here the photomultiplication observed in the bulk of a unipolar disordered organic semiconductor is reported. The proportion of extracted carriers to incident photons is experimentally determined to be in excess of 3000 % in a single‐layer device of the air‐stable, n‐type organic semiconductor F₁₆CuPc (Pc: phthalocyanine). This effect is explained in terms of exciton quenching by localized charges, the subsequent promotion of these detrapped charges to the high‐mobility energy band of the density‐of‐states (DOS) distribution, and subsequent slow equilibration within this broad intrinsic DOS. Such a mechanism allows multiple replenishment of the optically released charge by mobile carriers injected from an Ohmic electrode. Also shown is photomultiplication in double‐layer devices composed of layers of donor and acceptor small‐molecule materials. This result implies that, apart from exciton dissociation at a donor/acceptor interface, exciton energy transfer to trapped carriers is a complementary photoconductivity process in organic solar cells. This new insight paves the way to cheap, highly efficient organic photodetectors on flexible substrates for numerous applications.     

基于卤化物钙钛矿的法布里-珀罗微腔中激子极化激元（特邀）

当激子与腔光子间的相互作用强于激子和腔光子的衰减时,激子能级与腔模之间产生强耦合,形成的准粒子被称为激子极化激元。激子极化激元有效质量小,同时具有较强的非线性,在慢光和低功耗发光器件等方面具有巨大的应用前景。传统Ⅲ-Ⅴ族无机半导体材料激子束缚能较弱,而有机半导体材料非线性系数较小等问题限制着室温条件下激子极化激元的应用。卤化物钙钛矿材料具有高吸收系数、长扩散长度、高缺陷容忍度以及低非辐射复合率等一系列优异的光电性质,并且具有高的激子束缚能和振子强度,成为研究光与物质强相互作用的理想材料。文中从卤化物钙钛矿结构和法布里-珀罗（Fabry-Pérot, F-P）微腔类型两方面介绍了近年来卤化物钙钛矿与F-P微腔强耦合在激子极化激元方面的研究进展。首先回顾了极化激元的研究背景和卤化物钙钛矿的基本光电特性,其次介绍了三维钙钛矿和二维层状钙钛矿各自的特点以及与F-P微腔强耦合的相关研究,随后对钙钛矿的自构型和非自构型F-P微腔激子极化激元的调控与相关应用进行了讨论,最后总结和展望了卤化物钙钛矿激子极化激元面临的挑战以及未来研究方向。     

Polarization independent optical phase conjugation with pump-signalfiltering in a monolithically integrated Mach-Zehnder interferometersemiconductor optical amplifier configuration

Demonstration of polarization independent optical phase conjugation by four-wave mixing in a monolithic Mach-Zehnder interferometric semiconductor optical amplifier (SOA) configuration. An integrated filtering mechanism for the pump-signal enhances the conjugate-to-pump ratio by 15 dB compared to the single SOA case     

Exciton–Exciton Annihilation in Mixed‐Phase Polyfluorene Films

Singlet–singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of β‐phase chains using time‐resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time‐independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time‐dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the β‐phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low‐energy sites. The annihilation rate in the mixed‐phase films increases with increasing fraction of β‐phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersedβ‐phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line‐dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy andβ‐phase chromophores are intimately mixed rather than clustered or phase‐separated.     

Nonresonance Phase Conjugation of Light in Optically Pumped ZnO Films

Semiconductors - The possibility of the nonresonance phase conjugation of light occurring in an excited semiconductor medium is shown theoretically and experimentally. In epitaxial ZnO films pumped...     

缀饰激子在半导体微腔中的辐射

本文建立了半导体微腔的缀饰激子模型,在ＶＣＳＥＬ器件量子阱中的激子首先通过内电磁场与腔耦合,形成缀饰态。而后作为多粒子过程,缀饰激子与腔内真空场耦合产生辐射。通过ＱＥＤ方法,我们得到偶极子辐射密度方程和系统能量衰 变方程。从方程解的讨论中,我们得到超辐射和偶极子微腔方向 效应的结果,同时预言民当内场耦合足够强时,缀 激子可以直接辐射到一个很的激光模中。     

Role of Cooper pairs for the generation of entangled photon pairs from single quantum dots

Generation of entangled photon pairs from semiconductor quantum dots (QDs) is highly desirable for realizing practical solid-state photon sources for quantum information processing and quantum cryptography. However, the energy splitting of exciton states in QDs almost prevent the generation of entangled photon pairs. This paper discusses the new possibility with the injection of electron as well as hole Cooper pairs into QDs.     

Theoretical investigation on magnetic field effect in organic devices with asymmetrical molecules

A mechanism of organic magnetic field effect (OMFE) based on Lorentz effect in organic light-emitting devices with asymmetrical molecules is suggested and the magnetoconductance (MC) value is calculated. By considering the collision of a positive and a negative charged polaron and exciton formation in the organic layer through a non-adiabatic dynamic process, it is found that the exciton yield can be changed by applying a magnetic field due to the Lorentz effect on the moving polarons’ phase factors. By calculating the current through the device and the MC, we obtain that the calculated MC is well consistent to some experimental observations. It is also found that the MC value is sensitive to the asymmetrical structure and the electron–phonon (e–ph) coupling of the organic material, which explains why magnetic effect in an organic semiconductor is much more apparent than its inorganic counterpart.     

相位共轭反馈半导体激光器混沌动力学

本文研究了相位共轭反馈对半导体激光器的动力学行为的影响。结果表明，在弱反馈条件下，系统处于稳态;增加反馈量，系统由周期经周期三、周期四进入混沌。当相位共轭反馈量一定时，通过改变注入电流的大小，可以达到所期待的周期或混沌状态。     

Renormalization of energy spectrum of quantum dots under vibrational resonance conditions

The problem of modification of the energy spectrum of electronic and phonon excitations in semiconductor quantum dots (QDs) under conditions of vibrational resonance is considered. Analytical expressions for the energy of polaron-like states in QDs in the form of a sphere or rectangular parallelepiped, taking into account the size dependence of the electron-phonon interaction, are derived. Experimental data on renormalization of the lowest exciton states in QDs based on CuCl in a NaCl matrix have been obtained with the use of two-photon secondary emission resonance spectroscopy. Comparison with the calculated results demonstrates quantitative agreement indicating the adequacy of the employed theoretical model. It is established that the vibrational resonance strongly modifies the energy spectrum of small QDs.