题 目：Strong Light-Matter Interactions in Halide Perovskite Materials 报告人：熊启华教授主持人：李志远教授时间：2018年3月28日10:30地 点：物理楼二楼第一会议室欢迎广大师生参加!物理与光电学院2018年3月26日附件：内容摘要： Halide perovskites have recently attracted tremendous attention due to their remarkable properties as optical gain materials, which have shown high performance in solar cells, light-emitting diodes, photodetectors and many other optoelectronic applications. Deeply rooted in their large oscillator strength, exciton binding energy and accessible synthesis approaches, halide perovskites have shown even broader horizon in optical and optoelectronic perspectives. In this talk, we will start with a historical overview of polariton as a quasiparticle involving phonon or exciton with strong light-matter interactions. Then, we will discuss how the halide perovskites offer new ground to study strong light-matter coupling at room temperature, leading to robust room-temperature new quasiparticle exciton polariton in a few selected all inorganic halide perovskites, demonstrated by experimental realization of room-temperature polariton lasing and spontaneous spatial/temporal coherence. Such realization of polariton BEC at room temperature further manifested the coherent manipulation of polariton condensate in 1D microwire cavity, suggesting ultrafast polaritonic applications. From a perspective point of view, our results suggest the possibility of investigating quantum fluids of light inside perovskite materials at room temperature. In a final recap, we will share our recent measurements of optical properties and strong light-matter coupling in 2D hybrid perovskites, which feature multi-quantum well structures exhibiting considerable promises in manipulating light-matter interaction in both weak and strong coupling regime.References:1. R. Su et al., “Room temperature one-dimensional polariton condensate propagation in lead halide perovskites”, Science Advances, DOI: 10.1126/sciadv.aau0244 2. K.B. Lin et al., “Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent”, Nature 562, 245-248 3. G.K. Long et al., “Spin control in reduced-dimensional chiral perovskites”, Nature Photonics 12, 528-533 4. R. Su et al., “Room temperature polariton lasing in all-inorganic perovskite”, Nano Lett. 17, 3982–3988 5. T. Thu Ha Do et al., “Optical study on intrinsic exciton states in a high-quality CH3NH3PbBr3 single crystal”, Phys. Rev. B 96, 075308 6. Q. Zhang, et al., “Room-temperature near-infrared high-Q perovskite whispering-gallery planar nanolasers”, Nano Lett. 14, 5995-6001 报告人简介： 熊启华是南洋理工大学数理科学学院和电气电子工程学院双聘教授。美国物理学会会士。1997年本科毕业于武汉大学物理系，2000年从上海应用物理研究所获得硕士学位，2006年于宾夕法尼亚州立大学获得博士学位，师从Peter C. Eklund教授。2006-2009年在哈佛大学Charles Lieber研究组从事博士后研究。2009年初获得新加坡国立研究基金研究员项目资助并于当年6月加入南洋理工大学，任南洋助理教授。2014年获得终身教职，2016年升正教授。熊启华教授的主要研究领域是以稳态和瞬态光谱学为主要实验手段，以光和物质相互作用为研究主题, 着重研究低维半导体纳米材料基于光子-声子-电子耦合作用的物理机制和量子调控。他在纳米光子学和表面等离子体学，激光制冷，以及二维半导体材料光学性质等一系列前沿课题做出了一系列有影响的工作。在《自然》及子刊, 《纳米通讯》《先进材料》等一系列国际知名杂志上发表了200多篇文章，并被世界知名杂志及大众媒体所报道，总引用次数超过9400次，H-因子57。其出色的研究获得了一些奖励和认可，比如新加坡物理学会纳米科技奖，新加坡国立研究基金NRF Investigatorship奖，和南洋理工大学南洋研究卓越奖等。 2018年4月起，担任美国光学学会旗舰杂志《Optics Express》副主编。2018年12月起，担任Wiley信息材料领域新创刊杂志《InfoMat》的副主编。
报告题目：Molecular orientation as key parameter in organic optoelectronics
关于芯片本身的散热是thermal design的一个更复杂的话题，涉及到基础科学，材料科学。它与系统散热是thermal design行业的两端。如今，bf586.com ，新加坡南洋科技大学的研究者在激光对固体进行制冷的研究上有了新进展。这一研究成果发表在了新的《自然》杂志上。
The process of cooling materials to cryogenic temperatures is often expensive and messy. One successful method is laser cooling, where photons interact with the atoms in some way to dampen their motion. While laser cooling of gases has been standard procedure for many years, solids are another issue entirely. Success has only come with a few specially prepared materials.
Having a laser annihilate something isn't usually associated with chilling anything down. But a new experiment reduced the temperature of a semiconductor by about 40°C using a laser. Jun Zhang, Dehui Li, Renjie Chen, and Qihua Xiong exploited a particular type of electronic excitation: when the photons interacted with this excitation, they canceled it out, damping the thermal fluctuations in the material.
The cooling of materials using light was first proposed in 1929 by P. Pringsheim, well before the advent of lasers, but technical difficulties prevented its implementation. The principles were successfully combined with magnetic traps in subsequent decades, leading to the 1997 Nobel Prize in physics. Today, optical cooling is widely used in a number of applications, including Bose-Einstein condensation and atomic clocks.
Laser cooling of gases transfers some of the kinetic energy of the atoms into photons they interact with. Successful laser cooling was achieved in glasses—solids without an ordered, coherent crystal structure—by embedding rare-earth atoms in the matrix. As with gases, the excitation of the rare-earth atoms produced the cooling. However, that method won't work for every solid.
For solids, the thermal motion of the atoms takes the form of phonons: vibrations moving through the material. Being quantum excitations, phonons behave like particles: they can collide and scatter. One way to optically cool solids, therefore, would be to "annihilate" the phonons with laser light.
The authors of the new study used cadmium sulphide (CdS), a material known as a group-II-VI semiconductor. Commonly used in digital electronics, semiconductors are insulators under normal conditions, but can be induced to conduct electricity when impurity atoms are added. Group-II-VI semiconductors host both strong phonons, and an additional type of particle-like excitation known as an exciton. Excitons are created through interactions between electrons and "holes" that the electrons left behind.
The researchers fabricated narrow strips of CdS, deposited on a substrate of silicon and silicon dioxide at room temperature. They used an optical-wavelength laser, tuned to the precise wavelength to interact with multiple modes of phonons in the semiconductor. This interaction acted resonantly, canceling the phonons out—which means the material cooled rapidly, exhibiting a nearly 40°C drop in temperature.
The phonons in this material depend on temperature, so if it was colder to begin with, the laser wavelength needed to be longer-corresponding to lower energy. The researchers tested this and, while the temperature drop was less (about 15°C), the cooling process was more efficient.
To make sure it was resonant interaction between phonons and photons, the researchers used different laser wavelengths, and found they heated the CdS instead. Since these excitation modes are present in all group-II-VI semiconductors, the cooling method could be applied to other materials as well. Whether other, more common semiconductor materials can be cooled in similar ways isn't clear, but this experiment is a big step in the direction of rapid refrigeration of solids.
Three decades after the first publications on efficient light-emitting diodes and solar cells, organic optoelectronics has become part of our everyday life, e.g. as displays for smartphones or television screens. Furthermore, owing to their unique features, like low-cost large-area processing or the compatibility with various kinds of substrates in almost arbitrary shape, organic semiconductors can lead to new kinds of applications. One of the remarkable differences to their inorganic counterparts is that the majority of molecular semiconductors exhibit orientational degrees of freedom due their anisotropic shape. The microscopic orientation of molecules in thin films has strong impact on macroscopic properties such as charge carrier transport and optical properties as well as on the efficiency of optoelectronic devices. However, even lead halide perovskite nanocrystals have recently been found to exhibit anisotropic light emission which is expected to influence LED properties.
This talk will discuss the driving forces for molecular orientation in neat films and guest-host systems and give examples for the influence of molecular orientation on optoelectronic properties in different types of structures and the consequences for device functioning. In the first place, the orientation of the optical transition dipole moments of emitter molecules in organic light-emitting diodes controls the light outcoupling efficiency of these devices. Secondly, we discuss interfacial polarization caused by partial alignment of the permanent dipole moments of polar molecules. However, while interfacial polarization is well investigated in neat materials, there is a lack of studies evaluating the behavior of two-component mixed systems. Hence, we have investigated guest-host systems with varying concentrations of a polar species in a non-polar matrix. Using dipolar doping we could, on the one hand, achieve a better understanding of molecular interactions leading to a net orientation of the permanent dipole moments. On the other hand, we have utilized this approach to pinpoint molecular orientation of Iridium emitter complexes.